Comparative Pathology Update


This webpage is dedicated to a comparison of naturally occurring equine and human diseases where relevant.  The database is obtained from a weekly biopsy conference presented by veterinary residents and staff at University of California at Davis School of Veterinary Medicine.  Cases outlined in blue are briefly  described below and a representative histopathology slide(s) is provided as a link.






E-2      8/24/01     spinal cord         wallerian degeneration         Equine    "Wobbler"horse

E-95    3/14/03   skin             vasculitis/purpura         Equine  American Paint     1 yr

E-10    5/5/00    skin             photoactivation vasculitis Equine  Thoroughbred       15 yr

E-1     8/24/01   mandible         osteodystrophy fibrosa     Equine  Thoroughbred       3 mo  bran disease or bighead       

E-10    6/13/03     placenta            placentitis/vasculitis/?lepto  Equine    fetus                 1 mo

E-11    2/23/01   endometrium      endometritis               Equine  Quarter horse      20 yr

E-12     7/27/01     femur               osteochondrosis                Equine    Quarter horse         yearling

E-13     7/13/01     gingiva             ossifying fibroma              Equine    Thoroughbred          2 mo

E-14    7/6/01 lung                hyaline membrane disease   Equine  Foal               48 hr

E-15     7/6/01      lung,small bowel,   infarction/ DIC/clostridia     Equine    Quarter horse         48 hr foal

E-16     7/6/01      liver               hemolytic disease              Equine    Foal                  48 hr

E-17     6/15/01     placenta            placentitis/yeast              Equine    Thoroughbred          mare

E-18     6/8/01      colon               aganglia/Lethal White Syndrome Equine    Amerilcan Paint        neonatal

E-19     6/8/01      cerebrum            malacia/yellow star thislte    Equine    Appalloosa              3 yr

E-20     12/15/00    brain               malacia/ toxic?                Equine                          13 yr

E-21     12/15/00    skin                folliculitis/dermatophytosis   Equine

E-22    1/21/00   lung             ARDS                       Equine   Arabian foal

E-23     1/21/00     penis               habranema                      Equine

E-25     10/8/99     spinal cord         herpes vasculitis(EHV-1)       Equine                                 from Marty

E-27     3/2/01      max-sinus           hematoma                       Equine

E-28     2/2/01      skin                fungal granuloma               Equine

E-29     5/12/00     tongue              squamous cell ca               Equine                          15 yr

E-3      8/17/01     skin                fungal dermatitis              Equine    Arabian gelding       8 yr

E-30     1/28/00     small bowel         coccidiosis                    Equine    Mule                  3 mo

E-31     7/14/00     skin                habranema                      Equine    Apaloosa              13 yr

E-32     5/26/00     ethmoid sinus       hematoma                       Equine

E-33     ?           adrenal             pheochromacytoma               Equine    Quarter horse         25 yr

E-34     2/18/00     oropharynx          mycetoma/circularia            Equine                          6 yr

E-35     4/7/00      joint               giant cell tumor               Equine                          15 yr

E-36     1/26/01     nasal cavity        mast cell tumor                Equine

E-37     8/18/00     penis               habranema                      Equine    Arabian               10 yr

E-38     8/18/00     skin                mastocytoma                    Equine    Apalloosa              10 yr

E-39     8/25/00     skin                granulomatous inflamation      Equine    American Paint        2 yr

E-4      8/10/01     skin                pemphigus folliacious          Equine    Foal                  6 mo

E-40     8/4/00      lung                brochopneumonia/rotococcus     Equine    Thoroughbred          17 yr

E-41     9/21/00     kidney              myoglobinuria                  Equine                          11 yr

E-42     9/14/01     penis               habranema                      Equine    Arabian               6 yr

E-43     9/14/01     lung                mesothelioma                   Equine    Quarter horse         6 yr

E-44     9/14/01     skin                glomus cell tumor              Equine    Quarter horse         18 yr

E-45     10/1/99     skin                folliculitis/alopecia          Equine    Paint                 15 yr

E-46     9/17/99     penis               squamous cell ca               Equine    Appalloosa

E-47     8/27/99     nasal cavity        rhinitis/aspergillus           Equine

E-48     8/20/99     nasal sinus         hematoma                       Equine

E-49     8/20/99     spleen              melanoma                       Equine                          12 yr

E-5      6/1/01      skin                basal cell ca                  Equine    Appalloosa              24 yr

E-50     9/?/99      urethra             urethritis                     Equine    Arabian               14 yr

E-51     10/19/01    skeletal muscle     B-cell lymphoma                Equine    Quarter horse         13 yr

E-52     10/19/01    LN                  T-cell lymphoma                Equine    Quarter horse         6 yr

E-53     11/16/01    skin                mastocytosis                   Equine   National show horse   11 yr

E-54     11/30/01    skin                sarcoid/habronema              Equine

E-55     12/21/01    skin                habronema                      Equine

E-56     1/25/02     eyelid              angiosarcoma                   Equine    Quarter horse         22 yr

E-57     1/4/02      subQ                amyloidosis                    Equine                          8 yr

E-58     2/1/02      skin                sarcoid                        Equine

E-59     2/1/02      skin                chronic granulomatous disease  Equine    Arabian               17 yr

E-6      4/27/01     colon               adenocarcinoma                 Equine

E-60     12/1/00     liver               alkaloid toxicity              Equine    Arabian               9 yr

second picture
E-61     12/1/00     ovary               granulosal thecal cell tumor   Equine    Arabian               1 yr

E-62     10/6/00     lung                COPD/heaves                    Equine    Shetland              30 yr

E-63    11/17/00  eye              chronic uveitis            Equine                     20 yr

E-64     2/9/01      bowel               amoebiasis                     Equine                                 from Chili

E-65     2/9/01      skin                sporotrix                      Equine    Paint gelding

