Rabies and Pseudorabies

Chapter 15 Rabies and Pseudorabies



Robert G. Sherding



RABIES



Etiology



• Rabies virus is a rhabdovirus (genus Lyssavirus) that can cause fatal infection in all warm-blooded mammals. The virus primarily attacks the nervous system and salivary glands, and it is shed in saliva.

• Rabies has public health importance as a zoonotic infection that causes highly fatal encephalitis in humans. The incidence of human rabies in the United States is very low. Several countries are considered rabies free, including Great Britain, Australia (except for a bat variant), New Zealand, and the Scandinavian countries. Hawaii is also free of rabies.

• Rabies is transmitted in saliva from the bite of an infected animal. For both humans and domestic animals, the usual source is the bite of a rabid domestic animal or wild animal, especially reservoir species such as skunks, raccoons, bats, foxes, or coyotes. Some of these wild animals can shed rabies virus for prolonged periods in their saliva without evidence of clinical signs.

• Rabies virus is very labile outside the host, and it is readily inactivated by many disinfectants.



Key Point


Rabies is rare in dogs and cats in the United States; however, cats are more susceptible, and the incidence in cats is higher than in dogs. Wild animals are the principal reservoirs of infection.



Pathogenesis


Rabies virus is transmitted in saliva into a deep bite wound, where it enters the peripheral nervous tissue and spreads retrograde (centripetally) along peripheral nerves to the spinal cord and brain. Centrifugal spread then occurs along peripheral nerves from the central nervous system (CNS) to other tissues, such as the salivary glands. The incubation period before CNS signs occur is extremely variable, but it is usually 2 to 8 weeks. Incubation periods of up to 24 weeks have been reported in dogs and cats. Virus shedding in saliva begins a short time (less than 10 days) before neurologic signs appear and continues until the animal dies. The clinical course is 3 to 10 days.



Clinical Signs


The clinical progression of rabies, although variable, is divided classically into three phases: prodromal, furious, and paralytic. Death usually occurs within 3 to 10 days from the onset of signs; however, some animals develop atypical rabies and do not progress through these stages.



Prodromal Phase (2–3 Days)


This phase often passes unnoticed, but there may be subtle signs of erratic behavior, fever, slow corneal and palpebral reflexes, and chewing at the bite site due to pruritus.



Furious Phase (2–4 Days)


Initially, the forebrain region of the CNS is invaded, resulting in signs of erratic and unusual behavior (i.e., “mad dog syndrome”) such as irritability, restlessness, barking, episodic aggression, vicious attacks on inanimate objects, pica, unexplained roaming, and abnormal sexual behavior. Ataxia, disorientation, seizures, and paralysis may develop.



Paralytic Phase (2–4 Days)


This form is most common. Progressive lower motor neuron paralysis develops, causing signs of ascending paresis or paralysis of the limbs (often affecting a bitten extremity first) or signs of cranial nerve paralysis, such as laryngeal paralysis (change in bark, dyspnea), pharyngeal paralysis (drooling, dysphagia), and masticatory paralysis (dropped jaw). These are followed by generalized paralysis, depression, coma, and death from respiratory paralysis.



Atypical Rabies


Some animals develop subclinical, chronic, or recovered infections rather than the typical furious and paralytic forms. This has been observed in dogs, cats, bats, and skunks. These animals may survive and shed virus for extended periods.



Diagnosis




Key Point


Rapid laboratory confirmation of animal rabies is essential so that exposed humans can receive proper prophylaxis as early as possible. Definitive diagnosis requires laboratory testing of postmortem tissues.


Antemortem tests for rabies are too unreliable to be recommended; thus, always evaluate postmortem tissue specimens for diagnosis, especially the brain. For laboratory analysis of brain and salivary tissue for the presence of rabies virus or antigen, submit the animal’s head chilled on wet ice in a leak-proof container, along with appropriate information and biohazard labeling. Specimens can be stored by refrigeration but not freezing, because thawing will ruin the specimen for subsequent virus detection.



Direct Fluorescent Antibody Test


This is the test of choice used by most laboratories for rapid, reliable confirmation of rabies antigen in tissues. Brain tissue is most often used for routine testing, but salivary glands can also be evaluated. The direct fluorescent antibody (DFA) procedure has also been used for antemortem detection of rabies antigen in skin biopsies; however, a high percentage of false-negative results limits the usefulness of the skin test.



Histopathology


This older, less sensitive test identifies intracytoplasmic neuronal inclusions called Negri bodies, which are found in 75% of rabid dogs but rarely in cats.



Mouse Inoculation Test


This is a backup confirmatory test in which DFA-positive brain suspensions are inoculated intracerebrally into mice; the mice then are sacrificed and their brains are examined by DFA testing 5 to 6 days postinoculation. The correlation between DFA and mouse inoculation tests is reportedly 99.9%.



Tissue Culture Inoculation Test


This test is similar to the mouse inoculation test except that cell cultures are inoculated and then examined by DFA testing 24 to 72 hours later.

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Aug 27, 2016 | Posted by in SMALL ANIMAL | Comments Off on Rabies and Pseudorabies

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