Minnesota Department of Health

News Release

June 28, 2001

Contact information

Season's First Case of LaCrosse Encephalitis Prompts Call for Preventive Measures
West Nile virus is on health officials' "radar screen"- but threat from new encephalitis strain is not imminent

The season's first probable case of mosquito-borne LaCrosse encephalitis has been reported to the Minnesota Department of Health (MDH), prompting a reminder from MDH officials about the need to protect yourself from the LaCrosse virus.

What appears to be the year's first laboratory-confirmed case of the illness was reported in a one-year-old boy from the southwestern Twin Cities area. The child had been hospitalized, but has since been released and is now recovering.

A possible case had also been reported earlier in the Winona area, but it was later determined that the illness wasn't actually mosquito-borne encephalitis.

Minnesota doesn't usually record its first cases of LaCrosse encephalitis until later in the summer, according to Dr. Harry Hull, Minnesota State Epidemiologist. But while officials are concerned that the illness may be making an early appearance this year, Dr. Hull emphasized that it's too early to predict whether this will be a "bad year" for LaCrosse.

Every summer, MDH officials routinely recommend a number of preventive measures for people who live in parts of the state where the LaCrosse virus is commonly found. Those recommendations include eliminating mosquito habitat, and protecting children from mosquito bites.

Like many other states, Minnesota is also on the alert for a possible appearance by the West Nile encephalitis virus. But as of last year, West Nile had only been found in 12 northeastern states and the District of Columbia. So far this year, evidence of the virus has only been found in Connecticut, Maryland, New Jersey, New York and Rhode Island, and no human cases of illness have been reported.

But MDH officials say the most immediate health threat here is from the LaCrosse strain, which has been a fixture of life in the upper Midwest for many years. LaCrosse also remains the most common form of mosquito-borne encephalitis nationwide.

Minnesota's experience with LaCrosse may help it cope with any potential threat from West Nile, according to David Neitzel of the MDH Acute Disease Investigations and Control Section.

Minnesota's ongoing surveillance for the LaCrosse encephalitis virus should help state officials detect any West Nile activity, Neitzel noted. Minnesota officials already monitor for possible cases of mosquito-borne encephalitis in humans, as well as virus activity in mosquitoes and other animals.

The main addition to the state's encephalitis surveillance has been the collection and examination of dead birds who may have been infected with West Nile. Crows and Jays, in particular, tend to be very vulnerable to West Nile, and die very quickly once they become infected.

MDH has been working with the Minnesota Department of Natural Resources to collect and test birds that may have died under unusual circumstances.

"It's very possible that West Nile will eventually make its way to Minnesota," Dr. Hull said. "But the question is when. And in the meantime, the ongoing effort to prevent LaCrosse encephalitis is still at the top of our agenda."

Like all forms of the illness - including West Nile - LaCrosse encephalitis affects the brain and central nervous system. Severe cases of LaCrosse - which occur primarily in children and adolescents under the age of 16 - are characterized by symptoms like high fever, headache, confusion and other neurological symptoms.

Since 1985, 89 cases of LaCrosse have been reported to MDH, including eight cases last year. Half of the 89 reported cases occurred in children six years of age or younger. Almost all of the cases occurred in a relatively small area of the state, stretching from just west of Lake Minnetonka in the Twin Cities through the hardwood forests of southeastern Minnesota, along the Mississippi River.

Most cases of LaCrosse - like most cases of West Nile - tend to be mild, and may have no symptoms at all. However, severe cases of West Nile tend to occur in older people, rather than children or adolescents.

The "high risk" areas for LaCrosse activity in Minnesota include portions of 16 different counties, including Anoka, Blue Earth, Carver, Dakota, Dodge, Fillmore Goodhue, Hennepin, Houston, Olmsted, Ramsey, Rice, Wabasha, Washington, Winona and Wright. For people who visit or live in these areas, there are two major strategies for preventing LaCrosse encephalitis, according to Neitzel:

Eliminating potential breeding sites. Ochlerotatus triseriatus, the mosquito that carries LaCrosse, breeds in wooded or shaded areas, in locations where small amounts of water and debris can collect. That can include natural features like a hole in a hollow tree, but it can also include a variety of "artificial" items - old tires, buckets, clogged rain gutters, birdbaths, cans, or anything else that will hold a small pool of water. Because this particular mosquito doesn't tend to travel very far, eliminating these potential breeding sites from your own property or your immediate neighborhood can greatly reduce the risk of exposing children to this illness. People living in high risk areas of the state are advised to get rid of potential breeding sites immediately, so they will be ready well ahead of the peak season for LaCrosse, which usually begins in late July and extends through the month of August.

Protecting your children from bites. Appropriate protective clothing and use of mosquito repellents will help protect children against exposure to the LaCrosse virus. Health officials recommend using a product that contains no more than 30 percent DEET - the active ingredient in most insect repellents.

For more information, contact:

Buddy Ferguson
MDH Communications
(651) 215-1306

David Neitzel
Acute Disease Investigation & Control Section
(612) 676-5414

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