E-66     2/9/01      lung                herpes pneumonitis             Equine                          fetus

E-67     2/9/01      skin                pemphigus folliacious          Equine

E-68     2/8/02      liver               amyloidosis                    Equine    Arabian mare           9/yr

E-69     3/15/02     max-sinus           ethmoid hematoma               Equine                          16yr

E-69     3/15/02     maxillary sinus     hematoma                       Equine    Thoroughbred          16 yr

E-7      5/4/01      skin                Schwannoma                     Equine    Quarterhorse          15 yr  

E-70     3/29/02     skin                melanoma-amelanotic            Equine    White mustang         22 yr

E-71     3/29/02     mandible            lumpy jaw/actinomycetes        Equine    Arabian gelding               
E-72     4/5/02      placenta            bacterial placentitis          Equine    mare
E-73     4/5/02      liver               serum hepatitis                Equine    Quarter horse         13 yr       

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E-74.    4/12/02     rectum              eosinophilic colitis           Equine    Thoroughbred          18 yr

E-75    4/19/02   skin             leukocytoclastic vasculitis Equine  Quarterhorse      16 yr                      

E-76     4/26/02     lung                fibrosing interstitial pneumonia  Equine                                                  

E-77     5/17/02     ethmoid sinus       cyst+metaplastic bone          Equine    Thoroughbred          12 yr

E-78     5/17/02     sheath              aspergillus granuloma          Equine                  

E-79    8/9/02    skin             vasculitis                 Equine  Thoroughbred       23 yr

E-8      4/28/01     endometrium         chronic  endometritis          Equine    Peruvian Passo        15 yr

E-80     9/6/02      skin                flat wart                      Equine                           6 yr

E-81     10/11/02    skin                allergic dermatitis            Equine

E-82     10/11/02    ovatestis           hermaphrodite                  Equine                         

E-83     10/18/02    skeletal muscle     glycogen storage disease-type 4 Equine    Quarter horse         10 mo

E-84     10/18/02    kidney              pyelo/Klossiella equi (coccidiaEquine

E-85     10/25/02    skin                junctional melanoma            Equine    Mustang , white       23 yr

E-86     11/1/02     skin                squamous cell carcinoma        Equine

E-87     11/8/02     skin                eosonophilic granuloma/habranema Equine

E-88     11/15/02    brain               amoebic encephalitis           Equine    Thoroughbred          21 yr

E-89     1/17/03     scapula             sarcoma                        Equine    racehorse             3 yr

E-9      4/21/00     skin                pemphigus folliacious          Equine    "warm blood"

E-90     1/24/03     small bowel         GI stromal tumor               Equine    Morgan horse                ……

E-91     3/7/03      heart               toxic necrosis -oleander       Equine

E-92     2/28/03     plancenta           Aspergillus placentitis        Equine

E-93     3/14/03     liver               alkyloid toxicity              Equine    Quarter horse        10 yr

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E-94     3/14/03     liver               serum hepatitis                Equine                          2 yr

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E-96     3/28/03     skin                fungal dermatitis/lymphoid hyper Equine  Quarterhorse          4 yr
E-97     5/23/03     plancenta           placentitis/Candida            Equine
E-98     5/23/03     thymus              thymitis/herpesvirus(EHV-1)    Equine                         fetus
E-99     5/30/03     testis              male pseudohermaphrodyte       Equine    Quarterhorse          2 yr
E-100    6/13/03     plancenta           plancentitis/vasculitis        Equine                          10 mo fetus
E-101    7/18/03     liver               cholangiocarcinoma/portal fibrosis Equine Thoroughbred         19 yr
E-102    7/26/03     colon               colitis/C.difficile            Equine    
E-103    9/5/03      cerebellum          cerebellar abiotrophy          Equine     Arabian              2 yr
E-104    9/12/03     skin                melanoma                       Equine     Thoroughbred         10 yr
E-105    9/26/03     skeletal muscle     rhabdomyosarcoma               Equine     Quarterhorse         5 yr
E-106    9/26/03     skin                Sporothrix                     Equine     Thoroughbred mix     19 yr
E-107    10/10/03    skin                pemphigus folliacious          Equine     Paint                4 mo
E-108    10/24/03    lung                pneumonitis/DIC                Equine     Thoroughbred         2 yr
E-109   10/31/03  spinal cord      West Nile Virus .          Equine                             from Oklahoma
E-110    12/19/03    larynx              arytenoid chondritis           Equine     Thoroughbred         adult
E-111    12/19/      lung                exercise-induced pulm.hemorr  .Equine     Racehorse
E-112    2/13/04     skin                basal cell carcinoma           Equine     Quarterhorse         20 yr              
E-113    2/13/04     lung                diffuse interstitial fibrosis  Equine     Warmblood            17 yr
E-114    2/20/04     skin/frog           canker                         Equine     Standardbred         18 yr
E-115    4/16/04     lung                Adenovirus + bacterial pneum,  Equine     Arab                 4 mo     ? SCID
E-116    4/23/04     small intestine     hypogangliosis                 Equine     Quarterhorse         4 days    ? lethal white syndrome
E-117    5/28/04     parotid gland       adenocarcinoma                 Equine     American Paint       15 yr
E-118                                    coccidiomycosis                Equine
E-119    6/4/04      skin                leiomyoma                      Equine     Hanovarian           15 yr
E-120    6/25/04     lung                squamous cell ca, met from yhymoma Equine Quarterhorse         27 yr
E-121    7/9/04      skin                photosensitivity vasculitis    Equine     Thoroughbred         10 yr
E-122    8/27/04     skin                habronema                      Equine    
E-123    8/13/04     artery              strongylus vulgarus            Equine     Arabian              28 yrs
E-124    9/17/04     colon               torsion                        Equine     Quarterhorse         6/yr
E-125    10/1/04     liver               pyrolizidine toxicity          Equine     Gelding              6 yr
E-126    12/3/04     liver               acute toxic hepatitis          Equine     Arabian              18 yr
E-127    12/3/04     eye                 fungal keratitis               Equine     Thoroughbred         6 yr
E-128    12/17/04    colon               CLL/ T-cell                    Equine     Morgan               25 yr

