Zoonotic Disease Program
West Nile Virus Surveillance

People often ask, "Why do you test for this disease in animals when you know it is there?" The answer lies in the definition of the word "surveillance". Yes, we know it is there. But how much? Is it in epidemic proportions in wild animals? The term for that is epizootic. If the number of infected animals or mosquitoes, for example, is increasing, then we may be facing a spillover into the human population. Thus, knowing what is going on in animals and mosquitoes gives us a "heads-up" that people are at greater risk of becoming ill with these infections. This is most important regarding those diseases for which there is no vaccine or effective treatment for humans. Mosquito-borne viruses, such as West Nile virus, fall into this category. That's why we monitor these diseases in animals and arthropods such as mosquitoes. It gives a forewarning. People can then be advised to protect themselves, and mosquito and other vector control agencies can increase their activities to protect the public from illness and death.

Mosquitoes

Adult mosquitoes will be trapped from April to October throughout Ohio, concentrating on human population centers. The number of sites, the frequency of trap placement, and number of traps per site will vary depending on where the virus is most likely to be present. Mosquitoes will be collected using gravid mosquito traps, then sent to VBDP for identification and testing. Trapping will be expanded if West Nile virus is detected in mosquitoes, birds, other animals, or humans. Information on the total number, the type and the sex of mosquito (since only females spread viruses) will be collected.

Birds

Birds are the natural reservoir for West Nile virus. If a mosquito bites an infected bird and the virus is transmitted to the mosquito, it may then become a host itself. If the same mosquito then bites a human, it can pass the disease to the human. Because of this cycle, noticing dead birds could be the first sign of West Nile Virus in an area.

1. Wild bird surveillance

   Blood samples can be taken from wild birds to see if they are infected with West Nile and other viruses. During 2007, the ODH Vector-borne Disease laboratory tested 1,159 birds and found 26 positives.

    2.  Dead bird surveillance

• West Nile virus kills birds, and the most sensitive species in Ohio are American Crows and blue jays. They almost always die from infection with West Nile Virus. This is a new virus in the western hemisphere, but some studies have indicated that a dead crow rate of 1.5 per square mile per week is a prelude to human cases. Citizens should report dead crows and blue jays to their local health departments. Clusters of dead crows and blue jays may indicate increased risk of human disease. This is important to local health departments. Should they increase their prevention efforts or not? Citizens can help in this decision by reporting dead crows and blue jays to their local health department.
• While dead crows and blue jays can be valuable warnings of WNV activity, we are no longer accepting dead birds for testing. Please contact your local health department if you would like to report dead bird sightings.

Horses

Horses are known as a dead-end host of West Nile virus, that is, they can become ill with West Nile virus, but they do not maintain sufficient virus in the blood to infect either other mammals (including humans) or mosquitoes. Surveillance of horses has occurred in areas with a history of West Nile virus and in additional States on the eastern coast of the United States to monitor the possible spread of WNV. This surveillance consists of investigating suspect cases in horses. Because horses are not known to play a role in transmission of WNV, quarantines were never placed on any non-clinically ill horses in the outbreak area. However, some foreign countries have banned the importation of horses from New York. More information is available in the "Combating West Nile Virus: The Plan for the State of Ohio."

Last Updated: 5/20/2009

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