Rabies: A Rare But Important Disease
|Farm Manager Bob Kreft uses a balling gun to administer medication to a cow at the Michigan State University Dairy Cattle Teaching and Research Center. This common procedure is usually performed without wearing gloves and rarely has any consequences associated with contacting the cow’s saliva. However, in a rabid animal, the rabies virus is shed in the saliva, therefore it may be advisable to always wear protective hand coverings.|
Spread of Infection
In the United States, dogs were the most important animal host of rabies until the 1960’s.1 The gradual decline in domestic animal cases resulted from rabies control programs that included mandatory vaccination for dogs, stray animal removal and leash ordinances.2 Currently, in the United States, the majority of cases reported are found in bats, foxes, racoons and skunks.3 In Michigan, the bat is the number one wildlife species reported (Click to see Table 1).
The rabies virus is shed in high concentration in the saliva of infected animals and the infection is spread most commonly through a bite inflicted by the rabid animal. Unlike many other viruses that spread through the body by way of the blood stream, the rabies virus travels along nerves to the spinal cord where it incubates for several weeks to months and even longer. During the incubation period, clinical signs of rabies are not usually observed and the animal appears normal. Eventually, the virus moves into the brain and salivary glands, and typically, clinical signs appear once the brain and salivary glands are infected.
Although uncommon, the virus can also enter the body through open wounds, or mucous membranes including the mouth and eyes, if these areas are exposed to virus laden saliva. For example, opening a cow’s mouth to examine it, administer medication or pass a stomach tube without wearing protective gloves can expose the handler to a cow’s saliva. This is frequently performed in cattle that are ill for reasons other than rabies. Cautious handling of cattle with undetermined illnesses, especially if neurologic signs are observed, is recommended to avoiding exposure to saliva.
Rabies usually presents as an individual animal disease rather than as a herd outbreak. However, because cattle are naturally curious and explore their world with their noses, several animals in a herd may be bitten when they gather to investigate a rabid skunk or racoon displaying unusual behavior. In such situations, the length of time to the development of clinical signs is shortened when compared to an animal bitten on a hind leg because the virus has a shorter distance to travel to infect the brain.
The early clinical signs of rabies can be subtle, variable, vague, and easily missed. Milk production and feed intake may decrease gradually, before dropping dramatically. Cattle may appear very alert and stare intently at objects. A cow may become excitable and display exaggerated movement. She may become aggressive, charge objects and people and produce loud, distinctive bellowing. Aggressive behavior may be expressed as increased sexual activity, including vigorous mounting behavior. Bulls may have a persistent erection or prolapsed penis. Some cattle strain quite forcibly as if they are in labor, or trying to urinate and/or defecate.
As paralysis of the face and throat muscles develops, an animal’s face may appear expressionless. Because she is unable to swallow, she may drool, her tongue may protrude and uneaten food may hang from her mouth. These signs mimic choke (blockage in the throat or esophagus) and may prompt a caretaker to examine the mouth, often with bare hands. After such an examination fails to find an obstruction, the handler may think about rabies and his/her possible exposure to virus laden saliva.
Once clinical signs appear, the progression is rapid with increasing paralysis leading to the inability to rise, followed by death. In some cases, incoordination of the hind quarters appears before any others signs. In such cases, a cow will appear bright, alert, with normal facial expressions and eat habits. She may exhibit a swaying, unsteady gait, and her tail and anus may appear flaccid. As the paralysis ascends toward the head, she will become recumbent, lose her facial expression, drool, and eventually die.
Observations of the above signs are suggestive of several other diseases that are much more commonly encountered than rabies, including lead or organophosphate toxicity, nervous ketosis, milk fever, polioencephalomalacia, botulism, pseudorabies, listeriosis, brain tumor or abscess, and indigestion. Because rabies is so uncommon, it may rarely be thought of in cattle with signs associated with neurological disease. However, it should always be on the list of possibilities when an animal with an ill-defined problem, especially if neurological signs are noticed and the animal has had contact with a bat, skunk or racoon that displayed unusual behavior. It is very important to limit the number of people and other animals that come in contact with the rabies suspect, and, if possible, the animal should be isolated. Enlist the assistance of a veterinarian who is familiar with livestock diseases to help determine the cause of an animal’s illness.
Testing for Virus
There are no tests for rabies in a live animal. Determination of whether or not an animal has rabies requires a post-mortem examination of specific parts of an animals’ brain. In Michigan, animal rabies testing is performed by the Michigan Department of Community Health's Bureau of Laboratories located in Lansing and Houghton. Confirmed cases of rabies are reported to the state veterinarian and public health officials.
Currently, there are three companies, Pfizer (Defensor 3®), Schering- Plough (Rabdomun®) and Merial (Imrab 3® and Imrab Large Animal®) that produce rabies vaccines licensed for use in cattle. All of these are inactivated or killed vaccines that will not cause the disease, but will stimulate the formation of antibodies protective against the rabies virus. It is important to use only those vaccines that have been tested and approved for use in cattle. The efficacy of non-approved products has not been determined and may not provide adequate protection. In Michigan, routine vaccination of production-type animals is uncommon because the cost of vaccination outweighs the risk of exposure to rabies. The exceptions include very valuable purebred animals, cattle that are used in exhibits such as petting zoos because of their greater exposure to humans, and animals that are pastured in areas that have an abundance of bats, skunks or racoon or areas with a higher incidence of reported cases of rabies in wildlife, such as the southeastern and Thumb portions of Michigan.
To further protect cattle, as well as humans, farm dogs and cats should routinely be vaccinated. In fact, there have been more cats diagnosed with rabies in Michigan than racoons, and cats are more likely to come in contact with cattle, especially dairy cattle, and people more often than wildlife. In addition, although this is a challenge, any measures that can be taken to minimize exposure of your cattle to wildlife will decrease their risk of exposure to potentially rabid animals.
The rabies virus is inactivated by heat and consumption of cooked meat and/or pasteurized milk from a rabies positive animal is not considered a rabies exposure. It is however, theoretically possible for the rabies virus to be shed in milk and there are studies underway to isolate the rabies virus from udders of cattle that have died from rabies. Until that mode of transmission is fully proven or disproven, drinking unpasteurized milk from a rabid cow should be avoided since it is considered a rabies exposure.5
Keep in mind that rabies disease in Michigan cattle is rare. Effective and approved vaccines are available for cats, dogs, horses, sheep, ferrets and cattle and their use may be warranted in certain situations. Enlist the assistance of a veterinarian familiar with livestock diseases to help you determine if your sick dairy animal with neurological signs has rabies, or another more commonly occurring disease. Hopefully, you will never encounter rabies during your lifetime. However, if you do, you will want to be prepared.
2. Rupprecht, C. E, et al.: The Ascension of Wildlife Rabies: A Cause for Concern or Intervention? Centers for Disease Control and Prevention, Emerging Infectious Diseases, Vol 1(4), 1995.
3. Blanton, J. D., et al: Rabies Surveillance in the United States during 2008. Journal of the American Veterinary Medical Association, Vol 235 (6), Sept. 15, 2008, pp 676 - 689.
4. Viral Diseases of Cattle. Kahrs, R. F., ed., 2nd ed, 2001, Iowa State Press, Ames.
5. Compendium of Animal Rabies Prevention and Control, 2008, National Association of State Public Health Veterinarians.
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