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Pets & Horses: Mosquito-Borne Diseases

Mosquitoes are a nuisance to everyone, including the non-human members of our families or businesses. You’re probably aware that West Nile Virus is carried by mosquitoes, and that it can make you quite sick and even kill you. Did you know that it could also harm your animals? It’s not the only disease hitchhiking with mosquitoes that may do so, either.

Dogs & Cats

Not all zoonotic diseases (i.e., the ones that can be transmitted between people and animals) are dangerous for all species. For instance, West Nile Virus can infect dogs but is unlikely to make them sick. Infected cats may or may not show signs of general sickness, including reduced activity and slight fever. Similarly, there’s likely no reaction to forms of equine encephalomyelitis normally found in the U.S. {1}

However, there is a major infection for cats and dogs that are transmitted by mosquitoes — heartworm disease. At best, this is a serious illness, at worst, it’s fatal. Dogs are more likely to become infected, but infected cats are much more likely to die. Mosquitoes can carry the larval form of the heartworm, which, once passed on to your pet, develops into a parasitic worm that lives in the large blood vessels around the lungs. The worms can live for years, causing problems not only in the lungs and heart but the liver and kidneys as well. {2}

Much more rarely, mosquitoes can carry Lyme disease. While this tends to infect dogs more than cats, it can infect both species, causing lethargy, depression, loss of appetite, and even lameness. {3}

 Horses Have Higher Risks

There are a higher number of diseases that can be passed to horses via mosquito than there are for dogs and cats. They’re much more likely to be harmful, as well. West Nile Virus is the most recognizable, becoming deadly once it’s capable of breaching the blood-brain barrier, causing inflammation of the spinal cord and brain. The disease that may surprise you is heartworm — horses are also susceptible, although infection is rarer. Having your horses on a regular de-worming schedule is their best defense. Other mosquito-borne diseases you ought to be aware of:

  • Eastern and Western Equine Encephalomyelitis – Caused by the Togaviridae virus, early stages of both illnesses include poor appetite, stiffness, a fever (103-106° F) that lasts as long as 48 hours, confusion, seizures, and stupor. Often, WEE won’t progress, but in most cases, EEE will. If the diseases do progress, a short incubation period precedes brain dysfunction and inflammation, behavioral changes, GTI dysfunction, and in as many as 15% of cases, death. While the diseases are zoonotic, they cannot be transmitted from horse to human.
    Treatment is available, but any brain damage is permanent. Vaccines are available and highly recommended by veterinarians.

  • Venezuelan Equine Encephalitis – Similar to EEE and WEE, VEE is considered an “exotic” form in the U.S. and contains some sub-types. However, VEE is more dangerous in that the disease can be passed from horse to human, and is especially dangerous to humans. Like EEE, VEE causes flu-like symptoms before progressing to brain dysfunction and inflammation, behavioral changes, GTI dysfunction, and in 40-90% of equine cases, death. The “leaning stance” and circling are classic signs of VEE and are caused by the inflammation in the brain.
    Treatment is available, but as with EEE, brain damage is permanent. Vaccines are available.

  • Equine Infectious Anemia – Also known as Swamp Fever or Coggins Test disease, EIA is usually transmitted via horsefly but can be transmitted via mosquito. Acute cases involve loss of appetite, lethargy, and fever. While some horses may recover, the disease is chronic and will reappear in times of stress, featuring anemia, weight loss, and ventral edema (swelling of the legs and lower abdomen). About 33% of infected horses will die within 1 month with an acute infection. China currently uses a vaccine called “Chinese Live Attenuated EIA vaccine” which has been widely used there since 1983. A vaccine in the U.S. is in development, but there is not one currently available {4}.

  • African Horse Sickness – Although this is found almost exclusively in Africa, outbreaks have occurred in India, Pakistan, parts of the Middle East, Spain, and Portugal. There are some sub-types, but acute forms (often pulmonary) can have a death rate as high as 95% in as little as a few hours after symptoms can be recognized. Symptoms may include difficulty breathing, sweating, fever, and in other sub-types, edema in the face, tongue, and throat, as well as heart failure. Vaccinations are available. Treatment may be available — quarantine and monitor horses traveling from high-risk locations. 

How to Impede Infection

At home, you should follow the basic steps of mosquito-prevention: remove or drain objects that could provide standing water to breed in, use screens to keep bugs out, and make use of repellents. Since you and your indoor pets inhabit the same space, you’re unlikely to be infected by your pet; however, you do share the same risk of infection from mosquitoes. Avoid outings at high-mosquito times, such as dawn, dusk, and after rain.

Any animals allowed to roam, especially over large spaces (e.g., horses) or with minimal restriction (e.g., cats) will have a higher risk of infection, especially if they have access to standing water. Talk to your vet about appropriate vaccines and preventatives and how often your animals need them, as well as what repellents are suitable. Most repellents that are safe for humans are not safe for animals; they have different systems than humans and also may ingest repellents when grooming.

Most importantly, talk to your vet about the signs and symptoms to watch for. If you suspect your animal may be infected, talk to your vet immediately.  Where prevention fails, early treatment will give your animals the best chances of survival and help prevent other animals from becoming infected.

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