Pesky mosquitoes plague animals as well as people

Farm Forum

This is not the time of year I usually write about mosquitoes. But as people in many parts of the region know, this has not been a usual year. Normally we dry up in August and September. Instead, it has gone the other way. Precipitation is just about always welcomed in the Dakotas, but sometimes undesirable side effects come with it. This year, millions of those side effects are apparent – mosquitoes.

Of course, mosquitoes can’t thrive without water. They lay their eggs in standing water or muddy areas around water. With ample moisture, a mosquito population boom results.

Not all mosquitoes are created equal. The pest we slap at could be any one of over 150 different species of mosquitoes found in the US. They all vary somewhat regarding their purpose in life, where they like to breed, and what kind of harm they can inflict. By far the majority of mosquitoes that typically aggravate us are of the Aedes vexans variety. They like to breed next to floodwater situations (standing water in areas that are typically dry) and are appropriately referred to as “nuisance” mosquitoes. These bugs generally don’t have the capacity to carry serious diseases to animals or people. They simply proceed to drive us nuts.

However, other species of mosquitoes have evolved to infect people or animals with nasty diseases. Prime examples are the mosquitoes that carry West Nile Virus, Culex tarsalis and Culex pipiens. The eggs of this type of mosquito need to stay continually moist in order to hatch (nuisance mosquito eggs can survive short dry spells). Out in the yard or pasture, there’s no way to tell the difference between the disease-carrying bugs and the nuisance species.

The list of diseases that people can catch mosquitoes seems to be getting longer. These maladies include West Nile virus, dengue fever, and lately, chikungunya (a disease much more fun to pronounce than to catch). West Nile has established itself rapidly and robustly in the United States since its introduction in 2000. It remains a serious threat to us here, and is the main reason we should protect ourselves from mosquito bites. Fortunately, dengue and chikungunya are only carried by mosquito species that aren’t yet able to sustain themselves in the northern US. But travelers to the tropics can bring those diseases back home with them.

Animals are not exempt from mosquito-borne diseases. West Nile Virus hit our horses severely in the early 2000’s. Back then, about a third of the horses that caught West Nile died or had to be put down. Also blamed on mosquitoes are Eastern and Western Equine Encephalitis, better known as sleeping sickness. We can see either Eastern or Western strains here in our region. Similarly to West Nile, people can become sick from these diseases if they’re bit by a mosquito carrying the virus.

Heartworm disease is another common mosquito-based animal illness. As the name implies, heartworm disease is caused by a microscopic worm transmitted between dogs by mosquitoes. Left undetected and untreated, the worms can multiply sufficiently in the infected dog’s bloodstream to cause heart failure.

While our animal friends don’t get to take a pass on mosquito-borne diseases, they do have a huge advantage over us humans – the diseases mentioned are readily preventable. West Nile Virus vaccine is tremendously effective in horses. In fact, the only equine West Nile cases seen anymore are in animals that weren’t vaccinated. Likewise, sleeping sickness vaccine is a successful preventive measure for horses against those viruses. Heartworm disease in dogs is easily prevented by a monthly medication available through your veterinarian.

Beyond disease prevention, our animals don’t enjoy mosquito bites any more than we do. Sprays and other products can be used to repel mosquitoes on pets or horses. Be aware though, that DEET-based products popular with us humans should not be used on dogs or cats – if they lick it off their fur, it can make them sick.

Maybe the best thing about mosquitoes in October is that they aren’t long for this world in our part of the country. Until that hard freeze takes care of them, here’s hoping you can avoid the bites – or worse – that these pests can inflict on us.

Russ Daly, DVM, is the Extension Veterinarian at South Dakota State University. He can be reached via e-mail at or at 605-688-5171.