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genic than the DEV, although nonfatal allergic reactions do occur with a frequency of 1:625 vaccinees (Shope, 1984). In addition, a type of serum sickness may occur after administration of a booster dose following a completed primary immunization series (American Public Health Association, 1985).

Limitations on the use of the HDCV include the expensive production technology, difficulties with large-scale production, and the comparatively low yield of the method (Barth et al., 1984; World Health Organization, 1984). HDCV production may be too expensive for transfer to developing countries at this time.

Modified live virus (MLV) vaccines for immunization of animals are in use worldwide. These vaccines give about 3 years of immunity per series of injections. Adverse reactions include both immunological and neurological disease. Rabies itself can be produced by these live virus vaccines in animals immunosuppressed by steroids or by hematologic malignancies, such as feline leukemia. MLV vaccines recently have been field tested successfully in Europe as oral (enteric) vaccines for wild animals (World Health Organization, 1984). Previous attempts to use inactivated vaccines in this way were not successful.

There is a continuing need for a safe, easily produced, inexpensive vaccine for rabies in the developing world.

PATHOGEN DESCRIPTION

Rabies virus is a bullet-shaped particle 175 to 180 nm in length and 60 to 75 nm in width. Its capsid consists of five proteins designated G, N, M1, M2, and L. These include a glycoprotein (G), two matrix proteins, and a nucleoprotein (N). They are arranged helically and are enclosed in a lipid envelope through which the glycoprotein molecules extend 6 to 8 nm. In the center of the particle is a negative-sense RNA virion constituting the genetic material of the virus. A virion-associated RNA transcriptase is required to produce an active mRNA molecule from which protein translation can occur.

Attenuation of rabies virus has been shown to be related to replacement of arginine in position 333 by either isoleucine or glutamine in the viral glycoprotein (World Health Organization, 1984). Virus adapted to laboratory conditions is characterized by a fixed and shortened incubation period and by a tendency for viral particles to bud from the plasma membranes rather than from intracytoplasmic membranes as is typical of wild virus (Shope, 1984).

The nucleocapsid protein, N, is the antigen detected in immunofluorescence and complement fixation tests. The glycoprotein, G, induces neutralizing antibody (Shope, 1984). Four serotypes of the rabies group of Rhabdoviridae are recognized, and typing can be done using monoclonal antibodies. Differences among serotypes are apparently small enough that a vaccine made with a single type can protect against all types.

Rabies virus is able to maintain itself in an enzootic condition in many mammalian species, including dogs, foxes, raccoons, and bats. In part, this broad susceptibility results from an adaptation of the virus



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