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.
end
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