The family
Rhabdoviridae comprises more than 200 viruses which infect mammals, reptiles,
birds, fishes, insects and plants. It has two genera-Vesiculovirus,
which causes vesicular stomatitis in horses, cattle and pigs,rarely infects
humans & Lyssavirus, which comprises rabies virus.
Poxviruses
Poxviruses are the largest viruses
that infect vertebrates. This group include the human viruses – Variola (Small
Pox), Molluscum contagiosum, animal viruses, bird viruses and insect viruses.
Variola virus is the causative
agent of small pox. The vaccinia virus was used as the small pox vaccine.
Vaccinia virus is unique in that it is an artificial virus and does not occur
in nature as such. It is employed as a vector for the development of
recombinant vaccines. The genome can accomodate about 25,000 foreign bps, but
it is not suitable as a vector for human use due to its pathogenic effects.
Picornaviruses
The Picornaviridae family comprises
of a large number of very small RNA (pico:small, rna;RNA) viruses with a size
of 27-30 nm. They are nonenveloped viruses, resistant to ether and other lipid
solvents. Two groups of picornaviruses are of medical importance, the enteroviruses that parasitise the
enteric tract and the rhinoviruses that infect the nasal mucosa. Enteroviruses
include Polioviruses type 1, 2, 3, Coxsackie viruses A & B, Enterovirus
type 68-72.
Polioviruses causes poliomyelitis, a very ancient disease.
Myxoviridae
The
name Myxovirus came from their ability to adsorb onto mucoprotein receptors on
erythrocytes causing haemagglutination. It is classified into two families –
orthomyxoviridae consisting of the influenza virus and paramyxoviridae consisting
of the Newcastle disease virus, mumps virus, parainfluenza viruses, measles
virus and respiratory syncytial virus.
Human Immunodeficiency Virus – AIDS
The
emergence and pandemic spread of the AIDS- Acquired Immunodeficiency Syndrome-
have posed the greatest challenge to public health in modern times. It was
first recognized in the United
States in 1981, as a sudden outbreak of two
very rare diseases – Kaposi’s sarcoma and Pneumocystis carinii pneumonia
in young adults who were homosexuals or addicted to injected narcotics. They
appeared to have lost their immune competence, rendering them vulnerable to
fatal infections with relatively avirulent microorganisms, this condition was
given the name acquired immunodeficiency syndrome. Isolation of the
etiological agent was first reported in 1983 by Luc Montagnier and colleagues
from Pasteur Institute, Paris. They isolated a retrovirus from a West African
patient with persistent generalized lymphadenopathy and called it
lymphadenopathy associated virus (LAV). In 1984 Robert Gallo and colleagues
isolated a retrovirus called HTLV-3. two antigenic types of HIV– HIV-1 & HIV-2.
Virions
of the family Retroviridae possess reverse transcriptase enzyme, hence the
name. This family has been divided into 3 genera - Retrovirus (HTLV-1 &
HTLV-2, oncogenic viruses), Lentivirus (HIV-1 & HIV-2 causes AIDS) and
Spumavirus (Human foamy virus).
Herpes Viruses
The Herpesviridae family contains
over a 100 species of enveloped DNA viruses that affects humans and animals.
They are characterised by the ability to establish latent infections, enabling
the virus to persist indefinitely within infected hosts and to undergo periodic
activation.
The herpesvirus capsid is icosahedral, composed of 162 capsomeres,
and enclosing the core containing the linear dsDNA genome. The nucleocapsid is
surrounded by the lipid envelope derived from the host cell nuclear membrane.
The envelope carries surface spikes about 8nm long. Between the envelope and
capsid is an amorphous structure called the tegument, containing several
proteins and enzymes which aids in replication. The enveloped virion measures
about 200nm and the naked virion about 100nm in diameter. Herpesviruses
replicate in the host cell nucleus. Like other enveloped viruses, herpesviruses
are susceptible to fat solvents like alcohol, ether, chloroform and bile salts.
They are heat labile and have to be stored at -700C.
Hepatitis Viruses
‘Viral
hepatitis’ refers to a primary infection or inflammation of the liver by any
one of a heterogenous group of hepatitis
viruses which consists of types A, B, C, D, E and G. Hepatitis viruses are
taxonomically unrelated. All the human hepatitis viruses are RNA viruses except
for HBV, which is a DNA virus. The features common to them are their
hepatotropism and ability to cause a similar icteric illness, ranging in
severity.
The
most common viral hepatitis is hepatitis A, formerly called infectious
hepatitis, caused by hepatitis A virus (HAV), a single stranded RNA virus
usually transmitted by the faecal-oral route. Hepatitis B, formerly
called serum hepatitis, is caused by the hepatitis B virus(HBV), a
double stranded DNA virus usually transmitted via blood. Hepatitis C,
formerly called non-A non-B (NANB) hepatitis. Hepatitis E, (HEV)
transmitted by the faecal-route and formerly called non-A non-B non-C
hepatitis. An especially severe form of the disease hepatitis D or
delta hepatitis, is caused by the presence of both hepatitis D virus (HDV) and
HBV.
Subscribe to:
Posts (Atom)