Retroviruses
RNA tumor viruses have been known for many years. These viruses are widely distributed in nature and are associated with tumor production in a number of animal species, such as fowl, rodents and cats. The most familiar of viruses is the human immunodeficiency virus (HIV) which causes acquired immune deficiency syndrome (AIDS)
The single stranded RNA tumor viruses, which also include retroviruses (oncoviruses), are spherical in form, about 100nm in diameter and enveloped by host plasma membrane, although a few retroviruses are non specific that is they can infect any cell, most of them can infect only host cells that possess required receptors. In the case of AIDS virus, the host cell possesses a receptor that allows the viral adsorption and penetration in several types of leukocytes (white blood cells) and tissue cells. The retroviruses have a special enzyme called reverse transcriptase, which can convert a single stranded RNA genome into double stranded viral DNA. Not only this DNA can infect host cells, but it also can be incorporated into host genome as a provirus that can be passed on to progeny cells. In this way some of retroviruses can convert normal cells into cancer cells.
Acquired Immune Deficiency Syndrome (AIDS)
The AIDS was reported by some physicians in early 1980’s in young males having one or more of complex symptoms such as severe pneumonia, a rare vascular cancer, sudden weight loss, swollen lymph nodes and general loss of immune functions. All these young patients were homosexuals. Soon after the disease was discovered in nonhomosexual patients who were given blood (blood transfusion) or blood products. In 1984 the agent causing the disease was identified b research teams from Pasteur institute in France and national institute of health in USA. In 1986 the virus was named as human immunodeficiency virus (HIV) (Fig 5.8). the major cell infected by HIV is the helper T-lymphocyte, which is major component of immune system. As the HIV is the infection continues in the host, the decrease of helper T-lymphocytes results in failure of the immune system and the infected person becomes susceptible to other diseases. Cells in central nervous system can also be infected by HIV Fig 5.9 recent studies on HIV reveal that the virus infects and multiplies in monkey but does not cause disease in them, which means that HIV is host specific.
Fig. 5.8 Human immunodeficiency virus (HIV)
The HIV is transmitted by intimate sexual contact, contact with blood and breast feeding. Healthcare workers can also acquire HIV during professional activities. Avoiding the direct contact with HIV is important measure for preventing the disease. Prevention of intravenous drugs with common syringes and use of sterile needles/ syringes and utensils is important. Now vaccine against HIV has been synthesized and its experimental administration in humans started in early 2001 in south Africa.
Fig 5.9 Infection cycle of HIV
Hepatitis
Hepatitis is an inflammation of the liver. It is usually caused by viral infection, toxic agents or drugs. It is characterized by jaundice, abdominal pain, liver enlargement, fatigue and some times fever. It may be mild or can be acute and can lead to liver cancer. The different types of viral hepatitis are Hepatitis A (formerly called infectious hepatitis is transmitted by contact with faeces from infected individuals). Hepatitis B (serum hepatitis). Hepatitis C (formerly called non-A, non-B hepatitis) passes through blood, from mother to child during pregnancy and afterward and by sexual contact. Hepatitis D (delta hepatitis), Hepatitis E (a virus transmitted through the faeces of an infected person), Hepatitis F, G (caused by viruses yet unidentified). Viruses of hepatitis A, B and C are better known. Hepatitis A virus (HAV) is an RNA virus (non enveloped), which causes mild short term, less virulent disease. Hepatitis C virus (HCV) is also RNA virus (enveloped) causes infusion hepatitis, which is less severe than hepatitis A or hepatitis B, but hepatitis C often leads to chronic liver disease. Most recent work of Halbur and coworker (2001) reveals that pig could be the source of infection of hepatitis E.
Hepatitis B (HBV) is the second major form of hepatitis. It is caused by DNA Virus which is very common in Asia, china, Philippines, Africa and the Middle East. Hepatitis B is transmitted by the exchange of body fluids, for example blood serum, breast milk and saliva, from mother to child during birth of afterward and by sexual contact… during acute attacks of Hepatitis B fatigue, loss of appetite and jaundice are reported. Infected persons can recover completely and become immune to the virus. People with chronic hepatitis infection are at the risk of liver damage. Hepatitis can be controlled by adopting hygienic measures, with routine vaccination and screening of blood/ organ/ tissue of the donor.
Not: genetically engineered vaccine is available for HBV. Vaccine is also available for HAV but not for HCV.