CHAPTER 46 THE RHABDOVIRUS, REOVIRUS, AND OTHER RNA VIRUS FAMILIES THE RHABDOVIRUS FAMILY The only med ica lly imp ort ant spe cies of rha bdo vir us in the uni ted states is the rebiesvirus. Rabies is rare among humans in the united states, there are less than 2 confirmed cases per year since 1980. But animal rabies is still quite prevalent. About 5000 cases were reported in 1987 in the united states. Since rabies is transmitted by animal bites and is uniformly fatal and since post exposure immunoprophylaxis is quite effective, the possibility of expos ure to the rabie svi rus must be consid ered by the attending phys icia n in every instance of animal bite. Because human rabies is extremely rare in the united states, the diagnosis is difficult especially in the absense of an exposure history. It is important to consider rabies the differential diagnosis of acute encephalitis FAMILY CHARACTERISTICS Morphology Rhabdoviruses are enveloped and have helical capsid symmetry. These viruses have a bullet-shaped morphology that is unique for this family. Genomic Chemistry The genome ois a single-stranded negative-sense RNA. Replication Strategy This virus is presumed to be similar to all other enveloped virions with negative-sense RNA genome. The syn thetic pha se is ini tiat ed by a vir ion-ass oci ated pol yme ras e tha t transcribes the negative-sense into positive-sense RNA(mRNA) Species Ther e ar e numerous isol at es of rhabdovi ruses, ma ny of which have not been sufficiently characterized for classification. Only one species the rabiesvirus is important medically in the united states. There is only one immunotype of rabiesvirus (i e vaccine prepared from a standard strain induces immunity against all others). By antigenic analysis
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THE RHABDOVIRUS, REOVIRUS, AND OTHER RNA VIRUS FAMILIES
THE RHABDOVIRUS FAMILY
The only medically important species of rhabdovirus in the united states is the
rebiesvirus. Rabies is rare among humans in the united states, there are less than 2 confirmed
cases per year since 1980. But animal rabies is still quite prevalent. About 5000 cases were
reported in 1987 in the united states. Since rabies is transmitted by animal bites and is
uniformly fatal and since post exposure immunoprophylaxis is quite effective, the possibility
of exposure to the rabiesvirus must be considered by the attending physician in every
instance of animal bite. Because human rabies is extremely rare in the united states, the
diagnosis is difficult especially in the absense of an exposure history. It is important to
consider rabies the differential diagnosis of acute encephalitis
FAMILY CHARACTERISTICS
Morphology
Rhabdoviruses are enveloped and have helical capsid symmetry. These viruses have a
bullet-shaped morphology that is unique for this family.
Genomic Chemistry
The genome ois a single-stranded negative-sense RNA.
Replication Strategy
This virus is presumed to be similar to all other enveloped virions with negative-sense
RNA genome. The synthetic phase is initiated by a virion-associated polymerase that
transcribes the negative-sense into positive-sense RNA(mRNA)
Species
There are numerous isolates of rhabdoviruses, many of which have not been
sufficiently characterized for classification. Only one species the rabiesvirus is important
medically in the united states. There is only one immunotype of rabiesvirus (i e vaccine prepared from a standard strain induces immunity against all others). By antigenic analysis
Specific antiviral treatment for rabies is not available.
Epidemiology and prevention
An epidemiologically important source of rabiesvirus is the saliva of infected animals.
Distribution of 4729 repoted cases of rabies among animal species in the united states in 1987
is shown in figure 46.2. key points to note are (1) skunks, raccoons, foxes and bats accounted
for 87% (2) dogs and cats only 7% and (3) none among small agile mammals such as mice,
chipmunks, squirrels and rabbits. In many developing countries most of the animal rabies are
among dogs. For example in 1987 there were about 15.000 reported cases of dog rabies in
mexico a number about 30 times higher than in the united states.
Prevention of rabies in humans is achieved by (1) mandatory immunization of dogs
and cats (2) preexposure immunization of persons at high risk of animal bites (e.g
veterinarians,park rangers,animal handlers, etc ) and (3) postexposure immunoprophylaxis of
persons exposed to rabies. Postexposure immunoprophylaxis consists of the local infiltration
and parenteral administration of human rabies immunoglobulins and a course of rabies
vaccine. Rabies vaccine for human use contains inactivated rabiesvirus. The alum-adsorbed
type induces acceptable level of rabies neutralizing antibody in over 99% of the vaccinated
persons.
THE REOVIRUS FAMILY
Viruses of this family are unique in that their genomes consist of double strandedRNA. There are many species in this family, but only the rotaviruses produce significant
infected monkeys acquired the infection 7 died. There were also 6 secondary cases but none
of the animal caretakers became infected. In the importation of infected monkeys from the
philippines to the united states, many of the imported monkeys died during quarantine. The
ebolavirus was detected in many of the dying animals. No human was infected in this
importation. It appears that these filoviruses are of importance to workers in facilities that
handle imported monkeys.
The filoviruses are negative-sense single stranded RNA viruses. These viruses are
enveloped and have a unique bacilliform morphology.
ARBOVIRUSES
Arboviruses is a term frequently used in medical literature. This term is derived from
arthropod-borne viruses. Arboviruses include all viruses that are transmitted between
vertebrate hosts by the bite of arthropod vectors. To be a “ successful” arbovirus, the virus
must have a veremic phase of high intensity in the vertebrated host and the capability of
multiplying in the athropod vector, this grouping of viruses by the mode of transmission is an
epidemiologic classification. Arboviruses include species in five families of RNA viruses
(togavirus,flavivirus, rhabdovirus,reovirus and bunyavirus). The more important arboviruses
in the united states cause acute encephalitis in humans. Theses viruses are St. Louis , LaCrosse, Western and Eastern equine encephalitis viruses, belong to the togavirus , flavivirus
and bunyavirus families and are controlled by mosquito abatement program.
SUMMARY
1. Rhabdoviruses have a unique morphology ( bullet shaped )