By Samantha R. Selman
Photo by Wanda King
Last Wednesday, the mayor of Dallas declared a state of emergency in the ninth largest U.S. city to combat the spread of West Nile virus infections, which have been more prevalent than usual in Texas and Oklahoma this year. There have been more cases of West Nile virus reported so far this year than any year since the disease was first detected in the United States in 1999, according to the Centers for Disease Control said on its website. Nearly half of the 693 human cases of the mosquito-borne West Nile virus infections reported this year to the CDC have been in Texas, along with 14 of the 26 deaths confirmed by the federal agency as of Tuesday.
The Texas health department said the number of cases of West Nile in the state had reached 465 and there had been 17 deaths. There is a lag in the CDC confirming cases and deaths. The emergency declaration by Mayor Mike Rawlings followed a similar action last week by Dallas County officials and paves the way for aerial pesticide spraying to begin this week. Aerial spraying also is being used elsewhere, including in neighborhoods in New York City and Sacramento, California, to combat the spread of West Nile virus. Officials say such spraying is the most effective way to fight the mosquitoes that carry the disease despite safety concerns about exposing people to chemical pesticides.
We are on track to have the worst year ever for West Nile virus in the United States. It is unclear why the number of West Nile cases is so high. Scientists believe it could be related to a warmer winter and abnormally rainy spring. Wildfires, which have terrorized many of the states also suffering from an epidemic of West Nile Virus, are thought to be a contributing factor. Mosquitoes, which are infamous for their tendency to spread the disease, thrive in warm climates and places where water is readily available. Experts in the Dallas area suggest watering less, turning off sprinklers, and removing any standing water from around your home will help keep WNV-carrying mosquitoes away from your home.
West Nile Virus mainly infects birds, but is known to infect humans, horses, dogs, cats, bats, chipmunks, skunks, squirrels, domestic rabbits, crocodiles and alligators. The main route of human infection is through the bite of an infected mosquito. It should be noted that approximately eighty percent of West Nile virus infections in humans are without symptoms. The West Nile virus produces one of three different outcomes in humans. The first is an asymptomatic infection; the second is a mild febrile syndrome termed “West Nile fever”; the third is aneuro-invasive disease termed West Nile meningitis or encephalitis. The population proportion of these three states is roughly 110:30:1. The febrile stage has an incubation period of two to eight days followed by fever, headache, chills, diaphoresis (excessive sweating), weakness, lymphadenopathy (swollen lymph nodes), drowsiness, pain in the joints and symptoms like those of influenza. Occasionally, some patients experience short-lived gastrointestinal symptoms including nausea, vomiting, loss of appetite, or diarrhea. Symptoms are generally resolved within seven to 10 days, although fatigue can persist for some weeks and lymphadenopathy up to two months. The more dangerous encephalitis is characterized by similar early symptoms, but also a decreased level of consciousness, sometimes approaching near-coma. Deep tendon reflexes are hyperactive at first, later diminished. Recovery is marked by a long convalescence with fatigue.
The virus is transmitted through mosquito vectors, which bite and infect birds. The birds are amplifying hosts, developing sufficient viral levels to transmit the infection to other biting mosquitoes which go on to infect other birds (in the Western Hemisphere, the American robin and the American crow are the most common carriers) and also humans. The infected mosquito species vary according to geographical area; in the US, Culex pipiens(Eastern US), Culex tarsalis (Midwest and West), and Culex quinquefasciatus (Southeast) are the main sources.
There is no vaccine for humans. A vaccine for horses based on killed viruses exists; some zoos have given this vaccine to their birds, although its effectiveness is unknown. Dogs and cats show few if any signs of infection. There have been no known cases of direct canine-human or feline-human transmission; although these pets can become infected, it is unlikely they are, in turn, capable of infecting native mosquitoes and thus continuing the disease cycle.
Avoiding mosquito bites is the most straightforward means to avoid infection; remaining indoors (while preventing mosquitoes from entering) at dawn and dusk, wearing light-colored clothing that covers arms and legs, and using insect repellents on both skin and clothing (such as DEET, picaradin, or oil of lemon eucalyptus for skin and permethrin for clothes). If one becomes infected, generally, treatment is purely supportive: analgesia for the pain of neurologic diseases, and rehydration for nausea, vomiting, or diarrhea; encephalitis may also require airway protection and seizure management.
The use of pesticide spraying to combat mosquitoes and curb outbreaks of West Nile virus has sparked concern regarding the health effects of the chemicals used, but experts say, in these cases, the benefits of spraying far outweigh the risks. Aerial pesticide spraying began on Thursday in Dallas, where an outbreak of West Nile virus has infected 200 people and killed 10. Pesticide spraying also recently began in parts of New York City. There are several reasons why aerial pesticide spraying for West Nile is considered safe. For one, these sprays use very small amounts of pesticides — much lower than the amounts used on agricultural crops, said Robert Peterson, a professor of entomology at Montana State University. Even if someone was outside during the spraying, “the amount of insecticides that they would be exposed to is below any amount known to cause any adverse effects,” Peterson said. Because the exposure to these pesticides is negligible, the risks to people’s health are negligible.
During a spraying, a tiny cloud of aerosolized pesticide is released from a plane. The droplets are very small, and intended to fall on, and kill, mosquitoes. Even larger insects are typically not affected by the spraying, because the droplets bounce right off them, according to Peterson. In addition, the modern pesticides used in these sprays have a very short life in the environment, and are degraded by sunlight into non-toxic chemicals. “It will kill the things you want it to, and disappear very quickly thereafter,” David Savitz, an environmental epidemiologist at Brown University, said. When public health is threatened, authorities must balance the risks of an action — in this case, exposure to pesticides — with the benefits.
In Dallas, where West Nile cases have reached a high level, authorities have made a sound judgment to use pesticides. To avoid direct exposure to pesticides, the New York City Department of Health recommends people stay indoors during the spraying when possible, and bring children’s toys and belongings inside. Generally, there have been concerns over pesticide exposure for pregnant women, because it’s known that the fetus is especially sensitive to environmental chemicals. Exposure to a large amount of any substance, including pesticides, can be harmful. People exposed to large amounts of pesticides, can experience acute neurological problems.