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Modified:
Jul 13, 2005
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West Nile Virus
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Frequently Asked Questions
Questions
For Response click on Question.
Responses
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What is the health risk to people from the West Nile Virus?
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handout: [pdf - 44KB]
- SYMPTOMS of SERIOUS ILLNESS: fever, disorientation, muscle weakness,
neck stiffness, headache, nausea
- RISK of SERIOUS ILLNESS: greater for older people and the immuno-compromised
- CHILDREN ARE NOT PARTICULARLY VULNERABLE (few if any cases of serious
illness have involved children)
- INCUBATION PERIOD: 3-14 days after being bitten by an infected mosquito
- CALL a DOCTOR if you or someone you care about shows symptoms of
serious illness, whether or not caused by WNV
- TREAT the SYMPTOMS -- there is no vaccine or medication specific
to WNV
- AUGUST & SEPTEMBER are when WNV has been most likely to bridge
into the human population in the Northeast US. Human illness has occurred
earlier in the Southeast and Gulf Coast states of the US in 2002
- "FLU-LIKE" SYMPTOMS have been reported by about 30% of infected people,
but most of those infected do not get sick
- In US outbreaks, about 1 infected person in 150 has become seriously
ill with central nervous system infection (encephalitis &/or meningitis)
- About 12% of hospitalized cases have been fatal (7 of 62 hospitalized
in 1999, 2 of 20 in 2000, 9 of 64 in 2001, )
- Most mosquitoes bites will not lead to a WNV infection
- BIRDS are far more likely than people to become infected & sickened
by West Nile Virus
In some areas where West Nile Virus is showing up for the third and
fourth year, there is some complacency about taking precautionary and preventative
measures. Although fortunately WNV has not caused widespread human mortality--as
was initially feared by some--it can cause unpleasant (flu-like) and sometimes
long-lasting symptoms in people. It also presents a serious risk to unvaccinated
horses and has had a not-yet-fully-understood impact on susceptible wildlife (e.g., crows).
See Petersen and Marfin. 2002.
West Nile Virus: A Primer for the Clinician for more information about
how to prevent, treat and evaluate WNV as a health risk for people.
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How can I reduce my personal risk and my community's risk from the West Nile virus?
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"Mosquito hygiene" includes source reduction of mosquito breeding
sites and avoidance of biting mosquitoes--Both are key to reducing
risk from WNV. Precautions should continue for the duration of the mosquito
season. In the North this is into the Fall until after there have been two
hard frosts. [See "Mosquito Hygiene throughout Life Cycle of Culex Mosquitoes"
available as a full color 11 x 17 poster and in simplified black and white copy for handouts
and presentations (4-page pdf)].
Mosquitoes breed in wet areas, and Culex are found particularly
where there is decaying organic matter (e.g., leaves, grass clippings,
animal wastes). There does not have to be much water and the water does not
have to be left standing for very long - Some species can reproduce within
a week! Check for standing water especially after each rainstorm, drizzle,
watering of the garden or washing the car). Reduce mosquito breeding opportunites
by:
- Eliminate or empty the "artificial water-collecting containers" that
are prime breeding spots for the mosquito species implicated in transmission
of West Nile Virus
- Clean out rain gutters
- Aerate swimming pools and ponds (and perhaps stock with mosquito-eating
fish)
- Empty unused buckets, water troughs, etc.
- Keep unused tires under cover so they do not collect water
- Drill drainage holes in tires and other containers used in construction
sites, farms, gardens and play areas
- Clean bird baths and animal water bowls at least once a week
Avoid mosquito bites by wearing long clothes and/or by using insect
repellent when out after dusk or in shaded areas (such as woods) during the
daytime. This is when and where most vector species are more likely to bite.
People should be especially careful when in "mass gatherings" where the CO2
given off by the crowd attracts more mosquitoes from a greater distance.
The New York State Department of Health is urging people to
take common sense precautions to reduce their exposure to mosquito bites and
to continue to eliminate stagnant water where mosquitoes breed. The NYS DOH
WNV materials do not advocate widespread application of mosquito adulticide
pesticides. However, in NYS, decisions about mosquito control methods are
made by local jurisdictions.
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What should I know about mosquitoes?
