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Mosquito Populations Grew After Recent Hurricanes, West Nile Didn't

By Pippa Wysong, Access Excellence

New Orleans, LA (10/27/05)- Hurricanes Katrina and Rita left devastation in their wakes, but they didn't lead to increased rates of West Nile Virus (WNV) or other mosquito-borne illnesses as some feared. This was in spite of vast amounts of standing water left by the storms and booming mosquito populations.

Indeed, in the US there has never been any sort of marked increase in viral transmission of mosquito-borne diseases after hurricanes or other events leading to flooding, according to Roger Nasci, PhD, a research entomologist at the US Centers for Disease Control and Prevention. But that doesn't mean people are in the clear after such storms.

"We have monitored arboviruses (mosquito transmitted viruses) after hurricanes and floods for some years. We haven't seen any evidence that there is a massive increase in the amount of virus transmission following a hurricane or flood or other large natural disaster events," Dr. Nasci said.

The fact that WNV rates did not go up likely had to do with two things, he told Access Excellence. One is that "these are relatively fragile transmission cycles. Something which is that ecologically disruptive is going to displace birds, kill some mosquitoes in the hardest hit areas, and probably not be conducive to increasing the amount of virus transmission," he said. Birds are an important vector for WNV, and an integral part of the transmission cycle. It shows how close the links between environment, climate and disease are.

The second factor is that the background rate of WNV in the region was low to moderate in the areas most damaged by the storms. In fact, the state of Louisiana reported only 81 cases this year, most of which occurred before the hurricanes hit. Plus, most cases were in the northern parts of the state. Across the US there has been a total of 2,316 cases of West Nile disease this year, with the largest cluster, 711 cases, in California.

As well, most hurricanes usually occur late in the year when arbovirus transmission cycles are naturally declining due to falling mosquito densities and lower temperatures. But, it's possible that rates of West Nile disease where Katrina and Rita hit could go up in the future. "We continue to monitor the situation. We're not drawing any final conclusions," he said.

There is much that is not well understood about the general patterns of WNV because it is a relatively new disease in North America, and likely is still adapting to its new environment. The disease was originally identified in a woman in the West Nile District of Uganda in 1937 and is common throughout Africa, West Asia and in the Middle East. WNV is a flavivirus that is closely related to the St. Louis encephalitis virus which is also found in the US.

The first human case of WNV disease in North America was reported in 1999 in New York City. Since then, only six years have passed during which the virus has spread across the continent, but this is not enough time for scientists to understand just how the virus is adapting to the North American climate. This is one reason why WNV is so closely monitored through surveillance systems and reporting, Dr. Nasci said.

Researchers don't even know exactly what a typical pattern for disease occurrence in North America should be like with WNV. In the years 1999 to 2001 there were a total of 149 cases across the US, which jumped to 4,159 cases in 2002. The number of cases jumped again to 9,862 in 2003, but dropped to 2,313 in 2004 with a similar number in 2005.

Katrina is providing researchers with "our first real opportunity to observe what's happening with WNV after a hurricane," said Dr. Nasci. Monitoring was done after other large hurricanes in Florida during 2004, but the areas where they hit didn't have much in the way of West Nile activity. "While we saw really large mosquito population increases, there wasn't a pre-existing West Nile cycle to feed the system," he said.

Prior to Katrina and Rita, the southern parts of Louisiana and Mississippi both had evidence of WNV transmission, though not at high levels. Still, this meant there was a possibility that virus transmission could continue. Because of all the extra standing water that resulted from the flooding, mosquito populations underwent an explosion in some areas. This was one reason why mosquito control, in the form of spraying was undertaken fairly quickly.

However, not all mosquito species carry WNV. One key vector in the US is Culex pipiens, though a total of 60 species are known to carry the virus. C. pipiens has a preference for feeding on birds, but will feed on people too. While people, horses, dogs, cats and other animals can carry WNV they are considered dead-end hosts. This is because they don't develop a sufficiently high viral load to become a reservoir for the virus, or to transmit it.

Flood-Water Mosquitoes

The key mosquito population that had the biggest and fastest population growth in areas affected by Katrina and Rita were of a sort known as flood-water mosquitoes, according to Dr. Nasci. These insects lay their eggs, which can remain dormant for months or years, in areas that are prone to flooding. The eggs hatch when flooding occurs. But with the hurricanes, numerous areas flooded at the same time leading to massive broods of mosquitoes all hatching at once. These particular mosquitoes, while highly annoying and disruptive, fortunately are not good WNV vectors.

Even without disease, mosquito control is still an important aspect of recovery. While the risk for WNV appears to be low in this case, massive numbers of biting insects can impede rebuilding efforts. Researchers recorded landing rates from 50 to 200 mosquitoes per minute landing on people and trying to bite.

"Mosquitoes of that density can have a profound impact on your ability to perform recovery operations or just be there... people restringing electrical lines, or clearing roads, or a home owner trying to carve up a tree that's fallen on the house can't readily avoid the mosquitoes. The normal levels of mosquito control that one would expect as a service that would make your life more tolerable, suddenly becomes essential," Dr. Nasci said.

To date, the largest number of WNV cases coming out of the area was an epidemic in Louisiana in 2002 when 329 humans cases were reported. Dr. Nasci clarified that "epidemic means a number of cases higher than the background level."

While part of the recovery and health efforts post-Katrina and Rita are dealing with include mosquitoes and possible WNV, numerous other health-related issues have surfaced. There are concerns about human exposure to toxins and sewage in flood waters, and there were small outbreaks of diarrheal diseases in evacuation centers. Flooding also led to a massive growth of mold and mildew, which can cause other health issues. Post traumatic stress syndrome, depression, grief, and related psychiatric problems are additional important post-storm issues.

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