Rabies is a neurological disease of mammals that is almost invariably fatal once the clinical signs develop. Humans are usually infected when they are bitten by an infected animal, or exposed to its saliva or central nervous system (CNS) tissues.

Etiology

Rabies results from infection by the rabies virus, a neurotropic virus in the genus Lyssavirus, family Rhabdoviridae.

Transmission

The rabies virus is readily transmitted between mammals, whether they are the same or different species. This virus is usually spread in the saliva, when an infected animal bites another. Less often, an animal or person is infected by contact with infectious saliva or neurological tissues, through mucous membranes or breaks in the skin. The rabies virus is not transmitted through intact skin.

Infections in Humans

Incubation Period

In humans, the incubation period is a few days to several years. Most cases become apparent after 1 to 3 months. In one study, approximately 4-10% of cases had an incubation period of 6 months or more.

Clinical Signs

The early symptoms may include nonspecific prodromal signs such as malaise, fever or headache, as well as discomfort, pain, pruritus or sensory alterations at the site of virus entry. After several days, anxiety, confusion and agitation may appear, and progress to insomnia, abnormal behavior, hypersensitivity to light and sound, delirium, hallucinations, slight or partial paralysis, hypersalivation, difficulty swallowing, pharyngeal spasms upon exposure to liquids, and convulsions. Either an encephalitic (furious) form with hyper excitability, autonomic dysfunction and hydrophobia, or a paralytic (dumb) form characterized by generalized paralysis, may predominate. Death usually occurs within 2 to 10 days; survival is extremely rare.

Infections in Animals

The initial clinical signs are often nonspecific and may include apprehension, restlessness, anorexia or an increased appetite, vomiting, a slight fever, dilation of the pupils, hyperreactivity to stimuli and excessive salivation. The first sign of post-vaccinal rabies is usually lameness in the vaccinated leg. Animals often have behavior and temperament changes, and may either become unusually aggressive or uncharacteristically affectionate.
The furious form is associated with infection of the limbic system. It is characterized by restlessness, wandering, howling, polypnea, drooling and attacks on animals, people or inanimate objects. Animals with this form often swallow foreign objects such as sticks, stones, straw or feces. Wild animals often lose their fear of humans, and may attack humans or animal species they would normally fear.

Treatment

There is no treatment once the clinical signs appear