Eastern Equine Encephalitis Rare but Serious in Dogs


A recent report describes several fatal cases in puppies from Michigan and New York.

First discovered in 1933, Eastern equine encephalitis virus (EEEV) causes a highly lethal mosquito-borne disease affecting a wide range of mammalian and avian hosts. EEEV is distributed throughout the eastern half of North America, with estimated mortality rates in humans and horses reaching 75% and 90%, respectively. Recent research suggests that the host range for EEEV may be broader than previously thought. As an example, 1 study found that 35% of wild snakes examined in the southeastern United States were seropositive for EEEV.

A recent report in the Journal of Veterinary Diagnostic Investigation described the diagnosis of EEEV in several puppies in Michigan and New York, highlighting a rare but serious risk of infection in dogs.

Puppy Encephalitis Cases

The first puppy, an 8-week-old female boxer, exhibited a 2-day history of lethargy, weakness, suspected seizures, diarrhea, and aggression toward its littermates. All other littermates were apparently healthy. On presentation, the dog was ataxic, had cranial nerve deficits, and began having seizures. The owner elected euthanasia.

The second and third puppies were 4-month-old St. Bernard littermates that developed signs 2 weeks apart. Both puppies died or were euthanized after an acute history of fever and anorexia, and 1 of the puppies was also obtunded and exhibited seizure-like activity.

Necropsies were performed at the Michigan State University Veterinary Diagnostic Laboratory and Cornell University College of Veterinary Medicine. The boxer puppy was thin, dehydrated, and had mucopurulent discharge and evidence of diarrhea; the St. Bernard puppies lacked significant gross lesions.

Histology revealed that all 3 puppies had cerebral necrosis, fibrin thrombi in small blood vessels, and inflammatory infiltrates in the perivascular spaces. The 2 St. Bernard puppies also had inflammatory infiltrates in the leptomeninges and cerebellum.

EEEV infection was confirmed using the following diagnostic methods:

  • Immunohistochemistry of the cerebrum (all puppies)
  • Polymerase chain reaction (boxer puppy)
  • Plaque reduction neutralization test (St. Bernard puppies)

Some or all of the puppies were tested for other neurologic pathogens, including rabies virus, West Nile virus, canine distemper virus, and Toxoplasma gondii. All results were negative.

What to Know About EEEV in Dogs

EEEV infection is reported rarely in dogs, and cases most commonly occur in puppies. Although early signs typically consist of fever and gastrointestinal upset, severe neurologic signs usually develop 1 to 2 days later. While gross anatomic lesions are rare in infected dogs, humans, primates, and horses, histopathology can include neuronal necrosis, hemorrhage, vasculitis, edema, and encephalitis.

In the case of the St. Bernard puppies, 4 additional puppies from the same kennel eventually developed EEEV infection, raising the question of whether horizontal transmission plays a major role in canine cases. Thus far, horizontal transmission and fecal shedding of the virus have been documented only in birds. The St. Bernards were also infected during an active equine epidemic in the same area of New York, demonstrating that increased surveillance across species is critical during outbreaks.

Dr. Stilwell received her DVM from Auburn University, followed by a MS in fisheries and aquatic sciences and a PhD in veterinary medical sciences from the University of Florida. She provides freelance medical writing and aquatic veterinary consulting services through her business, Seastar Communications and Consulting.

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