VIRAL DISEASES

Rabies INTRODUCTION Rabies is a viral disease of the central nervous system characterized by acute encephalomyelitis, change in behavior, hydrophobia, paralysis and death.

AETIOLOGY:- Rhabdovirus of the genus Lyssavirus. an RNA virus, bullet shaped and about 180mµ by 75mµ in size. . It is enveloped with outer projections. It has a helical nucleocapsid. Other members of the genus: Mokola, Kotonkan, Obodhiang, Duvenhage and Lagos bat viruses.

Structure of the Rabies Virus

EPIZOOTIOLOGY Incidence:- Worldwide occurrence except in a few places like Great Britain, New Zealand, New Guinea, Alaska, Hawaii, Oceania and Holland . Susceptible Hosts All worm blooded animals are susceptible. Experimental infection possible in birds Natural infection had been reported in carrion birds

Transmission Through the bite of an infected animal. Through mucosae (but cannot penetrate unbroken skin). Through fresh wound Aerosol

Pathogenesis Consequences of infection depend on the amount of virus inoculated, the virulence and the site of the bite. Bite on the face or head region will result in faster and more severe signs because of the abundance of nerve supply and nearness to the CNS. Serverity also depends on the susceptibility of the animal species. About 15% of the people bitten by rabid animals may die of rabies if treatment is not instituted promptly. dogs are moderately susceptible.

In the dogs, incubation period varies between 10 days to six months sometimes about 15 months. In other species, the incubation period varies from 6 days to 1 year or more according to the species Following the inoculation of the virus at the bite site, it multiplies in the myocytes and spreads to the neuromuscular junctions and neurotendinal spindles and enters the exon sheath of afferent nerve within 5 hours. By 10 hours it disappears from the site of bite and also from the lymph nodes, travels up to the spinal ganglion where it multiplies for a few days, after which it enters the dorsal horn of the cord and soon reaches the ependymal cells.

From the segment of the cord first involved, the virus ascends to the brain by cell to cell infection. Once the brain is involved, infection spreads centrifugally down the cranial nerves to reach the salivary glands and other organs. In very susceptible animal like the fox, death may occur before the virus leaves the brain to infect the salivary glands. In less severe infection, the virus reaches the salivary gland and further multiplies. Virus can also multiply in the lungs and kidneys. The virus is shed in the saliva for a few days before death. Virus excretion in saliva occurs only in about 50% of infected dogs.

The virus induces fatal encephalomyelitis affecting mainly the midbrain, basal ganglia, hippocampus, pons, medulla and spinal cord. The cranial, spinal and sympathetic ganglia may also be affected. There is marked accumulation of microglia (Babes nodules) and perivascular deposits of lymphocytes, neurons are destroyed and may be infiltrated by microglia. Also present are eosinophilic, intracytoplasmic inclusion bodies in the neurons (Negri bodies) of various sizes between 2-8µ. The inclusion bodies have distinctive matrix of basophilic granules. These are pathognomonic of rabies.

Course The course varies between 3-11 days with an average of 7 days. It may be so short as to be regarded as sudden death without any sign suggestive of rabies infections.

Clinical signs The signs are classified as Prodromal Furious Dumb Atypical.

Prodromal stage Change in behavior Affectionate dog may become stand-offish Bad-tempered dog may become docile. The dog is usually restless, barks unnecessarily at objects. The appetite and thirst may be normal at this stage. There may be profuse salivation and the dog may lick or scratch at the bite wound. The stage may last up to 36 hours.

Furious Rabies:- The dog bites anything or anybody in its path at this stage. Animal may look anxious or stupid or just starring. There is also strabismus, protrusion of membrane nictitans and mucopurulent exudates in from the eye. The pupils are dilated at times they may be constricted. There is depraved appetite, (pica) eating stick, stone, and shews every object in its path including faeces. At this stage the animal can still drink.

There is excessive salivation and injury to the mucous membranes of the mouth. The tongue is later dry from exposure and has dirts or debris on it. There is frequent urination, penile erection and licking of the genitals. Females often accept males. There is hyperaesthesia. If confined it attacks the cage and tears the bedding. Periods of stupor and alertness alternate, trembling and incoordination, and general paralysis occurs with respiratory distress. The voice may be changed with persistent howling and barking referred to as rabies “howl”. As the dog becomes more restless, its roams and usually becomes highly irritable and vicious. It becomes ataxic, disorientated and with periods of seizures. There is a short period ascending paralysis before death.

Dumb Rabies:- Paralysis is the dominant feature, the lower jaw drops. The animal tends to choke or retch and cannot eat or drink. Hydrophobia is rarely recorded. Persistent howling and barking are often present until the dog becomes paralysed and unconscious. The voice becomes husky. On confinement if the degree of paralysis is not marked the animal may show some restlessness. The breathing becomes slower and deeper. Muscular tremors develop. The anterior part becomes paralysed and finally the dog becomes prostrate.

Atypical form:- This is a combination of furious and dumb types. Hysterical (running) fits and epileptiform convulsions have been described in rabies but these are different from the convulsions of distemper.

Differential diagnosis:- Distemper Encephalitis Poisoning Fracture, sprains, lameness, neuritis, peritonitis, pneumonitis and other organic diseases. Pseudorabies

Treatment None: Suspected rabid dogs should be confined for 10 days

Diagnosis:- Negri bodies in the brain (2 days) Isolation of the virus from the brain or saliva (about 3 weeks) FAT to dermostrate the virus in the brain smear 95% efficient (same day) Inoculation of baby mice (24-48 hr old) paralysis in 6-21 days..

Prophylaxis By Vaccination

First Aid for human bite:- Mechanical washing with soap and water is very effective. 2% Quarternary ammonium disinfectant or 50-70% alcohol are specifically rabicidal. Anti rabies serum or gamma globulin can be infiltrated under and around the wound

Animal control:- Vaccination Quarantine and vaccination of imported dogs and cats Catching and humanely killing strays dogs During epidemic animal can be confined to their homes or exercised on a leash while muzzled. Vaccination of wildlife with oral vaccine.