Wondering why you haven’t heard much about West Nile virus (WNV) this summer? In this case, no news is good news.
Thus far in 2004 there have been no new cases of humans contracting WNV in Mississippi. That compares to 83 confirmed cases of West Nile virus in Mississippi in 2003, with the largest number in Hinds County (18) and Harrison County (15), according to the Mississippi State Department of Health (MSDH).
WNV is a mosquito-borne illness that affects birds, animals and humans. Symptoms in humans are similar to the flu. Occasionally, illness can be severe, leading to meningitis or encephalitis. In 2003, two people who were infected in Mississippi died.
Birds that have tested positive for WNV in 2003 include eight in Adams County, one each in Rankin, Marion, Newton and Montgomery counties and two in Neshoba County. No horses have tested positive for WNV in 2004. That compares to 143 birds and 111 horses with WNV in Mississippi in 2003.
Dr. Jerome Goddard, MSDH medical entomologist, said Mississippians should be aware that WNV is still present in the state, and they should take proper precautions to reduce the risk of infection. Avoid mosquito bites to prevent infection and apply insect repellent containing DEET. When possible, wear long sleeves, long pants and socks when outdoors, and also when possible minimize outdoor activities during peak mosquito biting hours, usually dusk and dawn.
Goodard said most people who are infected with WNV do not become ill. However, approximately 20% of the people who become infected will develop West Nile fever. The symptoms include fever, headache, tiredness and body aches, occasionally with a skin rash on the trunk of the body and swollen lymph glands. These symptoms generally last three to six days and resolve spontaneously.
The symptoms of severe infection (West Nile encephalitis or meningitis) include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness and paralysis. It is estimated that approximately one in 150 persons infected with the West Nile virus will develop the more severe form of the disease. The most significant risk factor for the severe form of the disease is advanced age.
While the abnormal amount of rainfall seen in many areas of the state in June probably contributed to large numbers of mosquitoes, Goddard said the predominant type of mosquito resulting from heavy rains are “floodwater” species, which are painful biters but not believed to be involved in WNV transmission. The primary carriers of WNV are members of the Culex group and often are breeding right up around people’s homes and patios in the back yard.
“Their presence or absence around a home is largely unaffected by heavy rains,” Goddard said. “High numbers of huge floodwater mosquitoes are a nuisance, but irrelevant in regards to WNV. That goes to demonstrate the importance of prevention and protection measures we all must take to avoid contracting WNV.”
The worst may have passed as far as WNV in Mississippi, said Dr. James Jarratt, Mississippi State University Extension entomologist. When WNV first entered the eastern U.S. and moved across the country, it was common to see a lot of WNV in the areas affected.
“But as it progressed, birds become more tolerant of the disease,” Jarratt said. “They don’t develop it to the point that once a mosquito feeds on bird, it is enough to give it to mosquitoes and then to people. It is beginning to look like the worst cases have passed this area of the country. There may be a few cases but not as many as there were a couple of years ago.”
Jarratt said he hasn’t had as many questions this year from cities and utility districts requesting advice on mosquito controls. He believes many went through a steep learning curve the first year WNV emerged as a major threat.
“They have picked up all the things they need to know, and plus are probably not getting the pressure from the general population about the disease and control of mosquitoes,” Jarratt said. “They know how to calibrate the machines, and how to maintain their machinery. The types of chemicals used have been standard over the past several years. There has not been a good deal of change in that area.”
Some people who work outdoors a lot have reported that tick populations seem to be especially heavy this year. But Jarratt said abundant rain could harm tick population by washing away immature ticks.
“From the standpoint of ticks, the rain may not have been real detrimental to them,” Jarratt said. “But I don’t think it added to the numbers.”
Could a mild winter be a factor behind increased tick and mosquito populations? Most times Mississippi winters are not very cold.
“Every now and then we will have a hard winter that will exert some pressure on the insect population,” he said. “For the most part, things like mosquitoes and ticks that have developed for thousands of years in this area are pretty much adapted to the weather. If we had a winter recorded that was the coldest in a hundred years, I’m sure that would hurt the population. Most years it doesn’t get cold enough to kill insects. And, of course, the more who survive winter create potential for more numbers of insects over the spring and summer.”
Cases of tick diseases are rare in Mississippi. He estimated that cases of Lyme disease in Mississippi average less than 10 to 15 per year, and there are usually a few cases of Rocky Mountain spotted fever— primarily in the northern quarter of the state, although they can occur anywhere.
Despite the threat of WNV diminishing, and relatively few cases of the tick diseases, Jarratt recommends people still be concerned about bites and take precautions.
“As the probability of getting disease dies down, people don’t seem as concerned about it,” he said. “Try to reduce the number of bites. But if you work outside all day long, it is difficult to 100% protect yourself from mosquito bites. Wear clothing that at least gives you some protection from bites. But on a hot, humid day, you sometimes have to choose between heat prostration or a mosquito getting you.”
Another strategy is to avoid being outside in the late afternoon, early evening and early morning when mosquitoes like the Culex species are most active.
Jarratt recommends that people limit sources of water around their homes and businesses to reduce the mosquito populations. If you have a pond, make sure the water is running, and do not let it become a breeding source for mosquitoes.
For the most up-to-date information on West Nile virus, the public can visit the Website www.msdh.state.ms.us or call the WNV hotline at 1-877-WST-NILE.
Contact MBJ contributing writer Becky Gillette at firstname.lastname@example.org.