[Vertebrate Pests] Rodents & Public Health

There is something incredibly disturbing about being bitten by a rat. Although a fairly uncommon event these days, during the 1960s, rat bites were common enough for the Centers for Disease Control and Prevention to release a report detailing ways to reduce the incidence of rat bites in urban centers. Since that time, the incidence of wild rodent bites has decreased while the occurrence of bites from domesticated pet rats has increased.

The illnesses associated with rat bites, and rat body fluids such as urine, fecal matter, and saliva, include rat bite fever (RBF), salmonellosis, cowpox, lymphocytic choriomeningitis, and other Staphylococcus, Bacillus and diptheroid bacteria. Pest management professionals are not considered to have a greater risk of being bitten by a rat as compared to the general public or laboratory workers (unless they have an active rodent infestation in their home). However, pest management professionals frequently contact rodents and their fecal matter, urine and body fluids that potentially harbor the same pathogens transmitted through rat bites, so an understanding of rat bites and associated pathogens is warranted. The remainder of this article will focus on the transmission and prevention of rat bites and rat bite fever.

In the United States, two million people each year are bitten by animals, with the most common bites caused by dogs and cats. Wild animal bites occur less frequently than bites from domestic animals, but thousands occur each year. Rat bites are considered to be the most common wild animal bite.

The CDC conservatively estimates that large metropolitan areas experience rat bites at the rate of 10 bites per 100,000 people. Using this number as a guide, urban centers would account for 4,000 to 5,000 rat bites per year. This is a considerable number since many rat bites go unreported and no formal system exists for collecting rodent bite data. Information on rodent bites is only collected if researchers in a city are interested in the problem. Some diseases carried by rodents, such as rat bite fever, are not considered nationally notifiable diseases so there are no statistics available for the disease at a national level. All estimates of disease prevalence are extrapolated from smaller studies.

DISEASE TRANSMISSION. Worldwide, rats are the dominant natural reservoir for two pathogens, Streptobacillus moniliformis and Spirillum minus, which both cause RBF. In the United States, the majority of rats are asymptomatically infected with S. moniliformis, whereas rodents in Asia are asymptomatically colonized with S. minus. In times of stress, rats may demonstrate signs and symptoms of disease including watery eyes, leaking nasal fluids and sneezing.

Transmission of rat bite fever is primarily associated with the bite of an infected rat, but urine or secretions of the mouth, nose or eyes also can  introduce pathogens. Infections have occurred in people living or working in rat-infested buildings and laboratory workers have become infected through contact with infective blood from laboratory rats.

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Trapping Squirrels with the Kania2000

For many pest control professionals, squirrel complaints constitute their No. 1 wildlife control call. As such, PCOs are always looking for techniques and tools to make these contracts more profitable. Although trapping with cages is commonly used, cages often aren’t that efficient.

First, cage traps consume vital truck space, thereby reducing a vehicle’s ability to carry profitable add-on products (like chimney caps). Second, cage traps force technicians to spend time either relocating or euthanizing the catch. Additionally, live cage traps need to be visited daily. While body-gripping traps are an option, their design increases the likelihood of hip catches that fail to kill the squirrel in a timely manner. The reason for the increased chance of non-lethal strikes stems from the way the trap permits too much variability in the squirrel’s speed and approach to the trap. That variability means the squirrel has more chances to be in the wrong position when the trap fires. Even though the property owner may “hate” squirrels, it is unlikely he wants his 7-year-old daughter to see an injured squirrel struggling in a trap.

The Kania2000 resolves that problem. Its cubby design guides the squirrel into the best position for the strike bar to have the maximum humane effect. But the Kania2000 saves time in ways other than providing humane kills.

First, the trap reduces the risk of capturing non-target animals. The only thing worse than no catch is the wrong catch. Researchers in a study on Vancouver Island, British Columbia, experienced 100 percent target captures. Second, the trap has tabs that make anchoring the trap simple for PCOs. A portable drill and two screws are needed for installation. The removable side panel allows easy access to the baiting area. Curious squirrels cannot resist peeking inside and once the firing spring goes off, there is no handling of a distraught captured squirrel or relocation such as with a cage or box trap.

The Kania Trap 2000 should be secured at least 5 to 7 feet above ground to avoid accidental encounters with curious children and pets. Choose locations where squirrels travel and are less visible to the public. If complete privacy is needed, place traps inside attics or garages near where the squirrels are residing or build extension tunnels 24 inches long so that the trapped squirrel’s body and tail are hidden from view. Painting the trap is an option for further concealment.

Bait the traps with unshelled peanuts, hazelnuts (filberts), walnuts or other nuts native to your region. Peanut butter is also excellent bait, but it should be checked often as it has a tendency to become rancid with time.

To learn more about the trap visit http://www.kania.net. — Stephen Vantassel, CWCP, ACP and project coordinator, Internet Center for Wildlife Damage Management, School of Natural Resources, University of Nebraska-Lincoln

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Rat bite fever that is contracted by consuming milk and water contaminated with rodent urine is called Haverhill fever. Direct person-to-person transmission has not been demonstrated.