E-129    1/21/05     sub Q               eosinophilic gran./mast cell t.Equine     Arabian              14 yr
E-130    1/28/05     nasal mucosa        chronic rhinitis/org.hematoma  Equine     Morgan               15 yr post gun shot in nose

E-131    2/04/05     colon               gi stromal cell tumor?         Equine     Arabian              20 yr
E-132    2/11/05     skin                pemphigus folliaceous/Herta    Equine     Quarterhorse         6 yr
E-133    2/25/05     skin/liver          hepatic cutaneous syndrome     Equine     American paint pony  15 yr
E-134    3/4/05      nasal turbinate     adenocarcinoma                 Equine     Thoroughbred         17 yr
E-135    3/4/05      spinal cord         EPM/sarcosystis neurona        Equine     Quarterhorse         10/yr
E-136    4/8/05      liver               lymphoma/T-cell                Equine     Quaterhorse          22 yr
E-137    5/20/05     endometrium         endometritis/EHV-1 +           Equine     Thoroughbred         9 yr
E-138    6/3/05      skin                BCG granuloma                  Equine     Swedish Warmblood    6 yr
E-139    6/17/05     subcutis           ? hemangioma/granulation tissue Equine     Arabian              6 yr
E-140    6/17/05     liver               fibrosis of Glisson's capsule  Equine     Arabian              23 yr
E-141    6/24/05     oviduct             chronic salpingitis            Equine                                from "endometrial" Bx
E-142    7/15/05     lung                granular cell tumor            Equine     Warmblood            22 yr
E-143    7/15/05     skin                photosensitivity vasculitis    Equine     Apalloosa            5 yr
E-144    8/19/05     skin                eosinophilic granulomas        Equine     Mule                 22 yr
E-145    8/26/05     lung                organizing pneumonia           Equine     Thoroughbred         20 yr
E-146    8/26/05     skin                wart                           Equine     Swedish warmblood    8 yr
E-147    9/2-05      skin                ?lymphoma                      Equine     Quarter horse        7 yr
E-148    9/9/05      colon               hemorragic colitis/clostridia  Equine     Welsh/American Paint 9 yr
E-149    9/23/05     spinal cord         acute myelitis/?EPM            Equine     Warmblood            16yr
E-150    9/30/05     skin                nerve sheath tumor             Equine     Quarter horse        12 yr
E-151    10/14/05    skin                pemphigus folliacious          Equine     Morgan               5 mo
E-152    10/21/05    skin                photosensitivity dermatitis    Equine     Arabian              18 yr
E-153    11/5/05     skin                histiocytic sarcoma            Equine     Arabian              20 yr
E-154    11/18/05    hoof wall           keratoma                       Equine                          8 yr
E-155    12/9/05     bone                osteomyelitis/rhodococcus      Equine                          4 mo
E-156    12/9/05     colon               GI stromal cell tumor          Equine                          old
E-157    1/13/06     skel m/masseter.    rhabdomyolysis                 Equine     Minature horse       8 yr
E-158    1/20/06     skin                angiomatosis                   Equine     Show Jumper          8 yr
E-159    1/27/06     liver               hepatitis/toxic?/DIC           Equine     Quarter horse/preg.  22 yr
E-160    1/27/06     liver               hepatitis/toxic?               Equine     Quarter horse X      17 yr
E-161    2/17/06     small bowel         entrapment/post surgery        Equine
E-162    3/3/06      bone/mandible       odontogenic tumor/myxoma       Equine     Quarter horse        6 yr
E-163    3/10/06     lung                EHV pneumonitis                Equine     Thoroughbred         16 yr
E-164    3/17/06     skin                pemphigus/bullous              Equine     Apallosa             26 yr
E-165    4/21/06    
E-166    4/28/06     skin
E-167    5/5/06      skin                sporothrix                     Equine      Donkey              5 yr
E-168    6/2/06      endometrium         endometritis/cryptococcus      Equine      Thoroughbred        12 yr
E-169    6/23/06     skin                photosensitivity dermatitis    Equine
E-170    8/4/06      soft tissue         mast cell tumors/eosin, gran.  Equine      Arabian             15 yr
E-171    8/18/06     hoof                canker.hairy foot wart         Equine                          foal
E-172    9/1/06      colon               torsion                        Equine      Thoroughbred        11 y

E-173    9/15/06     skin/ear            sarcoid+ squamous cell ca      Equine      Quarterhorse        3 yr

E-174    9/29/06     oropharynx          chronic arytenoid chrondritis  Equine      Thoroughbred        3 yr

E=175    9/29/06     3rd eyelid          adenocarcinoma                 Equine      Morgan              14 yr
E-176    9/29/06     liver               toxic hepatitis                Equine                          adult
E-177    10/6/06     mass between eyes   sinus cyst/reactive bone       Equine      Quarterhorse        16 yr
E-178    10/27/06    prepucial sheath    ruptured cyst                  Equine      Mustang             5 yr
E-179    12/1/06     skin                P.F.                           Equine      Arab/Quarterhorse x adult
E-180    12/15/06    thyroid/adr/panc    autoimmune polyglandular syn   Equine      Percheron           6 yr
E-181    1/19/07     mass/cheek          sarcoma                        Equine      Quarterhorse        17 yr
E-182    2/2/07      placenta            adenomatous cystic hyperplasia Equine                          10yr
E-183    2/9/07      colon               adenoca/occult/reactive fibrosis Equine
E-184    2/16/07     skin                pseudosarcoma/ fungal infection  Equine     Arab               adult
E-185    3/16/07     brain               moldy corn poisoning           Equine      Palomino            1 yr
E-186    3/16/07     brain               moldy corn poisoning           Equine
E-187    4/13/07     lung                EHV pneumonia                  Equine                          stillborn
E-188    4/20/07     liver               hepatosplenic lymphoma         Equine      Arabian             27 yr