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Mosquitoes are insects. They go through 4 life stages (complete metamorphosis)
and look completely different at each stage: egg, larva, pupa and adult.
The first 3 life stages are spent in water or wet places. Adults emerge
from the pupal stage and are full size and able to fly. Adult females bite
to get a blood meal that provides the nutrients they need to form each
brood of eggs. (For great pictures and descriptions of each life stage, see ERAP's "Mosquito Hygiene" poster.)
Culex pipiens (Northern House Mosquitoes) are the type (species)
of mosquitoes that have been most closely associated with transmitting West Nile Virus
in the Northeast US. C. pipiens breed in standing water, especially
in water containing decaying organic matter (plant debris, animal wastes).
These mosquitoes "prefer" to bite birds, but it has been thought that if
breeding sites are available near people's homes and domestic animal enclosures,
Culex pipiens may bite people and domestic animals. Culex
pipiens are most active dawn and dusk. Culex quinquefasciatus
(Southern House Mosquitoes) fill this niche in the Southern United States
but are more likely to bite people.
Although Culex mosquitoes of several species (Cx. pipiens, Cx. restuans, Cx. salinarius, Cx. quinquefasciatus and Cx. tarsalis) accounted for 55% of the WNV-positive mosquito pools collected in 2002, 36 different mosquito species have been reported WNV-positive since 1999. As the range of WNV has spread across the US, into the habitats of different types of mosquitoes, additional species are added to the roster of WNV vectors. In 2002, 7 species were reported for the first time: Aedes aegypti, Anopheles walkeri, Cx. erraticus, Cx. tarsalis, Cx. territans, Culiseta inornata and Psorophora ciliata. Of greatest concern in terms of risk to human health (and horses) are vector species that feed readily on large mammals e.g., Culex salinarius, Cx. tarsalis, and Aedes vexans. However, CDC's Dec 20 MMWR states that although other species may contribute to human WNV transmission, control of Culex mosquitoes continues to be the most important strategy to reduce risk for WNV transmission to humans.
WNV-infected mosquitoes are difficult to catch and are therefore not a good sentinel of WNV activity. WNV-positive mosquitoes were the first indicator of WNV activity in only 3% of US counties reporting cases in 2002!
For more information about mosquitoes, see ERAP's "Mosquito Hygiene"
poster; ERAP's West Nile Virus Resources
for the General Public, Section III, and Bibliography of Scientific Literature--use the search
term "mosquito" or the name of a specific group of mosquito.
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Should I (or someone in my family) be tested for West Nile Virus if bitten by a mosquito?
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No. Testing for West Nile Virus is a complex process. It takes a
long time to get test results and only a very small percentage of the mosquitoes
that might bite you are infected. While test results are important to scientists
and medical doctors studying the disease, they would not be very useful
to you as an individual. There has not been* a particular cure for West
Nile Virus--so a sick person does not become better off by being tested for
WNV infection. If you become seriously ill, you should see your doctor,
no matter what the cause or name of the illness. If you show signs of WNV,
you would be treated for relief of your symptoms. (* In late Aug 2002, the
US Food and Drug Administration approved clinical trials to see how useful
the drug interferon might be in treating WNV patients, but this drug is not
widely available for this purpose and its efficacy in treating WNV is unknown
[More].
If you are bitten by mosquitoes, you should not assume that you are
at high risk for WNV. See West Nile Virus
as a Health Risk for People for more information about likelihood of
infection and illness for different groups of people and other animals.
In previous years of the WNV outbreak in North America, for the most
part, only people participating in a scientific study or those who had become
very ill were tested to see if they had been infected by WNV. This year some
people with a wider range of symptoms have also been tested. The purpose of
the scientific studies involving testing for WNV infection are to answer questions
like: "How many people in a certain area were infected?" "Of the number
infected, how many had mild symptoms?" "What percentage of those infected
get seriously ill?" (For more information about such epidemiological studies,
see ERAP's West Nile Virus Bibliography
of Scientific Literature
- Can I be vaccinated against West Nile Virus?
Is there a vaccine for horses? For birds? For other animals?
