West Nile Virus
West Nile virus - What’s New
Our 2-page “The Basics” for display or distribution!
This downloadable document summarizes what you need to know, and the steps you can take, to reduce the chance of exposure to WNV.
Current information on human cases and geographic occurrence
Because the situation can change rapidly, we will not attempt to post such data here. If you seek information on West Nile virus’s effects on humans, the Center for Disease Control and Prevention’s WNV site is a good place to start; for current cases, click on “Case Count” on the upper left. If you seek information regarding geographic occurrence within the United States, the maps available at the United States Geological Survey’s WNV site are current and depict the distribution for: birds, humans, mosquitoes (where testing occurs), sentinel chickens (where they exist), and horses (the primary animal composing their category “veterinary”). Maps depicting WNV’s detection in the categories above for the years 1999-2002 are also available at the USGS site. Information on the occurrence of WNV in Canada is available on Health Canada’s webpages for humans and birds (requires Adobe Reader).
2004 WNV season underway, and raises more questions
A blue jay found dead in Houston, TX in late January, an overwintering mosquito found in Union County, PA in February, and 2 live house finches trapped in Orange County, CA in March, tested positive for WNV1,2,16. In NY, 2 crows found dead at a roost site in Putnam County in late November, a Red-shouldered Hawk found alive on the ground in Albany in December, and 1 crow killed by a train in Amsterdam in January also tested positive3. Possible explanations for the cold-weather cases include, as surmised by Ward Stone of the NYS Department of Environmental Conservation: latent viral persistence in infection survivors, current infection through the consumption of infected prey or having been bitten by an emergent, infected mosquito, and the evolution of less pathogenic strains of WNV3.
Researchers uncover one possible reason for WNV’s unusually large host range and rapid geographic spread
In northern Europe, two types of Culex pipiens, a mosquito believed to be an important vector of WNV in North America, differ genetically and behaviorally; one feeds on birds and hibernates during the winter, and one feeds on mammals – including humans – potentially year-round by seeking winter refuge in sewers and subways. The two types do not interbreed in Europe, but they do in the United States; hybrids across the country are “ubiquitous”4. The hybrids may bite both birds and mammals, and transmit the virus among them. This finding may help to explain some the unusual characteristics of WNV (see “The Virus”).
At least a couple of North American reptiles and one amphibian unlikely to be important reservoir hosts for WNV
Green iguanas, Florida garter snakes, red-eared sliders, and North American bullfrogs were experimentally exposed to the virus, and most of the individuals of all four species didn’t even become “infected;” the virus was not reproduced to detectable levels in their blood5. Some of the iguanas and bullfrogs had detectable viremias, but at levels too low to infect Culex pipiens. A small number of iguanas, bullfrogs, and garter snakes had infectious viral particles present in tissues and organs. Infectivity levels for mosquitoes that feed on reptiles and amphibians are unknown, as are those for predators or scavengers of these animals5.
Cats and dogs and WNV
Four cats and four dogs were experimentally exposed to the virus via mosquito bites, and four cats were fed infected mice. The four dogs developed viremias of low magnitude and short duration, but all eight cats developed viremias possibly capable of infecting mosquitoes6. That cats infected through eating infected prey might contribute to local transmission of WNV is all the more reason to KEEP YOUR CATS INDOORS.
NIAID begins examination of possible treatment for WNV infection
The National Institute of Allergy and Infectious Diseases (of the National Institutes of Health) has announced that the clinical trial for an experimental treatment for WNV infection is underway. An Israeli company developed the treatment from the plasma of Israeli donors with high levels of antibodies to WNV (WNV has existed in Israel for decades, and many Israelis have antibodies in their blood). By giving the treatment to infected patients in the United States, researchers will determine if the treatment is safe, well-tolerated, and effective in preventing death or neurologic disability from encephalitis. For more information on the trial and participating sites, see NIH’s ClinicalTrials.
Alexander the Great may have died from West Nile virus infection
In a recent paper, John S Marr and Charles H Calisher make a plausible case for WNV as the cause of Alexander the Great’s demise; supporting evidence comes from a description of ravens exhibiting unusual behavior and dying in front of him upon his return to Babylon in May of 323 BC7.
Purdue team uncovers structure of WNV surface proteins
Purdue University biologists have determined the orientation of the three surface proteins of WNV8 – the ones involved with binding to and entering host cells. Such knowledge will help shed light on how WNV operates, thereby contributing to efforts to devise ways of interrupting the viral assembly process (for more information on viral characteristics, see our section “The Virus” on these pages).
WNV evolving in North America
The strain of WNV accidentally introduced into New York City in 1999 was closely related to the strain isolated from the brain of a dead goose in Israel in 1998. Throughout 1999 and 2000, as WNV spread through northeastern states of the U.S., little genetic divergence occurred; the sequence of nucleotides making up its RNA molecule remained pretty much the same. Yet WNV particles isolated from different regions of the U.S. in 2001 and 2002 indicate that WNV is evolving; mutations occurring as WNV spreads are being incorporated into its genetic material. Different variants have been found to be clustered spatially – isolates of similar genetic structure tend to occur in the same geographic area, but across geographic areas the strains differ. Evidence suggests that WNV has been introduced into Texas at least two separate times9.
