Reducing Risk

Pest management professionals face three kinds of pests: those that are a nuisance, those that can potentially cause significant economic damage to one’s property and those that can affect personal health.

In almost every field of science there is an exponential growth of information being generated. It has become nearly impossible to keep up with such developments. For medically important arthropods and rodents, as well as the diseases they can transmit to humans, this is also the case. I can in no way cover all of the health issues that PCOs may come into contact with here. Many of these are of concern in only certain parts of the U.S. Here is a review of the major diseases associated with rodents and mosquitoes.

RODENTS. The term "rodent" is a catch-all phrase for many rats, mice, squirrels and other lesser mammals that reach pest status when they infringe upon humans’ homes, possessions and foodstuffs. Just as with all insect scenarios, knowing the behavior and biology of your target species is of utmost importance.

The two major rodent diseases that are of great concern in certain parts of the U.S. that the PCO should be familiar with are hantavirus and plague. I would like to mention that although there are certainly beliefs and tales about rabies associated with rodents or from rodent bites or association with rodent droppings or living areas, the risk of this particular disease is low to non-existent and one must consider other mammals when this disease is of concern.

Hantavirus. An outbreak of unexplained illness occurred in May 1993 in the "Four Corners," an area of the Southwest shared by New Mexico, Arizona, Colorado and Utah. Several healthy young adults suddenly developed acute respiratory symptoms; shortly thereafter about half of these individuals died. Researchers soon suspected that they were dealing with a form of hantavirus, which is transmitted by rodents. Researchers then investigated the possible rodent connection, trapping rodents in the affected area, doing tissue studies both of rodents and hantavirus victims, until the virus and its principal carrier — the deer mouse — were positively identified. Early on, it was also established that person-to-person spread was unlikely. It was also determined that this "new" hantavirus had actually been present, but unrecognized, at least as early as 1959.

Since the 1993 outbreak, hantavirus pulmonary syndrome (HPS) has been identified in more than half of the states in the United States. As of February 2001, 281 cases of HPS had been reported in the United States since the 1993 outbreak. Thirty-eight percent of all reported cases have resulted in death. Of persons ill with HPS, 60 percent have been male, 40 percent female. More than half of the confirmed cases have been reported from areas outside the Four Corners area.

Which rodents are the culprits? Deer mice in the West and cotton and rice rats in the southeastern states, as well as the white-footed mouse in the Northeast. So far, neither dogs nor cats have been shown to carry the disease. Furthermore, farm animals cannot transmit the disease nor can it be transmitted between humans or between insects and humans.

The above rodents pass the virus from their bodies through their urine, droppings and saliva. Humans acquire the virus when they breathe in air contaminated with the virus. This happens when fresh rodent urine or feces are disturbed or encountered by humans. Very small particles containing the virus become airborne by a process called aerosolization. Who is most at risk to contract the disease? According to experts, little is understood about the activities that lead to a greater risk of infection. Of course, contact and disturbing areas where rodents are habitating (especially if these areas are enclosed and have little circulating air) are potential hot spots for the disease to exist. Moreover, an early case-control study suggests that increased numbers of rodents in the household is the strongest risk factor for infection. Entering rarely opened or seasonally closed buildings may also contribute to infection. PCOs who work in enclosed areas with rodent droppings should take precautions to avoid HPS exposure.

Plague. Humans are most likely to acquire plague by being bitten by a rodent flea that is carrying the plague bacterium (Yersinia pestis) or by handling a plague-infected animal. This disease is not a new one — it is referred to as the "Black Death" that resulted in the death of millions of people (one-fourth of the population) in Europe during the Middle Ages. During these times, flea-infested rats inhabited human homes and workplaces. Today, antibiotics are effective against plague, but if an infected person is not treated promptly, the disease can cause illness or death.

Wild rodents in certain areas around the world are infected with plague and outbreaks in people still occur in rural communities or in cities. They are usually associated with infected rats and rat fleas that live in the home. In the United States, the last urban plague epidemic occurred in Los Angeles in 1924-25. Globally, the World Health Organization reports 1,000 to 3,000 cases of plague every year. In North America, plague is found in certain animals and their fleas from the Pacific Coast to the Great Plains and from southwestern Canada to Mexico. Most human cases in the United States occur in two regions: 1) northern New Mexico, northern Arizona and southern Colorado; and 2) California, southern Oregon and far western Nevada.

To understand the cycle of this disease you must first start with an infected rodent. The bacteria circulate in the bodies of such creatures. Rats, as well as some squirrels, can harbor the deadly bacteria. Enter the flea. Fleas that are normally associated with rodents may feed on an infected individual; the bacteria is transferred to the flea’s gut during feeding and can remain viable there for some time. In many cases, infected mammals die as a result of the bacteria, leaving fleas no hosts to feed on. This is where the problem arises. In these cases, starved fleas will feed on hosts other than their normal rodent hosts and in doing so, the flea can transmit the bacteria to other mammals, including humans.