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People: Two independent research teams have taken similar
promising pathways to develop a WNV vaccine for people. Both efforts involve
creating a hybrid "modified live virus" that include genes from the West Nile virus spliced
into the "backbone" genome of a distantly related virus. When a vaccine is made from a modified life virus it has to replicate within the vaccinated person or animal. A government research
group is splicing two genes for West Nile coat proteins into a dengue virus
backbone that has been stripped of its own corresponding genes. The other
research group, from the Acambis Company, are swapping the same two WNV genes
into the backbone genome of an attenuated yellow fever virus. Acambis has
already demonstrated that its hybrid vaccine protects horses and primates
from WNV infection. They hope to start human clinical trials by summer 2002
[More] [ Still More].
Top US health officials from several agencies (CDC, NIH, FDA) reported
at a Sept 24, 2002, Senate Committee Hearing that a nucleic acid blood test
may be ready by summer 2003 and a vaccine based on the yellow fever vaccine
may be ready in three years [Senate Hearing] [Summary of Senate Hearing].
While the vaccines may be developed rapidly, it will likely take 3-4 years for them to be approved for human use. The use of a vaccine that itself poses risk is controversial, especially given the typically very low risk to people of serious illness from WNV.
Horses: On August 1, 2001, USDA issued
a one-year conditional license for development, manufacture and distribution
of a killed-virus vaccine intended to prevent WNV in horses. Such vaccines are
regulated under the Virus-Serum-Toxin Act, which permits conditional licenses
to be issued to meet special circumstances--such as presented by West Nile
Virus--if the product is pure and safe, and has reasonable expectation of
efficacy. The conditional license was issued to Fort
Dodge Laboratories, Inc., previously a division of American Home Products,
Inc. and now a division of Wyeth (NYSE:WYE), for the 1-year period Aug 1,
2001-Aug 1 2002. At the end of the conditional period (which has now been
extended), data obtained in support of the product efficacy, potency, and
performance are to be evaluated to determine if the conditional license should
be renewed or if a regular product license may be issued. During the first
year of distribution CDC reported that more than 3 million doses were distributed
nationwide (Aug 21, 2002 CDC News release). The vaccine is apparently recommended
for use on other equine species as well as horses. This is the only one of several equine vaccines under development that has received government approvals. Very little information about the other vaccines is publicly available.
Use of the vaccine is restricted to veterinarians in those states
where the product has been approved by the appropriate state regulatory authorities.
The vaccine is administered to healthy horses in 2 (1 ml) intramuscular (IM)
doses 3 to 6 weeks apart, followed by a yearly booster. Since the vaccine is not effective for 5 weeks after it is administered, the first dose should be given at least 6 weeks prior
to expected mosquito activity in the area. Horses bitten by an infected mosquito
before receiving the second dose of vaccine may become infected and ill.
Texas Animal Health Commission recommends that 'booster' shots are given
yearly or every 6 months in warmer areas with large mosquito populations in
order to maintain maximum immunity.
Preliminary observations of efficacy look promising: e.g.,
of the 20,000 equines vaccinated in Florida in 2001, only one developed WNV
infection. Efficacy and other information about the vaccine are discussed
in WestNileVirus-L listserve postings on April 1, 2002, and
in a series of postings the week of Sept 9 2002. It is possible that the much lower rate of WNV equine infections in East Coast states in 2003 as compared with Midwest states is reflective of the efficacy of the equine virus (assuming that more East Coast horses were vaccinated than in states where WNV was not expected).
Other Animals: The Ft Dodge horse vaccine has been experimentally used (off-label) on other zoo animals. It does not seem to cause harm, but its efficacy for many species is uncertain. Tests of the vaccine on some species of birds (e.g., crows) have shown that it is ineffective.
Birds: Impetus for developing vaccines protective of birds has come from zoos, captive breeding facilities, and conservationists concerned about threatened and endangered species in the wild and exotics in captive collections. In Israel concern about WNV impact on commercial flocks of geese provided impetus. According to a Sept 20 2002 American Bird Conservancy (ABC) press release, birds vaccinated with a new recombinant DNA vaccine prior to inoculation with WNV showed a 60% increase in survival over unvaccinated birds in trials by the CDC. CDC is also collaborating with other institutions to develop an oral vaccine for wild bird populations. Meanwhile a number of zoos were reported to be inoculating birds and other animals in their collections with the experimental horse vaccine (e.g., the National Zoo in Washington, D.C., July 2002
[More]). The efficacy of the equine vaccine on birds is not resolved and probably differs by species, according to rates of infectivity and seroconversion. Recent
research by Dr. Nick Komar, an arbovirus specialist with CDC found that
the vaccine did not protect American crows challenged with the West Nile 99 strain of virus
(i.e., vaccinated crows became ill and died). Another unpublished study, however, showed significant antibody response in four species of birds, which could confer protection.
