Mosquito-borne illnesses are some of the deadliest in the world today, causing more than 300 million clinical cases each year of illness in humans, including encephalitis, malaria, and dengue fever, according to the World Health Organization. But humans are not the only ones who can suffer from the effects of mosquito-borne disease. Mosquitoes can also carry a variety of illnesses that dogs and horses are susceptible to as well, including heartworm, equine encephalitis, and the relatively new but deadly threat of West Nile Virus.
The American Mosquito Control Association says there are more than 2500 different species of mosquitoes throughout the world, with around 150 species living in the United States. As one of the most diverse species on earth, each species of mosquito is adapted to live in specific habitats, exhibiting unique behaviors and biting different animals. There are species of mosquitoes that bite in the morning, and those that bite after dark. There are mosquitoes that bite in the shade and those that prefer the brightness of day. There are some species that prefer to bite mammals like humans and those that prefer birds, horses, dogs and livestock.
Mosquitoes are found in literally every climate in the world, from the jungles of Africa to the Arctic. And they are not getting any less dangerous or easier to control.
The West Nile Threat
With the emergence of the West Nile Virus in New York in 1999, the common house mosquito known as the Culex pipiens has become a carrier of an exotic, sometimes deadly virus that previously had been unheard of on American shores. The West Nile virus was first identified in Africa in 1937, and since that time has begun a slow steady spread throughout the world, including the United States. Since the first case of West Nile virus was identified in New York in 1999, the virus has spread from Maine to Florida, and as far west as Ohio. The West Nile virus causes West Nile encephalitis in humans, a fatal brain infection, and while the disease in humans has been proven deadly but rare, West Nile has quickly become established as a real threat to horses, with 40 percent of horses that contract the disease dying from the illness.
West Nile is a flavivirus, a member of a large group of viruses that are called arboviruses. Arboviruses are transmitted by blood sucking vectors, such as a mosquito or even a tick. Arboviruses require a host, which in West Nile’s case are birds. Mosquitoes act as a vector, and during periods of adult mosquito blood feeding, mosquitoes become infected when they feed on infected birds. Because birds tend to collect in flocks, it is easy for the disease to quickly spread. After an incubation period of 5 to 15 days, infected mosquitoes can then transmit the virus to humans and animals.
Just as in humans, following a bite by an infected mosquito, the West Nile virus multiplies in the horse’s blood system, and crosses into the brain, where it infects the brain, causing inflammation and interference with the central nervous system. Clinical signs of the disease in horses include fever, stumbling/tripping, muscle weakness/twitching, partial paralysis, inability to stand, convulsions and coma. There is no documented evidence of person-to-person or animal-to-person transmission of the West Nile virus, so an infected horse cannot infect a human or other horses.
In addition to birds and horses, West Nile virus has been shown to infect cats, bats, chipmunks, skunks, squirrels, and domestic rabbits, although unlike in horses and birds, it does not appear to cause extensive illness.
There is now a vaccine for West Nile virus, available from Fort Dodge Animal Health, to prevent infection of the deadly illnesses in horses. For more information on Fort Dodge’s West Nile vaccine visit their website at, www.equinewestnile.com. For other sources of information on the West Nile Virus go to, www.cdc.gov/ncidod/dvbid/westnile/index.htm and www.aphis.usda.gov.
Heartworm-A Growing Threat?
In addition to being carriers of viruses like West Nile, mosquitoes also carry deadly parasites, or filarial disease, including heartworm, a potentially fatal disease in dogs.
Heartworms live in the heart and pulmonary arteries of infected dogs. A disease that occurs all over the world, heartworm was once thought to be limited to the south and southeast region of the United States, but is now found in most regions of North America, particularly where mosquitoes are prevalent.
Coinciding with mosquito season, heartworm disease is carried by the mosquito as an intermediate host, who when it bites a dog, transmits the microscopic worm into the dog’s bloodstream. Although it takes a number of years before dogs show signs of infection, once infected, it is a particularly deadly and nasty illness, with worms that have grown up to 10 inches in length clogging the heart, causing severely infected dogs to die suddenly during exercise or excitement.
Heartworm preventatives have been available for some time through veterinarians to help prevent the deadly illness. However, these preventatives are only as effective as the pet owners who administer them, and many dogs have become infected due to pet owner noncompliance. A new injectable, ProHeart 6, is now available through a veterinarian that can give a dog complete protection against heartworm for up to six months.
One reason mosquitoes are prolific and difficult to control is because of their rapid life cycle, which spans from egg to adult in some species in as little as four days. An adult female (the only mosquito that can draw blood) can lay more than 200 eggs at a time, and in the perfect weather conditions, the eggs will hatch sometimes in as little as four days. Although mosquitoes generally only live a few weeks as an adult, one species of mosquito that has been found to carry West Nile virus can survive through the winter, hibernating until warmer temperatures to emerge again.
For all mosquitoes, however, water is the critical component of a successful habitat when laying their eggs. For this reason, pet owners should be aware of any sources of stagnant water around their property, in buckets, rain spots, clogged gutters, birdbaths, etc. Because mosquitoes like to rest on weeds and in other vegetation, pet owners can also help reduce mosquitoes by cutting down weeds near their homes’ foundations and mowing the lawn regularly. There are also a number of insecticides available that can be applied to trees, shrubs and walls.
Other effective measures include keeping pets inside at dawn and dusk, when mosquitoes are most active, and installing bug tight window and door screens, even on stables, and replacing outdoor lights with yellow “bug” lights.
Mosquito-borne diseases are a serious threat to both pets and humans, but with control and prevention, it is possible to protect our pets from the deadly diseases they carry.