Steps to prevent West Nile Virus

The following information comes from a June 12, 2019 Montana Department of Health and Human Services (DPHHS) press release. You can find the original press release here.

State and local public health officials are reminding Montanans to take steps to avoid mosquito bites and prevent infection with West Nile Virus (WNV). In Montana, WNV season usually begins in July and ends in October, as this is when the mosquitoes that transmit WNV emerge.

The best way to prevent mosquito-borne diseases, is to protect yourself from mosquito bites. While mosquitoes found in Montana are unable to transmit diseases like Zika virus, they do spread WNV. The virus can also infect horses and birds, with birds serving as the source of infection for most mosquitoes who then pass the virus along to humans by biting them.

“As we approach WNV season, we encourage everyone to take the proper precautions to prevent mosquito bites when outdoors,” stated DPHHS epidemiologist Erika Baldry.

Since WNV surveillance began in 2002, the 2018 season was the third highest in terms of the number of WNV cases reported in Montana. Forty-seven people were diagnosed and reported, including one death, which was the 14th WNV-related death in Montana since 2002. Humans are not the only ones that can be infected with WNV and 50 Montana horses were also diagnosed in 2018.

The average number of human cases reported in the three years prior (2015-2017) was 8, while the average number of equine cases reported in the three years prior (2015-2017) was 6 cases. A vaccine for horses is available and highly recommended but no vaccine is available for human use. According to MSU’s Equine Extension Program,

  • Horses are affected by West Nile virus much more often than any other domestic animal.
  • The percent of infected horses that go on to develop symptoms and illness is not known.
  • Of those horses that do present clinical signs, about one-third die or need to be euthanized. The clinical signs for horses include weakness or paralysis of hind limbs, muzzle twitching, impaired vision, incoordination, head pressing, aimless wandering, convulsions, inability to swallow, circling, hyperexcitability, or coma.
  • The incubation period for horses is 5 to 15 days.

When infected with WNV, about 4 out of 5 people will not have symptoms and will develop immunity after clearing the infection. Among the 1 in 5 individuals who develop illness, they will generally experience mild symptoms that may include: headache, body aches, joint pain, vomiting, diarrhea, or a rash.

Serious symptoms can develop in rare cases with fewer than 1 in 100 of those infected developing infections in or around the brain, also known as neuroinvasive disease. Of the 47 cases of WNV reported in 2018; 22 were mild cases, while 25 were neuroinvasive cases. Out of the total 47 cases reported, 51% (24) occurred in individuals over the age of 60 years. Severe illness can occur in people of any age; however, people over 60 years of age are at greater risk.

People with certain medical conditions, such as cancer, diabetes, hypertension, kidney disease, and people who have received organ transplants, are also at greater risk. Currently, no vaccine or specific treatment exists for a person at risk or ill with WNV. Anyone who develops any of the serious symptoms listed should see their healthcare provider for evaluation and care.

The 4 D’s of West Nile Virus prevention should be followed to reduce the chance of mosquito bites.

  1. DEET: Use insect repellent such as DEET or picaridin
  2. Drain: Drain standing water around your house to prevent mosquito breeding
  3. Dawn/Dusk: Mosquitoes are most active during dawn and dusk. Stay inside or take precautions to prevent mosquito bites during these times
  4. Dress: When possible, wear long sleeved shirts and pants to protect yourself from bites

For more information about WNV, please visit the DPHHS website or call CCHD at 406-454-6950.