The first human case of West Nile virus disease in 2012 in a Minnesota resident has been confirmed. State health officials are urging Minnesotans to protect themselves each morning and evening from mosquitoes until the autumn frost.
A St. Louis County man is the state’s first laboratory-confirmed WNV case of the year. He became ill with West Nile encephalitis and meningitis in late May after traveling to south-central Minnesota. He was hospitalized and is recovering. To protect themselves from this potentially life-threatening disease, Minnesotans should routinely use mosquito repellents and take other simple precautions against mosquito bites, Minnesota Department of Health (MDH) officials urged.
David Neitzel, an MDH epidemiologist specializing in diseases carried by mosquitoes, said that the highest risk for WNV is expected from mid-summer through early autumn.
“Sporadic early-season cases can occur, such as this first 2012 case,” Neitzel said. “However, the species of mosquito that transmits the virus to humans is most abundant in July and August. Anyone not already using repellents should begin doing so now to prevent this severe disease.”
About 1 out of 150 people bitten by WNV-infected mosquitoes will develop central nervous system disease (encephalitis or meningitis). Approximately 10 percent of people with this severe form of infection die from their illness, and many survivors suffer from long-term nervous system problems. Fortunately, most people bitten by infected mosquitoes develop West Nile fever, the less severe form of disease, or fight off the virus without any symptoms.
For those who do become severely ill, the disease can be devastating. “Each case is one too many, and the disease can be a huge burden for patients and their families,” said Neitzel. “WNV infection can lead to lengthy hospitalization or even death, and some survivors do not fully recover for many months after becoming ill.”
Illness from WNV can occur in residents throughout Minnesota and among all age groups. However, WNV risk is greatest in western and central counties, which typically have the greatest number of Culex tarsalis mosquitoes, the primary mosquito carrier of the virus in Minnesota. Elderly people or people with weakened immune systems face the highest risk of developing severe or even fatal illness from a WNV infection.
“They need to be especially diligent about protecting themselves from mosquitoes,” Neitzel noted.
The risk for WNV-related disease typically increases during late July and August as more Culex tarsalis mosquitoes begin to feed on people instead of birds, according to Neitzel. People can reduce their chance of WNV infection by using mosquito repellents at dusk and dawn when this mosquito is most active. Several safe and effective options are available:
• Repellents containing DEET (up to 30 percent concentration) are still the most widely used and can provide long-lasting protection against mosquito bites.
• Permethrin is a strong repellent and will kill mosquitoes that come into contact with treated clothing. Repellents containing permethrin are applied to clothing (not skin). Treated clothing can be worn after the repellent dries.
• Effective alternatives to DEET or permethrin include repellents containing picaridin, oil of lemon eucalyptus, or IR3535.
• For maximum effectiveness and safety, all mosquito repellents should be used according to label directions.
Other important steps to prevent mosquito bites and serious illness from WNV include:
• Minimize outdoor activities at dusk and dawn, as this is prime feeding time for WNV-carrying mosquitoes. If you go outside at these times, take precautions even if mosquito numbers seem low; it only takes one bite from an infected mosquito to transmit the virus.
• Wear loose-fitting, long-sleeve shirts and long pants if you must spend time in an area where mosquitoes are biting.
Culex tarsalis mosquitoes, which carry WNV, prefer open, agricultural areas of central and western Minnesota and other Great Plains states. While the majority of Minnesota’s WNV disease has occurred in people from these areas, cases are possible anywhere within the state. Since WNV was first found in Minnesota in 2002, 465 cases (including 15 fatalities) of WNV disease have been reported to MDH.
Symptoms of WNV disease usually begin 3 to 15 days after being bitten and can include headache, high fever, rash, muscle weakness, stiff neck, disorientation, convulsions, paralysis and coma. Information on WNV can be found at the MDH website: http://www.health.state.mn.us/.
Further information on repellents is at the CDC website: http://www.cdc.gov/ncidod/dvbid/westnile/RepellentUpdates.htm.