High prevalence of antibodies to WNV in birds of the UK
Research designed to survey resident birds of the UK for the presence of several virus strains common in Africa (to and from where many UK migrants travel annually) has uncovered a surprisingly high prevalence of antibodies to WNV: more than 60% of the individuals of 30 different species were found to possess antibodies to a strain of WNV very similar to the “New York strain” (as the strain spreading through the Americas has come to be called)10. The sample of birds included many juveniles that had yet to migrate, thus the authors surmise that the virus is being carried into the UK by migratory birds. Because there is no evidence that birds are dying in large numbers in the UK because of WNV, and because the birds carrying antibodies (including crows and magpies) were asymptomatic, the authors speculate that either the specific strain of WNV in the UK is less virulent than the one circulating in North and Central America, or that UK birds have actually been exposed to this virus for many years and have evolved immunity10.
Farmed alligators hit hard by WNV
In addition to the hundreds of alligators presumed to have died as a function of WNV infections in Florida and Georgia in 2001 and 200211,12, hundreds more died on farms in Louisiana in 200313. With regard to their possible role in the amplification cycle of WNV in the Western Hemisphere, a recent study of crocodiles in Israel demonstrated a high rate of infection with WNV14, suggesting they may serve as hosts for the virus. Given the high levels of virus found in many alligators that had died in Florida in 2002 – high enough to infect mosquitoes15 - and the role of various reptiles in the epidemiology of other New World arboviruses (references 13-15 in14, below), research into the possible role played by reptiles in the transmission cycle of WNV in the Americas will likely pick up.
Genetic basis for mosquito insecticide resistance discovered
Research performed in France has elucidated the basis for the resistance of some mosquitoes to the effects of commonly used pesticides. A single point mutation (a change in one nucleotide) in the gene that codes for acetylcholinesterase (ACHe) – the enzyme that breaks down the neurotransmitter acetylcholine – renders the pesticides useless. For mosquitoes without the mutation, organophosphate- and carbamate-type insecticides inactivate ACHe, which disrupts normal nervous transmission and results in paralysis and death of the mosquitoes. The ACHe of mosquitoes with the mutation is not inactivated by organophosphates and carbamates, thus the mosquitoes are not affected by the insecticides – they are “resistant.” This new information will likely lead to the development of new insecticides. A summary of the work can be found in a press release from the Universite de Montpellier II. (4/2/04)
Copepods – small crustaceans – control mosquitoes!
Researchers at the University of Florida have discovered that microscopic crustaceans called copepods are extremely effective at controlling mosquitoes. These native organisms pose no threat to people or other animals and can survive year-round in any size body of fresh water, including the stagnant water in discarded tires and other containers. Copepods feed aggressively on larval mosquitoes and will even kill the larvae when not looking for food; the survivorship of mosquito larvae is reduced up to 90% in the presence of certain copepods. These crustaceans do not occur in all bodies of water and would have to be introduced, but once established they reach numbers high enough for effective mosquito control. Information on obtaining and culturing copepods is available at the website of the University of Florida’s Florida Medical Entomology Laboratory. (4/2/04).
1. Pennsylvania Department of Health. PA West Nile Encephalitis Surveillance Program. 2004 News Releases. (4/2/04) 2. ProMED-mail. West Nile virus, bird – USA (Texas). ProMED-mail 2004; 12 February: 20040212.0464. (4/2/04) 3. Cornell University. Environmental Risk Assessment Program. WestNileVirus-L, February 25 2004: Cold weather cases of WNV in NYS. 4. Fonesca, DM, et al. 2004. Emerging vectors in the Culex pipiens complex. Science 303: 1535-38. For a summary: Nature. Science update, March 5 2004. Mosquitoes give clues to West Nile’s spread. (4/2/04) 5. Klenk, K and N Komar. 2003. Poor replication of West Nile Virus (New York 1999 strain) in three reptilian and one amphibian species. Am. J. Trop. Med. Hyg., 69(3): 260-62. 6. Austgen, LE, et al. 2004. Experimental infection of cats and dogs with West Nile virus. Emerg. Infect. Dis. 10(1): 82-86. (4/204) 7. Marr, JS, and CH Calisher. 2003. Alexander the Great and West Nile Virus Encephalitis. Emerg. Infect. Dis. 9(12): 1599-1603. (4/2/04) 8. Purdue University press release. October 9 2003. (4/2/04) 9. Davis, CT, et al. 2003. Genetic variation among temporally and geographically distinct West Nile virus isolates, United States, 2001, 2002. Emerg. Infect. Dis. 9(11): 1423-1429. (4/2/04) 10. Buckley, A, et al. 2003. Serological evidence of West Nile virus, Usutu virus and Sindbis virus infection of birds in the UK. Journal of General Virology 84: (4/2/04) 11. ProMED-mail. West Nile Virus, Reptiles, Alligators – USA (Florida). ProMED-mail 2002; 14 Nov: 20021114.5797. (4/2/04) 12. Miller, DL, et al. 2003. West Nile virus in farmed alligators. Emerging Infectious Diseases 9(7):794-799. (4/2/04) 13. ProMED-mail. West Nile Virus update 2003 – USA. ProMED-mail 2003; 6 November: 20031106.2757. (4/2/04) 14. Steinman, A, et al. 2003. West Nile virus infection in crocodiles. Emerg. Infect. Dis. 9(7): 887-889. (4/2/04) 15. Cornell University. Environmental Risk Assessment Program. WestNileVirus-L Archive, August 8 2003: Update: Range and Cases US, 2003. (4/2/04) 16. Cornell University. Environmental Risk Assessment Program. WestNileVirus-L, April 2 2004: WNV Detected in S California.
Last Update: 04/05/04