Geese: In August 2001, the State Laboratory for Vaccine Control
in Israel began field testing of a killed-virus vaccine for geese. About 300,000
geese in commercial flocks were vaccinated twice, at 2 and 4 weeks of age,
using virus replicated in suckling-mice. Field observations, as well as
laboratory challenge trials carried out in sampled vaccinated geese, confirmed
the efficiency of the vaccine in field conditions. Source: Veterinary Services
and Animal Health, Israel, Jan 2002 [More].
- Was West Nile Virus brought to the United
States by bioterrorists?
-
In the Sept 13, 2002 New York Times, Senator Patrick Leahy is quoted* as asking for investigation into the possibility
that the current WNV outbreak is a result of biological terrorism. This
question was asked before, at the time of the initial WNV outbreak in NYC
in 1999.
Evidence (and logic) indicated that the answer is "no," although we
do not know if the rejection of this hypothesis
has been expressed formally and/or officially. In press briefings in that
first year, CDC listed bioterrorism as one of the possible means by which
WNV might have entered this country, but spokespersons did not seem to support
the hypothesis nor to bring it up more recently.**
An article in the New Yorker -- West Nile Mystery: How did
it get here? The C.I.A. would like to know. Richard Preston. Oct 18
& 25, 1999 -- focused attention on this hypothesis, but presented a body
of evidence that essentially rejected it. For example:
"An Army expert on bioweapons told me [Richard Preston] that the military
has known for some time that Soviet biologists working for the USSR's biowarfare
program had evaluated the West Nile virus for use as a biological weapon.
'The Russians did this kind of crap back in the seventies...They abandoned
it because it didn't work very well...'"
"I [Richard Preston] spoke with a person at the CDC who said 'We're
taking it seriously. We'll see where the data takes us. It could be done.
You'd have to bring in a lot of mosquitoes....' He went on, 'But West
Nile is not a great biological weapon because it doesn't hurt most people
very much.' "
Before this call for investigation goes too far, in terms of allocating
resources or feeding panic, some key characteristics of WNV should be recalled:
- WNV is primarily a disease of birds that cycles between infected birds
and mosquitoes;
- historically only sporadic outbreaks have affected people (and <
1% of people infected become seriously ill);
- with the possibility of only rare exceptions, people are "dead end hosts"
to the virus and can't pass it on to other people or to mosquitoes that
bite them (i.e., it is not highly infectious);
- with the exception of horses, WNV does not seem to affect domestic animals
nor animals of commercial value (livestock, poultry), so is unlikely to
disrupt the domestic food supply or to affect food security;
- the elderly population at greatest risk from serious WNV illness would
generally not be considered a key target of one's enemies.
*NYTimes Sept 13, 2002
Vermont Senator Wants Study of Terror Link to West Nile Virus
By CHRISTOPHER MARQUIS
WASHINGTON, Sept. 12 - Senator Patrick J. Leahy, Democrat of Vermont,
said today that the authorities should examine whether the spread of the
West Nile virus in this country is a result of biological terrorism. "I
think we have to ask ourselves: Is it a coincidence that we are seeing such
an increase in West Nile virus, or is that something that is being tested
as a biological weapon against us?" Mr. Leahy, who is chairman of the Senate
Judiciary Committee, said in a radio interview in Waterbury, Vt. "There are
some people, credibly, who feel that it is a test of our defenses and is
a biological weapon or somebody doing this for commercial purposes."