Between 10 percent and 30 percent of rodent bites develop into active infections. (As a comparison, wounds from dog and cat bites have a much higher likelihood of becoming infected with a variety of bacteria.) This is a rather low percentage considering 100 percent of wild and domestic rodents are colonized with the bacteria that causes rat bite fever. Every bite from a rodent, and all rodent body fluids, should be considered potentially infective.

In the event that pathogens are transmitted from a rat bite, the lesion often heals without residual inflammation or drainage. About three to 10 days after a bite, if pathogens were transmitted through the bite wound, symptoms will develop. At onset of illness, patients report symptoms suggestive of an upper respiratory illness such as headache, nausea, vomiting, sore throat and body pains. As the disease progresses, stiffing joints are accompanied by a high fever that may resolve in three to five days only to relapse again.

As the fever progresses, more than 50 percent of patients will develop temporary arthritis that moves to various joints of the body and is considered incredibly debilitating. The arthritis will lessen with antibiotics but can persist for a few weeks to a few months after treatment. In a small percentage of the population, the arthritis can persist for years even after the illness has been treated. Another symptom of disease is a rash occurring on the hands and feet, which is present in 75 percent of cases. Left untreated, mortality from rat bite fever is approximately 13 percent.

The California Department of Public Health Microbial Disease Laboratory has been tracking cases of rat bite fever since the 1990s. A recent review of cases from 1999-2007 illustrates that rat bite fever can be transmitted in body fluids as well as bites. Of seven identified patients, whose ages range between six and 68, four patients were 12 years old or younger. Two patients had a recognized bite from a pet rat, two had contact with a pet rat, two had no known exposure to rats (suggesting a possible body fluid exposure) and one worked in a pet shop where he cut his finger on a cage two weeks prior to onset. One case developed complications from the illness and died.

RAT BITE CHARACTERISTICS. The brief summary of California cases highlights trends seen in rodent bites across the United States. The majority of persons bitten by rodents are children younger than six years of age who are bitten at home while they sleep. The parts of the body bitten most often are the hands, arms, face, legs and feet. Since most bites occur between midnight and 6 a.m., the areas of the body usually not covered with apparel are most often bitten. Others hypothesize that the hands and face could emit odors that attract rodents.

Seasonality of rat bites also varies and likely is affected by weather, food supply and vector control impact. Various studies note that the frequency of rat bites increase in the summer months. One study, conducted in Philadelphia from 1974-97, found 10 percent of rodent bite cases had a prior rodent bite within the previous 12 months. Researchers believe that once a rat has bitten a human, it tends to bite again and again. Additionally, a single rat has been known to bite several persons over the course of a night.

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! Do Rats Transmit Rabies? !
Rabies can be transmitted by any mammal, but transmission by roof rats and Norway rats in the United States is not known. As reported to The New York Times by Jean Smith of the Division of Viral and Rickettsial Diseases at the CDC, scientists are not sure why rats are not found with the rabies virus, but the most likely explanation is that rats probably would not survive an encounter with an infected carnivore. Small rodents such as squirrels, mice, rats, hamsters, gerbils and chipmunks are almost never found to be infected with rabies and are not known to cause human rabies cases in the United States.

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The type and physical condition of the buildings in which rat bites occurred was analyzed in the previously mentioned Philadelphia rat bite study. A majority of bites occurred in single-family dwellings (67 percent) as compared to multiple- family dwellings (25 percent) with the remaining cases occurring in schools and research laboratories. Almost 90 percent of bites occurred inside of someone’s home.

A majority of rat bite victims lived in a residence that was rated “poor” in terms of its physical condition, sanitation inside, sanitation outside, adjacent structures and the block where the residence was located. All studies illustrate that a relationship exists between the physical condition of the structure, level of sanitation and the number of rat bites that occur.

PREVENTION. Prevention of rodent bites is both a community and an individual responsibility. Improving neighborhood sanitation and community-wide rat eradication campaigns reduce populations of rodents. Rodent infestations inside of human dwellings need to be eradicated. Structures should be rodent-proofed to prevent rodents from gaining access to the inside.

Pest management professionals should keep in mind that although normally a reclusive animal, if a live rat is cornered or feels threatened it will attack and bite. Live traps should not be used when abating rodents as live traps become contaminated by rodent body fluids and removing live rodents puts pest management professionals  at risk of bite injury. When disposing of dead rodents, gloves should be worn at all times. Traps containing dead rodents should be considered potentially infective and gloves should be worn when traps are handled.

In the event someone is bitten by a wild or pet rodent, be sure to wash the bite wound immediately and seek further medical attention.

The author is assistant vector ecologist with the Orange County Vector Control District in Garden Grove, Calif. She can be reached at lkrueger@giemedia.com.