Eastern Equine Encephalitis: It’s Rare but Serious in Dogs

The recent news about Eastern Equine Encephalitis (EEE) being discovered in the geographic area around Manchester-by-the-Sea has us very concerned about our family, friends and our pets.  As a practicing veterinarian, I wanted to research any evidence about the affect mosquito bites might have on my patients in this geographical area. To date, I have not reported any cases of EEE at the Manchester Animal Hospital.  However, as the disease is becoming more widespread this season, we all will need to be vigilant.

The risk for the virus will continue until a hard frost, according to public health researchers. Until then, despite falling temperatures, the risk and recommendations for mosquito-borne illnesses remain in place.

EEE 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 Massachusetts, this has been one of the most active years for EEE in over half a century.  In addition to 12 human cases, seven horses and one goat have contracted EEE, and the health department has detected the virus in 422 mosquito samples. First discovered in 1933, Eastern Equine Encephalitis is a highly lethal mosquito-borne disease affecting a wide range of mammalian and avian hosts. EEE is distributed throughout the eastern half of North America, with estimated mortality rates in humans and horses reaching 75 percent and 90 percent, respectively.  Recent research suggests that the host range for EEE may be broader than previously thought. As an example, one study found that 35 percent of wild snakes examined in the southeastern United States tested positive for EEE.

A recent report in the Journal of Veterinary Diagnostic Investigation described the diagnosis of EEE in several puppies in Michigan and New York, highlighting a rare but serious risk of infection in dogs.  The first puppy, an 8-week-old female boxer, exhibited a two-day history of lethargy, weakness, suspected seizures, diarrhea and aggression toward its littermates. All other littermates were apparently healthy.  On presentation, the dog was uncoordinated, had nerve deficits, and began having seizures.  Sadly, the owner had to have the puppy put to sleep.  The second and third puppies were four-month-old St. Bernard littermates that developed symptoms two weeks apart.  Both puppies died or were euthanized after an acute history of fever and anorexia, and one of the puppies was also obtunded and exhibited seizure-like activity.

Autopsies 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 a pus-like discharge and evidence of diarrhea; the St. Bernard puppies lacked significant observable lesions.  Microscopic examination revealed that all three puppies had their brains affected, blood clots in small blood vessels, and inflammatory spaces.  The second St. Bernard puppies also had inflammation of brain tissue.

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.

In the case of the St. Bernard puppies, four additional puppies from the same kennel eventually developed EEE 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. Lamb is the Veterinarian at the Manchester Animal Hospital.