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West Nile Virus detected in 48 states

The proboscis of a female mosquito – here a Southern House Mosquito (Culex quinquefasciatus) - pierces the epidermis and dermis to allow it to feed on humanblood from a capillary: this one is almost fully engorged. The mosquito injects saliva which contains an anesthetic, and an anticoagulant into the puncture wound; and in infected mosquitoes, the West Nile virus.

The proboscis of a female mosquito – here a Southern House Mosquito (Culex quinquefasciatus) – pierces the epidermis and dermis to allow it to feed on human blood from a capillary: this one is almost fully engorged. The mosquito injects saliva which contains an anesthetic, and an anticoagulant into the puncture wound; and in infected mosquitoes, the West Nile virus. © CDC/Jim Gathany

Infections by the dangerous West Nile Virus (WNV) have now been reported in people, animals or mosquitoes in all states except Alaska and Hawaii.

That distribution was reported in figures from the Centers for Disease Control and Prevention, as of September 4.

Of 1993 human cases, 54 per cent involved the nervous system.

Dr Roberta Dwyer, of the Maxwell H. Gluck Equine Research Center at the University of Kentucky, noted that both horse people and mosquitoes tended to be most active at dawn and dusk.

“If WNV is reported from 48 states, people and horses in 48 states are at risk of being bitten by infected mosquitoes,” she said.

The only US-approved WNV vaccines are for horses, and the American Association of Equine Practitioners lists WNV as a core vaccine, emphasizing its importance.

“Compare the cost for WNV vaccination versus costs for veterinary examination, diagnosis, and treatment of a neurological horse,” Dwyer noted in the latest issue of Gluck’s Equine Disease Quarterly.

Electron microscopy of West Nile virus.

Electron microscopy of West Nile virus.

“And, let’s not forget that any neurological horse needs to be handled as a rabies suspect until proven otherwise.

“While extensive WNV data is available at the CDC ArboNet website, equine specific data is not.

“According to the Kentucky State Veterinarian’s website, as of September 11, 2012, 10 WNV-positive horses were reported from nine counties. Seven horses were unvaccinated; two were partially vaccinated and one was reportedly vaccinated.

“A summary of Kentucky WNV equine cases (2005- 2011) shows that 91 per cent of positive horses were not vaccinated; others were partially vaccinated or had unknown vaccination history. How complete is your equine vaccination program?”

Understanding WNV vectors was important to human and equine health, she said.

Mosquitoes in the genus Culex are primary vectors of WNV, with multiple species being able to transmit the virus throughout the continental United States.

The house mosquito (Cx. pipiens complex) is common in areas across Kentucky, noted Dr Lee Townsend, from the University of Kentucky’s Department of Entomology.

Inseminated females spend the winter in protected places near a moisture source and mortality is usually low following mild winters. Surviving females become active, laying rafts of eggs on the surface of water with moderately to very high organic content including grassy ditches, waste lagoons, and polluted ground pools.

The life cycle from egg to adult takes about eight to 12 days during the summer.

Female Cx. pipiens usually remain within a half-mile of their breeding habitat. These night-flying mosquitoes feed primarily on birds but will enter buildings and feed on a variety of mammals and humans.

They are particularly important in amplification of the virus in susceptible birds and along with other mosquito species can be “bridge vectors” that enable the virus to move from avian to mammalian species.

The two major preventive strategies in reducing risk of arthropod-borne viruses are:

  • Using measures to reduce contact with mosquitoes, including insecticides and repellents; and
  • Reducing, eliminating or treating known breeding sites.

The success of both strategies is variable for many practical reasons.

Insect-transmitted diseases rarely disappear, Dwyer and Townsend noted, concluding: “People need to take the WNV risk to themselves and their horses seriously.”

 

Gluck’s Equine Disease Quarterly is funded by underwriters at Lloyd’s, London, brokers and their Kentucky agents.

 

 

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