Most WNV illness is mild and not diagnosed
and reported. WNV appeared in Southern California in 2003, causing 3 cases
of human illness and no fatalities. California was severely affected in 2004,
when WNV activity was detected in all 58 California counties and caused 830
reported infections and 28 deaths.
WNV activity continued to persist in 2005, when 54 of 58 counties reported
positive mosquitoes, animals, or humans, with 935 infections and 19 fatalities
reported. In 2006, WNV activity was detected in 54 of 58 California counties.
There were fewer cases of human illness, with 292 infections from 36 counties,
and 7 reported fatalities. As of April 27, 2007, there have been no reports
of human illness due to WNV this year, yet West Nile virus has already been
detected in 8 California counties. The early season detection of WNV, triggered
by unusually high temperatures throughout the state in March, indicates that
WNV will continue to pose a statewide public health threat in 2007.
There is currently no vaccine for WNV, and local health departments must rely
on preventing mosquito bites and local mosquito abatement measures to prevent
transmission. Because infection with WNV results in a higher rate of serious
complications and death in the elderly, it is important that facilities such
as nursing homes and assisted living centers take appropriate measures to
reduce mosquito populations and prevent mosquito bites. To the extent possible,
residents should be informed about the disease and advised and assisted in
taking the appropriate precautions to prevent infection.
The California Department of Health Services (CDHS), Division of Communicable
Disease Control (DCDC) in consultation with the Licensing and Certification
Program have developed the following recommendations for reducing the risk
of West Nile virus infection for residents of skilled nursing facilities and
other congregate living facilities.
Questions related to mosquito control in a specific city or county should
be referred to the local mosquito and vector control agency or health department.
Information about West Nile virus in California is available at the CDHS West
Nile virus web site at http://www.westnile.ca.gov.
Additional information about West Nile virus, including West Nile virus activity
elsewhere in the United States, is available at the Centers for Disease Control
and Prevention West Nile virus web site at http://www.cdc.gov/ncidod/dvbid/westnile/index.htm.
What is the epidemiology of West Nile virus?
West Nile virus is a single-stranded RNA virus of the family Flaviviridae,
genus Flavivirus and is closely related to other human flaviviruses that cause
encephalitis. In the summer of 1999, 62 cases of WNV were reported in New
York. In 2004, there were almost 2500 human cases of WNV detected nationally,
including 88 deaths. In 2003, there were over 10,000 cases with 263 deaths.
In cold climate northern states, the number of human WNV cases peaks between
August and September. In warmer climates, WNV activity may be year-round,
with the number of human cases peaking from July through September.
How do people get infected with West Nile virus?
West Nile virus is transmitted to humans by infected mosquitoes. Mosquitoes
become infected when they feed on the blood of infected birds. During later
blood meals (when mosquitoes bite), the virus may be injected into humans
and animals, where it can multiply and possibly cause illness.
West Nile virus is generally NOT transmitted from person-to-person. For example,
you cannot get West Nile virus from touching, providing health care, or other
close contact with a person who is infected, or from a health care worker
who has treated someone with the disease. However there have been reports
of transmission from donors of infected blood and organs to recipients and
one documented report oftransplacental (mother to child) transmission. Donated
blood is now routinely screened for the presence of WNV.
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 will get severely ill.
Although the vast majority of infections have been identified in birds, WN
virus has been shown to infect horses, bats, chipmunks, skunks, squirrels,
and domestic rabbits. 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.
What are the symptoms of West Nile virus infection?
Most people who are infected with the West Nile virus will not have any type
of illness. It is estimated that 20% of the people who become infected will
develop West Nile fever. About 1 in 150 persons infected with the West Nile
virus will develop a more severe form of disease, such as West Nile encephalitis
or meningitis. The most significant risk factor for developing severe neurological
manifestations requiring hospitalization is advanced age. The incubation period
ranges from 3-14 days. Symptoms of mild disease will generally last a few
days. Symptoms of severe disease may last several weeks, although neurological
effects may be permanent.
West Nile fever consists of mild to moderate symptoms, including fever, headache,
and body aches, occasionally with a skin rash swollen lymph glands, eye pain,
and nausea and vomiting. Other symptoms may include prolonged malaise, anorexia,
and fatigue. West Nile encephalitis or meningitis symptoms include headache,
high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions,
and muscle weakness.
Severe neurological manifestations include ataxia, cranial nerve abnormalities,
myelitis, optic neuritis, polyradiculitis and seizures. WNV can also cause
an acute flaccid paralysis syndrome that may be misdiagnosed as atypical Guillain
Barre Syndrome.
WNV should be considered in anyone presenting with a paralysis especially
if the paralysis is asymmetric, often monoplegic, often bowel/bladder dysfunction.
The EMG may show decreased of absent compound muscle action potentials.
Source: California Department of Health Services
