How Can West Nile Virus be Prevented?
How Can West Nile Virus Be Prevented?
Knowledge about the factors that cause the spread of West Nile virus has enabled public health agencies to devise methods to prevent the virus from infecting more people and animals. Since the 1999 outbreak of West Nile virus in the United States, health officials on the national, state, and local levels have formulated extensive plans of action to try to contain the current epidemic and to prevent future outbreaks of the disease.
Prior to the appearance of West Nile virus in the United States, public health departments did not have trained personnel who could handle an outbreak of vector-borne disease. But the federal Centers for Disease Control, along with other agencies like the United States Department of Agriculture, formulated lengthy recommendations once the virus posed a widespread threat. They issued these recommendations to state and local jurisdictions to try to help these departments train the necessary people and coordinate an effort to be ready when or if the virus became a local menace. The recommendations included guidance for cooperation and sharing of information between federal, state, and local health departments; vector-control agencies; agriculture departments; and wildlife monitoring facilities. This required a wide range of specialists, including arbovirologists (experts on insect-borne viruses), epidemiologists, laboratory experts, vector-control experts, veterinarians, and physicians, to be educated
West Nile Virus in Horses
According to the United States Department of Agriculture (USDA), horses are infected with West Nile virus more often than any other domestic animal. Some affected horses develop no symptoms, but about one-third of those that become ill die or must be euthanized. Because of this, many veterinarians now recommend that horse owners and breeders have their horses vaccinated against West Nile virus. Although there is no vaccine available for humans yet, in 2001 the USDA authorized use of a new equine vaccine.
Experts suggest that horses be housed in quarters with screens to reduce their exposure to mosquitoes. There are also some insect repellents designed for horses that can be applied to the animals' skin to protect them from getting bitten by mosquitoes.
and assigned to programs aimed at controlling this new challenge to public safety.
The Centers for Disease Control reports that the first step in prevention and control efforts is surveillance of birds, mosquitoes, and, in some instances, horses. Because horses are commonly bitten by mosquitoes, they can be used as an index of increased West Nile virus activity. Horse breeders, owners, and veterinarians in certain areas are therefore being encouraged to report any instances of horses that get sick or die from West Nile virus.
Surveillance of birds consists primarily of monitoring bird deaths and illness in a particular area and conducting laboratory tests to find out if West Nile virus is responsible. Residents have been encouraged to call public health agencies to report such dead or ill birds, and, in addition, these agencies are actively sending biologists and other experts into the field to search for birds that may be affected. Scientists then bring dead birds to a laboratory and remove blood or organs such as the brain to test for the presence of West Nile virus. They also regularly check blood samples from live chickens and other birds to see if they carry the virus.
If birds in a particular region are infected, biologists use this as an indication that West Nile virus is active in the area. This information is reported to public health agencies so that the next step in surveillance efforts—the search for nearby mosquito breeding grounds—may be initiated.
Once biologists locate mosquitoes, they then determine which species of these insects are spreading the virus. This is achieved by trapping mosquitoes and larvae, identifying the species, and then testing for West Nile virus. The scientists who trap and test the mosquitoes generally keep track of how many mosquitoes per thousand show evidence of the virus. This is an efficient method of detecting increases in virus activity; the tests are usually repeated weekly or monthly, depending on the numbers of sick or dead birds in an area. If there are large numbers of infected birds, testing on mosquitoes is done more frequently to try to keep virus outbreaks under control.
In 2000 in the New York City area, for example, scientists collected more than three hundred thousand mosquitoes and tested them for West Nile virus using a technique called polymerase chain reaction. This method of copying DNA enables the investigators to identify a virus present in a sample of living tissue.
Polymerase Chain Reaction
Polymerase chain reaction is a laboratory technique that enables investigators to identify a virus or other living organism by its DNA fingerprints. The procedure was developed in 1985 by chemist Kary Mullis, who received a Nobel Prize for his work. It is sometimes called DNA amplification because it entails replicating a DNA segment to produce a large sample that can easily be analyzed.
