West Nile Virus
Peter S. Sakas DVM, MS
Niles Animal Hospital and Bird Medical Center
(847)-647-9325 FAX (847)-647-8498
The 1999 outbreak of West Nile Virus (WNV) infections in the New York City vicinity was the first introduction of this virus into the Western Hemisphere. During this outbreak, West Nile Virus disease killed seven people and infected thousands of birds and other animals. During the year 2000, more than 4300 WNV-infected birds (predominantly crows and blue jays) were reported from twelve eastern states. On Wednesday September 5, 2001, two crows from the Chicago area were confirmed as positive for WNV. This is the first time this disease was diagnosed in Illinois. It has been diagnosed in 21 other states and Canada, including Wisconsin, Michigan, Indiana, and Kentucky.
The rest of the continental U.S. will be at risk if the virus further expands to the Caribbean and Central and South America, which is extremely likely. If the virus expands to southern locations, it could be transmitted year-round. Infected mosquitoes have been tracked down in Florida this year, the furthest south WNV has been found in the U.S. It has been also recently reported in dead crows in Louisiana.
WNV has been detected in many birds, horses and in at least nine humans this year, from New England to Florida. The first human infection this year was detected in a 73-year-old man from Madison County, Florida in early July. A second case in Florida – an elderly woman – was found to be infected in late July. Both are recovering. The virus also afflicted three horses in Jefferson County, Florida, killing one – the first infection of any non-bird animal species this summer.
The WNV has not known to have infected any humans in Illinois. A total of 58 birds have tested positive for the disease as of early September and this number will most certainly increase as more samples are evaluated.
Because WNV arrived on the scene earlier than in previous summers, government agencies, such as the Center for Disease Control (CDC) and the U.S. Geological Survey are stressing the importance of state surveillance programs.
Humans, horses, and birds are most commonly affected by WNV. The virus can cause pregnant sheep to abort and experimental tests suggest that chickens and pigs could be affected. However, at this time there have been no cases of WNV in chickens or pigs in the U.S.
The virus, native to Africa, is spread when mosquitoes have bitten infected birds and pass it on to other birds and eventually people. Wild birds are the reservoirs for the virus, meaning that they carry the virus in their system and are a source for transmission. When a mosquito feeds on an infected bird that is harboring the virus, the virus is taken in during the blood meal and is then stored in the salivary glands of the mosquito. When the mosquito bites or feeds the virus is injected into the bloodstream where it multiplies and may cause illness.
The virus cannot be transmitted by human contact with an infected dead bird or from an infected person. Only 1% of the mosquito population carries the virus, and less than 1% of the people infected become severely ill, generally the elderly or people with weakened immune systems.
The mosquitoes that transmit the virus to humans still are not known for certain. If the biology and habitat of the mosquitoes are known they would be easy to control. If the mosquitoes could be knocked down early in the season then the virus amplification would not go on.
One of the mosquito species that is known to be a culprit in the transmission of WNV is the Culex mosquito. The Culex mosquito is the most common species in urban areas. The mosquitoes attack at dusk and after dark, prefer birds over humans and thrive in stagnant water, from birdbaths in yards to sewage treatment plants. The peak mosquito breeding season is not usually until late summer. Adult Culex mosquitoes go dormant in the late fall, hibernate until the winter and reappear in the spring. Because of this hibernation, health officials believe that mosquitoes infected with WNV will appear next year.
Horses will exhibit listlessness, stumbling and incoordination,
weakness of limbs, ataxia, partial paralysis or death. A fever is not usually observed. Treatment would usually be supportive and
consistent with standard treatment for animals infected with a viral agent.
During the outbreak of 2000, 36% of the affected horses died or were euthanized. It is likely that many horses recover from the infection without clinical illness. Since horses are incidental (accidental) hosts of the virus, affected animals do not need to be quarantined or euthanized. Through August 22, 2001, there have been 35 confirmed clinical cases in horses with 12 fatalities.
As mentioned earlier the risk for a human contracting disease is quite
small however, the elderly and people with weakened immune systems are at more
risk for developing the disease. WNV can cause flu-like symptoms that range
from mild to serious. The most common
symptoms are a fever and headache, but can include a rash, swollen lymph nodes
and conjunctivitis, to the rapid onset of a severe headache, high fever, stiff
neck, disorientation, muscle weakness, coma or death. Although there were seven fatalities reported due to WNV in 1999,
as of September 6, 2001, there has been only one human fatality this year, in
Georgia.
The WNV attacks the nervous system of birds causing neurological
disorders and death. Some birds
affected with WNV may be unable to fly and they may be found on the ground (or
possibly struck by a car because the bird was unable to fly properly).
Although WNV can infect virtually any species of bird, when crows are infected, unlike birds, horses, or people, they tend to die. The crow seems to be especially sensitive to the virus with thousands found dead since the virus first struck the U.S. Of all birds tested over the past three years, 88% of the positive cases have been crows, according to the National Wildlife Health Center. Crow deaths may be an indicator of WNV activity in an area. To take advantage of this, many states have developed systems to collect selected dead crows for testing for WNV. Other birds that have shown a susceptibility to WNV, but not to the degree of crows, are blue jays and raptors (hawks).
