Rabies
General information
Rabies is one of the oldest viral infections known to man. It is a dangerous zoonotic disease that occurs worldwide and can infect mammals, and in rare cases even birds.
The WHO estimates that tens of thousands of people, many of them children, still die of rabies every year, especially in African and Asian countries. The majority of documented cases occur in Asia, Central and South America, Africa, and Eastern Europe.
Depending on the epidemiological progression and the species of animal affected, the disease can be classified as follows:
- the urban variant (dogs, cats). Dogs are the main reservoir.
- the sylvatic variant (foxes, wild animals). This type is transmitted by wild carnivores.
A special type of rabies is that occurring in bats. This variant is transmitted by bloodsucking vampire bats and causes significant losses among grazing cattle, especially in Central and South America.
Rabies viruses are neurotropic (meaning they target the nerves) and bring about a disease of the central nervous system which is characterised by loss of consciousness, behavioural changes (uncontrolled excitement, hydrophobia (fear of water), aerophobia (fear of fresh air), photophobia (fear of light)), and progressive paralysis. Once clinical symptoms appear, the result is virtually always death. The pathogen that causes classic rabies is the rabies virus (RABV), for which mesocarnivores (dogs, cats, coyotes, raccoons, raccoon dogs, mongooses) are the natural reservoirs.
Contagion
Incubation period
The virus is primarily excreted via the saliva of infected animals. It can also be spread through urine, faeces, and exhaled air. Viral excretion can occur 1–13 days before symptoms arise.
The three phases of classic clinical progression:
- Typical symptoms develop within a few hours to several days
- Lasts 2–5 days
- Behavioural changes, sometimes irritability or aversion, increased salivation and difficulties swallowing, hydrophobia (excessive aversion to any liquids)
- Dogs are moody, fearful, restless, excessively friendly or dismissive, try to escape, bark or bite without cause, may develop pruritus (itching) at the site of the bite, and snap at imaginary flies
- A special symptom of rabies in wild animals is that they lose their natural fear of humans or become excessively aggressive
2. Excitative stage
- Characterised by aggression and an excessive urge to pace
- When the excitative stage is in the foreground and dominates the other stages, it is commonly referred to as “furious rabies”
- This stage lasts 1–4 days
Symptoms:
Increased restlessness and moodiness, anorexia (loss of appetite), biting objects to pieces, salivation, elongated barking, running around and escaping, stubborn wandering around, attacking other dogs, potential incoordination and epileptic episodes
3. Paralytic and depressed stage
- Lasts until death 3–4 days later
- Restlessness descends into exhaustion and signs of paralysis (e.g. laryngeal paralysis)
Special types of rabies
“Paralytic (or dumb) rabies”
- Paralysis occurs without prior excitation
- Lasts 2–4 days
- More typical in cattle, horses, and poultry
- vapid and stupefied expression, dog sits around impassively, salivation, paralysis of the lower jaw, hoarseness, inability to eat food, the dog appears as if it had swallowed a foreign object, prolapse of the third eyelid (cherry eye), unevenly sized pupils, dog is cross-eyed, paralysis of torso and extremities, and eventually death
- The main symptom of dumb rabies is signs of paralysis
Atypical rabies
- Chronic subclinical progression lasting up to three months or longer, possible expression of suboptimal antibody production
- Diarrhoea, enteroparesis (intestinal paralysis), hyperaesthesia (hypersensitivity), paresis (paralysis), motility dysfunction, depression, followed by temporary or lasting improvement
Pathogenesis
The severity of the symptoms depends on the infection dose and the site of the perforating bite.
Most cases are triggered by a traumatic injury, such as a bite by an infected animal. The virus now multiplies at the site of entry.
It then starts travelling through the infected dog’s body, as is typical in rabies. The virus first travels in a centripetal manner (from the periphery to the centre), and once it has reached the brain, it turns around and becomes centrifugal (from the centre to the periphery).
What happens in the body?
After multiplying at the site of infection for 24 hours, the virus traverses the muscular end plates and neurotendinal spindles to enter the nerve fibres. Once inside the nerves, the virus travels through the interaxonal fluid at a speed of max. 7 cm/day towards the spinal cord or brain (min. 18 days). Especially inside the brain, the virus multiplies exponentially and spreads from cell to cell, followed by a general centrifugal proliferation to all organs (especially the salivary glands and eyes) via the nerves.
The symptoms that occur are an expression of neuronal damage to the peripheral and motor neurons (paralyses) and the central limbic system (behavioural changes).
Diagnosis:
It is generally not possible to perform a laboratory diagnosis in a diseased animal. In case of doubt, the medical course of action is therefore to kill the animal. The dead animal can then be examined for the presence of rabies virus antigens in the brain using the immunofluorescence method.