West Nile Virus: Transmission, Infection, & Symptoms
West Nile Virus Transmission Cycle
WNV is typically transmitted between birds and mosquitoes [Transmission cycle]. Mammals can become infected if bitten by an infected mosquito, but individuals in most species of mammals do not become ill from the virus. Mammals are generally considered to be "dead-end hosts" because the virus does not generally build up (amplify) in their blood to levels high enough to be infective. Thus mosquitoes that bite an infected person or horse cannot pick up the virus from them and spread it further. A research article in the April 2002 issue of the journal Emerging Infectious Diseases supports the "conclusion that horses infected with WNV develop viremias of low magnitude and short duration and that infected horses are unlikely to serve as important amplifying hosts for WNV in nature" ((Bunning et al. 2001). I.e., mosquitoes fed on the infected horses did not become infected with the virus [Full text]. Typically, people (or horses) can not get the virus from contact with sick people or horses. However in Fall 2002 convincing evidence indicated that WNV could be transmitted via organ transplant, blood transfusion, from infected mother to fetus, and possibly from breast milk of an infected woman [More]. Research by the USGS National Wildlife Health Center and the Wildlife Conservation Society has shown that bird-to-bird transmission is possible, though the extent and full significance of this is not yet known [Text of News Release, Questions arising from News Release]. It may also be possible (though not field-verified) that predaceous birds may become infected with the virus by eating infected prey (e.g., other birds, small mammals, frogs).Birds
In some bird species--particularly Corvids (crows, blue jays, ravens)--the virus causes disease (often fatal) in a large percentage of infected birds, but birds of many other species can carry the virus (seroconvert) without becoming sick (e.g. chickens). In some infected birds viremia (i.e., virus in the blood) lasts 1-4 days, then birds seroconvert (i.e., produce antibodies) and likely develop immunity if they do not die of encephalitis, myocardial degeneration or pancreatitis. Sick birds may show neurological symptoms, paralysis, depression, anorexia and weight loss, weakness, tremors, blood in urine, disorientation, etc. Supportive treatments for sick birds seem to be most useful at earlier stages of illness. These could include antibiotics to prevent secondary bacterial infections and non-steroidal anti-inflammatory drugs to counteract encephalitis [More on symptoms, treatment and pathology: Steele et al., Redig, Kramer and Bernard, Malkinson et al.].Because crows are large and noticeable near human settlements, dead crows have been an important and sensitive "indicator" of WNV presence in an area. Research is underway to better understand how WNV is affecting the crow population (e.g. see webpages of Dr. Kevin McGowan at Cornell). Much remains to be learned about how WNV is affecting populations of other species. Vis-a-vis long term effects on bird health and ecology, antibodies produced after exposure to infection may lead to long term immunity or--as some veterinarians are speculating--WNV may cause chronic infection in birds that recrudesces (i.e., reappears) during times of stress or birds may die of secondary organ damage in months or years following the initial infection (source: Amador, 2002, unpublished).
Death of several exotic and domestic birds in the collections of the Bronx Zoo in September 1999 was a signal event important in highlighting and then unraveling the mystery of the human and animal disease that turned out to be caused by West Nile Virus (for more information see Cook 2000;Calle et al. 2000a, 2000b, and 2001; Drexler, Chapter 2 2000; Miller 2001; Steele et al. 2000). Since then WNV has been detected in the captive collections of many zoos, including in 2002 in 3 birds at the US National Zoo in Washington DC. In hopes of preventing more bird deaths, the zoo's veterinarians vaccinated the 300 or so birds in their captive collection with the experimental equine vaccine that was used already on the zebras and tapirs at the zoo. The efficacy of this vaccine on birds is uncertain [More information on WNV vaccines].
The University of Minnesota's Raptor Center reports increasing numbers of raptors picked up sick or dead, infected with WNV but perhaps also with other confounding disease [See Sept 13 and Sept 18 listserv postings]. The National Wildlife Health Center (NWHC) has tested some of the raptor cases admitted to raptor rehabilitation centers. These cases underwent full diagnostic evaluation to determine whether the deaths were due to WNV or to other causes. The NWHC WNV testing protocol consists of isolating virus from organ tissues (such as kidneys, spleen, brain, etc.) and then performing RT-PCR on the virus isolates to confirm WNV. Other tests include looking for other infectious causes (bacterial, fungal or other viral) and toxin exposure. As of December 18, 2002, the NWHC had received 73 raptor carcasses from several states. Diagnostic evaluation has been completed for 30 cases : 9 died due to WNV, 6 were infected with WNV but did not show any or sufficient brain damage to explain symptoms, 4 were WNV-negative but had sufficient brain damage suggestive of viral encephalitis, and 11 died from other causes. In sum, NWHC has diagnosed 9 confirmed and 10 possible cases of WNV. Further confirmatory testing is planned for the 10 possible cases (source of information on NWHC tests is Emi Kate Saito, VMD, MSPH, WNV Surveillance Coordinator, USGS NWHC, Dec 18, 2002).