E-189    4/20/07     hoof                laminitis                      Equine                          14 yr
E-190    5/4/07      skin                purpura hemorragica            Equine      Thoroughbred        14 yr
E 191    5/18/07     maxillary sinus     myxoma                         Equine      Quarterhorde        14 yr Dan Gardner
E-192    6/1/07      hock                mastocytosis+eosinophilic granuloma Equine                     23 yr
E-193    6/8/07      penis               flat wart/ BPV+                Equine      Oldenburg?          18 yr
E-194    6/8/07      skin                eosinophilic granuloma         Equine      Trotter             4 yr
E-195    6/15/07     skin                dermatitis/pyogranulomatous    Equine      Arabian             17 yr
E-196    6/15/07     premaxilla          ossifying fibroma              Equine      Quarterhorse        12 yr
E-197    6/15/07     skin                spiroadenoma                   Equine      Icelandic pony      18 yr
E-198    6/15/07     nerve trunk         amputation neuroma             Equine      American paint      6 yr
E-199    6/29/07     eye                 ocular choriostoma             Equine                          foal  born without eye
E-200    6/29/07     placenta            fibroadenomatous hyperplasia   Equine      Perceron

E-201    7/6/07      skin                photosen.dermatitis            Equine      American paint      6 yr
E-202    7/6/07      lung                acute broncho pneumonia        Equine      American paint      6 yr
E-203    8/3/07      skin                dermal vasculitis              Equine      Mustang        `    27 yr
E-204    8/3/07      skin                flat sarcoid                   Equine      Warmblood           10 yr
E-205    8/24/07     kidney              renal dysplasia                Equine      Trakeyner           3 mo
E-206    9/7/07      skin                purpura hemorragica            Equine      Quarterhorse        19/yr
E-207    9/7/07      cerebellum          lysosomal storage disease      Equine      Morganhorse         10 mo
E-208    9/21/07     skin                interface dermatitis           Equine      Morganhorse         10 yr-?post herpes vaccine
E-209    9/21/07     skin                interface dermatitis           Equine      Appaloosa           12 yr?-post herpes vaccine
E-210    9/28/07     ovary               granulosal thecal cell tumor   Equine                          7 yr-estrus for 1 yr
E-211    10/12/07    small bowell        peritonitis/>eosinophilic enter.Equine     Apalloosa           12 yr
E-212    10/19/06    skin                eosinophilic granuloma         Equine      Warmblood           6 yr
E-213    11/02/07    hoof wall           malignant melanoma             Equine      American paint      11 yr
E-214    12/21/07    skin                folliculitis/fungal            Equine                          1 yr
E-215    1/11/08     stifle              hygroma/angiomatosis           Equine      mule                6 yr
E-216    1/25/08     lung                granular cell lymphoma         Equine      Thoroughbred        20 yr
E-217    2/8/08      nasal sinus         myxosarcoma                    Equine      Arabian/Quarterhorse 27 yr
E-218    2/8/08      colon mucosa        lymphoma                       Equine      Paint horse         8 yr
E-219    2/22/08     eye                 squamous cell carcinoma        Equine      Thoroughbred/Apallosa 29 yr
E-220    2/29/08     skin                sarcoidosis?                   Equine
E-221    3/14/08     lung                fungal pneumonia/aspergillus   Equine      Arabian foal         2 weeks
E-222    1/4/08      colon               hemorrhage/stasis              Equine
E-223    4/4/08      epididymis          epididymitis/strep equi        Equine      Morgan               6 yr
E-224    4/11/08     jejunum             adenomatous polyp              Equine      Arabian              18 yr
E-225    5/2/08      colon               ganglioneuritis/?viral         Equine      Lipizzan stallion    10 yr
E-226    5/9/08      cerebellum          cerebellar abiotrophy          Equine      Arabian              foal
E-227    5/9/08      eye                 chronic uveitis                Equine
E-228    5/30/08     lung/liver          EHV-1                          Equine                           aborted fetus  
E-229    5/30/08     gi tract            acute enterocolitis/Clostridia Equine      Thoroughbred foal    1 day
E-230    6/6/08


Equine Serum Hepatitis------Theiler’s Disease

Case No:  E-73, E-94


This disease occurs in horses that have been injected with equine serum or tissue emulsions 6- 10 weeks earlier. Most commonly it is seen in horses passively immunized against C. perfringes or tetanus toxins to prevent grass sickness or tetanus, respectively. Postpartum mares are especially at risk. Vaccines against equine herpesvirus - 1 prepared from equine fetal tissues have produced the disease.  Whether the disease is infectious or immunogenic , or both, has not been established and the disease has not been experimentally reproduceable. The disease damages the liver but usually presents with neurologic disturbances and is often fatal. Microscopically, the liver shows swollen, vacuolated degenerating hepatocytes, cholestasis, scant acute and chronic inflammatory cells, bile ductular proliferation and slight fibrosis.( see pictures,1, 2).  Healed lesions have not been reported, but there is a possibility that subclinical cases may occur. There is no specific treatment. The main differential diagnosis is pyrrolizidine alkaloid toxicity. From the comparative standpoint this disease most closely resembles human viral hepatitis; one would certainly suspect a viral etiology here but extensive efforts to prove this experimentally have been negative to date.


