A Brief Description of the Natural History of the West Nile Virus
West Nile virus appeared in this country in the fall of 1999, coming in through New York and subsequently spreading throughout the country. As of late 2002, the virus was identified in all states but Oregon, California, Arizona, Nevada, Utah, Alaska and Hawaii. The spread of this disease has been dramatic since its arrival on this continent, and has been identified in Canada, the United States and Mexico. It is expected to appear in California this summer.
The West Nile virus resides primarily in birds, and is spread by mosquitoes. The common hosts for the virus have not been completely identified, but probably include common song birds such as sparrow s and robins. Ravens, crows and blue jays are examples of birds that may be infected but then frequently die from the disease. The appearance of large numbers of these dead birds may signal the appearance of the disease in a new area, and was in fact one of the early clues that West Nile Virus had arrived in the United States.
Birds that become infected and survive but still carry the virus in their blood are fed upon b y mosquitoes which then feed on uninfected animals. While many species of animals may be exposed to the virus in this way, very few actually develop obvious disease. The horse is the most susceptible mammal, with about one third of sick horses dying or being euthanized. People may also become ill and can die from this infection. Infections in other mammalian species have been occasionally reported. Fortunately infections in mammals are not transmissible by casual contact and would at the least require the transference of significant quantities of blood (blood transfusions or organ transplants in people, for example). Most animals that are exposed to the virus will develop antibodies to the virus but never show signs of disease. However, of those that show signs, many will die or be euthanized. Of the species of animals that rarely show signs, many of those that become ill have been determined to be either young, old, or have a weak immune system.
West Nile Virus Disease in Lamas
West Nile virus disease has been diagnosed in both llamas and alpacas. Compared to the number of cases confirmed in horses, very few cases have been documented in lamas . The lama species are very resistant to infection and subsequent disease following exposure. Very few llamas or alpacas have died from this disease since its introduction into North America, although those who develop signs are quite likely to die. Because so few cases have been reported and confirmed, reliable numbers are hard to come by.
The West Nile virus affects the central nervous system, causing the death of neurons and local inflammation. Llamas have been reported to die within a few days of developing signs. If recovery occurs, it may result in residual neurologic defects. Clinical signs include fever, weakness, muscle twitching, incoordination, depression, head tilt, seizures, altered mental state, circling, stumbling, inability to stand, coma and death. The signs can be similar to those of other diseases of the central nervous system, such as rabies, Parelaphostrongylus tenuis (meningeal worm) infection, bacterial infections, or even trauma.
Diagnosis is currently based on typical clinical signs and evidence of antibody production. Two tests for antibody levels taken 10-14 days apart and showing increasing levels is typical of a current infection. The specific blood test developed to diagnose this disease in horses does not work in lamas. Post-mortem exam of the brain and spinal cord show typical but not diagnostic lesions. However, necropsy is helpful to rule out other causes of neurologic disease.
Prevention of the disease is based on vaccination and control of mosquito populations. One vaccine has been licensed for use in horses. This vaccine has been shown in research testing by the manufacturer and widespread field use in the last 3 years to be safe and effective for use in horses if used according to the manufacturer’s recommendations. The vaccine is given to horses in 2 doses 3 to 6 weeks apart as an intramuscular injection, followed by yearly boosters. Full protection is not expected until at least 3 to 4 weeks following the second dose. Vaccination should be done in the spring, before mosquito season begins. It has been used on a twice-a-year basis in areas with year-round mosquito populations.
Limited research with the equine vaccine suggests that the vaccine will induce antibody formation in vaccinated llamas and alpacas, and 2 doses are required to stimulate any significant response. Maximum antibody levels are not reached for several weeks after the second dose. The antibody level also declines relatively quickly, so revaccination every 2 to 3 months may be required during the mosquito season. Remember that vaccinated animals will only be protected approximately 6 weeks after the first dose of vaccine is given. If the decision is made to vaccinate after the disease is present in the area, animals may become ill before they are fully vaccinated.
There has been no research into the safety of this vaccine in pregnant females, young lamas, and breeding males. In horses, all these groups have been safely and successfully vaccinated.
Control of the mosquito population is considered critical in control of the spread of West Nile virus. Since mosquitoes require standing water to breed, periodic emptying of any water containers (watering troughs, buckets, tires, etc .) is helpful. Other suggestions include adding mosquito larvae feeding fish to ponds and the use of insecticides and insect repellents.
One Veterinarian’s Opinion on Vaccination of Lamas
The decision to vaccinate llamas and alpacas for West Nile virus should be made only after careful consideration of the risks and benefits. In favor of not vaccinating are the following:
? Lamas are not very susceptible to developing disease after exposure to the virus.
? No vaccine has been developed, thoroughly tested, and licensed for use in lamas.
? The safety of the available vaccine in pregnant females, youngsters, and breeding males has not been established.
? The available vaccine has only been thoroughly tested and licensed in horses.
? The vaccine is not inexpensive, and multiple vaccinations for a large herd may be cost prohibitive.
? Multiple doses are required to induce production of high levels of antibodies, and frequent vaccination may be required to maintain these antibody levels.
In favor of vaccinating animals are these points:
? Lamas can get West Nile virus infection, develop neurologic signs and die of the disease.
? A vaccine is available which may aid in the prevention of disease, and appears to be safe for use in lamas.
? While West Nile virus has not yet been diagnosed in animals in California, it will likely be here this summer.
? Following a relatively wet winter, mosquito populations are likely to be higher that in recent years.
The decision to vaccinate for this new disease should not be made without consideration of the current and expected exposure risk and the costs involved. If the decision is made to vaccinate, the available vaccine does seem to confer some protection without serious side effects. However, I believe that the negative points tend to outweigh the positive points regarding the vaccination of lamas.
Even though I highly recommend vaccination of horses, I am currently not recommending vaccination of lamas, with the possible exception of any lama valuable enough (economic or sentimental) that the owner is not willing to take any avoidable risk, especially if that animal resides in an area with significant mosquito populations. As there is no official recommendation on vaccination of lamas, you should discuss the pros and cons with your veterinarian, who is in the best position to know your situation and the local conditions.
