|
Rhabdoviruses
GENERAL CONCEPTS:
- The Rhabdoviruses are uniquely bullet-shaped. They contain a negative stranded RNA genome and are very stable to drying.
- This group of viruses has a broad host range but there is only one that affects humans.
- The viruses are generally introduced through a bite wound.
DISTINCTIVE PROPERTIES:
- Rhabdoviruses possess a lipid envelope displaying a surface glycoprotein. The RNA-binding nucleocapsid protein surrounds the RNA genome. The virion itself contains an RNA-dependent RNA polymerase (RDRP).
- During replication, the virus first makes short positive strand RNAs (mRNAs) which are translated to produce proteins. Later, a full length positive RNA is transcribed and this is used to produce the full length negative strand RNA that is packaged into progeny.
PATHOGENESIS:
- Rhabdoviruses generally enter via a bite or a wound infected with saliva.
- Initially, the virus replicates at the site and then infects central nervous system (CNS) tissue. Following a variable (6 days up to 1 year, average 30-70 days) incubation period, the virus spreads rapidly via the nerves. CNS damage produces the symptoms of disease.
- Neurons accumulate ribonucleoprotein as intracytoplasmic inclusions (Negri bodies). Infection of the thalamus, hypothalamus or pons may occur.
- Symptoms include fever, excitation, dilation of the pupils, excessive lacrimation, salivation, anxiety, hydrophobia due to spasms of the throat muscles and, eventually, death.
HOST DEFENSES:
- Many factors come into play. Interferon, humoral factors and the cell-mediated response are all important.
- Only about half of those bitten by an infected animal actually acquire disease; however, once symptomatic, death is certain.
EPIDEMIOLOGY:
- Normally, rabies can be found in domestic and wild animals (dogs, cats, cattle, bats, foxes, skunks, raccoons). In the US, 8% of animals screened tested positive. Approximately 10,000 humans become infected per year worldwide.
DIAGNOSIS:
- Clinical: The patient's history along with symptoms of encephalitis are suggestive.
- Laboratory: Direct immunofluorescence is a highly specific and sensitive method. Also, confirmation in the animal (if available) is important.
CONTROL:
- Sanitary: Rigorous cleansing of a bite wound to reduce the number of viral particles can help to prevent disease. Use of the vaccine for all dogs (a modified live vaccine) and cats (a dead virus suspension) is important in preventing spread to humans.
- Immunological: Both active and passive vaccination may be used to prevent human disease. The active vaccines are inactivated virus grown in human diploid cell cultures (HDCV) while the passive vaccine uses immunoglobulin.
- Chemotherapeutic: None available.
|
|