Reference:  Pathology of Domestic Animals    4tth Ed. K; Jubb, P. Kennedy, N.Palmer Acad. Press pp 368-369 1993



Equine Herpesvirus – EHV

Case No:  E98, E25

Similar to humans and other species herpesvirus infections are extremely common in horses.  Virtually all horses (and humans) become infected at a young age and harbor a latent infection throughout life.  Various host and environmental factors may activate the latent infection and cause virus shedding and disease.  Several different strains of EHV cause different diseases, e.g. EHV-4 and EHV-1 are major cause of upper respiratory tract disease in adults and weanling foals, respectively.  Most of the adult animals recover in about 1 week.  EHV-3 causes genital "cold sores" and EHV-1 causes abortions and myeloencephalopathy.EHV-1 can evade the immune system and induce immunosuppression.  The diagnosis is based on clinical, serologic, PCR and virus isolation results.Immunity after natural infection or vaccination is brief,and horses may be repeatedly infected during their lifetime.  Treatment consists of isolation, supportive care and perhaps acyclovir.  Corticosteriods are probably counter- indicated because of their immune suppressive effects and the possibility of inducing laminitis.  Vaccination during an outbreak of EHV respiratory disease is not advised and can not be guaranteed to prevent new infections or activation of latent virus. 


Illustrated here are photomicrographs of fetal thymus(3) and lung(23) from an EHV-1 induced abortion and spinal cord (4) from an adult with ataxia an incontinence- one of many horses affected at a single pack station in the Eastern Sierras.  The fetal thymus shows necrosis and intranuclear inclusions in thymic epithelial cells. The  fetal  lung also shows  intranuclear inclusions.


The spinal cord shows perivascular cuffing with lymphocytes and swollen endotheliai cells with intravascular fibrin clots, but no discernable inclusions.  However, with anti-HSV-1 immunoperoxidase strains the endothelial cell nuclei were prominently labelled.  HSV-1 in this instance infects endothelial cells lining small vessels in the spinal cord leading to intravascular thrombi, small hemorrhagic infarcts and the resultant CNS symptoms.  Depending upon the severity of the CNS infection the horse may recover completely, and may survive with residual deficits or may die, as in this case.


EHV-1 induced abortion occurs worldwide and is the most important cause of infectious abortion stormes in the mare.  The fetus is systemically infected, and along with the infected placenta, is highly contagious.  The majority of infected mares abort in the last 4 months of gestation.  Vaccination of pregnant mares, even in the face of an abortive outbreak, may prevent infection in horses yet unexposed and may decrease the incidence of abortions but this cannot be guaranteed.


In humans, particularly, the immunocompromised, herpes virus can also cause a similar pattern of systemic potentially fatal infection, including the fetus, and lead to respiratory disease, abortion and CNS manifestations.  Herpes virus vaccines are not routinely used in humans. 


Ref.  Equine Herpesviruses:  Mark Donaldson :Chap 2.2 ,pp 38-42 In Current Therapy in Equine Medicine.  Ed M. Edward Robinson.  Saunders 2003





Case No:  E91, E19, E20, E93, E60


Many plants are potentially toxic to livestock.  Illustrated here are three characteristic examples.



Nigropallidal Encephalomalacia of Horses:


This disease follows prolonged ingestion of Yellow star thistle or Russian knopweed, plants that remain green in dry summer pastures.  About 1 month or more after ingestion the horse develops CNS symptoms including dysphagia, lip dystonia,  drowsiness and incoordination and soon dies.  The pathology features a dramatic degeneration and necrosis leading to softening (malacia) with secondary inflammation and gliosis(8), all nonspecific changes,but remarkably localized to  the globus pallidus and substantia nigra in the midbrain.  There is no specific treatment and no human counterpart.        


Ref: Jubb and Kennedy Vol 1 pp344-346



Toxic Myodegeneration of the Heart:


This occurs in horses and other animals including humans after ingestion of plants containing cardiac glycosides such as oleander.  It is an acute poisoning that causes death in a few hours.  The glycosides inhibit Na/K ATPase (sodium pump) that maintains the membrane potential of the muscle cells.  The pathology features a hemorrhagic necrosis of the heart muscle cells(5).  The accidendal ingestion of certain ionophores(rumensin or monensin) in cattle feed supplements can also cause lethal cardiac damage.Clinically, such horses present with exercise intolerance or sudden death due to heart failure with a dilated cardiomyopathy. 


Ref: Jubb and Kennedy Vol 3 pp28-29



Toxic Liver Disease:

This common fatal disease occurs in horses and many other species after the prolonged ingestion of pyrollizidine alkaloids contained in plant species such as senecio, crotolaria and heliotropium. Initial symptons include anorexia,lassitude and jaundice. Late symptoms are usually neurologic ("walkabout") secondary to ammonia toxicity from liver failure.  Microscopically, the liver shows enlarged hepatocytes ("megalocytosis")(6), fibrosis(7), especially around the central veins, bile ductular proliferation and nodular regeneration.  Other tissues are also involved.  Humans show similar changes in the liver after bush tea poisoning and to some extent after chronic alcohol ingestion.     


Ref: Jubb and Kennedy Vol 2 pp392-395




1.heart x40--oleander toxicity                              E-91

2.heartx400-- oleander toxicity                            E-91

3.brain x400--yellow star thistle toxicity               E19

4.brain x40-- yellow star thistle toxicity                E20

5.brain x400 -- yellow star thistle toxicity             E20

Most of the important clostridial diseases occur in herbivores and are caused by one or the other of the five toxigenic types (A-E) of Clostridium perfringes. However,this case(E-102) is an example of acute hemorragic enteritis caused by C. difficile, a gram positive spore forming obligate anaerobic bacteria,ubiquitous in the soil. C.difficile is linked to antibiotic associated enterocolitis in a number of species. Infection with C.difficile in horses,foals and humans presents with four different scenarios:  asymtomatic infection,mild diarrhea,colitis and pseudomembranous colitis. Foals can be infected in the absence of antibiotic treatment .Dams of foals treated for rhodococcus equi pneumonia with erythromycin and rifampycin are at increased risk. C.difficile secretes several toxins, Type A enterotoxin, Type B cytotoxin and a binary toxin, for which the gene has been identified in horses.