There have been few reports of pet birds being affected by WNV. A cockatiel and blue and gold macaw
reportedly died from WNV in the New York outbreak of 1999. The good news is that there have been no
other reported cases since then, although the risk exists. Our pet birds should be safe as long as we
prevent contact with mosquitoes.
However, a strong concern is that the disease has spread into the south,
where many aviculturists have outdoor aviaries. These populations of birds would be at risk, especially if there
are significant number of mosquitoes present in the area, and if they were able
to gain access to the inside of the aviaries.
WNV is a reportable disease in Illinois, as it is currently considered
to be a foreign animal disease. If you
suspect WNV in a horse, please contact the Illinois Department of Agriculture,
Bureau of Animal Health (217)-782-4944 or USDA/APHIS/VS Office (217)-241-6689
immediately.
Testing of dead birds, especially crows, blue jays and raptors, is a productive surveillance method for identification of WNV presence in an area. These birds can be necropsied and tested for WNV. Sick birds (crows, blue jays and raptors only) should be reported to the Illinois Department of Public Health (217)-782-6384. If you find a dead bird of the aforementioned species, contact your local health department during regular working hours. The local health department may send a representative to pick up the bird or may request that you dispose of the bird in an appropriate manner (check with your local health department for those details). Due to testing limitations the Illinois Department of Health will take only a set number of submissions from each municipality and only through that local health department. They had been flooded with submissions of dead birds of all kinds from private individuals during the initial mania and the laboratory was overwhelmed. In addition, most of these birds were not of the variety typically affected by WNV and they were usually improperly packaged.
Dead birds (of the appropriate species) shipped for WNV testing should be dead less than 24-48 hours, should be double-bagged and sent with refrigeration. Birds with obvious causes of death, such as poisoning, should not be submitted for testing. Wear rubber gloves when handling the dead birds. Crows cannot transmit the disease to humans; it can only be transmitted through an insect bite.
A vaccine for WNV in horses manufactured by Fort Dodge Laboratories has become available but is in short supply. The vaccine has been approved for use in Illinois and is only available to veterinarians. The vaccination protocol requires two doses two to three weeks apart for the prevention of disease caused by WNV. The telephone number for Fort Dodge is (800)-685-5656.
The principal action is to try to reduce the number of Culex mosquitoes by eliminating breeding sources such as stagnant pools of water, discarded tires, and other unnecessary water sources that do not drain well. Standing water sources can be treated with larvacides to kill the developing mosquitoes.
Preventing exposure to mosquitoes is essential. One of the best ways to do this is by
removing any potential sources of water in which mosquitoes can breed. These include:
· Disposing of water-holding containers, including discarded tires
· Drilling holes in the bottom of containers that are left outside
· Cleaning clogged roof gutters
· Turning over plastic wading pools or wheel barrows when not in use and do not allow water in bird baths to become stagnant
· Aerating ornamental pools or stocking them with fish
· Cleaning and chlorinating swimming pools that are not in use and being aware that mosquitoes can breed in the water that collects on swimming pool covers
· Landscaping property to eliminate standing water as mosquitoes can breed in any puddle that lasts more than four days
· Cleaning of livestock feeding troughs thoroughly on a monthly basis
Prevention for Horses include:
· Insect proofing stables
· Using insect repellant containing DEET
· Keeping animals inside during peak mosquito feeding hours (sunset to sunrise)
· Avoiding mosquito prone areas such as swampy areas and heavily forested areas, especially during sunset and sunrise
Prevention for humans include:
· Using insect repellant containing DEET
· Wearing long sleeves and pants when out-of-doors
· Avoiding mosquito-prone areas such as swampy areas and heavily forested areas, especially between sunset and sunrise
WNV at this time has not been shown to be a significant
threat to our pet birds. However, it is
prudent to follow the guidelines described above to eliminate any risks. If your bird remains inside there is
virtually no risk, unless mosquitoes can get into your house and the bird
area. If you like to take your birds
outside for short periods of time make sure that you eliminate any potential
breeding sites in your area, do not take your bird outside during periods of
mosquito feeding activity. If you do
notice mosquitoes when you are outside exercise caution and take your bird
indoors. If you have an outdoor
area/aviary for your bird then it is essential that is mosquito-proofed. Another threat that exists for birds that
are kept outside is Sarcocystis which
can cause death in pet birds (it is a problem in the south). The parasite is carried in opossum feces,
which could be carried into an aviary by insects. By insect-proofing your outside facility you can reduce the risk
of disease from both WNV and Sarcocystis.
Although the risk of humans and pets contracting West Nile Virus disease is quite low it is important to understand the organism, means of transmission, signs of disease and preventative measures so that can eliminate any potential danger. Hopefully, you have gained an understanding of WNV and have had some fears alleviated. Further information can be gathered from the following sources. For APHIS data on WNV, go to http://www aphis.usda.gov/oa/wnv/wnvstats.html. A guide to state and local health agency activities related to WNV is at http://www.cdc.gov/ncidod/dvbid/westnile/city_states.html.
The following discussion was gathered from several sources, publications from the Illinois Department of Agriculture, DVM magazine, Chicago Tribune 9/25/01 (article by Karen Mellen), and personal communication with David Phalen, DVM, assistant professor at Texas A&M College of Veterinary Medicine.