©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.

©June 2004

The success of a species on earth is a product of a balance between the forces that act to keep the organism confined and the capability of the organism to successfully disseminate.  Whether we are considering the magnificent simplicity of the helicopter wings attached to each seed of a pine cone, or the complex migration of sea turtles, this principle is the ultimate determinant of whether an organism will expand it’s territory of influence.  The same rules govern the ability of viruses and microbes to continue their invisible cohabitation with their hosts.  To control human disease it is appropriate to remain cognizant of our most effective weapons in keeping certain infectious agents, such as the West Nile Virus (WNV), at bay.   It is also well to respect the strengths of our foes in addition to our own weaknesses.

The relentless spread of WNV to California has made ingenious use of the migratory bird population as a mass transit system.  While members of the Corvid family (Ravens, Crows, Jays and Magpies) frequently succumb to infection with this virus, other birds often suffer little other than a headache.  Infected members of the wild bird population provide a source of the virus to be easily picked up by a mosquito and then transmitted to unsuspecting animals at the next mosquito meal.  It is testimony to the intimate relationship that we have with other creatures on this planet to realize that the over 9000 people in the U.S. who were diagnosed with West Nile Virus infections last year were bitten by mosquitoes which had also sipped the blood of a bird or two.

To our credit, birds and other animals have developed a formidable internal police force to keep watch against such devious invading microbes.  Through research devoted to understanding the Human Immunodeficiency Virus, we have increased our respect for the tremendous protection afforded to us by our immune systems.  We have taken advantage of that protection by developing vaccination programs that have effectively rendered  infections such as smallpox, measles, polio, and pertussis impotent.  Unfortunately, it does not appear that a vaccination will be developed in time to be deployed against the West Nile Virus epidemic.

It is also important to recognize that the majority of the 250 U.S. citizens who perished last year as a result of WNV infection were over 50 years of age.  This probably reflects a relative weakness of immune system function that occurs with older age.

As we tally our “assets” in preparation for battle against WNV, perhaps none is as valuable as the one which brings humans the greatest pride: intelligence.  Each of us should understand that West Nile Virus is incapable of being transmitted by handshake or sneeze.  This virus requires the bite of an infectious mosquito in order to do damage.  Therefore, if measures are taken to diminish exposure to mosquito bites, the virus will be less successful in causing human misery.  Such precautions as wearing long sleeve shirts and other protective clothing, avoiding outdoor activities during dusk, proper use of mosquito repellants containing 15% to 50% DEET (N, N – Diethyl toluamide), repair of torn window screens, elimination of standing water that can act as a breeding ground for mosquitoes, and reporting dead birds to the Health Department are all steps that will help reduce the risk of WNV infection.

While the march of the West Nile Virus across the country has been relentless, and only modestly influenced by vigorous  mosquito control programs, and while there is no likelihood that any one person will escape the season free of mosquito bites, there is no substitute for common sense.  Prevention of mosquito bites through the measures described by your local health department will go a long way to prevent a personal experience with the WNV epidemic.