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 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.
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.
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.
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).
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.
‘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.