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Transmission
Q.
How do people get infected with West Nile virus (WNV)?
A. The main route of human infection with West Nile virus
is through the bite of an infected mosquito. Mosquitoes become
infected when they feed on infected birds, which may circulate
the virus in their blood for a few days. The virus eventually
gets into the mosquito's salivary glands. During later blood meals
(when mosquitoes bite), the virus may be injected into humans
and animals, where it can multiply and possibly cause illness.
Additional
routes of human infection became apparent during the 2002 West
Nile epidemic. It is important to note that these other methods
of transmission represent a very small proportion of cases. Investigations
have identified WNV transmission through transplanted organs and
through blood transfusions. See Blood
Transfusions and Transmission: Questions and Answers.
There
is one reported case of transplacental (mother-to-child) WNV transmission.
This case is detailed in MMWR
Dec 20, 2002. There is also one reported case of transmission
of WNV through breast-milk. See Questions and Answers concerning
WNV and breastfeeding for more
information on this topic.
Although transmission of WNV and similar viruses to laboratory
workers is not a new phenomenon, two recent cases of WNV infection
of laboratory workers have been reported. These cases are detailed
in MMWR
Dec 20, 2002.
Q.
What is the basic transmission cycle of West Nile virus?
A. Mosquitoes become infected when they feed on infected birds,
which may circulate the virus in their blood for a few days. Infected
mosquitoes can then transmit West Nile virus to humans and animals
while biting to take blood. The virus is located in the mosquito's
salivary glands. During blood feeding, the virus may be injected
into the animal or human, where it may multiply, possibly causing
illness.
Q.
If I live in an area where birds or mosquitoes with West Nile
virus have been reported and a mosquito bites me, am I likely
to get sick?
A. No. Even in areas where the virus is circulating, very
few mosquitoes are infected with the virus. Even if the mosquito
is infected, less than 1% of people who get bitten and become
infected will get severely ill. The chances you will become severely
ill from any one mosquito bite are extremely small.
Q.
Can you get West Nile encephalitis from another person?
A. No. West Nile encephalitis is NOT transmitted from person-to-person.
For example, you cannot get West Nile virus from touching or kissing
a person who has the disease, or from a health care worker who
has treated someone with the disease.
Q.
Is a woman's pregnancy at risk if she gets infected with West
Nile virus?
A. There is one documented case of transplacental (mother-to-child)
transmission of WNV in a human. Although the newborn in this case
was infected with WNV at birth and had severe medical problems,
it is unknown whether the WNV infection itself caused these problems
or whether they were coincidental. More research will be needed
to improve our understanding of the relationship - if any - between
WNV infection and adverse birth outcomes.
Nevertheless,
pregnant women should take precautions to reduce their risk for
WNV and other arboviral infections by avoiding mosquitoes, using
protective clothing, and using repellents containing DEET (See
Using Repellent Safely). When
WNV transmission is occurring in an area, pregnant women who become
ill should see their health care provider, and those whose illness
is consistent with acure WNV infection, should undergo appropriate
diagnostic testing.
See
MMWR
Dec 20, 2002 for more information.
Q.
Can West Nile virus be transmitted through blood transfusions?
A. Please refer to Blood Transfusions
and Transmission: Questions and Answers.
Q.
Besides mosquitoes, can you get West Nile virus directly from
other insects or ticks?
A. Infected mosquitoes are the primary source for West Nile
virus. Although ticks infected with West Nile virus have been
found in Asia and Africa, their role in the transmission and maintenance
of the virus is uncertain. However, there is no information to
suggest that ticks played any role in the cases identified in
the United States.
Q.
How many types of animals have been found to be infected with
West Nile virus?
A. Although the vast majority of infections have been
identified in birds, WN virus has been shown to infect horses,
cats, bats, chipmunks, skunks, squirrels, and domestic rabbits.
Q.
Can you get West Nile virus directly from birds?
A. There is no evidence that a person can get the virus from
handling live or dead infected birds. However, persons should
avoid bare-handed contact when handling any dead animals
and use gloves or double plastic bags to place the carcass in
a garbage can.
Q.
Can you get infected with West Nile virus by caring for an infected
horse?
A. West Nile virus is transmitted by infectious mosquitoes.
There is no documented evidence of person-to-person or animal-to-person
transmission of West Nile virus. Normal veterinary infection control
precautions should be followed when caring for a horse suspected
to have this or any viral infection.
Q.
Can you get WNV from eating game birds or animals that have been
infected?
A. There is no evidence that WNV virus can be transmitted
to humans through consuming infected birds or animals. In keeping
with overall public health practice, and due to the risk of known
food-borne pathogens, people should always follow procedures for
fully cooking meat from either birds or mammals.
Q.
How does West Nile virus actually cause severe illness and death
in humans?
A. Following transmission by an infected mosquito, West Nile
virus multiplies in the person's blood system and crosses the
blood-brain barrier to reach the brain. The virus interferes with
normal central nervous system functioning and causes inflammation
of brain tissue.
Q.
How long does the West Nile virus remain in a person’s body
after they are infected?
A. There is no scientific evidence indicating that people
can be chronically infected with West Nile virus. What remain
in a person’s body for long periods of time are antibodies
and “memory” white blood cells (T-lymphocytes) that
the body produces to the virus. These antibodies and T-lymphocytes
last for years, and may last for the rest of a person’s
life. Antibodies are what many diagnostic tests look for when
clinical laboratories testing is performed. Both antibodies and
“memory” T-lymphocytes provide future protection from
the virus.
Q.
If a person contracts West Nile virus, does that person develop
a natural immunity to future infection by the virus?
A. It is assumed that immunity will be lifelong; however,
it may wane in later years.
See
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