Emerging Infectious Diseases


Lecture 7 & Sample Questions

John L. Robertson


Emerging Infectious Diseases in Horses: Serious Problems in the Past 25 Years




§      Equine Pathology, JR Rooney and JL Robertson, ISU Press, 1996


§      “The pathology of the equine respiratory system” JL Robertson and JR Rooney, 2nd Int. Equine Symp. Proceedings, 1997


§      Publications from USDA and State Depts. of Agriculture


§      “Neuroimmunology of the horse with special reference to EPM” MO Furr, PhD thesis


Common causes of morbidity and mortality in horses


§      Musculoskeletal disease (many use-related dysfunctions)


§      Digestive catastrophes (colic)


§      Infectious diseases (neurologic and respiratory diseases most common)


Frequency of disease by anatomic location

§       Locomotor system        52.2%

§       Skin                            29.8%

§       Foot                            12.7%

§       Eye                                   3.4%

§       Nervous system                 0.5%


Data from Prussian Army horses (10 years)(1886-1895)


278,323 horses with illness

•      94.6% recovered

•       1.3%  turned out

•        3.3% died

•        0.9% killed        

(Bartke,  in Bayer-Frohner, 1900)                        


Common infectious diseases


§       Cardiovascular - EVA, Streptococcus equi (zooepidemicus), Strongylus vulgaris

§       Respiratory - influenza, EHV-1, S. equi, S. pneumoniae, R. equi, Dictyocaulus arnfeldi

§       Digestive - Salmonella sp., Ehrlichia risticii, Strongylus sp.

§       Reproductive - EVA, EHV-1, Klebsiella genitalium

§       Nervous- Arboviruses (many), rabies, EHV-1, Sarcocystis neurona


Diseases of antiquity


§      Strangles and other respiratory diseases described by Greeks

•      Herbal preparations for therapy known in 1380 BC

§      Most ‘modern’ diseases (infectious and otherwise) described in German, Italian and French literature before 1920


Factors favoring development of infectious disease in horses


§      Cosmopolitan contact and transportation

§      Altered host immunity (iatrogenic, diet)

§      Altered age mix of population

§      Husbandry systems

§      Novel vectors in novel environments

§      ‘New’ pathogens ?


Contagious equine metritis (CEM)


§      Transmissible venereal disease of horses caused by Taylorella equigenitalis

§      Present in horses worldwide

•      UK, France, Germany, Ireland

§      First noted in US in 1977-78 in central Kentucky

§      Thoroughbreds more seriously affected


Contagious equine metritis (CEM)


§      First case diagnosed in UK in 1977

§      First cases diagnosed in US in 1978 in Thoroughbred mares and 1979 in Missouri

§      Disease eradicated from US


Contagious equine metritis (CEM)


§      Transmission


•     Natural sexual intercourse between CEM infected horses and susceptible horses

•     Indirectly through artificial insemination or fomites

•     An undetected carrier state in stallions is important in propagating disease


Contagious equine metritis (CEM)


§      Clinical signs


•     Infertility / failure to conceive

•     Spontaneous abortion (rare)

§      In mares


•     Acute infection leads to vulvar discharge

•     Chronic infection have less discharge

•     Carriers may be asymptomatic


Contagious equine metritis (CEM)


§      Lesions


•     Mild vulvar discharge

•     Mild to moderate suppurative endometritis and myometritis

•     In chronic cases, mixed inflammatory cell metritis

•     Few lesions in stallions


Contagious equine metritis (CEM)


§       Diagnosis

•      Bacterial culture

•      Serologic detection of antibodies

§       Treatment

•      Antibiotics and disinfectants

•      Rigid hygienic practices for breeding

§       Quarantine has kept pathogen from US


Potomac horse fever (PHF)


§      Also known as equine monocytic ehrlichiosis

§      Caused by infection with Ehrlichia risticii

§      First seen as epizootic in horses in Maryland and Virginia in 1979

•      Now seen in 32 states and Canada


Potomac horse fever (PHF)


§      Transmission


•      Precise mode of transmission is not known

•      Infectious organisms are found in feces

•      Seasonality of infection (late Spring and Summer) suggests transmission via arthropods and reservoir hosts

•      A freshwater snail may be involved in the complex transmission

•      Transplacental ?


