RABIES VACCINE
Disease description:
Rabies is an acute viral encephalomyelitis caused by lyssaviruses in the Rhabdovirus family. It is an enveloped, bullet shaped, RNA virus. Although all warm blooded animals are susceptible to infection, it principally affects carnivores and bats. Rabies is maintained in wildlife and thus is found throughout the world. A few countries claim to be rabies-free due either to successful elimination programs and/or due to their island status and enforcement of rigorous quarantine standards. Rabies is invariably fatal once clinical signs appear.
PATHOGENESIS
Transmission is almost always by introduction of virus-laden saliva into the tissues, usually by the bite of a rabid animal. Although much less likely, it is possible for virus from saliva, salivary glands, spinal fluid, or brain to cause infection by entering the body through other fresh wounds or through intact mucous membranes (e.g., being cut with an instrument used in surgery or necropsy of a rabid animal). Transplacental transmission has been recorded and human rabies cases have been reported from corneal transplants. Exposure to blood, urine or feces is not considered a rabies risk.
The incubation period is relatively prolonged from 3 weeks to over 6 months depending on the site of infection, the amount of virus deposited and the species involved. The unusual length of the incubation period helps to explain how postexposure treatment, including in humans the practice of locally infiltrating hyperimmune serum, is effective. Most cases in dogs develop within 21-80 days after exposure, but the incubation period may be shorter or considerably longer.
The virus travels along the peripheral nerves to the spinal cord and the brain. It replicates in the central nervous system (CNS), then migrates via peripheral nerves to the salivary glands. New viral particles are shed in the saliva. If an animal is capable of transmitting rabies via its saliva, virus will be detectable in the brain. Hematogenous spread does not occur but aerosol transmission has occurred under very specialized conditions such as in laboratories with less than ideal containment situations. Aerosol transmission has also been documented in 1 bat cave.
Usually, saliva is infectious at the time that clinical signs occur, but it is possible for dogs and cats to shed virus for several days before onset of clinical signs. Viral shedding in skunks has been reported for up to 8 days prior to onset of signs.
CLINICAL SIGNS
The most reliable signs, regardless of species, are acute behavioral changes and unexplained progressive paralysis. Commonly, rabid wild animals may lose their fear of humans; species that are normally nocturnal may be seen wandering about during the daytime.
Three phases have been described with irregular lengths and variability of signs. Unfortunately the overlap in both signs and duration limit the practical value of classification.
DIAGNOSIS
Pre-mortem clinical diagnosis is difficult; a suspected diagnosis is based on history and clinical signs.
Saliva testing has been suggested as a possible means of detecting rabies pre-mortem. Currently, there are no commercial labs offering this test.
Spinal fluid analysis results are compatible with encephalitis but because the virus could be in the spinal fuid, removal of fluid is not usually recommended. Suspected cases should be euthanized followed by head removal and shipment to laboratory that can perform rabies testing on brain tissue.
Immunofluorescence microscopy on fresh brain tissue (medulla oblongata and cerebellum) allows direct visual observation of a specific antigen-antibody reaction that results in the diagnosis. Immunofluorescence becomes positive before any clinical signs are evident. Round or oval cytoplasmic inclusion bodies form in affected neuronal cell and are known as Negri bodies. Brain tissues should be preserved by refrigeration with wet ice or cold packs. The virus remains viable without refrigeration in tissue for less than 24 hours, longer under refrigeration.
Treatment/Management/Prevention:
1) There is no known effective treatment; death is the expected outcome.
Preventive Measures:
Clients should not be allowed to participate in procedures where they could be bitten, e.g., holding a pet while it receives an injection. Animal handlers should be taught proper restraint technique and appropriate equipment to reduce their risk of being bitten or scratched.
RABIES VACCINATION
1)The AAHA or AAFP Vaccine Guidelines on rabies vaccination follow the recommendations put forth by the manufacturers and the Association of State Public Health Veterinarians. Administration of rabies vaccine is ultimately dictated by state or local statutes. 6
2) A rabies vaccine is usually administered at 12-16 weeks of age and again one year later; then at 3 year intervals. There is not much difference between the one year and three year rabies vaccines; what differences do exist are proprietary but may only involve higher concentrations of adjuvant in some 3-year vaccines.6
3) Whenever feasible, avoid the use of killed (inactivated) vaccines in cats. All of the major vaccine manufacturers do offer/sell MLV feline vaccines for each of the CORE vaccines except rabies. The only non-Adjuvanted Rabies vaccine on the market today (dogs/cats) is the recombinant feline rabies virus (rRabies) vaccine. This product is not licensed for use in dogs. 6
4) Infection site sarcomas are far less frequently seen in dogs than in cats. Despite the rarity, it is probably a good idea to record the site of the vaccine. 7
Special considerations:
Veterinarians should include rabies in the differential diagnosis for any dog or cat with clinical signs compatible with rabies regardless of vaccination history. Any case, including vaccinated ones, 1 with any remote suspicion of rabies should be handled with extreme caution including proper confinement and observation of the suspected animal. Contact the state veterinarian and local public health officials in suspected cases.
Rabies can be spread to humans by inoculation of skin wounds or mucus membranes and, under some circumstances, by respiratory or oral spread. Appropriate safety precautions must be taken when handling potentially rabid animals or material from possible cases of rabies. Cases of human exposure must be treated immediately.
As of August 3rd, 2011, there have been 11 rabies cases in Harford County; 1 bat, 2 foxes and 8 racoons.
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