    Clinically, this horse had bloody diarrhea, colic,weakness and dehydration.Although this horse recovered, such cases often die within 24 hrs after onset of clinical signs. The histopathology (16) shows diffuce mucosal hemorrhage of the colon with vascular congestion but no necrosis or pseudomembrane..Also present in this horse is an intralesional protozoa,Balantidium sp. which may have contributed to the pathology. The diagnosis of Clostridium established by detection using Elisa or PCR of the characteristic toxin in the feces and culture of the bacteria from the feces. Treatment of this disease in horses as in humans relies on using alternate antibiotics such as metranidazole and prevention involves avoiding antibiotics, isolation and improved hygiene, ie, avoiding infected feces.

             Ref. Jubb and Kennedy 2: 244-247         Robinson:  Chap 3.25  pp166-169

    As in humans a deficiency of the phospholipid surfactant is implicated as the major contributing factor to the development of this disease in premature foals. This is associated with severe respiratory acidosis, hypoxia-induced pulmonary arterial hypertension,reduced CO and tissue hypoxia. The lungs are not fully expended and fibrinogen in edema fluid is converted to fibrin which coats the inner wall of the alveoli(24)-thus the name hyaline membrane. In humans treatment with synthetic surfactant has dramaically improved survival and this treatment may also help premature foals but is usually prohibitivily expensive. 

             Ref. Robinson:   Chap 12.3  p643

   Similar to hemolytic disease of neonatal humans the foal inherits a specific red blood cell(RBC) surface antigen from the sire to which the mare is incompatable. After ingestion of antibodies--in contrast to transplacental transport of antibodies in humans --the antibodies bind to the foal's RBCs which results in hemolysis and consequent anemia .Exposure to incompatable RBC antigen(8 equine blood groups with 32 distinct RBC antigens) usually occurs during transplacental hemorrhage late in gestation or during parturition. Primiparous mares rarely give birth to affected foals because transplacental hemorrhage during the first pregnancy fails to stimulate an immune response of sufficient strengh. Once a mare has an affected foal  all future pregnancies are at risk. This disease is more common in mule foals than in horse foals because donkey RBCs have a unique antigen not present in horses. The severity of the disease varies and symptons begin from 5 hrs to 5 days of age.Signs and symptons include anemia,hypoxia.shock,icterus,hemoglobinuria,renal failure and CNS signs(kernicterus). Death often occurs from secondary septicemia. The lab diagnosis is based on X-matching showing RBC lysis in presence of complement. Prevention also depends on X-matching and use of alternate colostrum. Treatment is transfusion of compatable blood.
             Ref. Robinson:        Chap12.2    pp636-640

     This disease is caused by ptolonged  and excessive secretion of parathyroid hormone (PTH) due most commonly to nutritional or metabolic derangements such as deficiencies of dietary calcium and/or Vitamin D and excess dietary phosphorous. The same condition can arise from similar calcium and phosphorous changes secondary to chronic renal failure. The disease is characterized by extensive osteoclastic resorbtion of bone (22),often of facial bones ,with reactive formation of new bone and fibrous tissue. The disease affects horses and equine relatives ,goats,pigs,cattle,rarely sheep and dogs,cats,monkeys and humans.In horses and the other farm animals dietary calcium deficiency is usually the cause,whereas in humans,cats and dogs chronic renal failure is usually responsible for the secondary hyperparathyroidism. In the animals the condition usually occurs after prolonged diets consisting largely ofgrain,corn or grain by-products such as bran, thus the term "bran disease". Swelling of the jaws, at first soft and later hard, is a diagnostic feature leading to the terms "bighead" and "rubber jaw". Limb long bones and articular cartilage may be affected causing lameness and deformity. Laboratory confirmation of the diagnosis depends on detection of the calcium and phosphorous abnormalities and elevated levels of PTH. Treatment involves correction of the nutritional imbalance or renal failure , if possible.Fortunately, in humans the renal failure is usually corrected by dialysis and renal transplantation long before this condition arises.

            Ref. Jubb and Kennedy 1: pp72-79

        This condition is a common cause of infertility in multiparous mares, greater than 14 years of age and with poor perineal conformation. such mares accumulate fluid in the uterine lumen, probably due to an intrinsic defect in myometrial contraction and failure to reabsorb interstitial edema fluid via uterine lymphatics. Transient inflammation is a normal physiologic response to mating or artificial insemination (A .I.).Mares that fail to clear the semen-induced inflammation within 36 hours accumulate fluid in  the uterine lumen. This fluid contains neutrophils, immunoglobulin, protein,serum, bacteria and other by-products of inflammation. The common infecting organisms are B-hemolytic streptococcus,E.coli and Taylorella equigenitalis,the organism responsible for contagious equine metritis (very uncommon nowdays).Histologically,the endometrial changes are minimal consisting of light infiltration of neutrophils and accumulation of plasma cells and lymphocytes in the stroma (14) and dilated lymphatics. Desquamation of glandular epithelial cells with glandular dilatation and periglandular fibrosis are more chronic changes. A definative diagnosis can be made 12-36 hours after breeding by the the detection of intrauterine fluid by ultrasonography. A presumptive diagnosis can be made earlier by the mare's past reproductive history. A vaginal discharge of purulent fluid would also be tell-tail. Treatment consists of uterine lavage 4-8 hours after breeding and 10-20 I.U. of oxytocin I.V. to induce uterine contractions and expulsion of the lavage fluids. Antibiotics are given intrauterinally only if bacteria and neutrophils are present in large numbers. Mares susceptible to this condition should be bred no more than once per cycle.
         The closest counterpart in humans would be endometritis and salpingitis accompaning sexually- transmitted diseases such as gonorrhea. Were humans subjected to similar regimens of forced breeding or A.I., they would no doubt have a similar problem or worse.
            Ref. Robinson Chap 5.5    pp234-237       
Jubb and Kennedy  3: 379

