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Mosquito-Borne Disease: Is Your Horse at Risk?

Mosquito-Borne Disease: Is Your Horse at Risk?
By  Nancy S. Loving
These pests can transmit a variety of dangerous pathogens, depending on certain factors in your region
We’re all too familiar with the irritating sound of a tiny buzzing mosquito planning an approach for landing and a bloodmeal. This arthropod is not only annoying but also dangerous; in fact, it could be one of the world’s most lethal creatures. The many potentially fatal viruses it can transmit to humans and animals include encephalitis, West Nile virus (WNV), dengue, and now Zika, among others. It can also pass along the deadly parasites that cause malaria, yellow fever, heartworm disease, and elephantiasis.
For such a small insect, it packs a big punch of torment and suffering worldwide, with more than 1 million people dying each year from mosquito-borne diseases.
Horses are not immune to the equine pathogens mosquitoes carry. Unless immunized against specific mosquito-borne viruses, known as arboviruses, a horse is at risk of contracting Eastern, Western, and Venezuelan equine encephalomyelitis (EEE, WEE, and VEE) viruses and West Nile virus, which can cause fatal neurologic disease. As these names suggest, certain encephalitides are more prevalent in particular regions, whereas WNV is endemic, or found regularly, throughout the continental United States.
Mosquitoes get infected with these viruses by feeding on infected birds, after which they might turn to human or horse for another meal. Horses and humans are “dead-end hosts” for these viruses, meaning if a mosquito bites a horse or person that is infected with EEE, WEE, VEE, or WNV, it will not pick up enough viral load to be able to spread that infection to the next human or animal it bites.  
While most of these neurologic viral infections come from bird “reservoirs,” there is evidence that snakes and other reptiles could also harbor EEE, says Brian Foy, PhD, a professor in Colorado State University’s Department of Microbiology, Immunology and Pathology, in Fort Collins, who has a special interest in arthropod-borne disease. “For WNV, mosquito-infecting virus titers can be found in the blood of infected cottontail rabbits, some squirrels, and chipmunks,” he says.  
So just how vulnerable is your horse? Well, it depends on your region, along with weather and other factors that can change year to year. It also depends on vaccination status.
How Weather and Habitat Affect Mosquitoes
Mosquitoes love standing water because it’s the ideal place to breed and lay eggs. So, climate and habitat (the first often dictating the second) play key roles in the likelihood of EEE, WEE, or WNV outbreaks occurring. “Hot summers without drought are always important for breeding the most mosquitoes, but rainfall can also confound issues depending on its variability and intensity,” says Foy. “Large deluge storms can flush out mosquito larvae that were previously developing happily in ponds and filled ditches.”
Climate and weather patterns also greatly influence the birds that carry these viruses. “Susceptible hatchlings that can’t fly have poor defensive behavior against biting mosquitoes,” says Foy. “These individuals often develop high viremia (virus levels in the blood) because they have no immunity.”
Temperature ranges also determine which mosquito types are prevalent. “Larvae of many mosquito species in the mountains and northern states grow in melting snow pools in the spring, can tolerate being frozen for a bit, and emerge as adults in early to middle summer,” says Foy. “On the Plains, Culex tarsalis develops in spring rain puddles, but doesn’t emerge in large numbers until around late June, depending on the temperature.”  
Typically, mosquito-borne viruses spread to humans or horses toward the end of the transmission season, likely because of locally abundant “bridge vector” mosquitoes, which are species that transmit infection from the bird reservoir to their preferred feeding source, mammals.¹ In 2016, EEE outbreaks in the eastern United States began in Florida in April and inched northward as spring progressed to end up in Michigan in September. States between saw their first cases May through August. Also in 2016, West Nile virus appeared in June in Florida and by August and September reached as far north as Washington, North Dakota, and Ontario. ( Most of the affected horses were unvaccinated against those viruses.
Early summer to midsummer drought can prevent generations of mosquitoes from developing and lower the burden of mosquito-borne diseases. Foy says drought in late hot summer, however, might not affect mosquito numbers because they’re already adults by then. Whether well-timed (for the virus-susceptible mammals) or not, drought can impact the next year’s disease outlook.
In a 2015 study,² researchers from Mississippi State University reported that the estimated risk of equine WNV infections for 2002 was inversely related to the preceding year’s precipitation. “Dry weather conditions of the previous year might reduce mosquito competitors and predators and subsequently increase mosquito abundances and equine WNV risks in agricultural areas with low biodiversity,” they said. In other words, the more diverse an area’s insect and bird species, the lower the risk of equine WNV cases.
