Rabies update for practicing veterinarians
Apr 01, 2005
A recent report in The New England Journal of Medicine describes the epidemiology and prophylaxis for rabies. Fewer than three human deaths are reported each year in the United States due to rabies, but 15,000 to 40,000 people receive prophylaxis annually [C.E. Rupprecht, R.V. Gibbons: Prophylaxis against Rabies in The New England Journal of Medicine, 2004; 351:2626-35]. Veterinarians and physicians have a major role in recognizing and preventing this lethal zoonosis. In the report, the authors describe the current distribution of animal rabies cases in the United States.
Rabies is caused by a highly neurotropic RNA virus in the family of Rhabdoviridae, genus lyssavirus. Exposure occurs when a bite penetrates the skin or transdermal/mucosal contact occurs with infectious material such as brain or saliva. The incubation period may be as short as one week, but is usually between one to three months. Occasionally it exceeds one year. Once the virus successfully reaches the central nervous system by retrograde axoplasmic flow, a nonsuppurative polioencephalomyelitis with cranial ganglionitis develops. Replication occurs in neurons, internal organs and glands. Shedding may begin before detectable clinical signs develop [B.A. Summers, J.F. Cummings, A. deLahunta.: Veterinary Neuropathology, Mosby 1995].Prophylaxis should begin as soon as possible after exposure to a rabies suspect animal or material. This postexposure prophylaxis (if done correctly) is so highly effective that no failures have been reported in the United States since 1979. It consists of wound care, use of rabies immunoglobulin and vaccine administration. Immediate and vigorous washing of the site of the inoculation with soap solution significantly reduces the risk of contracting rabies. Human, or alternatively equine rabies immunoglobulin, can be used either locally (by infiltrating directly into the bite wound) or as a multi-site intradermal vaccination. These human rabies immunoglobulins are purified from the serum of vaccinated individuals. Finally, a serial vaccine administration should take place with inactivated preparations on days 0, 3, 7, 14 and 28 in patients who have not been previously vaccinated and on days 0 and 3 on patients who have been previously vaccinated.
Pre-exposure prophylaxis may be considered and/or offered to certain people who are at a higher risk of exposure such as veterinarians, their staff and animal-control officers. Booster vaccinations may be given if serologic tests (performed every six months to two years) do not show complete neutralization at a serum dilution of 1:5.
In North America, raccoons, skunks, bats and foxes are the primary reservoirs of rabies. Currently, the number of cats found infected with rabies is higher than the number of dogs, perhaps because cats are less frequently vaccinated. Table 1 shows the distribution and numbers of animal rabies cases between 1998 and 2002 in the United States.
Rabies vaccine induced poliomyelitis of the lumbosacral intumescence has been well documented in cats [A. DeLahunta: Neuroanatomy and clinical neurology, 2nd ed. Saunders 1983]. Initially, one can observe paresis in the limb where the vaccination was given with weak segmental spinal reflexes. The condition can then progress to paresis of both the pelvic limbs, tail, anus and the urinary bladder.
In summary, veterinarians and their staff always have to be alert for rabies. A differential diagnosis should include rabies in patients with an unknown vaccination history and neurologic signs.
Dr. Nanai is a resident of the European College of Veterinary Neurology/Neurosurgery at the Animal Emergency and Referral Center in Fort Pierce, Fla.
Dr. Lyman is a graduate of The Ohio State University College of Veterinary Medicine. He completed a formal internship at the Animal Medical Center in New York City. Lyman is a co-author of chapters in the 2000 editions of Kirk's Current Veterinary Therapy XIII and Quick Reference to Veterinary Medicine.