             WNV is a flavivirus that was endemic in other parts of the world until making it's first appearance in the Western Hemisphere in New York City in Aug. 1999. Since then thue virus has spread north and south and is now headed westward. The virus cycles between bird resevoir hosts ( crows ) and mosquitoes (Culex) that carry the virus in their salivary glands. Susceptible birds may become illor die of encephalitis. Humans,horses,and other mammals are dead -end hosts that do not transmit the virus except via contaminated blood transfusions.Infected horses may remain asymptomatic or develop clinical signs of myeloencephalitis, most commonally rear limb ataxia or paralysis. In 2003 WNV was diagnosed in 8567 humans (199 fatal ), among which two cases were documented in  both Nevada and California , respectively. In 1999-2000 about 35% of the horses with neurologic disease either died or were euthanized, Other cases resolved over several weeks or months thanks to an effective immune response.In 2003 WNV infection was diagnosed in  4146 horses in 41 states, but so far not in Nevada or California. However, plenty of infected humans and horses were detected in 2003 in Colorado so the virus appears to be on our doorstep ,ie, Nevada and California. In 2003 WNV was detected in 13277 sentinal chicken foocks in 15 states , 61 sentinal horses (43 in Illinois) and 7725 mosquito pools in 38 states, but again, none as yet in Nevada or California.
               Histopathologically, one sees the typical features of a viral encephalitis , consisting of perivascular cuffing with lymphocytes (18) ,reactive gliosis (the CNS patter of scaring) and variable degrees of necrosis and neuronal death.No viral inclusions are present. In this horse ( from Oklahoma ) the microscopic changes were concentrated in the anterior horns of yhe lower soinal cord.
               Diagnosis depends on serologic detection of WNV antibodies, particularly  high Ig M titers or rising Ig titers, in serum or CSF or viral nucleic acids in CNS tissue by PCR. Virus isolation can , of course , be used for conformation .High Ig G tiers alone could reflect recovered horses or the result of prior vaccination.The differential diagnosis includes other viral encephalitides ,e.g. Venezuelian equine encephalitis, Rabies, Equine herpesvirus or EPM( Equine Protozoal Myeloencephalitis ).
            A vaccine made from killed WNV is available for horses but it's efficacy is not yet determined.Current recommendations are initial vaccination followed by a booster 3 weeks later and annually thereafter. There is no specific antiviral treatment.

            Ref;  Robinson  Chap2.5   pp49-50        MMWR   Nov 28  52: 1160, 2003      Jubb and Kennedy 1: pp412-413

        This term applies to hemorrhage in the lungs occuring in horses during racing or training. The hemorrhage occurs in dorsocaudal lung regions.It appears to be a multifactorial condition that airway,vascular,platelet and locomotory components.Increased pulmonary vascular pressure seems to be a major cofactor along with a easy bleeding tendency.Bronchial endoscopic examination reveals that some degree of hemorrhage is present in the great majority of horses after strenuous exercise.The extent to which this event impairs exercise performance remains uncertain.The diagnosis is made by the appearance ,after exercise, of epistaxis or blood via bronchial endoscopy or lavage.Microscopically the lungs show recent hemorrhage
and hemosiderin-laden macrophages as evidence of previous bleeding(Fig- ).The mainstay of treatment is furosemide which reduces pulmonary vascular pressures both at rest and during exercise when given in doses of 200 to 500 mg 1 to 4 hours before exercise.Relief of upper airway resistance by the use of nasal strips is also practiced with some evidence of benefit in reduction of pulmonary bleeding after exercise.

             Ref:   Robinson   Chap 8.7   pp429-433      Jubb and Kennedy 2:  p587

         This is a major ophtalmic disease of the horse. It results in large economic loses to the horse industry because of a 8-25% prevalence rate across horse in the United States. The disease is  non-specific immune mediated and may be triggered by any injury to the horse eye, e.g. trauma, bacteria ,such as leptospirosis, viruses such as influenza, parasites such as onchocerca or miscellaneous causes such as endotoxemia. However, in most cases the responsible antigen(s) is unknown. Recurrent or persistent inflammatory episodes in the uveal tract lead to blindness,although not all horses with acute uveitis will develop the recurrent disease. Recurrent episodes are increasingly severe. The diagnosis is based on characteristic clinical signs indicating ocular discomfort, corneal edema or blindness. Histopathology features an infiltration of the uveal tract with lymphocytes, plasma cells and macrophges (13), along with many secondary changes involving the whole eye depending on the severity of the disease. Traditional treatment consists of corticosteroids and nonsteroidal antiinflammatory agents aimed at reducing inflammation during each active episode alyhough they are not effective in recurrence of disease. Two new surgical procedures,- intravitrial cyclosporin A devices and core vitrectomy- aimed at preventing disease recurrence are currently under evaluation.
              Ref:   Robinson      Chap9.5    pp468-473       Jubb and Kennedy 1:  pp480-482
           These conditions are probably  immune mediated and cause inflammation and necrosis of blood vessel walls of any size, location, sex or breed of horse.The most commonly  recognized form is secondary to  respiratory infection with  S.equi (strangles)  or  vaccination for S.equi and is called purpura hemorrhagica. S.equi antigen/antibody complexes are deposited in vessel walls leading to activation of complement and an acute inflammatory response with neutrophils infiltrating and damaging the vessel walls. Subsequent compromise of the lumen results in edema, hemorrhage and ischemic changes.Clinically, the major signs are edema in the skin of the face ,distal extremities or abdomen. RBC extravasation causes the petechiae and larger areas of hemorrhage( purpura) visible in the skin, but also present in mucosal membranes and elsewhere in the body. The diagnosis can be confirmed by skin biopsy which shows neutrophils infiltrating the vessel walls in the dermis along with neutrophil nuclear debris ( leukocyclasis) and fibrinoid necrosis of the vessel walls(20). Treatment  entails removing the antigenic stimulus, if possible, and systemic glucocorticoids and nonsteroidal antiinflammation drugs( NSAIDS). Most cases recover in about a month although some horses die or must be eutanized because of serious sequella.