Migratory birds are likely a key means by which the virus is transported to new areas along their migratory routes. (WNV is also likely transported in the process of international and regional trade and movement of goods, in containers such as wet, used tires that are carrying infected mosquitoes). For example, birds migrating southward from the northern Midwestern states--where intense transmission was occurring in 2001--have carried the virus into the Southern states. Northward migratory birds are likely responsible for reintroducing the virus to Northern states. Given the patterns of bird migration [maps], one hypothesis is that WNV will move westward in a zig-zag fashion, arriving on the West Coast only after it is established in places in Central America or Mexico where most of the west coast migrating birds pass during their northward migration. (Note that WNV may already be in Central and South America, but the only confirmed surveillance report we are aware of from points south of the US is from the Cayman Islands in the Caribbean. Probable cases in birds in the Yucatan and in a man who may have contracted the disease in Texas were reported Aug 28, 2002.)
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Source: DeGraaf, Richard M. and John H. Rappole. 1995. Neotropical
Migratory Birds: Natural History, Distribution, and Population Change. Cornell University Press: Ithaca, NY and London. 676 pages. |
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Mosquitoes
In the Northern US, WNV has been most closely
associated with Culex pipiens mosquitoes, a species that breeds in
standing water, especially in water polluted with organic matter. It has been
thought that these mosquitoes "prefer" to bite birds, but 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. Another hypothesis suggests that other species of mosquitoes, not Culex pipiens, acts as a "bridge" biting both birds (ornithophilic) and mammals (e.g., people and horses). Some recent evidence indicates, that
C. salinarius are responsible for WNV
transmission to people. Culex salinarius are found in fresh and saltwater
marshes, lakes, ponds and seepage areas, as well as in the many types of
artificial containers found around human residences and businesses. It is active
from sunset to sunrise. As WNV has moved westward, Culex tarsalis has become more important as a vector. In laboratory studies it has been shown to be the most efficient of WNV vectors, both in becoming infected and in transmission, and is known to seek human and large mammal hosts, as well as birds, for its blood meals.
The CDC list of mosquito species that have tested WNV-positive in the United States
includes, as of Oct 21 2002:
Aedes species: Aedes albopictus, Aedes aegypti, Aedes vexans, Aedes cinereus;
Anopheles species: Anopheles barberi, Anopheles atropos, Anopheles crucians/bradleyi, Anopheles punctipennis Anopheles quadrimaculatus, Anopheles walkeri
Coquillettidia species: Coquillettidia perturbans;
Culiseta species: Culiseta inornata, Culiseta melanura;
Culex species: Culex abominator, Culex erraticus, Culex nigripalpus, Culex pipiens, Culex quinquefasciatus, Culex restuans, Culex salinarius, Culex tarsalis, Culex territans;
Deinocerites species: Deinocerites cancer;
Ochlerotatus species: Ochlerotatus atropalpus, Ochlerotatus atlanticus/tormentor, Ochlerotatus canadensis, Ochlerotatus cantator, Ochlerotatus japonicus, Ochlerotatus sollicitans, Ochlerotatus taeniorhynchus, Ochlerotatus triseriatus, Ochlerotatus trivittatus
Orthopodomyia species: Orthopodomyia signifera. [Note that species in the genus Ochlerotatus were previously classified as Aedes].
In New York State, mosquitoes from the following species have been found WNV-positive. The species name is followed by the number of positive mosquito pools from that species found in
the state as of Jan 11, 2001. Culex spp. (271), Culex salinarius (22), Ochlerotatus
vexans (2), Ochlerotatus sollicitans (1), Ochlerotatus trivittatus (1), Coquillettidia perturbans (1), and Culiseta melanura (1).
