Page 1153 - Small Animal Internal Medicine, 6th Edition

CHAPTER 64 Encephalitis, Myelitis, and Meningitis 1125

with neurologic abnormalities that include progressive tet- RABIES
raparesis or vestibular dysfunction in the absence of systemic Rabies virus infection in dogs and cats is almost always the
VetBooks.ir signs. result of a bite from an infected animal with rabies virus in
its saliva. Most dogs and cats are infected through contact
CDV is diagnosed on the basis of history, physical exami-
nation, and laboratory findings. In most young dogs a history
Although the prevalence of wildlife rabies has been increas-
of mild to severe gastrointestinal and respiratory illness pre- with wildlife vectors (e.g., skunks, raccoons, foxes, bats).
cedes the onset of neurologic signs. Results of a CBC may be ing, cases of rabies in pet dogs and cats have been decreasing
normal or may reveal a persistent lymphopenia; distemper thanks to routine vaccination protocols. The incubation
inclusions can sometimes be found in the circulating lym- period from the time of the bite to the onset of clinical signs
phocytes and erythrocytes. Optic neuritis, chorioretinitis, is extremely variable (1 week to 8 months), with average
and retinal detachment are occasionally seen. incubation 3 to 8 weeks. Once neurologic signs develop, the
Early in an infection (first 3 weeks), immunofluorescent disease is rapidly progressive, with death occurring within 7
or immunohistochemical techniques using anti-CDV anti- days in most animals.
bodies may reveal CDV in cytologic smears prepared from Rabies can have a wide range of clinical signs, which
conjunctival, tonsilar, or nasal epithelium. Virus may be makes it difficult to differentiate from other acute progres-
detected past these initial stages in epithelial cells and mac- sive encephalomyelitis syndromes. Because of its public
rophages obtained from the lower respiratory tract by tra- health significance, rabies should be on the list of differential
cheal wash or in histologic samples of the skin, footpads, and diagnoses considered in every animal with rapidly progress-
CNS; thus immunohistochemical techniques can be applied ing neurologic dysfunction, and precautions should be taken
to biopsy or necropsy specimens for diagnosis. Biopsy of the to minimize human exposure. Rabies infection has classi-
haired skin of the dorsal neck can be used for antemortem cally been divided into two major types: furious and para-
immunohistochemical testing to confirm acute and subacute lytic. Dogs and cats typically undergo an early prodromal
infection with CDV. Reverse transcriptase–polymerase chain phase lasting 2 to 3 days during which they may be appre-
reaction (RT-PCR) can also be used as a sensitive and spe- hensive or nervous and may lick or chew at the site of inocu-
cific test to detect CDV RNA in whole blood, buffy coat lation. This can be followed by a furious or psychotic phase
preparations, CSF, and tissues of affected dogs. (1-7 days) in which animals are increasingly irritable and
MRI findings include poorly demarcated focal or multifo- excitable, often snapping at imaginary objects and biting at
cal hyperintense lesions on T2W in the forebrain, brainstem, their cage or surroundings. They become uncoordinated and
cerebellum, or spinal cord. Distemper meningoencephalitis may exhibit generalized seizures progressing to death.
characteristically causes an increase in protein concentration Animals that develop the paralytic or dumb type of rabies
and a mild lymphocytic pleocytosis in the CSF; occasionally develop generalized LMN paralysis progressing from the site
the CSF is normal or more indicative of an inflammatory of inoculation to involve the entire CNS within a few (range,
process (increased neutrophils). Increased protein concen- 1-10) days. Cranial nerve paralysis may be the first sign seen
tration in the CSF has been identified primarily as anti-CDV (especially if the bite was on the face). Difficulty swallowing,
antibody. Measured CDV antibody titer in the CSF may be excessive drooling, hoarse vocalization, diminished facial
increased relative to the serum titer. sensation, and dropped jaw occur, usually progressing to
Treatment of acute CDV meningoencephalomyelitis is respiratory paralysis and death.
supportive, nonspecific, and frequently unrewarding. Pro- Any inadequately vaccinated animal with an acute, rapidly
gressive neurologic dysfunction usually necessitates eutha- progressive course of neurologic disease should be suspected
nasia. Anticonvulsant therapy has been recommended to of having rabies. Ancillary testing should be performed with
control seizures. Antiinflammatory doses of glucocorticoids caution, minimizing exposure of personnel. CSF analysis
(predisone 0.5 mg/kg q12h PO for 10 days, then taper) may reveals increased mononuclear cells and protein concentra-
be used to control other neurologic signs in the absence of tion, as might be expected with any viral encephalomyelitis.
systemic disease, but their beneficial effects are not well Rabies antibody may be increased in CSF compared with
documented. serum. Biopsies obtained from the dorsal skin at the nape of
Prevention of CDV infection through routine vaccination the neck or the maxillary sensory vibrissae may be positive
is usually very effective. CDV can, however, develop with for rabies virus antigen, but although positive results are reli-
exposure after stress, illness, or immunosuppression, even in able, negative results are not. Definitive diagnosis of rabies
a currently vaccinated dog. Meningoencephalitis that was encephalitis is through postmortem demonstration of rabies
presumed to be vaccine-induced distemper was reported in virus antigen by immunohistochemical techniques in the
a few immunosuppressed puppies 7 to 14 days after vaccina- brain tissue (thalamus, pons, medulla) of an infected animal.
tion with modified-live virus–canine distemper vaccines Because of the risk associated with inadvertent human expo-
(MLV-CDV). Although this was likely a historical problem sure, it is recommended that all inadequately vaccinated
with particular batches of vaccines produced using old tech- animals euthanized or dying with progressive neurologic
nology, vaccination of immunosuppressed neonates, partic- dysfunction of unknown origin undergo postmortem evalu-
ularly those with a known or suspected parvoviral infection, ation, and persons performing these examinations should
should be avoided. be advised to take precautions to prevent rabies exposure.