CLINICAL SIGNS
It is difficult to observe the progression of rabies in naturally infected free- living wild animals. There are several anecdotal descriptions of rabid animals that were observed while clinically active, or interacting with people in the countryside.
They are probably biased towards the spectacular, with few observations of mute rabies. Most descriptions are usefully oriented towards clinical suspicion for surveillance purposes: alteration of normal behaviour (encephalitis), lack of muscular co-ordination or paralysis (myelitis) and swallowing difficulties due to pharyngeal paralysis are the most common findings reported in animal species. Clinical signs differ based on the route of administration; IM inoculation is deemed similar to the natural route (bite) for the administration of a lethal dose of virus.The Rabid Fox (and Rabies in Other Carnivores)
Canine rabies is classically depicted as following three successive stages: prodromal, excitative and paralytic, the last of these being pre-terminal(5). These stages can generally be transposed to describe the disease in other carnivores.
After a short incubation period (from 1 day up to 2 weeks, depending of the challenge dose and anatomical localization of virus inoculation, but believed to be about 4 weeks in the wild) a rabid fox is described as restless and pacing; it loses its appetite and has a typical phonation/ bark similar to that heard during the mating season; clonic and tonic convulsions are elicited by stimuli, but they rarely turn into the furious form; the third eyelid is frequently prolapsed. At the next stage of the disease progression, ataxia, paralysis, loss of locomotor function, and then coma, is followed by death.
I n the natural environment, aggressive behaviour or spontaneous attacks are reported in about 10% of cases, leading to diagnostic submission.
Rabid foxes are described as furious, inflicting injuries to themselves, or to animals and rarely humans. More frequently reported are constant prostration and complete apathy, with wild animals found in buildings, or at the edge of a street or path.The Rabid Bat
Inoculated bats have altered reflexes, loss of appetite, tremors and paralysis. Aggressive behaviour is rarely observed in bats infected by the peripheral route. Furious behaviour occurs with IC inoculation of virus. Bats found alive and subsequently diagnosed as affected by EBLV are immobile, seen in daylight, occasionally squeaking, or instead lying on the ground with ataxia resulting in ineffective wing flapping. Such abnormal behaviour, if investigated, can result in a bite or scratch, exposing a human to infection. Injury by a predator, or intoxication, can lead to a similar behaviour and are differential diagnoses.
Rabies in Spillover and Other Hosts
Fox rabies has been frequently reported in wild cats and mustelids, with clinical signs similar to those described above. Rabies in roe deer is not a rare event, but usually animals are found dead, with clinical disease rarely being observed.
Laboratory mice are a common rabies animal model, with experimental rabies signs mostly involving ataxia, tremor and paresis(5). However, rabies is not sustained in rodent populations, with the notable exception of sciurid species (such as marmots in America). Naturally acquired disease is reported as either lethargic (coma and death), or a furious (aggressive) stage preceding this.
The Dumb Form of Rabies
Rabies infection can frequently lead to death with no visible signs noted in animals that are not under constant human supervision. Silent excretion of virus has been described in bats and dogs(5); so far these events are of a limited occurrence and do not impact on public health regulation.
For the rabies disease in wild animals, no sign is pathognomonic or species-specific, and it can be said that ‘atypical is typical’1-11). In general, after a variable incubation period, a non-specific prodromal stage succeeds, which can only be observed in captivity; clinical signs of a progressive meningoencephalitis then will appear, characterized by abnormal behaviour (activity rhythm: diurnal movements for nocturnal species, loss of the usual fear of humans) plus disorders of locomotion (paresis and paralysis) and sometimes, excessive salivation. Death occurs after a few days. The severity of the signs appears to be related to the site of primary CNS lesions, as well as the viral strain, the dose and route of inoculation.