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August 19, 2013
The first reported equine case of West Nile Virus (WNV) was diagnosed in Colorado last week. Colorado State University's Veterinary Diagnostic Laboratory diagnosed the WNV positive horse, a 3-month-old colt from Montezuma County.
According to Hana Van Campen, professor of Microbiology, Immunology and Pathology at CSU, the clinical signs of WNV encephalitis are very similar to many other infectious diseases that affect the nervous system of horses such as rabies and Western equine encephalitis, as well as causes such as trauma and poisonings. If a horse becomes sick, owners should have their veterinarian examine the horse and select appropriate diagnostic tests and procedures to determine the cause of the illness.
“The majority of horses, like people, that become infected with West Nile Virus do not show any signs of illness,” said Van Campen, who also works at the Veterinary Diagnostic Laboratory. “In a small percentage of infected horses, the virus reaches the brain and spinal cord. These horses may have a fever, be lethargic, stop eating, and have muscle fasciculation (twitching), tremors, incoordination and weakness progressing to recumbency.”
There are three types of vaccines readily available to protect horses against severe disease caused by WNV.
“It is most important to vaccinate young horses against WNV thoroughly following the directions of the vaccine manufacturers,” said Van Campen.
Specific recommendations for vaccination can be found on the website of the American Association of Equine Practitioners at http://www.aaep.org/wnv.htm.
At CSU’s Veterinary Diagnostic Lab, a crew of 80 runs about 500,000 tests a year – about 2,000 a day – to help diagnose and monitor sick pets and livestock on behalf of practicing veterinarians, animal owners and government agencies such as the Colorado Parks and Wildlife, the U.S. Department of Agriculture, National Park Service and the Office of the State Veterinarian.
The state veterinarian’s office reports the illnesses that affect livestock and the Colorado Department of Public Health and Environment report diseases that are a potential threat to public health, but it is the staff at the Veterinary Diagnostic Lab that often make the diagnoses as a service to the state of Colorado, which is true to the university’s land-grant mission.
This summer's high temperatures, along with frequent and heavy precipitation, have created ideal conditions for the area’s mosquitoes that carry the West Nile virus according to Chet Moore, an infectious disease researcher at CSU.
Last year’s numbers of mosquitoes were low because of the severe drought, as well as generally cool temperatures. Lack of spring and summer rain meant fewer water-filled locations for the mosquitoes to lay their eggs. Cool temperatures slow the development of larvae and cause female mosquitoes to diminish their efforts to bite humans and animals. Moore’s lab at the CSU Foothills Campus works with the cities of Fort Collins and Loveland to test samples of mosquito populations for the virus.
“A spring with lots of snow and rainfall followed by warm weather tends to be a sign that these mosquitoes will be in higher numbers because the weather pattern provides them with more habitat,” Moore said. “Warm weather leads to runoff and flooding, which leaves standing water for mosquitoes to lay their eggs. Summer monsoon rains further contribute to the problem.”
Hot and dry weather also means an increase in irrigation, leading to standing water on irrigated land.
A mosquito lays her eggs in standing water and if the water sits for about five to six days, the larvae develop into adult mosquitoes. Some large areas of standing water can produce as many as 1 million new mosquitoes; even a birdbath, without preventative measures, can spawn hundreds of newly hatched mosquitoes every four to six days.
Not all mosquitoes carry the virus; in the United States, there are several species of mosquito that are considered by scientists as important in transmitting West Nile virus, and in the West, it is the Culex tarsalis and Culex pipiens mosquitoes that carry and transmit the virus.
And while temperatures play a large role in determining how much of a problem virus-carrying pests may be, scientists still are working to unravel some of the mysteries of West Nile virus. Bird populations, for example, also may impact the number of mosquitoes carrying the virus. Birds carry the virus after being bitten by an infected mosquito, then pass it on to uninfected mosquitoes when they are bitten, helping spread the disease through the mosquito population that in turn infect humans and other animals. Some species of birds, including crows, magpies, jays and many hawks and owls, are extremely susceptible to West Nile infection and suffer high mortality.
The virus has only existed in Colorado since 2002, but researchers don’t know if the virus survives in infected mosquitoes over the winter to re-emerge each spring, or if migrating birds return the virus to the area as they return each spring. The average lifespan of a mosquito is about a week, and although adult female mosquitoes can overwinter, the virus does not seem to survive the winter in mosquitoes.
Only the female mosquitoes bite and transmit the virus. Male mosquitoes feed on plant nectars. Females must take a blood meal to produce eggs and need to mate with a male only once in their lifetime. Once mated and fed, they produce as many as 200 eggs every three to four days in ideal conditions.
In their lifetime, some mosquitoes can easily travel up to 30 miles, sometimes covering up to as much as 15 miles in a night. Fortunately, the Culex vectors of West Nile virus fly only about half those distances. When cooler temperatures strike – in the low 50s, which occurs often at night in the spring and fall – mosquitoes are unable to fly, which contributes to diminishing mosquito bites in the fall and increasing bites when temperatures warm.
“Early prevention is the key to lowering mosquito populations,” Moore said. “Taking steps in the spring can reduce the numbers of mosquitoes through the summer.”