Mosquito-Borne Viruses

Potentially fatal viruses spread to dogs by mosquitoes are often misdiagnosed

By Deb Eldredge, DVM

Eastern Equine Encephalitis (Triple E) is a disease that dog fanciers need to keep an eye on. Known cases may only be the tip of a larger collection of ill or dead dogs. If you have a puppy die of neurologic signs, consider having a complete autopsy done.

In general, dog fanciers have paid minimal attention to some of the serious mosquito-borne viral diseases such as Eastern Equine Encephalitis (EEE) and West Nile Virus (WNV). These viruses are part of the arbovirus population. They are spread by mosquito bites.
Mosquitoes tend to have favorite species to feed on and these two viruses are generally found in bird species. So it is an “accident” when a mosquito picks up one of these viruses from a bird and then passes it on to a different host.

West Nile Virus is often detected in health department surveillance of mosquito pools during the summer throughout the US. While ill dogs do not seem to be common at all, 10 out of 139 dogs tested in New York City in 1999 were found to have antibodies to West Nile Virus. It appears that most dogs exposed to this virus will fight it off.

Both Eastern Equine Encephalitis and West Nile Virus are serious threats to horses. Horses are very susceptible to the viruses. They live outside or in barns, which are not mosquito proof. They are exposed to mosquito populations through all mild weather. The viruses cause neurologic signs in horses with uncoordinated gait, depression and eventually seizures or coma. Horses with Eastern Equine Encephalitis tend to have high fevers as well. Luckily, horses are protected by effective vaccines.

Eastern Equine Encephalitisis notoriously fatal in unvaccinated horses and is known to cause death in people as well. It has not been considered a major threat to dogs. Veterinary surveillance for 2010 up to May of 2011 shows 249 cases of Eastern Equine Encephalitis detected in animals — dogs included.

A recent case in upstate New York emphasizes that cases of Eastern Equine Encephalitisin dogs may be greatly under-reported. Nancy Nosiglia of BericDowns is known for her lovely Saint Bernards and Norwich Terriers. She was very concerned when one of her 4-month-old Saint puppies became ill. He was taken to the local emergency clinic that night.

Dr. Maureen Luschini, VMD was on duty at the Veterinary Medical Center of Central New York. She said the pup came in with a high fever, doing some lip smacking and drooling. Those signs can indicate neurologic involvement. The pup responded to symptomatic care and went home. He was basically normal for two days and then died. An autopsy by Nancy’s vet found nothing unusual on gross examination. Luckily the body was held there.

About a week later a second pup, female this time, came down with similar signs. Along with a high fever, there were some seizures. This puppy was euthanized when her fever could not be reduced. A high fever can cause seizures by itself, but by now there was concern about an infectious disease.

Certainly rabies and distemper had to be considered possibilities, especially since these were pups that might not have fully developed immune systems yet. Rabies is endemic throughout the US. It is a serious zoonotic disease and should always be considered when any animal shows drastic changes in behavior or neurologic signs.

These two puppies had what could be considered an ideal setup for a Saint Bernard in upstate New York. They had nice kennel runs so they could go inside a dog house or be outside at will. Unfortunately that meant they had more mosquito exposure than an indoor dog would have.

Around the same time, a 4-year-old girl living a short distance away died with a diagnosis of Eastern Equine Encephalitis, as did a horse a few towns away. Both Eastern Equine Encephalitis and West Nile Virus had been detected on routine health department checks of mosquito pools in the area. Nancy decided to take one puppy to Cornell for a complete autopsy. The diagnosis came back — Eastern Equine Encephalitis! As Dr. Luschini points out, if Nancy had not been willing and able to pursue a diagnosis with Cornell, these cases would have been missed.

Searching veterinary literature there is really only one main study of Eastern Equine Encephalitis in dogs. This study was done in Georgia, looking at 101 dogs from 1993 to 2005 that died with neurologic signs and had tissues submitted to the Veterinary Diagnostic and Investigational Laboratory in Tifton, Ga. Twelve of those dogs turned up positive for Eastern Equine Encephalitis. All 12 dogs were actually puppies under 6 months of age representing five of the seven AKC Groups.

As is usual with Eastern Equine Encephalitis, all the cases occurred during prime mosquito season. For Georgia that is late spring and summer. For upstate New York, late summer is the time when Eastern Equine Encephalitis cases are generally detected in horses. To diagnose viral disease, sometimes antibody titers can be used. It seems susceptible dogs die so quickly that most serum samples will be negative for an antibody response. The immune systems are overwhelmed so quickly that the puppy’s body hasn’t even had a chance to respond.

For a truly accurate diagnosis, in most cases, viral isolation is required. Samples of tissue are used to try and grow the actual virus in a lab setting. Depending on the virus this can take a week or more. In addition, some viruses do not grow well in lab settings. Simply evaluating tissue samples may not distinguish between the various viruses that can infect the brain.

Nancy’s willingness to pay for a complete autopsy at Cornell and to share her experience provides an invaluable insight into this disease in dogs. How many puppies have died with high fevers or neurologic signs that were dismissed as distemper? If seizures occur, there is a higher likelihood that samples may be sent off for a rabies rule-out, but if the pup is well vaccinated with no history of wildlife encounters, that may not be done.

The Wadsworth Center is the site in New York state for rabies testing. Sources there stated that if a rabies exam comes back negative on a dog with suspect neurologic signs, tissue samples may be shared with the lab that looks for viruses like Eastern Equine Encephalitis and West Nile Virus.

There is no vaccine for dogs at this time. The best way to avoid these viral diseases is to minimize mosquito exposure. Eastern Equine Encephalitis is more prevalent in the eastern half of the US. Dogs living there may need to be confined at night. There should be limited activity during the early morning and evening hours, which are prime mosquito feeding times.

With the very hot weather this summer, those are exactly the times most families have been walking, exercising and training their dogs. Conditioning a show dog means keeping that dog fit. This can be difficult on the road during really hot weather.

Mosquito repellants are another possibility. Many human repellants are not safe for dogs. Some of the topical parasite preparations do have mosquito-repellant claims. There are also natural formulas that can help repel mosquitoes. This is an area to discuss with your veterinarian, while realizing that no repellant is going to be 100 percent effective.