Description

West Nile virus (WNV) is a mosquito-borne virus that can cause encephalitis (inflammation of the brain) and/or meningitis (inflammation of the lining of the brain and spinal cord). WNV primarily infects and multiplies in birds, which serve as reservoirs for the virus. Corvids (crows, blue jays, and ravens) are the most susceptible to the disease. The virus is spread between birds mostly through the bite of an infected mosquito. When the level of virus transmission among birds and mosquitoes becomes high, horses and humans can be incidentally infected.

Distribution

Outbreaks of the disease caused by WNV have occurred in Egypt, Asia, Israel, South Africa, and some parts of Europe and Australia. The virus was first seen in the U.S. in the fall of 1999 in New York City, and has since spread west across the U.S. to the Pacific Ocean, into several Canadian Provinces and into Mexico. WNV was first detected in Michigan in 2001. Since then it has been found across the state.

Besides corvids, over 170 other species of birds have been positive for WNV in the U.S. In Michigan, 28 avian species have been detected positive. WNV has been detected in at least 30 domestic and wild mammals in the U.S. In Michigan, the eastern fox squirrel and the white-tailed deer are the only wild mammals detected positive for the disease. Humans, horses and other mammals are also susceptible to WNV, although they do not usually exhibit clinical signs.

Transmission and Development

In the wild WNV has only been known to be transmitted by mosquitoes. Mosquitoes become infected when they bite, or take a blood meal from a bird carrying WNV. The infected mosquito then transmits the virus to another bird through a second bite. Birds in the "enzootic cycle" are relatively resistant to the disease, and serve to maintain the virus in the avian population. Susceptible avian species and mammals may be infected when bitten by a "bridge vector" mosquito (one that feeds on birds and mammals), which essentially takes the virus out of the enzootic cycle. These bridge vector species of mosquitoes that bite mammals as well as birds may transmit the virus to a horse, human or other mammal.

Research has shown that it is possible for bird to bird transmission to occur as a result of oral and cloacal secretions contaminating water and food. There may be a direct bird to bird transmission, but the cause is unknown. Research has also shown that the virus may be passed to raptors by eating infected birds. However, the most likely route of transmission is still the mosquito.

In Michigan, the most common species of mosquito associated with WNV transmission is Culex sp. These mosquitoes primarily bite birds and breed in small collections of stagnant water. When levels of the circulating virus are high, horses and humans can become incidentally infected. Horses and humans are considered dead-end hosts because they do not produce enough virus in their blood to infect biting mosquitoes. There is no evidence that infected horses, people, or other mammals are able to transmit the virus through normal contact.

In 2002, a positive Coquillettidia perturbans mosquito pool was detected for the first time in the state. This species of mosquito may be important as a potential bridge vector species as it bites both birds and mammals.

Migrating birds carrying WNV in their blood have had a significant impact on the spread of WNV across the U.S. Because corvids have proven to be especially susceptible to WNV they have been the focus of WNV surveillance efforts across the U.S. and Canada. Residents in areas where WNV activity has been identified are at risk of contracting West Nile encephalitis or West Nile fever. However, very few mosquitoes actually become infected with the virus and less than 1% of people who are bitten become infected and severely ill. People over the age of 50 are found to be at the highest risk of developing severe disease. In addition, immunocompromised persons or people with underlying health conditions have an increased risk of developing more severe forms of WNV disease.

In 2002, the CDC confirmed additional routes of transmission which include infection through transplanted organs, transplacental (mother-to-child) infection, transmission through breast milk, and blood transfusion-associated transmission. Blood banks now screen donor blood units for WNV.

Clinical Signs and Pathology

Most corvids infected with WNV die within 3 weeks of infection. Clinical signs prior to death may include incoordinated walking, weakness, lethargy, tremors, and abnormal head posture. Although crows and blue jays account for between 50 and 90 percent of reported avian cases, WNV infection has been identified in over 170 other avian species across North America.

Clinical signs and pathologic findings of WNV have not been thoroughly described in most mammalian species. In cases where a description is given, both clinical illness and pathology involve the nervous system. Therefore, it seems that WNV associated disease in both horses and other mammals is primarily a neurological disease.

Most people who are affected with WNV either have no symptoms or experience mild illness with flu-like symptoms. In some people, particularly the elderly, WNV can cause serious disease that affects the brain tissue. In extreme cases it can cause serious neurological damage and can be fatal.

Diagnosis

Corvids are reported and collected by the health departments or DNR wildlife offices if found dead or observed displaying symptoms consistent with WNV infection. Oral swabs are collected from the bird and submitted to Michigan State University's (MSU) Diagnostic Center. A commercial kit, called VecTest West Nile Virus Antigen Assay is used to test for the presence of the WNV antigen. The assay is a rapid "immunochromatographic" assay (i.e. color change is detected when antibodies to WNV bind to virus antigen or protein in the specimen) and the test can be read in 15 minutes.

Non-corvid birds and mammals observed alive prior to death with signs of a central nervous system disorder are collected and submitted to the DNR where a complete necropsy is performed. Tissue sections of the kidney, heart, and brain are sent to MSU for immunohistochemical (IHC) examination. The tissues are placed in formalin for 24 hours and then are embedded in paraffin wax. Thin sections of the tissues are cut from the paraffin blocks and stained for WNV antigen using an antibody directed against the virus. A positive reaction is indicated by a reddish brown, granular staining in the cells of the kidney and/or heart.

Treatment and Control

There is no vaccine available for mammals and birds. Keeping pets indoors around dusk and dawn when mosquitoes are most active can help them avoid being bitten. There is no vaccine or specific treatment for humans. Treatment for humans is aimed at improving the symptoms of an infected person but does not shorten the course of the illness. Main treatment concerns involve lowering fever, avoiding dehydration, decreasing brain swelling, or dealing with a loss of automatic breathing activity. Mild symptoms will usually resolve in a few days. In more severe cases more intensive supportive therapy involving hospitalization is needed. Currently the only vaccine available is for horses (although it has been used on avian species in captive collections in the U.S. and Canada). The vaccine is available through a licensed veterinarian.

Reducing exposure to mosquitoes is the best method of control. These methods include: reduce time outdoors at dusk and dawn, wear long sleeves and pants when outdoors, apply insect repellent containing DEET (follow manufacturer's directions), and drain standing water in yards where mosquitoes may breed.

Significance

The impact of WNV on corvids in the U.S. is the greatest with between 50 and 90 percent of the reported avian cases being corvids. In 2002, significant numbers of raptor species (owls, hawks, eagles, and vultures) were found to be infected with the virus. Many mammalian species have also been shown to be susceptible to WNV. Human health is not at great risk with only 1% of those bitten actually becoming infected. People should still take the proper precautions in areas where WNV activity has been shown to be high.

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