Rabid animals are the only source of virus. Virus is shed in the saliva some days beforethe onset of clinical signs and virus is transmitted through a bite or a scratch of the skinor mucous membranes (eyes, nose, mouth). The blood of rabid animals is not consideredinfectious. The average incubation period in cats is two months but may vary from 2weeks to several months or even years depending on the dose of virus transmitted andthe severity and site of the wound [Jackson 2002, Charlton et al, 1997].
The incubation period is variable because the virus moves along peripheral nerves withthe normal axoplasmic flow from the inoculation site to the central nervous system(CNS): hence the greater the distance from the CNS, the longer the incubation period;and the greater the density of innervation of the inoculated tissue the shorter thisduration [Greene and Rupprecht, 2006]. Very long incubation periods have beendescribed in some experimental cases [Murphy et al, 1980], and this must be taken intoaccount when evaluating wound history, especially in free-roaming cats exhibitingsudden behavioural change and/or signs of motor neuron dysfunction that may initiatethe clinical phase.
The virus replicates in striated muscle and in connective tissue at the site of inoculationand then enters the peripheral nerves through the neuromuscular junction [Murphy et al,1973]. Alternatively, it can infect directly the peripheral nerves, spreading to the centralnervous system via the axonal route. The virus can then travel to the salivary glands bythe retrograde axonal route. At this time, the animal becomes infectious, i.e. about 3days before the first clinical signs appear. By the time clinical signs appear, the virus iswidely disseminated throughout the organs. In most cases, death occurs within 5 days sothat a cat or a dog will be shedding the virus in saliva for about 8 days in total.
Most clinical signs are related to the virus-induced central and peripheral nervous systemdysfunction rather than neuronal death and abnormalities in neurotransmission havebeen described [Jackson 2002]. Rabies glycoprotein probably plays an important role inthe trans-synaptic spread of the virus between neurons and in the topographicdistribution of virus infections through the nervous system [Etessami et al, 2000].