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West Nile Virus Moves Westward

In 1999, West Nile Virus (WNV) appeared for the first time in the state of New York. Until that time, the West Nile virus had never been seen in the Western Hemisphere. Even though virus outbreaks flared up sporadically in Africa, Asia, Europe and other parts of the world, the combination of vast ocean protection and strict livestock entry enforcement had stopped its invasion into this hemisphere.
Since the virus was first identified in New York, WNV has continued to spread across the United States. By the end of 1999, the virus was verified in four states. By the end of 2000, WNV had been identified in twelve states. In 2001, the West Nile virus was continuing to spread, and as of Sept. 4, 2001, 21 states had reported “verified” cases of West Nile Virus.

At an Oct. 9, 2001 meeting of the California Equine Medication Monitoring Advisory Committee, Dr. Boone of the University of California at Davis (UC-Davis) stated. “We had expected the West Nile virus to be seen in California in the year 2005. But now, we anticipate seeing cases of it in 2002.”

According to the Center for Disease Control’s Vector-Borne Infectious Diseases section, “The continued expansion of West Nile virus in the United States indicates that it is permanently established in the Western Hemisphere.”

West Nile Virus is probably one of the least-known diseases among horse owners, yet one of the most dangerous because not only do horses develop the disease…but also humans and other mammals. Generally causing flu-like symptoms, it can lead, in its most severe cases, to central nervous system infections, causing encephalitis and meningitis.

As with all equine and human diseases, the first line of defense is with the horse owner and individual himself. Knowing and education about and understanding what the disease is…its symptoms and its “course”…can head off more severe and even deadly infections. In western states, we have the advantage of knowledge learned from previous years and cases in the eastern states.

The outbreak of West Nile Virus in New York was considered “an unprecedented event”. After the initial outbreak in 1999, it was not certain if the virus had run its course or if the cycle of infection would begin again when mosquitoes re-emerged. It was hoped that the cold winter months would destroy the virus and mosquitoes carrying it. But not only did the West Nile virus persist through the 1999-2000 winter, but it spread rapidly throughout New York State and the northeastern and Mid-Atlantic states.

By the end of summer and fall 2000, all but one county in New York State had documented, confirmed cases. West Nile Virus was diagnosed in 14 New York City residents…one later died as also 1,263 birds. Also, 400 mosquito pools were confirmed with WNV and ten live wild birds, eight sentinel chickens, two bats, 28 horses, one domestic rabbit, one squirrel and one chipmunk, plus six other individuals in New Jersey were hospitalized in 2000 due to West Nile Virus infection.

Mosquitoes transmit the disease, and mosquitoes are common to every state in the nation. Although not every mosquito transmits WNV, mosquitoes are considered the “bridge vector” since they are the carriers of the virus from the infected to the non-infected. Since the West Nile virus typically circulates between birds and bird-feeding mosquitoes, the mosquito is referred to as a “bridge vector” because it carries the diseases outside its usual cycle, from bird to bird and into an unexpected or “peripheral” species…humans and horses.

So far, WNV has been detected in1 4 different mosquito species. But the good news is that even in areas where mosquitoes do carry the virus, less than one percent are considered infected. If the mosquito is infected, less than one percent of people who get bitten and become infected will get severely ill. People 50 and older are at the highest risk of becoming seriously ill of they are infected. Also, West Nile Virus cannot be transmitted from person to person.

According to the Center for Disease Control, symptoms of WNV in humans include flu-like symptoms. But a person who develops a high fever, confusion, muscle weakness or severe headaches should see a doctor immediately. Once infected, WNV multiplies in the person’s blood system and crosses the blood-brain barrier to reach the brain, which can be fatal. There is no current human vaccine for WNV, but several companies are working toward developing a vaccine.

Since birds are the carriers of WNV, many states, such as California, are setting up “sentinel flocks”…flocks of chickens located throughout the state that are monitored for illness and death. Plus, California and other states are tracking dead crow sightings.

Crows die from an infection of WNV. Consequently, most state health departments are asking individuals to report dead crows or dead crow sightings. Some areas will collect the dead birds, and they will be tested. Other areas are simply “tracking” sightings. Knowing where and how many dead crows there are within a county helps to forecast the human risk for West Nile Virus…and its spread pattern.