The good news is that fleas tend to be host-specific and therefore few can transmit the plague from one species of mammal to the other. Those most highly at risk are individuals that work around rodent burrows because if the rodent dies as a result of the plague, the infected fleas may seek other hosts if there are no other alternatives. Luckily, if diagnosed early enough, the disease can be treated with antibiotics.

Plague can be introduced to humans in three ways. Septicemic plague is the introduction of the bacteria into the bloodstream that results in eventual septicemia. Bubonic plague is named such for the characteristic occurrence of buboes (swollen lymph glands) in the groin, armpit or neck area. The third (and most deadly) introduction route is pneumonic plague whereby the bacteria enter the body through the respiratory system. Symptoms are short-lived as most cases of this form of the disease result in death in as little as four days. General plague symptoms include high fever, delirium, rapid or irregular pulse and severe headaches or nausea.

Common sense is the best preventive tool. Steer clear of areas marked as potential plague hot spots. If you know you are going to be working around ground squirrel or prairie dog areas, wear clothing that can be tucked in and is light so that fleas will show up. Finally, when working near a potential hot spot, if you have been bitten and develop any symptoms, seek medical attention immediately.

MOSQUITOES. Mosquitoes are bothersome pests almost everywhere. In most situations, they are a nuisance pest that causes annoying, itchy bites. In other parts of the country where there are many mosquitoes, coupled with the threat of potential disease transmission, local town, county and state governments get involved and coordinate massive treatment programs. There are some pest control companies in these areas that offer their own services using either adulticides or larvicides to affect mosquito control. Below are some of the more prevalent mosquito-borne diseases currently being monitored in the United States.

St. Louis Encephalitis (SLE). SLE is a mosquito-transmitted virus that was first recognized in 1933 in St. Louis, Mo. The virus is thought to occur naturally in some common birds (sparrows, finches, blue jays, robins and doves) and can be transmitted between birds and other animals by certain species of mosquitoes. Transmission occurs when an infected female mosquito bites a human. SLE is not transmitted person to person, nor do pets transmit it. In order for a mosquito to transmit the virus to humans it must first feed on a bird that has the virus.

The virus is transmitted by certain species of the genus Culex, which functions as the most important mosquito vector of SLE. The severity of SLE in humans who come in contact with the virus varies according to age, with the elderly and young individuals most at risk. The diagnosis of SLE requires specific blood tests or examination of spinal fluid.

As mentioned earlier, both government and PCOs work together to abate mosquito problems as they arise. Such tactics involve habitat manipulation, biological control efforts, larval and adult mosquito surveillance, public education, and application of larvacides, and if necessary, spraying adulticides.

West Nile Virus. West Nile virus (WNV) became recognized as a cause of severe human enin-goencephalitis (inflammation of the spinal cord and brain) in elderly patients during an outbreak in Israel in 1957. In recent years, WNV has emerged in temperate regions of Europe and North America, presenting a threat to public, equine and animal health. The most serious cases of WNV infection involve fatal encephalitis (inflammation of the brain) in humans and in horses and death in both wild and domestic birds.

WNV is amplified during periods of adult female mosquito blood-feeding by continuous transmission between mosquito vectors and bird reservoir hosts. Infectious mosquitoes carry the virus in their salivary glands and infect susceptible bird species (in the U.S., it’s usually crows) during blood meal feeding. Infected birds can then sustain the virus for up to four days after exposure. WNV is transmitted by infected mosquitoes, primarily members of the Culex species; however, the virus has been isolated from almost 50 species of mosquitoes.

Humans, horses and most other mammals are not known to develop infectious levels of the virus often, however as mentioned earlier, birds are the major reservoir of the virus. According to the CDC, since September 2000, more than 70 species of birds (mostly crows) have tested positive for WNV either by virus isolation or nucleic acid testing. Birds infected with WNV can die or become ill. There is no evidence that a person can get WNV from handling live or dead infected birds. However, PCOs should avoid barehanded contact when handling dead animals and use gloves or double plastic bags to place the bird carcass in a garbage can or contact their local health department.

Arboviral encephalitis, including WNV and SLE, can be prevented in two ways: personal protective measures to reduce contact with mosquitoes and public health measures to reduce the population of infected mosquitoes in the environment. Personal protection measures include reducing time outdoors, particularly in early evening hours, wearing long pants and long-sleeved shirts and applying mosquito repellent to exposed skin areas. Public health measures include elimination of larval habitats or spraying of insecticides targeted at adult mosquitoes.

CONCLUSION. The PCO provides a myriad of services to the public. With these services the quality of life of many individuals are improved. It is always important to provide sound information to your clients, but when questions arise about public health pests and the diseases they carry, providing detailed, factual and sound advice becomes even more important. If you are asked questions and cannot provide the correct answer get back to the individual after some research. Your clients will appreciate the extra effort and will be impressed by your professionalism in today’s "information age."

The author is a research scientist at SC Johnson A Family Company. He can be reached at rkopanic@pctonline.com.