BOSTON -- Routine surveillance shows a larger population than normal of the type of mosquitoes that can carry the West Nile virus, the Massachusetts Department of Public Health announced Tuesday.

VIDEO: More Mosquitoes Carrying West Nile Virus

Health officials said that positive West Nile virus samples have occurred earlier this year than last year. So far this summer, four mosquito collections have tested positive for the virus in Massachusetts.

Two samples were collected in Brookline, and the city of Worcester and the town of Millbury each have had a single positive sample. The first positive result this year was announced on July 2, almost one month earlier than the first result announced in 2007. Last year, the first positive West Nile virus result was not announced until July 26.

"We believe that rain patterns this year have provided conditions that have led to much higher populations of the type of mosquitoes that usually carry West Nile virus and the rain that occurred over the weekend would be expected to make this even worse," said state epidemiologist Dr. Alfred DeMaria.

In 2007, there were six human cases of West Nile virus in Massachusetts. While the virus can infect people of all ages, people over the age of 50 are at higher risk for severe disease. West Nile virus is usually spread to humans through the bite of an infected mosquito.

"West Nile virus will be with us for the remainder of the summer and into the fall, so it is important to protect yourself from mosquito bites by using clothing to cover up or applying bug spray. Getting rid of any accumulated water around your yard will also help keep more mosquitoes from breeding," DeMaria said.

Residents were urged:

To be aware of peak mosquito hours. The hours from dusk to dawn are peak biting times for many mosquitoes.

Consider rescheduling outdoor activities that occur during evening or early morning. Otherwise, take extra care to use repellent and protective clothing.

Although it may be difficult to do when it's hot, wearing long-sleeves, long pants and socks when outdoors will help keep mosquitoes away from your skin.

Apply insect repellent when you go outdoors. Use a repellent with DEET, permethrin, picaridin, oil of lemon eucalyptus or IR3535 according to the instructions on the product label. DEET products should not be used on infants under two months of age and should be used in concentrations of 30 percent or less on older children. Oil of lemon eucalyptus should not be used on children under 3 years of age.

Drain standing water. Mosquitoes lay their eggs in standing water. Limit the number of places around your home for mosquitoes to breed by either draining or getting rid of items that hold water. Check rain gutters and drains. Empty any unused flowerpots and wading pools, and change water in birdbaths frequently.

Install or repair screens. Some mosquitoes like to come indoors. Keep them outside by having tightly fitting screens on all of your windows and doors.

Dead crows or blue jays may be a sign that West Nile virus is circulating among the birds and mosquitoes in an area. Call 866-MASS WNV to report a dead bird. By reporting dead birds, you can play an important role in monitoring West Nile virus.

Information about West Nile virus and Eastern Equine Encephalitis is also available by calling the DPH recorded information line at 866-MASS-WNV (866-627-7968) or the Epidemiology Program at 617-983-6800.