** Other candidates for carrying the WNV
into the US that were mentioned by CDC at that time included: infected mosquitoes,
people, legally or illegally imported birds, or birds migrating across
the Atlantic Ocean. Ornithologists have said that the last mentioned of
these would be extremely unlikely, and would have caused a very excited
stir in the bird-watching community had it been seen. More recently, in
a March 5, 2001 National Public Radio interview, a CDC scientist reported
on results of a simulated probability model that pointed to infected frogs
from Western Asia as the most likely means of WNV entry into the US. Little
has been heard about this hypothesis since. Most persons talking about this
subject in recent years have repeated that the answer is simply not known--e.g.,
CDC's Petersen and Marfin introduce their recent publication West Nile Virus: A Primer for
the Clinician (2002) by saying "It remains unknown how the West Nile
virus came to North America."
- What are the issues and the risks of human-to-human West Nile Virus transmission
(e.g, via organ transplant, blood transfusions, or from mother to baby)?
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(revised Dec 23, 2002)
Health officials have become convinced that WNV can be transmitted via organ transplant and blood transfusion, as well as from mother to fetus and possibly to babies in breast milk.
As of
Labor Day Weekend 2002 there was a
growing body of evidence that WN virus can be transmitted via organ transplant
from an infected person (and an associated risk that it might also be transmitted
by blood transfusion). By end of the first week in September, during a
Sept 5 telebriefing, and then in the Sept 6 MMWR, CDC confirmed with a high level of certainty
that organs from an infected donor had transmitted the infection to 4 organ
recipients.
Research to determine if WNV can also be transmitted via blood transfusion
was prompted by trying to understand how the organ donor had become infected,
since blood tests before she received blood transfusions did not show she
was WNV-positive, while later tests did. Interest was spurred when it was
learned that at least 4 other people had been diagnosed with WNV infection
after receiving blood transfusions. Since all patients resided in areas
with high levels of WNV activity, CDC and others needed to investigate further
to find evidence that WNV was in fact transmitted through
blood transfusion.
During the period Aug 28 to Oct 26, CDC received reports of 47 persons
with possible transfusion-related WNV infection (17 reports received since
Oct 16), 14 of whom were later found not to have WNV infection or did not
acquire WNV infection through transfusion. Of the remaining 33 (reported
from 17 states), 6 cases now provide evidence that WNV can be transmitted
through blood transfusion. Investigations are ongoing for the other 27 cases.
These 33 cases are people who had confirmed or probable WNV infection who
had received blood components in the month before illness onset (Oct 28 2002 CDC dispatch).
Of the 30 cases tallied in the Oct 16 2002 MMWR (9 fatalities with WNV the likely
cause of death), 25 patients had meningoencephalitis, 4 had other WNV-associated
illnesses, and demographic and clinical information was not yet published
for 1 patient. As of Oct 16, 4 investigations provided evidence of transmission
via blood transfussion: 2 of the 4 developed WNV-encephalitis after receiving
different blood components derived from a single blood donation. In follow-up
testing, the donor seroconverted and developed WNV IgM antibody. In the 3rd
case (Investigation 3, detailed by CDC in the Sept 13 and Sept 20 MMWR) WNV was isolated from a withdrawn unit
of frozen plasma from the suspected donation, indicating that the virus can
survive in some blood components, likely for several days. The donor of this
plasma subsequently developed an acute febrile illness and seroconverted following
the suspect collection.
Three of the cases providing evidence of WNV transmission via transfusion
are people who received transfusions while under treatment for malignancies
(cases 4-6, detailed in the Oct 28 CDC dispatch). Case 4, an adolescent hospitalized
for 65 days, developed WNV-meningoencephalitis after receiving a component
derived from a donation that contained WNV RNA. Follow-up found that the donor
had developed a febrile WNV-illness within days of the suspect donation.
In another of the 7 initial investigations conducted by CDC, it was shown
that not all recipients of WNV-contaminated units will become infected with
WNV (Investigation 7: a 55-year-old woman received contaminated blood after
an orthopedic procedure in July but has not tested WNV-positive). Note that
as recently as the Sept 20, 2002 MMWR, evidence for transfusion-associated
transmission was "highly suspicious" but not conclusive. A similar
case is under investigation in Ontario, Canada re: a 47-year-old woman who
received many blood transfusions during treatment for cancer and was found
to be WNV-positive around the time of her death in Toronto, Nov 21, 2002.
