The unusual summer heat across the United States this year could mean that the risk of West Nile virus will be worse than usual, health officials are warning.

The the Centers for Disease Control and Prevention is warning that a significant outbreak among humans is likely.

West Nile virus, which is structurally similar to the dengue fever virus -- both belong to the same virus family -- primarily infects birds. However, it also can infect humans, horses, cats, bats, chipmunks, skunks, squirrels and domestic rabbits.

The recent death of a young horse in Montana has spurred public health officials there to urge residents to take steps to lower their risk of exposure to mosquitoes, since their bite is the main route of human infection.

Hot weather is a problem, because it encourages the virus to spread faster.

West Nile virus spreads when remale mosquitoes bite infected birds, carry the virus in their salivary glands, and then infect other bird or mammals, including humans, when they bite again.

Originally, it was thought that direct human-to-human transmission did not occur, but the CDC discovered in 2002 that the virus could be spread trough blood transfusions,organ transplants, breastfeeding, prenatal infection or occupational exposure. Blood banks in the US now routinely screen potential donors for the virus.

In the vast majority -- 80 percent -- of people infected with the West Nile virus, no symptoms appear. Some victims experience a mild, flu-like fever. However, because the virus is capable of passing the blood-brain barrier, a small number of infected people suffer such serious, potentially fatal, effects as encephalitis and meningitis.

Those over 50 years of age are at greater risk of serious consequences from infection.

No cure, or even effective treatment, is known. There is no vaccine for humans, although a vaccine for horses does exist.

The number of human cases of West Nile infection typically peaks in August and early September, according to the CDC.

To minimize the chance of contracting West Nile virus, people should cover up during "mosquito weather," even if that means wearing long sleeves and pants when it's hot. Screens should be kept in good repair, and places that mosquitoes like to congregate -- such as gutters, birdbaths, garden ponds -- should be cleared out or avoided.

Insect repellants such as DEET can be useful, though experts advise caution in using any type of toxin, which carry their own risks. In Canada, officials have banned products that contain more than 30 percent DEET.