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West Nile virus increase corresponds to heavy rains

Recent rains have provided the environment needed to increase Louisiana mosquito populations, which also increases the likelihood of more cases of West Nile virus.

Recent rains have provided the environment needed to increase Louisiana mosquito populations, which also increases the likelihood of more cases of West Nile virus.

For the past few weeks the number of West Nile virus cases has risen to numbers not seen in recent years, and standing water seems to be the main reason, according to LSU AgCenter entomologist Tim Schowalter.“Female mosquitoes require standing water to lay their eggs. And if we can deny this, there is a better chance of keeping their numbers low.”

When standing water is available near the home, the mosquito has all it needs to reproduce.

“She has to have a blood meal or she will die before laying her eggs. But if humans or other animals are around, and the standing water is available, the conditions are favorable for the increase in populations that we’ve seen recently,” he said.

It normally takes about two weeks for mosquitoes to go through their breeding cycle in standing water. Then they become the biting pests that are both feared and hated.

Schowalter said the recent statewide mosquito count shows that about one percent of the mosquitoes tested were positive for the virus.

Louisiana’s subtropical climate allows mosquitoes to be active year-round, which increases the chance of being bitten.

Of the nearly 70 species of mosquitoes found in Louisiana, there are only about 10 that people have to worry about, Schowalter said. 


In 2002, Louisiana saw its first outbreak of the West Nile virus, which is carried by the mosquito from birds to humans.Since that time, LSU AgCenter scientists have been looking for ways to keep mosquito numbers down and have enlisted the public’s help in the process. 

West Nile cases in humans had decreased in the past few years, but a combination of factors can be blamed for this year’s increase, Schowalter said.“One problem is the decreased habitat for birds that don’t allow for the spread of the disease. After Hurricane Gustav a lot of trees were cut in the area, and that was not good for some of the birds that we need.”

The birds that are most likely to carry the virus are urban birds like crows, cardinals, jays and sparrows.

“It’s interesting that in areas where there is greater bird diversity, the West Nile gets damped out because mosquitoes are going back and forth between birds that don’t carry the disease, so that’s a dead end.”

Other animals susceptible to the virus include horses, squirrels, domestic rabbits, bats, chipmunks, skunks and alligators.

The number of cases in horses has steadily decreased since the development and widespread use of an effective equine vaccine.

The disease does not cause most people to become seriously ill.

“People over 50 years of age or with impaired immune systems are most likely to develop serious illness from the disease,” Schowalter said.

Symptoms occur three to 15 days after the initial infection and can range in mild cases from slight fever to headaches to extreme cases that can result in paralysis or death.

Things that can be done around the home to reduce the mosquito population include:

  • Remove containers that hold standing water, such as old tires and other debris.
  • Empty flower pots and other yard and patio containers.
  • Drain fountains, ornamental ponds and swimming pools no longer being properly maintained or treat with Bt discs, which contain a bacterial pathogen of mosquito larvae.
  • Fill low-lying areas to avoid standing water.
  • Provide drainage ditches to promote rapid runoff of rainwater.
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