The technique begins by immersing DNA in a solution that contains the enzyme DNA polymerase, a series of DNA's chemical building blocks known as nucleotides, and primers that bind with the ends of a DNA segment. The solution is heated to break apart the DNA strands. When it cools, the primers bind to the separated strands and the DNA polymerase builds new strands by joining the primers to the nucleotides. The process is repeated so that billions of copies of a small piece of DNA can be fabricated in several hours. The procedure produces a large enough sample of DNA for scientists to identify the organism from which it came. They have used this technique to identify West Nile virus in blood and tissue samples.
Once data shows increases in virus activity, "appropriate and timely response to surveillance data is the key to preventing human and animal disease associated with WN [West Nile] and other arboviruses. That response must be effective mosquito control without delay; if increasing levels of virus activity are detected in the bird or mosquito surveillance systems,"10 according to the federal Centers for Disease Control.
Mosquito control can be accomplished in several different ways. These techniques can focus on any of the four stages of mosquito development. After a female mosquito obtains a blood meal from a person or animal, she lays her eggs in stagnant water or in another wet place. The eggs hatch after about two days, and larvae emerge. The larvae live in water and develop into a third state known as the pupa after one to two weeks. A young mosquito lives in the pupa for one to four days. Then a mature mosquito crawls out of the pupa and flies away.
Female mosquitoes can lay eggs every ten to fourteen days during their lifetime, so ongoing efforts at eradication must be targeted toward a rapidly reproducing population of insects. One method of eradication uses mosquito traps containing substances that attract mature mosquitoes. Once the insects are trapped, they are destroyed with poison. Another method involves draining standing water or marshes where mosquito eggs, larvae, and pupae grow. Without water, mosquitoes in these stages of development will die. Still another method of eradication involves biologists placing mosquito-larvae-eating fish such as gambusia into marshes or other wet breeding areas.
The most prevalent method of mosquito control uses insecticides to kill these insects or their larvae. These chemicals can be applied by a licensed contractor either using ground-based trucks or airborne hovercraft. The trucks drop or spray insecticides over a carefully targeted area; hovercraft airplanes are used to spray the substances over larger areas containing mosquito breeding grounds. The insecticides can be in the form of liquids, granules, or solids, depending on the particular situation. If a particular location is windy, for example, exterminators will use granules or solids because they are less likely to blow through the air and expose people and animals to the poison.
Although chemicals used to destroy mosquitoes and their larvae must comply with state and federal safety regulations, they can still be dangerous to humans and animals. For this reason, most experts employ these methods of eradication only when it is absolutely necessary to protect the public's health.
Individual Measures to Prevent Mosquito Bites
These mosquito-control techniques applied by contractors and biologists are only some of the measures that can be taken to prevent mosquito bites. Experts say there are also several things that individuals can do to reduce the number of mosquitoes in an area and to avoid being bitten.
Doctors recommend using insect repellents applied to the skin as the best method of keeping mosquitoes away from people. Studies have shown that repellents containing the compound known as DEET make the skin smell unattractive to the insects and therefore provide the best protection. These repellents achieve this end by blocking smell receptors on the mosquitoes' antennae. Normally, mosquitoes are attracted to people and animals because of odors on the skin and by the smell of carbon dioxide, the gas that comes out of the mouth or nose when the lungs exhale. Female mosquitoes then bite people and animals because a protein in the blood they eat helps develop mosquito eggs. Male mosquitoes do not need this protein, so they do not feed on humans or animals.
In a recent study reported in the New England Journal of Medicine, investigators found that products with a 23.8 percent concentration of DEET gave complete protection from mosquito bites for about three hundred minutes, compared with about ninety-five minutes for a soybean-oil-based product, about twenty minutes for a product containing the chemical IR3535, and less than twenty minutes for other plant-based repellents such as citronella, cedar, eucalyptus, peppermint, lemon grass, and geranium oils. The study found that wristbands marketed as insect repellents offered no protection whatsoever. Neither did devices that emit sounds that manufacturers claim are aversive to mosquitoes.
Doctors caution that the true effectiveness of any insect-repellent product varies somewhat according to an individual's skin chemistry, the outside temperature, humidity, wind speed, and how much the person sweats. However, among all the repellents tested in this research, products containing DEET consistently provided more protection than other products.
