Rhabdoviridae Viral Profiles:
Comparing Rabies and VSV

In the order Mononegavirales, Family Rhabdoviridae, there exist five genera, two of which infect humans: Lyssavirus and Vesiculovirus. Lyssavirus human pathogens include rabies virus and two rabies-like viruses, Mokola and Duvenhage, that can cause human infection. Vesicular Stomatitis is a disease caused by Vesiculovirus types 1-4 that is zoonotic in humans and causes subclinical, flu-like infections.

+ Incubation
Rhabdoviridae has a relatively long incubation period of 14-90 days, with symptoms usually beginning 4-8 weeks after inoculation, although it may be longer. For instance, in Australia there was a case in which the last opportunity for infection occurred 6 years earlier.

+ Epidemiology
Animal rabies is present in all continents except for Australia and Antarctica. It is a disease with no natural reservoir, transmitted via the saliva of infected mammals. The principal vector of the virus is the dog although wild mammals can also infect humans, either through direct bite or indirectly, via the infection of domestic animals. The primary mode of transmission is through the bite of an infected animal, albeit licking and scratching can also transmit the disease. In some cases, rabies can be transmitted via aerosol (such as in bat caves) or via corneal transplants from infected individuals.

In the U.S. bats are increasingly implicated as important wildlife reservoirs and vectors of rabies. Recent epidemiological data suggest that transmission of rabies can occur from minor or unrecognized bites from bats. Since 1980 there have been 21 bat-associated cases of human rabies out of a total of 36 cases in the continental U.S. (Hawaii remains rabies-free). Foxes, skunks, raccoons and coyotes are also implicated with rabies transmission. Fox rabies is enzootic in Western Europe and North America. It is evident that rabies among wildlife has become more prevalent since 1950, accounting for more than 85% of all reported cases of animal rabies in the U.S. every year since 1976. Each year, roughly 16-39 thousand people receive post-exposure prophylaxis. In Latin America, Africa, and Asia, dogs are the most common source of rabies transmission to humans. In Latin America, vampire bats are also a problem for humans and livestock. All in all, the World Health Organization attributed >36,000 deaths world-wide to rabies in 1992. In regards to VSV, it is a zoonotic disease transmissible from mammals to humans from the vesicular fluids and tissues of infected animals. In humans it resembles the flu, with the characteristic chills, fever, and muscle pain, but in 7-10 days it runs its course without further complications.

+ Symptomology and Outcome
Rabies virus is neurotropic. Once rabies virus enters a suitable host, it replicates in muscle and in cells of the subepitheleal tissues until it reaches a sufficiently high concentration to infect sensory and motor nerves in skin or muscle by binding to the acetylcholine receptors and entering the nerve endings. Movement of viruses within neuronal axons delivers virus to the Central Nervous System, usually via the spinal column. An ascending wave of neuronal infection and neuronal dysfunction later occurs. After a prodromal phase of fever, malaise, and parasthesia around the site of infection, the patient's muscles become hypertonic (stiff, rigid) and s/he becomes anxious, with hydrophobia, hyperactivity, convulsions, aggression, and eventually paralysis. Encephalitis is the most common pathological effect. There are two possible clinical consequences of rabies infection: "furious" or "dumb" rabies. When the virus reaches the limbic system it causes "furious rabies" due to the extensive proliferation of virus and the subsequent release of cortical control of behavior. Once the infection reaches the neocortex it causes "dumb rabies" as replication of virus here causes delirium, coma, and eventually respiratory arrest and death.

+ Pathology and Pathogenesis
Rabies Virus and Vesicular Stomatitis Virus share many common structural and functional characteristics, but they differ in their pathogenicity and speed. The Lyssavirus family (Rabies) usually causes a slow, progressive disease as compared with those of the Vesiculovirus family, which cause acute, self-limiting infections. Lyssaviruses invade the Central Nervous System producing encephalitis, while Vesiculoviruses tend to infect the epithelial cell in organs such as the tongue, causing vesicles (hence the name). Rabies can infect all mammals, while VSV has an even wider host range, with the ability to infect horses, cattle, swine, humans, and even insects. The VSV receptor recognized by neutralizing antibody is not a protein, but rather believed to be phosphatidyl serine (according to University of Rochester Medical Center web-site www.urmc.rochester.edu/smd/mbi/grad2/nns99A.html; according to Fields, Fundamental Virology 3rd Edition,the receptor is the G protein, as in rabies virus). The molecular basis for the pathogenicity of VSV has been studied extensively and seems to indicate that all vertebrate cells tested and to some degree, invertebrate cells, are susceptible to infection. Replication occurs in the cytoplasm, and assembly occurs through the budding off upon plasma (vesiculoviruses) or intracytoplasmic (lyssaviruses) membranes. Rabies virus produces conspicuous cytoplasmic inclusion bodies. VSV is often referred to as inducing rapid Cytopathic Effects (CPEs) in vitro, although it depends on the multiplicity of infection. At low doses of infection, VSV is not seen for 24hrs or longer, but at higher concentrations, cell rounding occurs in several hours. Within the first hours, VSV inhibits cellular macromolecular synthesis, resulting in the shutdown of cellular DNA, RNA, and protein production. In terms of genome, rabies is very similar to VSV, both containing an unsegmented negative strand of RNA with five to six genes in the order 3*-N-P-M-G-(X)-L 5*, but with the exception that the intergenic regions are longer and more divergent in rabies virus, and rabies also contains an untranslated pseudogene. This RNA is associated with nucleocapsid protein composing the Ribonucleoprotein core (RNP), the infectious component of VSV and rabies. In rabies, the major receptor-binding molecule and antigenic determinants are the G protein trimeric spikes on the surface which insert into cellular membranes and allows fusion to occur. G protein undergoes a conformational shift at pH<6.0 which stabilizes the trimer and exposes a hydrophobic domain that can insert into cellular mebrane. Fusion occurs in the endocytic vesicle, where there is acidic pH. Once the virus has adsorbed, penetrated, uncoated, transcribed, replicated, assembled, and budded off the cell, initiating a new round of infection by releasing more virus into nerves, do the clinical symptoms appear that characterize rabies (see next section). In VSV, only mild, flu-like symptoms are apparent.

