How to Manage Pests

Mosquitoes

Managing Mosquitoes on the Farm Section 2: West Nile Virus Published 2005 Sections of this publication:

Why You Should Care About Mosquito Control West Nile Virus You May Be Raising the Mosquito That Is Biting You! Mosquito Prevention Three Basic Principles of Mosquito Prevention Natural Waters Associated With Farms Managing Stagnant Waters Created by Agricultural Activities Irrigated Fields for Upland Crops Mosquito-Free Irrigated Pastures Rice Dairy Operations Biological and Chemical Mosquito Control Common Mosquitoes and Their Life Cycles Acknowledgments For More Information

What Is It?

West Nile virus is a virus carried by mosquitoes. Viruses are tiny microorganisms (smaller than bacteria) that inject their genes into cells. The infected cells produce more viruses. Antibiotics are not effective against viruses.

About 80 percent of human West Nile virus infections are so mild that there are no symptoms. About 20 percent of infected people experience West Nile fever, which produces flu-like symptoms (headache, nausea, fever, and sometimes a rash or swollen glands). However, one-half percent of those infected may experience life-threatening or disabling consequences including severe fever and headache, disorientation, paralysis, numbness, convulsions, vision loss, coma, or death. People with severe symptoms should seek medical care immediately. Persons over 50 years of age or those with compromised immune systems are at higher risk of complications.

How Is It Transmitted?

The virus is mainly transmitted by mosquitoes in the genus Culex, but other kinds of mosquitoes may transmit it as well. The primary life cycle of this virus is between birds and mosquitoes. Birds can produce high levels of the virus in their blood, and mosquitoes pick up the virus when they feed. Mosquitoes that feed on infected birds can transmit the virus to humans, horses, and other mammals.

During the brief period that the virus is circulating in the human bloodstream, it is possible (but rare) for it to be transmitted from human to human through blood transfusions, organ transplants, or even breast feeding of infants. In California, the blood supply and organ donations are now screened for West Nile virus, and the risk of infection from these sources is very low. Nursing mothers who think they have symptoms of West Nile fever should talk to their physicians. The virus is not transmitted through casual contact, such as touching, kissing, and coughing.

It is unlikely that mosquitoes would carry the virus from horses to humans or between horses, because the level of virus in mammal blood is too low. Therefore, euthanizing infected horses does not prevent the spread of the disease.

Who Is at Risk?

Anyone suffering mosquito bites is at risk of this disease.

What Animals Are at Risk?

Horses and wild birds are the animals that are most susceptible to the virus. Fortunately, domestic poultry and livestock other than horses very rarely show symptoms of infection. One-third of infected horses show severe symptoms and either die or are euthanized. A vaccine is available for horses. Vaccines against other encephalitis viruses do not work against West Nile virus.

Managing Mosquitoes on the Farm, UC ANR Publication 8158 Sharon P. Lawler and Gregory C. Lanzaro, Department of Entomology, University of California, Davis

Copyright © 2005 The Regents of the University of California, Division of Agriculture and Natural Resources. All rights reserved.