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How Concerned Should Michigan Residents Be?
Incidence of Rabies in Michigan
Human rabies is very rare in the United States today. On average, only 1-2 cases occur each year. The most recent human case of rabies in Michigan occurred in 2009. Several factors are responsible for the decline in human rabies cases over the past five decades. Beginning in the 1940's and 50's, the large-scale vaccination of domestic dogs led to a dramatic decrease in the incidence of human rabies in the U.S. As a result, rabies changed from being primarily a disease of domestic animals (dogs) to one of wild animals. In 2001, more that 93% of rabies cases reported to the Centers for Disease Control and Prevention were in wild animals.1 Improvements in the biologics used to prevent rabies in people have also been a factor in the reduction of human fatalities due to rabies. In addition, the development of standard recommendations to be followed in the case of an animal bite2, and requiring that animal bites be reported to local health authorities has led to improved treatment for people potentially exposed to rabid animals.
In Michigan, bats are the species most at risk for rabies infection, and of all the species of animals tested each year in the state, they are the most likely to be positive. Although it is estimated that fewer than 1% of the bats in Michigan carry rabies, a higher proportion (average of 6%) of those tested at the Michigan Department of Community Health Bureau of Laboratories (MDCH BOL) are rabid. This reflects the fact that sick bats are more likely to exhibit abnormal behavior, have an encounter with a human or another animal, and be submitted for rabies testing. Abnormal behaviors include being active during the day, being found in a place where bats are not usually seen (such as inside a home), or inability to fly.
Bats also appear to present a unique problem with regard to the transmission of rabies to humans. Of all of the human cases of rabies acquired in the United States since 1990, 27 of 29 cases were due to a bat variant, and of those, only 2 had a history of a bat bite.1 It is likely that the other cases were transmitted by bat bites that were either unnoticed or ignored. The sometimes-long incubation period that can occur can make recollection of bat exposure difficult. Also, by the time rabies is considered as a possible cause of a person's illness, they may be unable to answer questions about previous exposures to bats. Also, bats have very small teeth, and a bite inflicted by a bat may not be felt. As a result of these cases, PEP should be considered in cases where a bat is discovered in the room with a sleeping person, child, or a mentally disabled or intoxicated person.2
Recommendations with regard to bats are as follows:
¨ Never handle a bat with bare hands.
¨ Any dead, sick, or injured bat should be collected and tested for rabies if exposure to people, pets, or livestock has occurred.
¨ Prevent bats from entering occupied spaces in any building where contact with people, pets, or livestock might occur.
¨ If dead bats are found and there has been no known pet or human exposure, they should be collected by scooping them up with a shovel, placing them in a plastic bag, and disposing of them in the trash to go to a landfill.
Rabies in terrestrial mammals such as skunks, raccoons, foxes, and coyotes represent a greater risk to humans because their habits make them more likely to interact with humans and other domestic animal species, resulting in a "spilling over" of the particular strain of rabies into other types of animals. In Michigan, cases of rabies in skunks can be found in the southeastern Lower Peninsula. As a result, we see occasional cases of rabies in domestic livestock and pets due to exposure to rabid skunks. Bat and skunk strains of rabies are currently the only wildlife strains of rabies known to occur in Michigan.
Michigan law currently only requires that dogs be vaccinated for rabies. However, cats, especially those that are allowed to spend time outside, may encounter wildlife, and thus present a risk for rabies exposure to their owners or others they may encounter. Even cats housed indoors exclusively may encounter potentially rabid bats inside a home. Along with bats, cats are the species of animal most often submitted for rabies testing at the MDCH BOL.
Since the 1970's, raccoon rabies has spread rapidly up and down the east coast of the United States, and as far west as eastern Ohio. Raccoons are of particular concern because, unlike skunks, they prefer to live in proximity to human activity as it provides them with ready access to food and shelter, and humans more often tolerate their presence. As a result, they can represent a greater risk of rabies to both domestic animals like cats and dogs, and to people. The encroachment of raccoon rabies into Michigan is a great concern, and as a result, the Rabies Working Group was formed to address this concern and work to prevent the introduction of raccoon rabies into Michigan.
In general, rodents and rabbits are not considered to represent a risk for rabies. These types of animals have never been involved in the transmission of rabies to a human. Many rodents are kept as pets, and they often bite when handled. In any unusual circumstances involving a bite from a rodent or rabbit, consult your local health department.
In general, the following precautions can be taken to reduce your risk of exposure to rabies from terrestrial mammals:
¨ Never approach or try to handle a wild animal. If the animal is injured, call the local animal control agency nearest you to report the animal.
¨ Thoroughly wash any wound caused by an animal with soap and water and seek medical attention immediately.
¨ It is illegal to possess wild animals as pets or rehabilitate injured wildlife without a special permit issued by the Department of Natural Resources (DNR).
¨ All pets and domestic livestock for which a licensed vaccine exists should be vaccinated. This includes dogs, cats, ferrets, horses, cattle, and sheep. Vaccines should be administered by a licensed veterinarian, and boosters given according to the manufacturers directions.
¨ Animals for which a licensed rabies vaccine does not exist should be housed in such as way as to reduce the chance of contact with potentially rabid wildlife.
1 Krebs JW, Noll HR, Rupprecht CE, Childs JE, Rabies surveillance in the United States during 2001. J Am Vet Med Assoc. 2002;221:1690-1701.
2 Centers for Disease Control and Prevention. Human rabies prevention-United States, 2008: Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 2008; 57(RR-3):1-28.
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