+ Prevention and Management
In countries where canine rabies is enzootic, the principle way to control transmission of the disease to humans is via control of stray dogs and cats, with quarantines enforced if deemed necessary. Anti-rabies vaccination of dogs and cats, surveillance, and public education measures are all important to ensure a halt to the spread of disease. In nations such as the United States and Western Europe, where wild animals are the principal vector of rabies, control of wild-life rabies is more important, yet more difficult to achieve. Oral administration of vaccine to foxes has been successful in reducing the incidence of rabies in Western Europe.

Pre-exposure and post-exposure prophylaxis exists to treat actual or potential rabies inoculation. Rabies prophylaxis consists of local wound care, passive immunization, and active immunization. Pre-exposure prophylaxis should only be given to individuals at risk for contracting the virus such as laboratory personnel working with rhabdovirus, veterinarians, animal control and wildlife workers, and travelers to rabies-enzootic areas.

Types of Rabies Treatment available:
Two kinds of rabies treatment are currently available in the U.S.:
(1) Rabies vaccines (2) Rabies Immune Globulin (RIG)

Rabies vaccines induce an active immune response that includes the production of neutralizing antibodies. This humoral response usually requires 7-10 days to develop and usually persists for at least two years. RIG provides fast-acting, but passive immunity that persists for only a short time (half-life is estimated at 21 days).

Vaccines licensed in the U.S.:
Currently there are four formulations of three inactivated rabies vaccines licensed for preexposure and postexposure prophylaxis in the United States:

(1) Human Diploid Cell Vaccine (HDCV)- prepared from the Pitman-Moore strain of rabies virus grown on MRC-5 human diploid cell culture. It is supplied either through Intramuscular (IM) Administration or Intradermal (ID)Administration.

(2) Rabies Vaccine Adsorbed (RVA)- RVA is prepared from the Kissling strain of Challenge Virus Standard (CVS) rabies virus adapted to fetal rhesus lung diploid culture. RVA is adsorbed to aluminum phosphate and is approved for IM administration only.

(3) Purified Chick Embryo Cell Vaccine (PCEC)- PCEC became available in late 1997. It is prepared from the fixed rabies virus strain Flury LEP grown in primary cultures of chicken fibroblasts. It is only approved for IM administration.

The Fourth Formulation, Purified Duck Embryo Vaccine (PDEV) licensed as Lyssavac N, is available only outside the United States. All of the three aforementioned vaccines licensed in the Unites states are considered to be equally efficacious.

Rabies Immune Globulin (RIG) Licensed in the U.S.:
There are two RIG products, Imogam-Rabies HT and BayRabTM. RIG is an anti-rabies immunoglobulin preparation concentrated by cold ethanol fractionation from plasma of hyperimmunized human donors. This rabies neutralizing antibody is administered at 20 IU/kg body weight and is used only in conjunction with vaccine during post-exposure therapy.

Post-Exposure Rabies Prophylaxis:
The principle components of rabies prophylaxis are wound treatment and for previously unvaccinated individuals, the administration of the RIG and vaccine. Persons bitten by a rabid animal should be given post-exposure treatment immediately. In 1977, the World Health Organization recommended a regimen of RIG and six doses of HDCV over a three month period. Nowadays, one dose of RIG and five doses of HDCV over a 28-day period has been found to be safe and effective in inducing a humoral response. Trials with RVA and PCEC have been shown to be equally as effective.

Treatment of Wounds:
Bite wounds should be washed thoroughly with soap, water, and a virucidal agent. Thorough cleansing of the wound alone has been proven to reduce the likelihood of rabies. Concurrent tetanus prophylaxis and other bacterial control measures should also be implemented.

Post-exposure immunization should always include administration of both passive antibody (RIG) and vaccine, with the exception of people who have previously received complete vaccination regimens or those who have been vaccinated with other vaccines and have documented rabies antibody titers. These patients should only receive vaccine. RIG is administer only once at the beginning of the antirabies treatment to provide immediate antibodies until the patient responds to one of the three vaccines by producing his/her own antibodies (usually after the seventh day). It should be injected close to the site of infection.

At the beginning of therapy any of the three vaccines approved in the US (HDCV, RVA, or PCEC) can be administered along with RIG. A regimen of five 1-mL doses should be administered intramuscularly: the first immediately after infection occurs, and the remaining four doses on days 3, 7, 14, and 28. In adults, the vaccine should always be administered in the deltoid area intramuscularly, whereas in children the thigh is also acceptable.

White, David O. and Frank J. Fenner. Medical Virology. Academic Press: San Diego, CA 1994.
Fields, Bernard et al. Fundamental Virology. Lippincott-Raven: Philadelphia, PA 1996.
Center for Disease Control: http://www.cdc.gov.
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