It is considered highly unlikely that she contracted WNV through a mosquito
bite (source: The Edmunton Sun, Nov 29 2002).
While evidence that WNV can be transmitted through blood transfusions
adds a wrinkle to the set of assumptions about disease transmission, the general
population is at no greater risk of contracting WNV than previously
thought. The news does make us aware that a sub-population of people about
to undergo high risk organ transplants and/or receive blood transfusions (about
4.5 million persons in the US receive blood or blood products annually) may
be at still greater health risk due to the increased risk of contracting WNV.
However, CDC health officials point out that the additional risk of contracting
WNV is quite small in comparison with the much larger risks from these medical
procedures. The process of donating blood poses no risk to the blood donor
for contracting WNV.
Related to these investigations, research is also underway to develop
more rapid tests for determining if asymptomatic people are infected with
WNV. Such a test could enable screening for WNV in donated blood.
However, in a Sept 19, 2002 press briefing it was noted (a) that screening
would be quite difficult because there would be relatively small amounts of
the virus in blood of an asymptomatic infected person; and (b) it would be
necessary to detect the virus itself, not antibodies the body produces in
response to infection [More].
On Oct 25, the Food and Drug Administration issued a guidance
to the blood industry regarding procedures for screening and quarantine of
suspect blood donations [More]. CDC requests that cases of WNV infection in patients
who have received blood transfusions within the 4 weeks preceding illness
onset be reported to CDC through state and local public health authorities.
Serum or tissue samples should be retained for later studies. In addition,
cases of WNV infection in persons with illness onset within 2 weeks after
blood donation should be reported. Prompt reporting of these cases will facilitate
withdrawal of potentially infectious blood components.
On Sept 2 a 40 year old woman gave birth to an infant, then required
a blood transfusion for anemia. The woman began breast-feeding her child
on the day of delivery. On Oct 3 CDC announced that a sample of the breast milk tested positive for the WNV-specific IgM and IgG antibodies, indicating that NV genetic material was transiently present in the breast milk. The infant remained
healthy, but tests when the infant was 25 days old revealed presence
of the WNV-specific IgM antibody, which could not have been passed from mother to child. The infant had little contact with mosquitoes and thus is believed to have contracted the infection via virus in breastmilk.
Because the health benefits of breast-feeding are well established and the
risk for WNV transmission through breast-feeding is unknown, the CDC does
not suggest a change in breast-feeding recommendations.
Intrauterine, Transplacental (Vertical) Transmission (Dec 20 2002): An infant born in November to a woman who contracted WNV while pregnant was hospitalized in Syracuse, New York, (Onondaga County Health Dept, PR Dec 18, 2002). "WNV has not previously been associated with intrauterine infection or adverse birth outcomes" in the formal record, although at least one other human case of transplacental transmission has been described anecdotally. Details are described in the CDC MMWR Dec 20, 2002 and were also covered in the Dec 19 CDC press conference. CDC recommends that "pregnant women should take precautions to reduce their risk for WNV or other arboviral infection and should undergo diagnostic testing when clinically appropriate." [For additional information and background, see WestNileVirus-l listserv postings from Oct 13, 15, and 17 and Dec 20].
More on these topics: CDC's MMWR for Sept 6, Sept 13, Sept 20, Oct 16, and Oct 28 (Dispatch, Vol 51, "Investigations
of West Nile Virus Infections in Recipients of Blood Transfusions," 2002;
a Sept 27 CDC Advisory; transcripts from CDC's Sept 3 and Sept 19 2002 telebriefings; ERAP postings to the WESTNILEVIRUS-L
listserv Sept 3, Sept 4, Sept 6, Sept 19, Oct 28 and Oct 29, 2002.
Back to Top
ERAP's West Nile Virus education program has been supported by Smith-Lever funds from the USDA Cooperative State Research, Education and Extension Service (CSREES), through a grant from Cornell Cooperative Extension, and by a grant from the National Oceanic & Atmospheric Administration's Office of Global Programs (NOAA-OGP) for the project "Climate Effects, West Nile Virus Vector Development, and Transmission Risk" (Sept 1, 2004-Aug 31, 2007).
© 1999-2006 Cornell University Environmental Risk Analysis Program
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