Even with this awareness, it can be tough to know exactly when WNV and EEE will be a big problem. To help, public health agencies use sentinel chicken flocks to monitor the current circulation of certain arboviruses. Foy explains how this works: “The chickens are housed in outside cages where mosquitoes can access and bite them, and then the chickens’ blood is regularly screened for the development of antibodies against the arboviruses. The chickens don’t fall ill from the viruses, so it is safe for them. Sentinel screening can be more efficient than catching and testing mosquitoes.”
Eastern and Western Equine Encephalitis Virus Incidence
Eastern equine encephalitis virus and Western equine encephalitis virus are in the same virus family (Togaviridae) and genus (Alphavirus). This means they have the same viral proteins and genetic structure, says Foy, but their genome sequences—the order of their DNA nucleotides—are a bit different.
“Both viruses are known for causing encephalitis (inflammation of the brain) in horses,” he says. “EEE is an extremely virulent virus, with fatality rates of 70-90% in nonvaccinated horses, while WEE is less so, with fatality rates of 20-50%.
“The mosquito vectors and the host species they bite are what usually define the endemic regions of all arboviruses,” says Foy. “WEE in the USA is mostly transmitted by C. tarsalis mosquitoes limited to the states west of the Mississippi River, where they proliferate on the Great Plains.”  
Eastern equine encephalitis virus is transmitted by Culiseta melanura and other mosquito species that flourish in shaded freshwater hardwood swamps in eastern North America, especially near the Atlantic and Gulf coasts. The wood thrush, tufted titmouse, and a few other bird species are the main EEE reservoir hosts.  
Indeed, the Mississippi River seems to form a natural boundary between EEE- and WEE-carrying mosquito species, says Foy, while the Mexican border appears to be the northern stopping point for VEE. Areas with horses potentially at risk of VEE exposure are those along the southern borders of New Mexico, Texas, California, Louisiana, Mississippi, Alabama, and Florida’s west coast, where the various virus-carrying mosquito species reside.  
Each mosquito species has adapted to its particular habitat, says Foy. For example, C. tarsalis doesn’t range into woodlands but instead flourishes on dry, treeless plains.  
“Mosquito species certainly can become ‘invasive’ and colonize new habitats,” says Foy. “An example of this is Aedes albopictus that has invaded the southeastern U.S. and many other subtropical areas around the globe. It is a vector of Zika virus, chikungunya virus, and dengue viruses that are currently causing pandemics (widespread illness) in humans. Human movement (think cars, planes, and trains) has probably transferred C. tarsalis to the eastern United States innumerable times over the last hundred-plus years, but it has never taken hold, possibly because it isn’t able to adapt to that habitat.”
There have been no reported WEE cases since 2004; however, researchers continue to detect the virus in birds and mosquitoes in the western United States. In a 2016 study,³ New York Department of Health officials showed that from 1992 to 2002, New York saw no cases of EEE but experienced a big spike in cases from 2002 to 2012, with four in humans and 58 in horses, as well as infections in deer and dogs and in pheasants and other birds.  
“Some scientists are explaining this trend by suggesting summer warming linked to climate change, which could directly influence mosquito species abundance,” says Foy. “But, it also could be related to indirect effects of climate, which subsequently is changing reservoir animals’ movement, abundance, or behavior.  
“We also can’t discount the massive effects that humans have on terrain and environment,” he adds. “We move mosquitoes, reservoir animals, and viruses around with great efficiency because we move so much and so often.”  
West Nile Virus Incidence
Again, in contrast to EEE, WEE, and VEE, WNV is endemic throughout most of the contiguous United States. Mortality in horses is 30-40%, with 40% of survivors experiencing some residual neurologic effects on gait or behavior for at least six months.  
“WNV is not just transmitted by one type of mosquito,” says Foy. He explains that in the West, the main vector is C. tarsalis, but on the East Coast it is Culex pipiens and other species. How readily the virus spreads depends on:
  • Its ability to replicate in a particular type of mosquito’s gut, spread through its tissues, and invade its salivary glands to be transmitted in a bite;
  • The bird or mammal species (or diversity of species) that the mosquito species prefers to bite;  
  • The level of viremia in these particular birds or mammals. The higher or longer-lasting the viremia (particularly in birds), the more mosquitoes are likely to pick it up.  
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