UNDERSTANDING
WEST NILE VIRUS
West Nile virus is spreading among
cage bird and other animal - and it's here to stay.
Although we still have a lot to learn
about this potentially deadly virus, we may be able to minimize livestock casualties by
practicing good husbandry.
West Nile virus has been making
news in the United States since it was found in New York City in 1999. Since the North
American outbreak began, authorities have documented WNV infections in more than 8o animal
species, including wildlife and domestic animals.
Psittacines and other bird species
commonly kept as pets can become infected. Disease can occur, and WNV induced mortality
has been documented in both macaws and cockatiels. The birds also can develop asymptomatic
infections that help perpetuate the cycle of the virus. In addition, certain species of
frogs have been shown to become infected and develop large enough virus loads to
potentially infect feeding mosquitoes-the vector that transmits the virus.
The virus has been documented in
humans as well. Outbreaks of West Nile encephalitis occur sporadically in countries where
it has been identified, most recently in the United States and Israel. The effects of
illness can be severe, even leading to death. Seven human deaths were associated with the
initial outbreak, and one fatal case occurred in both 2000 and 2001.
For these reasons, WNV is an
important issue for those involved in the animal care and pet industries. Understanding
WNV can help you minimize animal casualties as-well as potential human exposure.
Discovery and Transmission
West Nile virus was discovered in
Uganda in 1937. Since then, it has been identified in additional countries in Africa, as
well as throughout parts of Europe, Asia and the Middle East. It was not reported in the
western hemisphere until the New York cases in 1999. WNV has spread further across North
America every year since then. The virus spread as far south as Georgia and Florida in
2ooi and seems to have firmly established itself in that region.
As of early October 2001, 921
positive specimens were recorded in New York, 614 dead birds, 28 live birds, 255 Mosquito
pools, 13 horses and humans. Judging by the number of documented cases and the apparent
spread of the virus, it is unlikely that WNV will be eradicated from North America.
West Nile virus is a flavivirus, a
member of a larger group of viruses collectively termed arboviruses. An arbovirus is a
general term for a virus that is transmitted by an invertebrate vector (usually a
blood-sucking insect or tick), which in turn circulates the virus among specific nonhuman
vertebrate hosts. The virus is maintained by cycling between the invertebrate vector and
the vertebrate host.
Humans and other vertebrate animals
besides the primary host can be infected with arboviruses, but usually are incidental,
dead-end hosts (and are generally thought to be incapable of transmitting the virus back
to the invertebrate vector). In the case of West Nile virus, the invertebrate vector is
usually a mosquito most commonly members of the Culex genus-and the primary vertebrate
hosts are birds.
The initial outbreak involved
American and fish crows. Although they remain the most commonly affected species,
accounting for more than 8o percent of documented deaths, more than 20 other types of
birds have died from the virus. The WNV isolate from the North American outbreak seems to
have an increased pathogenicity for birds. This observation is not unexpected, as these
types of viruses tend to cause asymptomatic infections in birds that co-evolved with
endemic strains of the virus. Conversely, the same virus can cause severe disease or death
in birds that are from areas where the virus is not endemic and the animals are
immunologically naive.
Scientists have no evidence that
the virus is transmitted between mammals, including humans. For a mammal to become
infected, it must be bitten by a mosquito that was previously infected by an avian host.
Most infected mammals probably do not develop virus loads high enough to transmit the
virus back to a mosquito or other potential vector.
There has been no evidence of
transmission from carcasses of infected animals (including birds) to humans or other
animals. Studies in which turkeys and chickens were experimentally infected failed to show
transmission of the virus to directly exposed control birds. The mosquito appears to be
the vital key in the transmission cycle.
Effects of WNV
Most animals and people infected
with West Nile virus do not develop clinical disease, and they recover from the infections
without incident. However, it can cause severe disease and even death in susceptible
individuals.
Less than is percent of people
infected with West Nile virus develop severe disease. These usually are elderly patients.
Prevalence studies from Egypt and Sudan show evidence of exposure (antibody titers) in 61
percent and 40 percent of the study population, respectively. These studies indicate that
few cases of severe human disease occur even in areas where the virus is endemic.
In humans and animals, West Nile
virus causes clinical disease by infecting the brain and causing inflammation, creating a
condition called encephalitis. Other potential outcomes of severe infection are hepatitis,
pancreatitis or myocarditis. Mild infections in humans usually result in flu-like symptoms
including fever, headache, sore throat, nausea, diarrhea, muscle aches, rash, and general
signs of respiratory disease.
Detection and Treatment
Although serological tests can be
used to screen for WNV, none are commercially available at this point. WNV infections are
confirmed by virus isolation, documentation of a rising antibody titer, and a variety of
immunologic and molecular procedures (usually post-mortem). Again, these tests are not
commercially available. However, state diagnostic laboratories as well as the National
Wildlife Health Center (Reston, Va.) and the National Veterinary Services Laboratory
(Ames, Iowa) can run WNV diagnostics.