There is no evidence that WNV is spread directly from dead birds to humans or horses. But never handle a dead bird…or any dead animal…with bare hands.

Since some horse owners are also dog owners, a dog or even a cat may become ill from WNV. They become infected the same way as humans and horses…by the bite of an infected mosquito. The virus is located in the mosquito’s salivary glands. During blood feeding, the mosquito becomes infected when it feeds on infected birds that may circulate the virus in their blood for a few days. But even if a dog or cat has WNV, there is no documented evidence of person-to-person transmission.

Horses also become infected with WNV by the bite if infected mosquitoes. While data suggests that most horses infected with WNV recover, there are deaths.

According to the Animal and Plant Health Inspection Service (APHIS) of the United States Department of Agriculture (USDA), 60 horses were confirmed with WNV in 2000. Of the 60 horses, 37 survived and 23 (38 percent) died or were euthanized. The horses ranged in age from four months to 38 years, with an average age of 14 years. Thirty-six cases were male; 24 female. Onset of the illness in the horses was from mid-August to the end of October.

According to the October 2001 issue of Horse Report from the UC-Davis Center for Equine Health, “This year (2001) so far, 81 horses have been affected in at least 20 states, with a 33-per-cent fatality rate.”

One of the biggest myths circulating among horse owners is that if a horse is vaccinated against Eastern Encephalitis (EEE), Western Equine Encephalitis (WEE), or Venezuelan Equine Encephalitis (VEE)… the horse will not “catch” the WNV. This is false! EEE, WEE and VEE belong to another family of viruses for which there is no cross-protection. So even if a horse is vaccinated against EEE, WEE and/or VEE, it could still become infected with WNV.

If a horse does become infected with the virus, there is no reason to destroy the animal just because it is infected. An infected horse cannot “spread” the disease to its owner or to another horse. Although no antibiotics or antiviral drugs have yet been identified as effective treatments for WNV infections, supportive and consistent standard veterinary practice data suggest most horses recover from the infection.

On August 1, 2001, the USDA granted a “conditional approval” to Fort Dodge Animal Health for an inactivated (killed) equine vaccine against the WNV. But some veterinarians are “unsure” how it will work, and “its effectiveness is unknown”, plus some individual state regulations are still pending. So, for the individual horse owner who wants to vaccinate a horse, the first course of action is to contact your personal equine veterinarian. If your vet has no information about the vaccine, the next step would be to contact your individual state veterinarian for comment and advice. If that fails, contact Fort Dodge directly for their information and help.

Meantime, prevention is the best course of action. Since mosquitoes are the “bridge vector”, mosquito control is vital! Some states have set up surveillance programs to monitor for WNV. Others have begun mosquito testing for infected “pools”, placed out “sentinel chicken” flocks, encephalitis surveillance, and dead bird testing. States with major flyways for bird migration, such as the Tulelake area in southern Oregon and northern California, are especially concerned with dead bird testing. In other areas, counties and cities have started plans for mosquito spraying.

But the average horse owner can help to control mosquitoes with some good old common sense:

Dispose of tin cans, plastic containers, ceramic pots or similar water-holding containers.
Remove all discarded or old tires on your property. Used tires have become the most important source of mosquito breeding in the nation.
Drill holes in the bottoms of recycling containers kept outside. The holes will allow water to drain out.
Make sure roof gutters drain properly, and clean clogged gutters. A standing tablespoon of water can support a “pool” of mosquitoes
Change the water in bird baths at least twice a week.
In a water trough, have a slight overflow to keep the water moving. Add a couple of fish…fish east mosquito eggs…and turn upside down all water containers not being used in corrals, paddocks or fields.
When turning water containers upside down, don’t forget to turn that wheelbarrow over and those water pails, too!
Clean vegetation and debris from pond edges. Standing water with a high organic content, such as leaves, grass or other plant material, is a prime location for mosquito breeding.
Clean and chlorinate swimming pools, outdoor saunas and hot tubs, and drain water from pool covers.
Use landscaping to eliminate standing water that collects on your property.

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West Nile Virus Moves Westward