Health officials announce West Nile Virus infection in an alpaca; increased risk levels in Massachusetts

Residents urged to take precautions to avoid mosquito bites.

The Massachusetts Department of Public Health (DPH) today announced the first animal case of West Nile virus (WNV) in the state this year. WNV infection was diagnosed in an alpaca exposed to WNV in Middlesex County.

Last week, DPH announced the first four human cases of WNV in Massachusetts this year. Boston and adjacent areas in Essex and Middlesex counties, and several towns in Bristol, Hampden, and Worcester counties are already at moderate risk for WNV.

But based on the occurrence of both animal and human cases, above-average populations of the Culex mosquitoes that carry WNV, recent rainfall, and continued weather favorable for mosquito activity, the WNV risk level is being raised in 27 communities from moderate to high. These communities are Lynnfield and Saugus, in Essex County; Arlington, Bedford, Belmont, Burlington, Cambridge, Everett, Lexington, Lincoln, Malden, Medford, Melrose, Newton, Reading, Somerville, Stoneham, Wakefield, Waltham, Watertown, Winchester, and Woburn in Middlesex County; Brookline in Norfolk County; and Boston, Chelsea, Revere, and Winthrop in Suffolk County.

“September is the month when we are most likely to see people get infected with West Nile virus,” said Acting Public Health Commissioner Margret Cooke. “While we advise everyone to take steps to avoid mosquito bites, this is especially important if you are over the age of 50 or have an immune compromising condition. It is also important to know that as overnight temperatures get cooler, mosquito activity right around dusk and dawn may be more intense.”

In 2020, there were five human cases of WNV infection identified in Massachusetts. WNV is usually transmitted to humans through the bite of an infected mosquito. While WNV can infect people of all ages, people over the age of 50 are at higher risk for severe disease. Most people infected with WNV will have no symptoms. When present, WNV symptoms tend to include fever and flu-like illness. In rare cases, more severe illness can occur.

People have an important role to play in protecting themselves and their loved ones from illnesses caused by mosquitoes.

Avoid Mosquito Bites

Apply Insect Repellent when Outdoors. Use a repellent with DEET (N, N-diethyl-m-toluamide), permethrin, picaridin (KBR 3023), oil of lemon eucalyptus [p-menthane 3, 8-diol (PMD)], or IR3535 according to the product label. DEET products should not be used on infants under two months of age and should be used in concentrations of 30% or less on older children. Oil of lemon eucalyptus should not be used on children under three years of age.

Be Aware of Peak Mosquito Hours. The hours from dusk to dawn are peak biting times for many mosquitoes. Consider rescheduling outdoor activities that occur during evening or early morning.

Clothing Can Help Reduce Mosquito Bites. Wear long sleeves, long pants, and socks when outdoors to help keep mosquitoes away from your skin.

Mosquito-Proof Your Home

Drain Standing Water. Mosquitoes lay their eggs in standing water. Limit the number of places around your home for mosquitoes to breed by either draining or discarding items that hold water. Check rain gutters and drains. Empty any unused flowerpots and wading pools and change the water in birdbaths frequently.

Install or Repair Screens. Keep mosquitoes outside by having tightly-fitting screens on all of your windows and doors.

Protect Your Animals

Animal owners should reduce potential mosquito breeding sites on their property by eliminating standing water from containers such as buckets, tires, and wading pools – especially after heavy rains. Water troughs should be flushed out to reduce mosquitoes near paddock areas. Horse owners should keep horses in indoor stalls at night to reduce their risk of exposure to mosquitoes.

Owners should also speak with their veterinarian about mosquito repellents approved for use in animals and vaccinations to prevent WNV and Eastern Equine Encephalitis (EEE). If an animal is diagnosed with WNV or EEE, owners are required to report to the Massachusetts Department of Agricultural Resources’ (MDAR) Division of Animal Health by calling 617-626-1795 and to the DPH by calling 617-983-6800.

More information, including all WNV and EEE positive results, can be found on the Arbovirus Surveillance Information web page at www.mass.gov/dph/mosquito or by calling the DPH Epidemiology Program at 617-983-6800.