The heat is on for West Nile Virus
West Nile Virus (WNV) has become a national health problem this year despite the fact that many parts of the United States are experiencing moderate to severe drought.
As of Aug. 28, there have been 48 states reporting West Nile Virus infections in people, birds or mosquitoes. A total of 1,590 cases of West Nile Virus in people, including 65 deaths, has been reported to the Centers For Disease Control and Prevention (CDC). The cases reported are the highest number reported to CDC through the last week in August since West Nile Virus was first detected in the United States in 1999.
Last year, rainfall in the Lehigh Valley was more than 26 inches above normal. This year, rainfall in the Valley is about three inches below normal.
You might wonder what a lack or abundance of rainfall has to do with WNV.
Whatever the amount of rain, it fills natural vessels like the holes in trees and hollowed-out areas in the ground, as well as man-made objects like birdbaths, wheelbarrows, flowerpots and other containers. Areas filled with stagnant water provide the perfect nursery to breed mosquitoes.
Moreover, the recent mild winter and spring prevented many of the adult Culex mosquitoes that normally die from the cold to survive through the winter. These mosquitoes, plus the population numbers that normally survive the winter, have multiplied the overall total amount of adult mosquitoes this summer.
According to Louise Bugbee, Lehigh County West Nile Virus Coordinator, there are more than 50 species of mosquitoes in Pennsylvania. She and her staff of one, Assistant Director Jeff Carroll, have identified 28 different species of mosquitoes in Lehigh County this year.
"Not every mosquito species can act as a vector for all the diseases that mosquitoes can carry. For example, Anopheles mosquitoes are the vectors for malaria. The Culex species acts as the main vector for West Nile Virus and also dog heartworm," Bugbee explains.
"Culex mosquitoes are peri-domestic - they live in close association with humans. These are the mosquitoes that breed in the water in old tires, rain gutters, tarps, and children's toys anything that can hold water. It is imperative that individuals dump all sources of standing water," Bugbee stresses.
Temperature is a big influence in mosquito development, according to Dr. Michael Raupp, an Entomologist at the University of Maryland. A recent study of Culex mosquitoes found that it took 27 days for a Culex mosquito to transform from egg to adult at a temperature of 59 degrees Fahrenheit. It took only seven days for the same mosquito species to grow from egg to adult at 93 degrees Fahrenheit.
The study shows that several generations of mosquitoes can be produced during the same time period at high temperatures as can a single generation at lower temperatures. High temperatures this summer have mosquitoes completing their lifecycles at a record pace.
Hot wet weather provides the ideal recipe for spawning massive numbers of mosquitoes. The males and females consume large quantities of carbohydrates during the first few days of their lifecycles. Plant nectar and aphid honey provide much needed carbohydrates. These sweet sources satisfy the males, but the females also desire animal blood. The animal blood provides the protein for females to produce eggs.
Says Bugbee, "Various mosquito species have different breeding habits. Some species lay their eggs in moist soil while some lay their eggs in standing water. Female Culex, also known as northern house mosquitoes, lay their eggs in water including water-filled containers and standing water on the ground."
The eggs are laid in clusters called rafts. Each floating raft contains about 150 eggs. Larvae hatch from the eggs and go through four molts as they increase in size. They then become a pupa from which adults emerge and fly off from the breeding site to carry on the reproductive cycle.
"Several WNV-carrying mosquitoes feed on bird blood. Birds are the reservoirs for WNV. Although humans are a dead-end host the virus cannot be passed back to the mosquito from an infected human birds can pass the virus back to the mosquitoes.
"WNV is an Old-World disease. The native birds in the Old World countries were immune to the disease. Crows and other North American native bird species do not have a natural immunity. WNV kills many native birds.
"Many of the Culex mosquitoes overwinter as adults in relatively even temperature areas such as caves, root cellars and other sites where they will not freeze," Bugbee continues.
Raupp adds, "The more mosquitoes there are, the greater the likelihood that WNV will be transmitted from bird to bird. Many of the bird-feeding mosquito species also bite humans. The incidence of human infection rises with the increase of infection in the birds and the larger mosquito populations.
"This scenario is underway in several areas of the country. Most of these cases have been in the southern and central regions of the United States."
We must all become aware of the WNV problem and do our part to eliminate the standing water sources for mosquito-breeding. Be sure to clean your rain gutters, recycle old tires, empty birdbaths at least twice a week and eliminate water-filled containers. This has to be an ongoing task each time it rains. If possible, it is a good idea to eliminate the containers completely.
"Late August is typically a time when the amount of human cases starts to escalate quickly. This annual trend is most likely due to gradual amplification of the virus over the course of the summer in bird populations, leading to the increases in infectious mosquitoes," says Laura C. Harrington, an associate professor of entomology at Cornell University.
"As a consequence, we are likely only at the beginning of a significant upward trajectory in human cases. With no vaccine and no specific treatment for WNV, the best prevention is to minimize exposure to mosquito bites through use of repellents, limit outdoor activity and eliminate mosquito breeding-sites," says Harrington.
Bugbee says WNV should not be taken lightly because everyone is susceptible.
The WNV program is funded by the Pennsylvania Department of Environmental Protection, which advises counties to undertake spraying to reduce the numbers of adult mosquitoes. Because Culex mosquitoes are crepuscular (most active at dusk and dawn), spraying is most effective between 8 and 10 p.m. Bugbee and Carroll perform mosquito surveillance throughout Lehigh County to determine areas of high risk. Spraying is conducted when necessary.
Mosquitoes tend to begin feeding around dusk. It is best to avoid being outside at this time. If that is not possible, it is advised to wear light-weight, long-sleeved shirts and long pants when working outdoors. Read directions carefully before applying mosquito repellants to clothing and bare skin.
If you dine outdoors, use a small fan to create a slight breeze. The breeze will greatly reduce the number of mosquitoes in the immediate area.
Bug lights are not a deterrent to mosquitoes because mosquitoes are not attracted to light. Most of the bugs attracted and killed by bug lights are harmless insects like moths, lacewings and craneflies.
Northampton County disbanded its WNV Department last year because of budget concerns. Carbon County does not have a WNV program.