Rhabdovirus is one virus causing but one disease. There are a number of viruses in the family Rhabdoviridae which infect various vertebrate and invertebrate hosts; however, only one, the rabies virus, causes serious disease in humans. The virus is an RNA with a helix shape, but, in the case of the rabies virus, the helix is wound in a rod-shape with one end tapered, giving it a “bullet” appearance.
Infection usually occurs from a bite from an infected animal, where the virus gains entry through the break in the skin. However, it can also be contracted through contact with infected saliva that enters through open sores or abrasions. The virus may remain for weeks at the site of entry, where some local multiplication may occur. From there, it moves along nerve fibers to the regional ganglia and the CNS. The incubation time from the bite to the onset of the disease is dependent on the severity of the bite, as well as some host factors. The period can range from a week to months and even a year or more, but the average is one to two months. The incubation period is longer when the bite occurs on an extremity as opposed to one on the neck or head.
Approximately 50% of the time, the virus fails to progress from the site of entry to the central nervous system. The rabies virus is so verocious in its seeking out nerves that it literally drives its host ‘crazy’ in order to jump to the next host. This explains the persistence of rabid animals in seeking out a new host. Once the virus enters the CNS however, it spreads rapidly throughout these tissues and on down the peripheral nerves to other tissues. In particular, the rabies virus proliferates in the salivary glands, and may be found in high concentrations in the saliva. When this stage is reached, the overt signs and symptoms of the disease begin with fever, headache, and loss of appetite. Once signs of the disease begin to appear in humans, it indicates that it has progressed to the central nervous system, and, in almost all cases, death results. Symptoms soon progress to convulsions, excessive flow of tears and saliva, insomnia, anxiety, muscle spasms triggered by swallowing, and sometimes maniacal behavior. Ascending paralysis may be seen, and death can result two to six days later, generally because of respiratory or cardiac failure. In other cases, the patient will lapse into a coma and die.
In animals, the pathogenesis of the virus is similar to that of humans but with some differences. The route of infection is often as a result of the bite of another infected animal, but it can also result from eating an infected animal. In many animals, the virus passes to the CNS during the long incubation period. As soon as signs appear, the animal dies within a relatively short period of time. A small percentage of these animals that become infected have been found to shed the virus in their saliva for up to two years without showing any clinical symptoms of having the disease. Bats which have the infection appear to tolerate the disease better than any other animal.
To test for rabies after an animal has been killed, the lab worker examines the brain cells for the appearance of clusters of viral materials (inclusion bodies) in the cytoplasm called Negri bodies. Negri bodies are rapidly and specifically identified by using specific fluorescent antibodies. A positive diagnosis can be made on most infected animals within a few minutes via this procedure. For human diagnosis, the clinical signs and symptoms of an animal that has bitten them is usually the first clue. After that, there may be a diagnosis made through the showing of the rabies virus antigens in corneal impressions or skin biopsies taken from the nape of the neck using fluorescent antibodies.
With few exceptions, treatment of clinical rabies has not been successful. There is a hyperimmune globulin known as human rabies immune globulin (HRIG) that is used when there is a strong possibility of exposure to a rabid animal. This is followed by post exposure immunization with HDCV (a killed purified human diploid cell vaccine) or RVA (rhesus lung diploid cell vaccine) with IM injections on days 0,3,7,14,and 28. The treatment regimen should be correlated with the diagnostic program on the animal that inflicted the bite. If tests show that the animal is not infected, the immunization program should be stopped to avoid further discomfort to the patient. It is for this reason that the suspect animal should be apprehended.