Suffolk County Health Services Commissioner Dr. James L. Tomarken announced on July 24 that six additional samples of mosquitoes have tested positive for infection with the West Nile Virus (WNV). This brings the 2015 total for the county to 13 positive samples. Samples are regularly collected from known mosquito breeding sites.
West Nile Virus has been detected in several areas of New York, according to the New York State Department of Health. In 2015, seven counties along with three boroughs in New York City, have reported WNV activity. Through July 23, no human illnesses from the virus have been reported in the state.
Through July 18, the state of Texas reports 221 mosquito pools that are positive for West Nile. Two birds and one horse have also tested positive for the disease. In humans, there has been one case of West Nile Fever and two cases of West Nile neurologic illness.
In Florida, also through July 18, seven counties have reported the finding of West Nile in sentinel chickens. A total of 19 chickens have tested positive, but no illnesses in horses or humans have yet been reported.
Nationally, the Centers for Disease Control and Prevention (CDC) has received reports of 23 patients with WNV illnesses. The most serious form of illness, neuroinvasive disease, has been reported in nine patients. In the past, the CDC has estimated that between 150 and 250 patients have sub-clinical West Nile illnesses or West Nile Fever for each case of neuroinvasive disease. That would place the number of total WNV illnesses in 2015 to date at between 1,350 and 2,250.
Culex pipiens and related Culex species carry the West Nile virus. The science of predicting what conditions are responsible for increases in the mosquito population, and the increase in mosquito borne illnesses that follow, is becoming better researched. A 2013 paper titled “Predicting Culex pipiens/restuans population dynamics by interval lagged weather data” laid out some of the factors involved.
According to the paper, Culex pipiens can carry “West Nile fever, St. Louis encephalitis, Japanese encephalitis, Western equine encephalitis, and Rift Valley fever.” For this mosquito, the primary factor influencing mosquito abundance is the length of day. Maximum mosquito abundance was found to occur about four weeks after the longest day.
Activity by female mosquitoes, which are the mosquitoes that bite, is heavily influenced by air temperature. It appears that a rapid increase in temperature will result in a rapid response by the females as they search for host feeders.
Rainfall has differing effects, depending upon the rate and volume of the precipitation. Heavy rain washing mosquito young from their breeding pools and can result in a temporary reduction in activity. A lighter rainfall that refreshes pools, such as sewer catch basins, or creates stagnant water that will last for several weeks, will increase the number of mosquito young. The increase in water volume reduces the mortality of the young during development.
Humidity has also shown the potential for predicting mosquito activity. This study found that a high relative humidity in the preceding month increases the number of mosquitoes captured from a mosquito pool.
Modeling can now be done, with the data from this and other studies, that can estimate mosquito population. Each of the variables involved has its own time delay, but modeling can overlay all of the variables and produce a composite picture of what is happening with the mosquito population or what may occur in the future. This allow mosquito control efforts such as fogging or larvacide application to be more effective and targeted.