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WEST NILE VIRUS DOES NOT APPEAR TO INFECT CATTLE

by: Stephen B. Blezinger
Ph.D, PAS

As in the past, occasionally it becomes necessary to address issues that don't really apply to the feeding and management of beef cattle. The events surrounding the occurrences of West Nile Virus (WNV) have made a discussion of this disease important here.

To begin, we need to relate apparent good news: West Nile does not appear to infect cattle. Thus, fortunately, this is one health problem that cattlemen do not have to worry about. It does, however, infect horses and people and that is an obvious cause for concern. The following will discuss this disease and hopefully help dispel some of the misinformation that seems to become so rampant when the news media latches on to an issue such as this.

Many of the health agencies have released great volumes of information pertaining to WNV. Agencies such as the Center for Disease Control and Cornell University Center for the Environment are both excellent sources of data pertaining to the disease.

Introduction and History

The Center for Disease Control (CDC) reports that WNV has emerged in recent years in temperate regions of Europe and North America, presenting a threat to public, equine, and animal health. The most serious manifestation of WN virus infection is fatal encephalitis (inflammation of the brain) in humans and horses, as well as mortality in certain domestic and wild birds. Unfortunately we have seen some of these more extreme cases, although they are not very common in humans.

West Nile virus was first isolated from a adult woman in the West Nile District of Uganda in 1937. The disease was actually better recognized and described in Egypt in the 1950s. As noted above, the virus became recognized as a cause of severe human meningoencephalitis (inflammation of the spinal cord and brain) primarily in elderly patients during an outbreak in Israel in 1957. Equine disease was first noted in Egypt and France in the early 1960s. The first appearance of WN virus in North America in 1999, with encephalitis reported in humans and horses, and the subsequent spread in the United States may be an important milestone in the evolving history of this virus.

Transmission Cycle

WNV is typically transmitted between birds and mosquitoes. This is essentially a two-way street. Infected mosquitoes bite birds and transmit the disease or clean mosquitoes bite infected birds with the disease and pick up the blood born virus. Mammals occasionally become infected if bitten by an infected mosquito, but there is no evidence that people (or horses) can get the virus from contact with sick people or horses. Mammals are generally considered to be "dead-end vectors" because the virus does not generally build up (amplify) in their blood to levels high enough to be infective. Thus mosquitoes that bite an infected person or horse cannot pick up the virus from them and spread it further. Research by the USGS National Wildlife Health Center and the Wildlife Conservation Society has shown that bird-to-bird transmission is possible, though the full significance of this is not yet known. It may also be possible (though not field-verified) that predatory birds may become infected with the virus by eating infected prey (e.g., other birds, small mammals, frogs).

Infections in Birds

In some bird species, particularly Corvids (crows, blue jays, ravens), the virus causes disease (often fatal) in a large percentage of infected birds, but birds of many other species can carry the virus without becoming sick (e.g., chickens). Because crows are large and noticeable near human settlements, dead crows have been an important and sensitive "indicator" of WNV presence in an area. Research is underway to better understand how WNV is affecting the crow population. Migratory birds are likely a key means by which the virus is transported to new areas along their migratory routes. For example, birds migrating southward from the northern Midwestern states--where intense transmission was occurring in 2001--have now carried the virus into the Southern states. Northward migratory birds are likely responsible for reintroducing the virus to Northern states. Given the patterns of bird migration, one hypothesis is that WNV will move westward in a zig-zag fashion, arriving on the West Coast only after it is established in places in Central America or Mexico where most of the west coast migrating birds pass during their northward migration.

Mosquitoes as “Vectors” (Carriers)

In the Northern US, WNV has been most closely associated with Culex pipiens mosquitoes, a species that breeds in standing water, especially in water polluted with organic matter. It has been thought that these mosquitoes "prefer" to bite birds, but if breeding sites are available near people's homes and domestic animal enclosures, Culex pipiens may bite people and domestic animals. Culex pipiens are most active at dawn and dusk. Another hypothesis suggests that other species of mosquitoes, not Culex pipiens, acts as a "bridge," biting both birds and mammals (e.g., people and horses). Some recent evidence indicates that Culex salinarius are responsible for WNV transmission to people. Culex salinarius are found in fresh and saltwater marshes, lakes, ponds and seepage areas, as well as in the many types of artificial containers found around human residences and businesses. It is active from sunset to sunrise.