Severe weakness and depression are the
most common clinical changes seen with disease induced by WNV. Other clinical changes
center around neurologic dysfunction. Specific symptoms can include ataxia
(un-coordination), circling and convulsions.
WNV is not treatable. Not all
clinically ill animals will die from WNV infections, however, and can be helped with
supportive therapy such as nutrition, fluids and so on while the virus runs its course.
If you suspect a bird has WNV (or any
illness), seek veterinary assistance. All animals that die should be necropsied.
Precautions
For those involved with the pet
trade, concerns revolve around both animal and human safety. According to the known facts
about WNV, it would appear that the danger posed by direct animal-to-animal or
animal-to-human exposure is minimal. Nonetheless, you still should practice standard
biosecurity precautions with animals suspected of harboring WNV or any zoonotic disease.
Minimal precautions include wearing
gloves when handling sick or deceased animals, maintaining sick animals in isolated
quarters, adequately disinfecting equipment and the environment following exposure to sick
animals, and arranging for complete necropsy examinations of all deceased animals.
In addition, follow general good
husbandry practices: Avoid overstocking cages, mixing incompatible species, and mixing
birds from different sources. For optimum health, house birds individually so they can be
monitored for signs of illness and not serve as sources of infection to others. Again,
these are all general recommendations since we do not have enough information to set
specific guidelines pertaining to WNV
Nor do we have sufficient information
about WNV to make specific recommendations about what sources of birds are safer than
others. The safest bird to buy would be from a closed indoor aviary in a region that has
not been documented to have WNV (yet), that has absolutely impenetrable biosecurity, and
that never allows its bird’s access to the outdoors (and mosquitoes or other
potential vectors). We all know this is simply not possible.
Therefore, the best recommendation is to
obtain your birds from a reputable aviculturist who practices sound husbandry and
biosecurity. Reputable aviculturists should be working with a veterinarian, and should
have necropsies performed on all deceased animals. This should help with disease
surveillance.
Mosquito Control
Since we don't have enough information to
set specific guidelines pertaining to WNV, the best approach is to control mosquitoes and
other biting insects. Mosquito control is essential to minimize the risk of exposure to
WNV.
Mosquitoes can breed in any puddle
of water that lasts for more than four days. For that reason, any man-made, stagnant body
of water that could support mosquito breeding should be eliminated or emptied regularly.
These can include clogged rain gutters, old tires, plastic wading pools, birdbaths and
animal water containers. Aerate ornamental ponds or stock them with fish such as Gambusia
that will consume mosquito larvae.
Use screens and insect repellants to
reduce exposure to adult mosquitoes. Well-maintained - mosquito-proof screening can be
effective in decreasing exposure as long as you take care to avoid trapping a viable
population of insects within the screened area. Insect repellants can be used cautiously
to aid in deterring feeding mosquitoes. Use human products as directed and seek the advice
of a veterinarian before using any product in an animal application.
Finally, decreasing outdoor exposure when
numbers of mosquitoes are high may be beneficial in protecting against WNV. Different
species of mosquito feed at different times of the day or night. Thus, specific
recommendations as to when to avoid outdoor exposure for animals are impossible to make.
However, bringing animals indoors after dark may be helpful in decreasing risk of
infection.
Many Unknowns
It is important for those dealing
with exotic animals to realize that the available information makes it impossible to
predict susceptibility of all species to WNV-induced disease. Different species'
capabilities of passing the virus back to the mosquito vector are also mostly unknown.
The bottom line is that there are many
unknowns regarding WNV infection in the majority of vertebrate species. However, by using
the facts that we do know, the risk of WNV-associated problems can potentially be
minimized.
Reprinted by Pretty Birds from Pet Age Magazine by Ryan S. De Voe, DVM
Ryan S. De Voe, DVM, is a resident in
avian and reptile medicine at the College of Veterinary Medicine at North Carolina State
University (Raleigh).
Additional Information:
http://cindi.usgs.gov/hazard/event/west_nile/west_nile.html
http://www.cdc.gov
http://www.aphis.usda.gov/oa/wnv/index.html
http://www.westnile.state.pa.us/
http://www.doh.state.fl.us/disease_ctrl/epi/alerts/alerts.htm
http://www.promedmail.org/pls/askus/f?p=2400:1000
Use of Insect Repellant w/ Deet:
http://www.cdc.gov/ncidod/dvbid/westnile/qa/insect_repellent.htm
Use Browser BACK Button
Pretty Birds: Copyright © 1997: Last Modified 02/11/03: Copyright Law and International Treaties protect these Web Site
Pages. Warning: Reproduction or Distribution of these images, program, text or any portion
of it, may result in severe civil and criminal penalties and will be prosecuted to the
maximum extent possible under the law.