by S. Todd Stolp MD
©May 2008
The relationship between seasonal cycles and such things as risks of wildfire, automobile accidents, and power outages are well known and often discussed. However, it is not as easy to appreciate how the biological world is similarly influenced, and how cyclic changes in living systems around us may have very real affects on people and health. It is worth reminding ourselves that we are immersed in a biological soup made up of plants, viruses, bacteria, fungi, protozoans, mammals and other kin, all of which are influenced by environmental fluctuations which may increase or decrease the periodic risk of crossing each others paths. One vivid example of this is the periodic waxing and waning of rabies infections in wildlife. Taking a look at such ecological cycles provides one way to visit the topic of the mysterious rabies virus.
The name, “rabies,” is Latin for “madness.” There are references to rabies in Babylonian records over 4000 years old. Treatment of animal bites in ancient Greece consisted of wound cauterization, or the burning of the bite wound with heated implements to kill both the virus and the surrounding tissue. This was long before the need to seek insurance authorization.
Bats and skunks are by far the most common animals in the United States to contract and transmit rabies, but rare incidents involving other mammals have occurred. In developed countries, rabies has been nearly eliminated in domestic animals through regular vaccination of pets and livestock. In Africa and Asia, rabies still causes tens of thousands of human deaths each year.
To classify behavior as “madness” requires a sound understanding of the range of “normal” behavior for a particular creature, even humans. Few would argue that a dog responding aggressively to having someone step on its tail is overresponding. But would one excuse a dog for bolting out of a yard to sink teeth into the foot of a passing stranger? Skunks are, by rule, nocturnal creatures, so finding a skunk wandering aimlessly in ones back yard in the light of midday would be suspicious. If you are not sure whether the behavior of an animal you encounter is normal, consult an expert.
The rabies virus is concentrated in the saliva of the infected animal and transmitted most often by bite wounds. Through an elegant mechanism, the virus travels from the bite wound along the nerves to the spinal cord and brain to begin infection in a new host. The virus then travels back out along the nerves to the salivary gland of the victim, to enter the saliva and prepare for transmission to the next host. While rabies infection is almost always fatal once the central nervous system (brain or spinal cord) is infected, treatment with a series of one dose of antibodies and four doses of rabies vaccine after being bitten by an infected animal is very effective in preventing disease. It is for this reason that it is very important to seek professional advice after suffering an animal bite wound.
A brain infection is called “encephalitis.” Many viruses besides rabies can cause encephalitis, including West Nile Virus, herpes viruses, poliovirus and rarely, even common respiratory viruses. Symptoms include confusion, visual disturbances, weakness, fatigue, muscle cramps and agitation. One peculiar type of agitation is called “hydrophobia,” or fear of water. This particular manifestation of rabies was often highlighted in the 1800’s, but in fact was more related to the inability to drink water and to generalized irritability. The goal of rabies control is not to identify these symptoms in people, because it is then too late to effectively treat the disease. The goal is to avoid exposure to the virus in the first place.
Diseases transmitted by mosquitoes obviously peak during the late spring, summer and early fall, when mosquitoes are actively breeding and most interested in finding meals. Bats are more likely to be found indoors when weather and outside temperatures drive them to seek a more comfortable habitat. Skunks do not hibernate, but they often spend cold winter months in their dens, appearing much more often during the warmer evenings of the year. Considering these patterns may help to distinguish expected from unexpected wildlife encounters.
Waxing and waning cycles of rabies infection in California tend to occur every seven to ten years. Be absolutely sure that your pets or livestock are up to date with their rabies vaccine and take steps to keep them from roaming. Do not handle dead or dying animals. Do not invite animals to your living areas by leaving them food. If you have questions about an animal that appears ill, or there has been a contact exposure to a wild animal, call your local animal control agency immediately. If you can safely contain the animal source of exposure, do so without touching the animal. While transmission of the virus requires contact with mucous membranes (eyes, lining of the mouth, etc…) or a break in the skin, any contact with the animal should be described to the health care provider and the decision whether an exposure has occurred should be left to those experts. Arrangements for preventative vaccination will be made if necessary.
However, this treatment can be avoided by avoiding close contact with wild and unvaccinated animals in the first place. If you encounter bats in the living areas of your home, call a pest control company for assistance. If it is not clear how long a bat may have been in a living area where family members have been sleeping, discuss the situation with your health care provider.