West Nile Virus as a Health Risk for People

Past and recent history has shown that WNV, in some cases, can be fatal to humans. It should be emphasized that this is very rare and occurrences have primarily been in older and very young individuals, most commonly in those with a less functional immune system or whom, for some reason are weaker or more susceptible to the effects of a disease. Some factors that need to be recognized include:

• Birds are far more likely to become infected than people (or other mammals).

• Birds are far more likely than people to become sickened by WNV.

• Most mosquitoes are not infected with WNV.

• Most mosquito bites will not lead to a WNV infection WNV in the Northeast US appears to most likely bridge into the human population in August and September.

• The virus incubation period is thought to be 3-14 days after being bitten by an infected mosquito.

• Most people who become infected do not get sick.

• About 30 percent of people infected in 1999 reported minor "flu-like" symptoms.

• In US outbreaks, fewer than one percent of infected people (about one infected person in 150) became seriously ill, with central nervous system infection (encephalitis &/or meningitis).

• Symptoms of serious illness: fever, disorientation, muscle weakness, neck stiffness, headache, nausea, altered mental status.

• Older people and the immuno-compromised appear to be at greater risk of becoming seriously ill from WNV.

• Children are not in a higher risk group.

• Whether or not these serious symptoms are caused by WNV, they should be treated. If you or someone you care about becomes seriously ill, go to your doctor.

• There is no vaccine or medication specific to WNV -- Treat the symptoms.

• Of 62 hospitalized in 1999 with WNV, 7 died. In 2000, of 20 hospitalized with WNV, 2 died. Eight persons are confirmed to have died in 2001, with at least 56 hospitalized due to WNV infection.

• One "model" for predicting a WNV outbreak in the human population uses 1.5 WNV-positive dead crows per square mile as the threshold number indicating likelihood of a human outbreak. Another model suggests that hot, dry summers with few birds are the best indicator of greater human risk.

Figure 1, taken from Cornel Universities Center for the Environment web page (www.cfe.cornell.edu/erap/WNV/), shows how the disease has spread across the United States as of August 12, 2002.

 

 

Reducing the Risk from West Nile Virus

"Mosquito hygiene" includes source reduction of mosquito breeding sites and avoidance of biting mosquitoes. Both of these factors are key to reducing risk from WNV. Eliminating mosquito breeding sites is also key to reducing risk of pesticide exposure. This is the case because most communities will not consider applying mosquito adulticides (a pesticide that will kill adult mosquitoes) unless WNV is detected in local bird or mosquito populations. Precautions should continue into the Fall until there have been two hard frosts. In the Southern United States, this may occur very late in the year. Mosquitoes breed in wet areas, and Culex are found particularly where there is decaying organic matter (e.g., leaves, grass clippings, animal wastes). There does not have to be much water and the water does not have to be left standing for very long -- some species can reproduce within a week. Thus, they can reproduce throughout the mosquito breeding season in your area (and especially after each rainstorm, drizzle, watering of the garden or washing the car):

• Eliminate or empty the "artificial water-collecting containers" that are prime breeding spots for the mosquito species implicated in transmission of West Nile Virus.

• Clean out rain gutters.

• Aerate swimming pools and ponds (and perhaps stock with mosquito-eating fish).

• Empty unused buckets, water troughs, etc.

• Keep unused tires under cover so they do not collect water

• Drill drainage holes in tires and other containers used in construction sites, farms, gardens and play areas.

• Clean bird baths and animal water bowls at least once a week.

• Avoid mosquito bites by wearing long clothes and/or by using insect repellent when out after dusk or in shaded areas (such as woods) during the daytime. This is when and where most vector species are more likely to bite. People should be especially careful when in "mass gatherings" where the CO2 given off by the crowd attracts more mosquitoes from a greater distance.

Conclusions

While WNV is receiving a huge amount of press, the risk for humans may not be as great as one might believe. However, it is important that precautions be taken as noted. On a positive note, vaccines are being developed for horses that should protect health animals against the disease and should be ready for release in the near future. Just remember, your best defense is to use common sense and reduce or eliminate mosquito exposure as much as possible.

Dr. Steve Blezinger is a nutritional and management consultant with an office in Sulphur Springs, TX. He can be reached at Route 4 Box 89 Sulphur Springs, TX 75482, by phone at (903) 885-7992 or by e-mail at sblez@peoplescom.net.

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