For most people, West Nile does not pose much of a threat

Originally published in The Ottawa Citizen April 8, 2003
Original Title: There is No Denial of West Nile

We are in the midst of several disease outbreaks of great concern to all. The lack of definitive information creates fear and anxiety. The Centers for Disease Control (CDC) posted alerts about Severe Acute Respiratory Syndrome (http://www.cdc.gov/ncidod/sars/) and information about the upcoming season of West Nile virus infection (http://www.cdc.gov/ncidod/dvbid/westnile/index.htm). Information puts risk in perspective.

Many physicians including myself have not seen any cases of West Nile virus infection. What is West Nile Virus, what is the risk of infection and what are its effects?

West Nile virus is found in Africa, the Middle East, Eastern Europe and West Asia. It can infect birds, mosquitoes, horses, cats, bats, chipmunks, skunks, squirrels, and domestic rabbits and humans. Up until 1999, there were no reports of its presence in North America.

Birds are the reservoir for the virus. Mosquitoes bite the birds and transfer the infection to other birds. This process (viral amplification) cycles between bird and mosquito beginning in the spring.

These infections usually arise at the end of August and early September in the temperate regions between latitudes 23.5° and 66.5° (Tropic of Cancer to the Arctic Circle). Mosquitoes emerge in the spring. They start the process of viral amplification in the bird-mosquito-bird cycle. This process peaks in the early fall. Thereafter the risk of infection decreases in humans when female mosquitoes begin semi-hibernation (diapause) and infrequently bite.

Reports out of New York City and Windsor, Ontario indicate the virus survives in diapausal mosquitoes residing in the sewer systems.

The mosquito bite transfers the virus into the blood stream. The virus incubates for three to 14 days before the onset of symptoms.

Most people infected with the West Nile virus will not have any type of illness. Twenty per cent of people infected with West Nile virus develop West Nile fever; a mild form of the disease. Symptoms of mild disease include fever, sore throat, nausea, vomiting, diarrhea, headache, body aches, an occasional skin rash on the trunk of the body and swollen lymph glands in the neck. The rash covers the chest, back and arms, is not itchy and occurs in 20 per cent of those with the mild illness.

Fever is present in at least 90 percent, with weakness, nausea, vomiting, and headache in approximately 50 per cent of the patients. The symptoms last about six days, recovery is rapid and the illness poses no long-term health risk.

A severe form of the disease, West Nile encephalitis/meningoencephalitis occurs in one in 150 infected people. It can cause inflammation of the brain (encephalitis) and possibly inflame the meninges; the protective coating overlying the brain and spinal cord. The symptoms of severe infection can last several weeks. They include headache, high fever, neck stiffness, confusion, disorientation, coma, tremors, convulsions, muscle weakness and paralysis. The damage to the nervous system and brain can be permanent.

To date, the risk of contracting the illness remains low. In regions where the virus resides, few mosquitoes are infected. The risk of severe infection from one mosquito is small. Severe infection occurs in less than one per cent of bitten people.

It is unclear why some people develop severe disease. The elderly seem to be more prone to nervous system damage. Advanced age is the most important predictor of death and patients older than 70 years are at particularly high risk. Less robust immune systems are a fact of life as we age. There is speculation that a weakened immune system is a risk factor for severe disease.

Person-to-person transmission does not occur with West Nile virus. Kissing or touching another person will not place you at risk of disease. Indeed, there is no evidence that you can directly contract the disease from birds, horses or other of the aforementioned mammals or by eating them. As with all food-borne infectious organisms, always fully cook the meat. There are reports of transmission of the virus through blood transfusions but to date it remains a rare occurrence.

The assumption from the CDC is that immunity to the virus develops after the infection and remains life-long.

Prevention is the only means of protecting the population at large. Liberal use of mosquito repellents on clothing or skin (depending upon the concentration of the solution), patio screens, mesh-covered tents, protective clothing and avoidance of mosquito-infested areas are recommended.

Over time, we will learn more about West Nile virus and perhaps develop a better means of controlling it.


© Dr. Barry Dworkin 2003

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