Potomac horse fever (PHF)


§       Ehrlichia risticii

•      Prefers to grow and sequester within monocytes (inside vacuoles)

•      Shares antigenic determinants with E. sennetsu, cause of human rickettsiosis in SE Asia

•      Variable morphology in different stages of disease


Potomac horse fever (PHF)


§      Clinical signs


•     Fever, anorexia and depression

•     Laminitis

•     Acute colitis (dorsal large colon) with profuse diarrhea

•     Edema, hemoconcentration

•     Mortality of 10-30%


Potomac horse fever (PHF)


§      Lesions

•     Acute nonspecific mixed inflammatory cell colitis

•     Acute laminitis

•     Difficult to differentiate from other causes and presentations of colitis (Salmonella sp.)


Potomac horse fever (PHF)


§      Diagnosis

•      IFA, ELISA and PCR methods are available

•      Culture is difficult in cell free media (grows in monocyte cultures)

§      Treatment

•      Generally symptomatic - antibiotics and fluids

§      Prevention

•      Bacterins are available 


Equine protozoal myelitis (EPM)


§      First described by Rooney in 1970 as a focal myeloencephalitis in 52 horses with asymmetrical gait and postural abnormalities


Equine protozoal myelitis (EPM)


§      Caused by infection of horses with Sarcocytis neurona

§      An intermediate host, opposums, have been identified

§      Infection appears to occur by consumption of contaminated food and water


Equine protozoal myelitis (EPM)


§       EPM is the most common cause of neurological dysfunction in horses

•      May affect (produce clinical disease) 1-3% of all horses in US (esp. Warmbloods)

•      20 - 50% of horses have seroprevalence

•      60% of affected horses <4 years old

•      No apparent sex predilection

§       Seen in North,Central and South America


Equine protozoal myelitis (EPM)


§      Clinical signs

•     Restricted to CNS

•     Asymmetrical ataxia most common

•     Proprioceptive deficits

•     Spasticity and stumbling

•     Muscle atrophy


Equine protozoal myelitis (EPM)


§      Lesions

•     Focal segmental hemorrhage

•     Necrosis of gray and white matter

•     Focal nonsuppurative myeloencephalitis with lymphocytes, eosinophils and focal gliosis and neuronal necrosis (micro)


Equine protozoal myelitis (EPM)


§      Diagnosis

•     Immunoblot assay (Western blot) using lysed cultured merozoites

•     PCR for protozoal DNA (KY and CA variants)

•     CSF analysis - Albumin quotient:IgG index


Equine protozoal myelitis (EPM)


§       Treatment

•      Sulfonamide antibiotics (sulfamethoxazole and trimethoprim) combined with pyrimethamine have been the mainstay of current therapy

•      Diclazuril, and other coccidiostats, have also been used but with variable results

•      Supportive therapy


On the horizon ...


§      A/equi/Jilin/1-89 - equine influenza virus

§      Zoonotic paramyxoviruses in Australia



Lecture 7 Sample Questions


1. A major factor which has facilitated the development of emerging infectious diseases in horses is:

A. Worldwide transportation of horses
B. Decreased immunity in horses in general
C. Many new pathogens in the last 10 years

2. The most common type of disease or injury in horses, according to large published studies is disease/lesions in what body system?

A. Urinary tract
B. Locomotor system
C. Respiratory system

3. The reason that contagious equine metritis is a problem in horses that have it is:

A. It causes staggering and blindness
B. It causes lameness
C. It causes sterility and abortion

4. Published studies now indicate that equine protozoal myelitis:

A. Is a common infectious disease affecting the nervous system
B. Is a common disease which causes pneumonia
C. Is easily curable