Perhaps the most-feared word in infectious diseases.
Rabies is found on every land mass on the planet except for a few islands.
While the average American’s risk of contracting rabies is extremely small, the danger lies in the fact that, once contracted, the disease is almost uniformly fatal. You may read about an exceptional case here.
Rabies is almost always contracted through the bite of an infected animal.
Rabies virus, present in the saliva of the attacker, is deposited into the tissues of the person or animal bitten. The rabies virus then begins to replicate, or reproduce itself. Replication most commonly infects nerve cells, which soon begin to transmit the virus “upstream” toward the brain.
Soon the virus reaches the spinal cord, which is part of the central nervous system (CNS). CNS is the ideal location for virus replication, and the quantity of virus in the body increases rapidly. In doing so, nerve conduction in both motor (nerves to muscles) and sensory (nerves which carry sensation to the brain) neurons is impaired. Now the victim may exhibit staggering, stumbling and low sensitivity to stimuli. For example, a person standing on a hot surface might not feel pain.
Virus continues toward the brain, where it can spreads “downstream” in nerves leading from the brain to every organ with a nerve branch. Infection of those tiniest nerves leads to intrusion of the virus into non-nerve structures, such as the salivary glands. Once rabies virus is in the saliva, a bite can transmit the disease to yet another victim.
When rabies virus enters the brain and begins to damage it, behavioral changes may be seen. Classic hydrophobia occurs because people become unable to swallow, and fear even attempting to drink water, as they may choke. People and animals may experience falling, seizures and loss of consciousness. They may behave in a bizarre fashion. For example, wild animals may lose their fear of humans and wander into a crowd, normally-docile dogs may attack, some animals may hide.
Incubation period, the time between the initial bite and the time signs appear, may vary from a few weeks to several years. The average, however, is a few weeks.
Treatment of affected humans has a low, but increasing success rate.
There is no protocol for treatment of animals. Poorly-documented reports exist of dogs surviving rabies.
Definitive diagnosis is made by examining the brain after death occurs. PCR (polymerase chain reaction) and virus isolation techniques are accurate and sensitive. Microscopic analysis for Negri bodies is definitive, but Negri bodies do not occur in every brain infected by rabies. In people, antemortem diagnosis can be made by skin sampling if the virus is in nerves serving that area of skin.
Rabies vaccine has one of the highest success rates of preventing disease among prophylactic medications. Traditional one-year-duration vaccines are used in many states, while an increasing percentage of states are approving use of three-year-duration vaccines. Even in three-year states a pet’s first rabies vaccination must be boostered in one year. Pet owners should follow the recommendations of their local veterinarians regarding rabies vaccination.
There are many reasons to protect your pets from wild animals, as they may suffer vicious bites, scratches, intestinal parasites and leptospirosis, in addition to rabies. Hunting dogs are at greatest risk, but living in a natural wonderland such as Brenda, Willie and I do leaves even the most domesticated pets susceptible to wildlife dangers.
Take your pets indoors at night. Any pet food, bird seed or other attractants to wild animals should be removed. Outdoor lights may discourage some animals, as well as allowing you to see them should they venture too close to your home.
OTHER MEANS OF TRANSMISSION OF RABIES
While biting is the most common way rabies virus is deposited into a recipient, people have been infected by inhaling virus aerosolized by bats while spelunking(exploring caves). Also, virus may enter mucus membranes or open wounds. Rabies virus cannot enter intact skin.
WHY TEN DAYS OF OBSERVATION?
When a person is bitten by an animal whose rabies vaccination status is unknown, it is common for the biting animal to be observed for ten days (regulations vary by locality). The reasoning is that the time period for rabies virus to travel from the brain to the salivary glands is, on average, ten days or more. Thus, if a biter does not exhibit symptoms of rabies virus in the brain within the observation period, it is unlikely that virus could have been in the saliva at the time the person was bitten.
See you Monday, Dr. Randolph.