WNV can cause illness varying from a mild fever to more serious disease like encephalitis or meningitis. WNV grows in birds and is spread from bird to bird by infected mosquitoes. If mosquitoes infected with the virus bite horses or humans, the animal or person can become sick. In the United States, WNV was first identified in New York during the summer of 1999. Since then, it has spread throughout most of the continental U.S. It's not known how WNV got to the U.S., but it has occurred naturally in Europe, Africa and Asia for many years.
Most WNV infections do not cause any symptom. Mild WNV infections can cause fever, headache and body aches, often with a skin rash and swollen lymph glands. In a small percentage of people infected by the virus, the disease can be serious, even fatal. More severe infections can cause headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, paralysis and, sometimes, death. Persons older than 50 years of age have a higher risk of developing severe illness.
There is no specific treatment for WNV infection. People with mild WNV infections usually recover on their own. Doctors can provide supportive therapy for people who have more serious complications, such as encephalitis or meningitis. However, approximately 10% of people with severe infections die.
WNV usually is spread by adult mosquitoes that are infected by the virus. Mosquitoes spread the virus by biting humans, horses, and other animals. WNV can sometimes be spread in other ways. For example, WNV can be spread to humans through blood transfusions and organ transplants from infected donors. Also, it is possible the pregnant women or breastfeeding mothers who become infected with WNV may pass the virus to their baby. Because of the unknown risk at this time and the fact that breastfeeding has well-established benefits, it is not recommended to discontinue breastfeeding. Horses that have WNV infection cannot spread the disease directly to humans. There is no evidence that a person can get WNV from touching live or dead infected birds. Still, basic safety precautions should be used when handling any dead animals, including a dead bird. If you need to move or dispose of a dead bird, use gloves or a shovel to handle it, and place it into two plastic bags (one inside the other).
Serious illness caused by WNV is uncommon and has been identified in a small number of people in Massachusetts for the past several years. Additionally, WNV has been found in horses, mosquitoes, and many species of birds throughout the state. The mosquitoes that carry this virus are common throughout the state, and these mosquitoes are found in the city as well as in the woods and other less populated places.
Massachusetts developed a Surveillance and Response Plan that serves to coordinate efforts of local officials and state agencies to reduce the risk of WNV. The plan involves checking for the virus in mosquitoes and birds, reducing the number of mosquitoes in the environment, and educating people on ways to avoid mosquito bites. Every year from May until the first hard frost, the Massachusetts Department of Public Health (MDPH) and local mosquito control projects collect mosquitoes from various locations around the state and send them to the State Laboratory for testing. MDPH collects information on the number and locations of dead birds reported in the state and tests selected birds for WNV. If WNV is identified in birds or mosquitoes, MDPH will tell local boards of health and mosquito control projects and ask them to increase their education and mosquito control activities. MDPH provides educational materials for physicians, veterinarians, local public health officials, and the public along with updates on WNV activity in mosquitoes, horses and humans online.