The outbreak of West Nile virus is soaring across the country, reaching unseen levels, according to a new report from the Centers for Disease Control.
Closer to home, Stanislaus County has more confirmed cases of West Nile virus in humans than any other county in California.
As of August 21, a total of 47 states have reported West Nile virus infections in people, birds or mosquitoes. Of the 47 states that reported any West Nile virus activity, 38 had human cases of the disease. A total of 1,118 cases of West Nile virus in people, including 41 deaths, have been reported to the CDC. Of these, 629 or 56 percent were classified as neuroinvasive disease, such as meningitis or encephalitis, and 489, or 44 percent, were classified as non-neuroinvasive disease.
The 1,118 cases and 41 deaths identified thus far in 2012 are the highest numbers of West Nile virus cases reported to the CDC through the third week in August since West Nile virus was first detected in the United States in 1999. In comparison, one month ago, there were only 25 people with West Nile virus reported to the CDC.
“These data show that the number of West Nile virus disease cases in people has risen dramatically in recent weeks. And indicate that we're in the midst of one of the largest West Nile virus outbreaks ever seen in the United States,” said Dr. Lyle Petersen, the director of the Division of Vector-borne Infectious Diseases at the CDC.
Stanislaus County has 10 confirmed cases of the West Nile virus in people, as of Wednesday, according to California’s West Nile virus website. The county has a West Nile virus rate of 1.92 per 100,000 residents and leads the state in human cases. Behind Stanislaus is Kern County with six human cases and Fresno County at five.
One fatality, which occurred in Kern County, has been reported to the California Department of Public Health.
West Nile virus is transmitted to humans and animals through a mosquito bite. Mosquitoes become infected when they feed on infected birds.
In the United States, most people are infected from June through September, and the number of these infections usually peaks in mid-August, according to the Centers for Disease Control. Seasonal outbreaks often occur in local areas that can vary from year to year. For example, Stanislaus County had at least one human case reported last year in October, said Monica Patterson, a vector biologist with the Turlock Mosquito Abatement District.
“The peak of West Nile virus epidemics usually occur in mid-August, however it takes a couple of weeks before people get sick, go to the doctor and get diagnosed and then are reported,” Peterson said. “Thus cases now being reported reflect infections from a week or more ago. Thus, we expect many more cases to occur and the risk of West Nile virus infection will probably continue through the end of September.”
Approximately 1 in 5 people who are infected with West Nile virus will develop symptoms such as fever, headache, body aches, joint pains, vomiting, diarrhea, or rash. Less than 1 percent will develop a serious neurologic illness such as encephalitis or meningitis (inflammation of the brain or surrounding tissues). About 10 percent of people who develop neurologic infection due to West Nile virus will die, according to the CDC. People over 50 years of age and those with certain medical conditions, such as cancer, diabetes, hypertension, kidney disease, and organ transplants, are at greater risk for serious illness.
The weather can be a significant factor in West Nile virus outbreaks. The CDC reported higher number of cases during periods of abnormally high temperatures.
“Hot weather, we know, from experiments done in the laboratory, can increase the transmissibility of the virus through mosquitoes and that could be one contributing factor. We have no information yet about whether the virus is mutated,” Peterson said.
A factor at the local level has been that the season appears to be longer. Last year the first dead bird to test positive for the virus was found in June. This year the first was found in May.
A high number of foreclosed properties could also be having an effect locally, if pools have been left unattended and are serving as breeding grounds for mosquitoes.
There are no medications to treat, or vaccines to prevent, West Nile virus infection. People with milder illnesses typically recover on their own, although symptoms may last for several weeks or months. In the neuroinvasive forms, patients can suffer severe and sometimes long-term symptoms.
The 10 people in Stanislaus County sickened with the virus range in age from 6 years old to 74 years old. Of the 10, five have been diagnosed with the neuroinvasive form.
“We are tending to see more of the severe forms of the virus,” said Stanislaus County Public Health Officer Dr. John Walker. “Typically we would expect 5 to 20 percent presenting with the more severe form, but we are already at 50 percent.”
In addition to the human cases in Stanislaus County, there has been one horse found with the virus and 28 birds that have tested positive.
The most effective way for individuals to prevent exposure to mosquito bites and West Nile virus is to remember the “Three D’s”:
1. DEFEND – Use an EPA-registered insect repellent with DEET, picaridin, oil of lemon eucalyptus, or IR3535 according to label instructions. Repellents keep the mosquitoes from biting you. DEET can be used safely on infants and children 2 months of age and older.
2. DAWN AND DUSK – Mosquitoes that carry WNV bite in the early morning and evening. It is important to use repellent and wear clothing that reduces the risk of skin exposure to mosquito bites during this time. Make sure your doors and windows have tight-fitting screens to keep out mosquitoes. Repair or replace screens with tears or holes.
3. DRAIN – Mosquitoes lay their eggs on standing water. Eliminate all sources of standing water on your property, including buckets, old car tires, and pet bowls. If you have a pond, use mosquito fish (available from your local mosquito and vector control agency) or commercially available products to eliminate mosquito larvae.
Residents can call their local Mosquito Abatement District to report a neglected swimming pools or ornamental ponds or with questions or concerns. In Stanislaus County, north of the Tuolumne River call East Side Mosquito Abatement District at 522-4098. All other residents may call Turlock Mosquito Abatement District at 634-1234. All horse owners are urged to consult their veterinarians about proper and timely West Nile virus vaccinations.
The public can report dead birds to the California Department of Health Services by logging on to http://westnile.ca.gov/cfm/deadbird.cfm or by calling toll-free 1-877-WNV-BIRD (1-877-968-2473).