More than 50,000 people in the world die each year of rabies, most of whom have acquired it from dogs. Rabies is a virus maintained in mammalian wildlife that enters the body by bite or saliva contaminated wounds.  After entering the body, it replicates in muscle and travels up nerves to the spinal cord, then to the brain, and down nerves to the salivary glands.  There is no effective treatment once the virus reaches the brain.  The disease is fatal.

Incidence

Although thousands of people die of rabies each year in other countries, only about 3 cases of rabies occur per year in humans in the U.S. despite the virus being plentiful in wildlife.  More than 90% of confirmed rabies in animals in the U.S. involve wildlife, particularly (in decreasing order) raccoons, skunks, bats, and foxes.  Raccoons have been the most frequently affected species since 1991.  However, rabid raccoons are currently found primarily in states east of Ohio.  Skunks and bats are more common hosts in Missouri.  

Among domestic animals the incidence of rabies is (in decreasing order) cats, cattle, dogs, and horses.  Cats have been the most frequently affected domestic species each year since 1987.

The reasons for a low incidence of human rabies in the U.S. are vaccination of dogs and post-exposure treatment of humans.  Globally, dogs are the major source of rabies in humans.  In 1944, more than 9,000 rabid dogs were diagnosed in the U.S.  Today, because of routine vaccination against rabies, approximately 100 dogs are diagnosed per year.  The last case of dog "strain" type rabies (the type that spreads easiest in dogs) in the U.S. was reported in 2004.  Preventive treatment of humans is also common in the U.S. Each year, about 18,000 people in high risk jobs receive pre-exposure vaccinations and 40,000 who have possibly been exposed go through anti-rabies treatment.    

Transmission

Rabies in humans in the U.S. is usually acquired from bat bites.  Some cases diagnosed in this country were acquired from rabid dog exposure while visiting another country.

Rabies is not believed to be common in the general bat population.  Less than 1% may be affected.  All species of bats can carry rabies, but of those that have been associated with human exposure nearly 3/4 have involved a woodland roosting bat, the silver-haired bat.

Reasons for concern about bats carrying rabies include nearly invisible bites and the ability to gain access and roost in homes.   

Small rodents (mice, gerbils, guinea pigs, squirrels) and rabbits are capable of carrying rabies in a laboratory environment.  However in nature, they are not reservoirs for rabies and their bites are not a risk for acquiring rabies.

Signs and Symptoms

Symptoms appear after the virus reaches the brain which usually takes 1 to 3 months.  This incubation period varies with proximity of the bite to the head and the amount of saliva in wounds.  Death usually occurs in 3 to 10 days after symptoms appear.  Symptoms may be depression ("dumb rabies") to rage ("furious") rabies and depends on the species affected and the stage of the disease.  The ability to swallow becomes painful and difficulty causing saliva to drool and foam.  The inability to swallow was thought to be a fear of water and led to rabies historical name of hydrophobia (fear of water).

Animals with rabies will act in inappropriate ways.  They will lose their inherent fear of humans or other larger animals (horses and cows).  They may not avoid other animals that they should, such as porcupines.  For example, a skunk, raccoon, or fox with porcupine quills in its face is likely to have rabies.  Rabid animals also become active during an unusual part of the day.  Skunks, raccoons, and bats that are active during daylight are likely to have rabies.

Diagnosis

The diagnosis is confirmed by laboratory tests on brain tissue after death.

Treatment

If bitten, immediately clean the wound with soap and water.  Apply compression, if bleeding.  Seek pmedical attention as soon as possible. 

Try to identify the animal.  If a wild animal, it should be trapped under a box or can, IF this can be done with safety, or it should killed without damaging its brain tissue

Prevention Measures

For more information, contact your local veterinarian or the MU Veterinary Medical Teaching Hospital 573-882-7821 or visit http://www.vmth.missouri.edu

C.B. Chastain, DVM
Professor, University of Missouri
College of Veterinary Medicine