Many people are worried about applying a dangerous chemical like DEET to the skin, but physicians say that adverse reactions are rare when the substance is used according to package directions. Using too much DEET can be toxic, though, so it is important to use only the recommended amount. This amount depends on the concentration of the compound—for example, a product containing 23.8 percent DEET provides about five hours of protection and should not be reapplied more frequently unless it is washed off by rain, swimming, or perspiration. A product with a 20 percent DEET concentration gives about four hours of protection, a 4.75 percent concentration provides about one and one-half hours of protection, and so on.
Other Precautionary Measures
In addition to wearing insect repellent when mosquitoes are active, experts say there are several other measures people can take to avoid mosquito bites. One is avoiding places where mosquitoes gather, such as areas with stagnant water. Another is staying indoors at dawn, dusk, and early evening when mosquitoes are most likely to bite. A third is not wearing perfumes or lotions that may attract these insects.
Wearing protective clothing can also help. Long pants, long sleeves, shoes, and socks may not be comfortable in the summertime, but are important during times when mosquitoes are out. Clothing can also be sprayed with repellents containing DEET or the chemical permethrin to prevent mosquitoes from biting through clothing. Doctors caution, however, that substances with permethrin should never be sprayed on the skin.
Although public health officials assert that it is critical to take steps to avoid mosquito bites, they also point out that it is not necessary or desirable to panic and stay indoors all day during warm or hot weather. During the 1999 West Nile virus outbreak in the New York City area, many people became alarmed about being outdoors at all, as revealed in an article published by the American Academy of Pediatrics: "When West Nile virus broke out in New York City, callers flooded public health phone lines asking questions such as whether children should play outdoor soccer. Those bitten by mosquitoes rushed to emergency departments. Schools cancelled outdoor field trips."11 Since West Nile virus is still relatively rare, though, and the chances of catching it from a mosquito bite are not overwhelming, doctors say that extreme measures such as these are not needed. They do emphasize that wearing insect repellent and trying to stay indoors during peak mosquito activity hours are prudent practices that should become a part of everyday life now that the threat of the disease is here. During the month following the New York City outbreak, health officials distributed more than 300,000 cans of DEET-based mosquito repellents through local fire stations and handed out more than 750,000 information leaflets about mosquito protection to help allay some of the widespread panic and to inform the public about mosquito protection practices.
Measures for Protection Around the House
Besides exercising care in protecting peoples' bodies from mosquito bites, public health authorities say there are also things to do around residences or businesses to cut down on mosquito assaults. Draining standing water in the yard is one thing that can reduce the chances of mosquitoes breeding in the vicinity. In addition, the Centers for Disease Control recommends the following:
At least once or twice a week, empty water from flower pots, pet food and water dishes, bird baths, swimming pool covers, buckets, barrels, and cans. Check for clogged rain gutters and clean them out. Remove discarded tires, and other items that could collect water. Be sure to check for containers or trash in places that may be hard to see, such as under bushes or under your home.12
It is also recommended that window and door screens be checked to make sure they are secure and do not have holes. This prevents any mosquitoes from getting indoors. Placing mosquito netting over infant carriers or buggies while outdoors is also a prudent practice, and replacing outdoor lights with yellow lights that do not attract bugs is recommended as another method of keeping mosquitoes away.
Preventing the Spread of West Nile Virus Through Transfusions and Organ Donations
Mosquito surveillance and eradication are not the only fronts on which public health experts are focusing their efforts to contain the spread of West Nile virus. Since doctors have discovered that the virus can be transmitted through blood transfusions and organ donations, the U.S. Food and Drug Administration (FDA) has issued guidelines on screening donor applicants to try to prevent the spread of West Nile virus through these avenues. But because only about 20 percent of infected persons show symptoms, authorities acknowledge that until laboratory screening tests are available for widespread use, it will be difficult, if not impossible, to prevent donors who have West Nile virus from donating blood or organs.
