Recent flooding along the Mississippi and Missouri rivers may be establishing optimum mosquito breeding conditions, but Karen Yates, coordinator of the state's vector-borne disease program, says chances are slim that the pests currently breeding along the riverways will carry the West Nile Virus.
"The mosquitoes that come right after flooding are really different than the mosquitoes that carry West Nile," she said. "They (floodwater mosquitoes) can become infected with it (WNV), but they're not very efficient at reinfecting other animals. You can't say they never get infected, but the chances of it happening are really low, because they're not good vectors. They are not good at turning around and infecting other mosquitoes."
Instead, she said floodwater mosquitoes are principally loathed for their nuisance factor. "The thing with these flood plain mosquitoes is that they can actually interfere with recovery efforts, because they are such aggressive mosquitoes," stated Yates. "Believe it or not, you can actually apply for federal assistance if the nuisance mosquitoes are actually hampering the recovery effort."
As annoying as the species is, a more egregious concern resides with varieties more likely to carry WNV. According to the Centers for Disease Control and Prevention, WNV is most frequently spread by the bite of an infected mosquito, especially the Northern House Mosquito. Mosquitoes become infected when they feed on infected birds, and can then spread the disease to humans and other animals. The illness – which cannot be spread between humans – is potentially serious, sometimes leading to fatality. The CDC estimates that about 1 in 150 people infected with the virus will develop severe illness, with symptoms including high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness, and paralysis. The symptoms may last for several weeks, but neurological effects can be permanent.
According to the CDC, no human cases have been reported in Missouri this year, although Yates noted "St. Louis County has been having quite a few mosquito pools test positive" for the virus. In 2007, the CDC reported a nationwide total of 3,630 human cases of WNV, resulting in 124 related fatalities. Last year in Missouri, there were 77 reported incidents, with five fatalities. Data likewise indicates that human cases tend to manifest in late summer, usually in August and September.
Meantime, general conditions would seem to be ripe for a high rate of overall mosquito activity later this summer, although various factors could still impact outcome. Rushing floodwaters typically hamper floodwater mosquitoes, which prefer stagnant, shallow puddles. But as floodwaters recede, saturated soil may not be able to soak up additional rainfall, leading to creation of a large number of puddles, and potentially, more mosquitoes.
"Just from reading the incident reports that have come out from the State Emergency Management Agency, some of the flooding has been very powerful and turbulent in certain areas," stated Yates, who also cited warm temperatures as a critical factor spurring mosquito growth. "And that has the potential to create new surface drainage areas that might actually not drain. Like in the '93 flood, there were several areas along the Missouri River where holes were carved out in the ground with turbulent water. So those don't drain, and then they can convert from flood water mosquito habitat to West Nile habitat. Especially as the summer goes on, and more organic matter gets concentrated in the water, and the water begins to heat up, the opportunity for breeding of flood plain mosquitoes dies down, and it becomes an opportunity for West Nile mosquitoes."
Yates noted that stagnant water conditions can similarly lead to proper breeding conditions in communities further inland from river flooding. "Even though you're not on the river (in Mexico), I watch the weather maps, and you guys have been getting a lot of rain. So I would say that there are probably ditches, and maybe containers in people's back yards that have been filled up, and they have got organic matter in them, and can break down and become a lovely soup. And mosquitoes will find their way to that and lay eggs."
To counter the potential dangers of disease-carrying mosquitoes, Yates said most rural residents will likely have to rely on personal wherewithal.
"Especially in rural areas, there are no mosquito control programs," she said. "People just have to be vigilant. If you have to be outdoors in the evening, maybe working in the garden – which is the only decent time of the day you can be out working in the garden – people need to be wearing an insect repellant that is effective against mosquitoes."
The CDC specifically recommends applying EPA-registered insect repellants when outside, featuring active ingredients such as DEET, Picaridin, or Oil of Lemon Eucalyptus. Other suggestions include wearing light-colored clothing, and long-sleeved shirts and slacks when outside; limiting use of colognes and perfumes, since the fragrances can attract mosquitoes; and minimizing mosquito breeding grounds by turning over unused wheelbarrows and plastic wading pools, cleaning clogged gutters, and removing discarded tires, unwanted tin cans, plastic containers, flower pots or similar water-holding containers.
Yates likewise noted individuals over age 50 are more at risk of vector-borne diseases. "It's probably declining immune system function," she said. "But also the fact that people over the age of 50 are more likely to have other kinds of chronic problems, like heart problems or disease. And sometimes medications that people take, or conditions that they have, they make it easier for the virus to get into the nervous system, and then that produces more serious disease."
For additional information about vector-borne concerns, visit the CDC Web site at www.cdc.gov/ncidod/dvbid/westnile/index.htm.
Mexico, Mo. —