Horses
HORSES: 14,358 equine cases in 40 states were reported to USDA APHIS by state health officials as of Dec 1, 2002--nearly 20-fold the case load reported by 20 states last year. During 2003, equine cases increased from:130 cases reported Aug 8,
644 cases reported from 25 states as of Aug 19,
2360 cases reported Sept 1,
3453 cases reported from 30 states as of Sept 8,
5380 cases reported from 33 states as of Sept 17,
9036 cases reported from 36 statesas of Oct 6, and
10,172 cases reported from 38 states as of Oct 13,
12,843 cases as of Nov 3, and
13,577 cases reported from 39 states Nov 12.
At least 4,300 horses are said to have died from WNV-associated illness (30%), a death rate similar to that of previous seasons and similar across geographic areas of the US. The 2003 WNV "attack rate" on horses in the Midwest was about 25% -- meaning that about 25 horses out of 100 showed clinical symptoms of WNV. This rate is much higher than what was seen in the WNV range 1999-2001.
In 2001 there were about 5% as many equine cases (738), and they appeared later in the year than in 2002. Numbers increased dramatically during Fall 2001, from 137 cases reported by Oct 1, 191 cases in 15 states reported by Oct 15, 347 cases reported by Oct 31, to the final tally of 738 cases in 20 states reported by USDA APHIS in its 2001 summary [More].
Of the 470 equine cases in 2001 for which an outcome was reported to APHIS, at least 156 (33.2 percent) died or were euthanized. The vast majority of equine cases (492) and deaths (82) were in Florida. Equine WNV was confirmed in Alabama*, Connecticut, Delaware, Florida*, Georgia*, Illinois*, Indiana*, Kentucky*, Louisiana*, Maryland*, Massachusetts, Mississippi*, New Hampshire*, New Jersey, New York, North Carolina*, Pennsylvania, Rhode Island, Tennessee* and Virginia* (asterisk* indicates that 2001 was first year with equine cases).
In 2000, there were 88 confirmed clinical cases of WNV in horses in 7 states: Connecticut, Delaware, Massachusetts, New Jersey, New York, Pennsylvania, and Rhode Island. In 1999 there were 25 cases with clinical signs of illness (9 deaths)--all from two areas of Long Island, NY. In 1999 and 2000, roughly 40 percent of the horses with confirmed WNV infections either died or were euthanized.
HORSE SYMPTOMS: Signs of illness in horses may include ataxia (lack of coordination, stumbling, staggering), difficulty walking, knuckling over, head tilt, muscle twitches or tremors, inability to stand, circling, weakness or paralysis of limbs, apparent blindness, lip droop, grinding teeth and death. However, these symptoms could also be caused by other diseases including rabies, eastern and western equine encephalitis virus infections, and equine herpes virus-1 infection. Currently, there are live-animal tests for WNV in horses and chickens, but none for other animals. Unlike some other encephalitis viruses such as Eastern Equine Encephalitis (EEE), horses that live through the 2 to 3 weeks of WNV infection generally recover fully with no long-term side effects.
HORSE VACCINE: On August 1, 2001, USDA APHIS granted a conditional 1-year license to the Fort Dodge Animal Health company in Iowa for manufacture and distribution of a vaccine to protect healthy horses from WNV infection. [More on horse vaccine]. The conditional license has now been extended and the vaccine is still available to veterinarians. A full hearing for permanent license is expected to be scheduled as soon as studies of its effectiveness are completed. Veterinarian Terry Conger of the Texas Animal Health Commission reported that 'booster' shots must be given yearly or every 6 months in warmer areas with large mosquito populations in order to maintain maximum immunity. [More resources re: WNV and horses]
Other Mammals
OTHER MAMMALS: Several recent reports have detailed WNV causing serious or fatal illness in other mammal species, including reindeer [More]; squirrel, wolf and dog in Illinois [More]--which leads the country in number of human cases--and mountain goat and sheep in Nebraska [More]--which leads the country in numbers of equine cases. Black bears in NW New Jersey, tested in Feb and Mar 2002 as part of a neutralizing antibody serosurvery, tested positive in 3 of 51 samples (Rutgers University) [More] The same Rutgers research team also tested 689 blood samples from white-tailed deer during the 2001 hunting season in NJ and found that 7 were seropositive for the presence of WN virus neutralizing antibodies, 11 were seropositive for the presence of SLE neutralizing antibodies and18 were seropositive to flavivirus in general (there was not sufficient difference in the titer levels between SLE and WNV to determine which). Implications of these findings, in terms of WNV transmission and disease ecology, are unclear since the bear and deer were not apparently sickened by the virus and it is believed that mammals are dead end hosts and cannot transmit the disease.
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).