Experts now hope to develop laboratory screening tests appropriate for use at blood donation sites in the near future. "Laboratory screening tests to detect donor infections with West Nile virus will be needed if the epidemic persists," states a guidance document issued by the FDA. "Our current thinking is that we would recommend routine use of licensed donor screening tests to detect acute donor infections with West Nile virus once such tests are available. If necessary, we would allow widespread use of appropriate tests under an Investigational New Drug Application."13 The FDA has the authority to allow new diagnostic medical procedures such as screening tests to be used before they have gone through the normal lengthy testing protocols if they deem it essential in emergencies like this epidemic. The organization has pledged to work with drug companies to make such a screening test available as soon as possible, and several companies are reportedly trying to develop such a test.
Until a screening test is available, the FDA has advised blood banks to turn away potential donors who have a fever or other symptoms indicating that they may have West Nile virus. However, authorities have stated that people who were previously infected but recovered can probably give blood if it has been several months since the infection went away.
In late 2002, the FDA also warned blood banks that blood and blood products frozen for later use should not be used if the blood was taken during a West Nile virus outbreak in a particular area. This meant that blood banks throughout the nation had to quarantine more than thirty thousand pints of plasma, the liquid part of blood routinely frozen for future use. The American Red Cross and America's Blood Centers, the major blood suppliers in the United States, have imposed the quarantine on frozen blood products collected between a week before each state's first case of West Nile virus and a week after the last documented case during a seasonal outbreak.
Special Laboratory Precautions
Since the recent West Nile virus infections of several laboratory workers due to being stabbed with contaminated needles or scalpels, public health officials have also emphasized stringent guidelines for laboratories that handle this pathogen. The U.S. Department of Health and Human Services, the Centers for Disease Control, and the National Institutes of Health are responsible for formulating rules and guidelines for laboratories that use a variety of dangerous pathogens for testing and research. These agencies classify hazardous germs according to the precautions necessary to protect laboratory workers and others from these harmful agents.
Regulations for handling infectious agents are divided into four levels based on how dangerous the pathogen is and on how it can be transmitted to people. Biosafety Level Four (BSL4) regulations are devised for working with the most dangerous pathogens. West Nile virus is classified as a Biosafety Level Three (BSL3) agent because it is considered a serious threat.
Biosafety regulations exist not only to protect laboratory workers but also to ensure that the infectious agent does not leave a laboratory to endanger those outside. This requires that the infectious agent and anything that touches it be confined to a secure place inside the laboratory.
Methods of ensuring safe handling of infectious materials within a confined space are referred to as containment. "The purpose of containment is to reduce or eliminate exposure of laboratory workers, other persons, and the outside environment to potentially hazardous agents… the three elements of containment include laboratory practice and technique, safety equipment, and facility design,"14 explain federal health authorities at the U.S. Department of Health and Human Services, the Centers for Disease Control, and the National Institutes of Health.
Containment regulations for BSL3 agents include comprehensive training for personnel so they are aware of the regulations and have plenty of experience using the required safety equipment and techniques. Such practices also include careful decontamination of anything, including air, that may have touched the pathogen; minimal use of needles, scalpels, or other tools that might puncture a worker's skin; not allowing any food or drink in the laboratory area; and not allowing any air to get out of the containment area.
Special equipment required for BSL3 containment includes protective clothing such as disposable masks, gloves, gowns, booties, and eye shields so the virus cannot be touched or inhaled in any manner. Handling of anything that touches the virus must be done in a biological safety cabinet. This is an enclosed container that has its own ventilation system so nothing the pathogen touches can get out. All work inside the safety cabinet is performed through attached arm-length rubber gloves.
Facility design regulations for BSL3 pathogens require that all surfaces be constructed so that they may be decontaminated frequently; the laboratory must have self-closing double doors separating it from outside areas; and the ventilation system must be outfitted so that any air going into the laboratory must not be allowed to get out.
Even with all these precautions, several laboratory workers have been infected with West Nile virus due to unfortunate accidents. Still, authorities hope the stringent regulations will help reduce the chances of infection for anyone who works with the virus and protect everyone outside from possible contamination as well.
The Goals of Prevention
The goal of all these preventive measures, whether in laboratories, blood banks, or in people's backyards, is, of course, to try to control the spread of West Nile virus throughout the United States. Because the disease is so new to this country, data on how well these measures are working is not yet available. However, public health officials hope that educating people about actions to prevent transmission of the virus will soon result in a reduced incidence of infection.