            Photoactivated vasculitis affects mature horses during the summer months and is limited to non-pigmented regions of the lower etremities,especially the heels. The lesions must be differentiated from the "greasy heel complex".  It is thought that immune complexes may be acting as photosensitizing agents. The acute lesions are well demarcated , erythematous, oozing and crusted. Chronic lesions are hyperakeratotic plaques. Histologically, the hallmark features are dermal edema, vascular dilatation and small vessel leukocytoclastic vasculitis confined to the superficial dermis (19 and 21).Treatment involves avoiding exposure to sunlight and systemic glucocorticosteroids. Topical applications of steroids or antibiotics are not effective.
             Ref:   Robinson       Chap6.10  pp363-365           Jubb and Kennedy 1:  p597
          Sarcoids are benign cutaneous neoplasms, the most common skin tumors in horses , donkeys and mules. They are most common in American Quarterhorses and Thoroughbreds whereas Standardbred horses appear more resistant. The tumors are commonly located on the head, ventral abdomen and limbs. They may have a wart-like (verrucous) or a firm, nodular form, sometimes with an ulcerated surface. They are locallly expansive, may be multiple, do not metastasize, and may regress spontaneously. Microscopically, they are composed of a locally invasive but benign proliferation of mature fibroblasts in the dermis. The overlying epidermis is often hyperplastic with long extensions into the dermis and vacuolated or balooned cytoplasm (12).The tumor is thought to be caused by infection with the bovine papilloma virus ( BPV) whose genes can be detected in the proliferating fibroblasts. However, infectious BPV is not produced by the sarcoid tumors. Flies may translocate the sarcoid cells into open wounds. The diagnosis is confirmed by biopsy but this procedure may stimulate further growth of the tumor. Treatment results are unpredictable and include surgical excision, cryosurgery, or local immunotherapy, chemotherapy or radiatiion. There is a high rate of local recurrance ( but no metastases ).The differenyial diagnosis includsd flat warts, habronemas and squamous cell carcinomas.
              Ref    Jubb and Kennedy 1: p709           Robinson  Chap9.8  pp484-485

HABRONEMA OR SUMMER SORE (E-23,31,37,42,54,55,87)
          These are ulcerative, pruitic tumorous masses in the skin most commonly at the medial canthus of the eye, the glans penis or any cutaneous wound.The lesion is caused by the aberrant deposition of stomach worm larvae( spirurid nematode) by transmitting flies. The horse ingests the worm , which lives in the gi tract, from which the eggs are passed in the feces, ingested by the flies and deposited in the skin. Microscopically,(10,11.12) the lesion represents a hypersensitivity reaction characterized by degenerating eosinophils , collagenous connective tissue , epitheliod macrophages and giant cells around degenerating larvae. Superficial ulceration and granulation tissue are often present.Secondary mineral deposits may give the lesions a gritty texture. The diagnosis is confirmed by biopsy or surgical removal. Treatment consists of Ivermectin, local or systemic steroids and fly control. The differential diagnosis includes sarcoids,squamous cell carcinoma, fungal or bacterial dermatitis and granulation tissue( "proud flesh").
             Ref: Jubb and Kennedy 1: pp692-693       Robinson  Chap4.7    pp195-197


            These are cutaneous nodules that are firm, non pruitic and non ulcerated, commonly located on the withers,back, or lateral neck.The overlying skin is normal. Although the cause is unknown , they are probably a hypersensitivity reaction to an insect bite. Microscopically, (Fig- ) the lesion shows degenerating collagen with secondary mineraiizatioin, eosinophils and granulomatous inflammation. Fly larvae are not detected. The lesions may regress soontaneously or may be surgically removed or injected with steroids. Fly control is indicated.
            Ref: Jubb and Kennedy 1:  pp700-701       Robinson   Chap4.11   pp206-208

               This group of disorders has several causes. Examples include Hyperkalemic Periodic Paralysis , a metabolic condition due to loss of potassium from muscle cells, Post-anesthesia Myopathy  and Ionophore Poisoning , both toxic events, Severe Equine Rhabdomyolysis Syndrome , triggered by exertion , Nutritional Myodegeneration and Atypical Myoglobinuria, both linked to dietary deficiencies (eg selenium and Vitamin E)). All of these conditions present with some degree of muscle weakness along with stiffness and rigidity , usually in the hind limbs. Severity may be mild to severe to fatal. In all of these conditions( except hyperkalemic periodic paralysis) , one can observe myogobinemia, myoglobinuria, azotemia,anuria and renal failure, and metabolic acidosis. These findings are related to the breakdown of skeletal and ,occationally, cardiac muscle cells with release of the intracellular myoglobin and enzymes such as creatine kinase. The case (E-41) shown here is an example of the exertional rhabdomyolysis syndrome following an endurance trail ride. The kidney tubules fill up with brown staining myoglobin casts(26) which block the outflow of urine.Muscle biopsies would show signs of muscle cell degeneration and necrosis and ,perhaps regeneration depending on severity and duration of the muscle damage.To the naked eye the muscles would have white streaks or be entirely white because of the loss of myoglobin pigment.  Elevated levels of creatine kinase would be found in the serum. Other names given to  manifestations of exertional rhabdomyolysis include" tying up", "azoturia", "paralytic myoglobinuria", "Monday morning disease", and "Capture myopathy" in wild animals.
            Ref:  Jubb and Kennedy 1: pp235-243        Robinson    Chap14.2   pp740-746


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