Rabies, Biology


Rabies, a fatal zoonotic viral disease of dogs, cats, man and other mammals, causes an acute encephalomyelitis. The rabies virus belongs to the genus Lyssavirus in the family Rhabdoviridae. This disease has been eradicated in some of the foreign countries by compulsory muzzling and vaccination of all dogs, quarantining of imported dogs, etc.

Epidemiology: In South America and Mexico, the blood sucking vampire bat has been found to carry the infection for months without showing any symptoms and is a source of danger for man and animals there. In India, the first record of rabies infective free- flying bats was made in 1954. Bat rabies, however, is not a problem in India. Among the reservoirs of rabies in India, jackal, fox and mongoose are the most significant apart from the domestic dog. The virus isolated from a natural case of rabies is known as 'street virus' and the one obtained by passaging in experimental animals viz. rabbits is termed 'fixed virus'.The disease in man is also known as hydrophobia, because the affected individual develops a fear at the sight or sound of water.

Clinical signs:
The first symptoms of the disease appear 2-8 weeks after the bite, though an incubation period of 300 days or more has also been recorded. The incubation period usually depends on the site where the bite is inflicted. The bite on the face is  considered the most dangerous. The disease, as usually observed, is caused by the street virus. In dogs it is of furious and dumb forms.

In this form, there are 3 different stages of development of the disease, viz. melancholic, excitable or furious and paralytic. In furious rabies the animal suddenly shows a marked change in its behavior. It wants to remain aloof and is irritable and erratic, bites at the flies, is characteristically irresponsible to its master and shows a disinclination for food. This is the stage of melancholy, which in the course of a week progresses to the furious stage. The animal is restless, excited and develops a hoarse voice as a result of paralysis of the muscles of the throat and larynx. It shows an acute desire to bite anything that it meets, while it runs about aimlessly even when it is in confinement. The animal, if survives this stage, soon becomes paralytic as a result of exhaustion. There is lowering of the jaw due to paralysis and the tongue protrudes out. The animal becomes weak on its legs and dies in 2 to 4 days.

Dumb rabies: The initial symptoms manifest themselves in the form of paralysis. The animal has a vacant look and is conspicuously inattentive. The paralysis soon extends to the muscles of the throat, larynx and pharynx, so that the animal is unable to bark or bite. At this stage, the lower jaw is depressed and it looks as a piece of bone is stuck in its throat. The animal is soon rendered unable to move and is dead in 1 to 4 days. In either form, the dog rarely survives for more than 7 days from the onset of symptoms. An animal suspected for rabies is, therefore, kept under observation for 10-15 days.In cats and wild carnivores, only the furious form is usually observed. In equines, the disease resembles acute colic. The animal bites at various objects, viz. the wall or wooden structure and kicks about. The hind legs are soon paralysed and death follows in about a week. In cattle, the disease occurs in both the forms described for dogs.In man, the disease is invariably fatal. After an incubation period of 10 to 90 days, which sometimes could be even 2 years, the patient complains of a severe headache, vomiting sensation, watering from the eyes and nose and extreme dryness of the throat. Paralysis of the throat soon sets in and he is unable to eat or drink. The paralysis soon extends to the entire body and the patient dies in about 7 days.

Diagnosis: The disease is diagnosed by clinical symptoms and laboratory examination. Swiss albino mice are susceptible when inoculated intracerebrally with test suspensions. Although the incubation period is variable, a majority of the inoculated mice show symptoms and die between 8th and 14th days. In guinea-pigs and rabbits, usually the incubation period is longer than in mice. Negri bodies can be found in the brains of animals inoculated with the street virus by the time symptoms become manifest. The virus neutralization test is performed with an anti -serum for rabies virus. Alternatively, the suspensions are seeded onto BHK-21 monolayer and the viral antigen is demonstrated by FAT on the third day. Immunoperoxidase and the fluorescent antibody techniques are becoming more and more popular, replacing the use of mice in the diagnosis of rabies.

It is generally recommended that the area of bite should be washed thoroughly with water and soap solution or any mild detergent to reduce the  concentration of the virus that may be present. A liberal application of tincture of iodine or cautery with concentrated hydrochloric acid is also advised. In emergencies, when no antiseptics are available, the juice crushed out of a fresh lemon or any beverage may be applied on the wound to flush the virus, if any. Hyperimmune serum or gamma globulin solution may also be applied, if available.
Any treatment carried out should invariably be accompanied by injections of Semple's antirabic vaccine, consisting of brain and spinal cord of sheep inoculated with a laboratory fixed virus and treated with 0.5%phenol for 48 hr at 37C to inactivate the virus. The vaccine is a 5% suspension and the dosage for different species of animals is as follows:

Dogs under 14 kg of body weight  :   5 ml for 7 days

Dogs over 14 kg of body weight    :   5 ml for 10 days, or 1 injection of 5 ml of 20% vaccine

Cats                                          :   5 ml for 7 days

Calves, goats, sheep and pigs       :   10 ml for 7 days

Mules, ponies and donkeys          :   20 ml for 14 days

Cows, buffaloes and horses          :   30 ml for 14 days

Camels and elephants                  :   60 ml for 14 days

Cell culture adapted viruses are being used as vaccine after inactivation, in recent years. These are relatively more purified and concentrated and are available as prophylactic and post-bite vaccines. The duration of protection following vaccination with such anti-rabies vaccine is nearly 2 years.

Prevention and control: Flury strain, developed in the USA, was serially passaged in non-mammalian hosts which resulted in the necessary degree of attenuation or modification. The vaccine is safe, cheap and immunogenic. It consists of 33% chick- embryo virus in lyophilized state. The dose is 3 ml.If live virus vaccine is used, immunization of 70% of the dog population over a large area controls the disease quite effectively. Long persisting immunity maintains an immune barrier so high that the virus cannot propagate itself. Such a procedure accomplishes the maintenance of a sufficiently high percentage of immunized dogs for effective control of the disease.

Possession of a certificate of vaccination of the dog against rabies, granted by a competent authority, should be made compulsory for each owner before the dog is allowed to move about among residents and animals in a locality. Education of the owners to observe restraint of movement of the pet dogs in public places and enforcement of rabies control measures through legislation often help in rabies control programmes.

Posted Date: 9/18/2012 8:08:25 AM | Location : United States

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