Viruses Health Article

Immunological response

When challenged by a viral infection, the human body responds with both anti-bodies and cell-mediated responses to counteract the virus. Antibodies, produced by B lymphocytes, are specific for surface proteins of the virus. When acting as a target for antibodies, such viral proteins are known as antigens. The binding of the antibodies to the viruses can inactive them or target them as foreign for destruction by other components of the immune system. Antibodies can also bind to viral proteins seen in the membrane of infected cells, directing their elimination by the immune system. Antibodies mediate the immunity to reinfection by the same virus. Unfortunately, many viruses have high rates of mutation that alter the surface antigens, rendering the host again susceptible to infection. This process is the reason that one cold does not make a person immune to all rhinoviruses, a virus with at least 95 different serotypes (a characteristic of a virus based on the antibodies that are produced against the surface antigens upon infection).

Non-specific cell-mediated responses are also important to the body's fight against viruses. The production of interferons and cytokines, in particular, is known to help control viral infections. However, the side effects of these molecules, including fever, malaise, fatigue and muscle pains, significantly contribute to the physical symptoms of viral infections.

Diagnosis

In general there are three methods of diagnosing viral disease in humans. Some viruses can be identified clinically, as the infection causes unmistakable outward signs. The blistery pox of the varicella-zoster or chicken pox virus is a good example of a clinically diagnosed viral disease. Viral diseases can also be diagnosed epidemiologically, through known exposure to certain viruses or virus-harboring hosts. However, many virus infections cannot be diagnosed definitively without diagnostic testing.

Diagnostic testing can involve direct detection, using electron microscopy, light microscopy of CPE seen in host cells, detection of viral antigen in patient samples, or detection of the viral genome using the polymerase chain reaction (PCR) test. Effective tests for some viral infections involve indirect detection, generally using cell culture systems to grow the virus in vitro (outside the organism). A final method of diagnosing viral illnesses is serological testing that involves the detection of antibodies against the virus antigen in samples taken at presentation and during convalescence. A serious drawback to traditional serological testing is the amount of time needed to obtain the results. New techniques are being developed, however, that may speed serological tests and make them more useful.

Treatment

Most viral diseases have no cure, so treatment involves easing symptoms and allowing the body's immune system to eliminate the virus. Viruses are not affected by antibiotics, which target bacteria. However, a handful of anti-viral drugs have been developed and many more are in the developmental and drug trial stage. In general, the development of anti-viral drugs has been hampered by the parasitic relationship between viruses and their hosts. It has been difficult to find pharmacological means to kill the virus without harming the host. The speed of viral infection has also been a problem, as viral numbers are so high by the time the infection has symptoms, the drugs have little effect.

Amantadine and rimantiadine are two drugs that have been used successfully against influenza A. These drugs appear to inhibit the absorption of the influenza virus into the epithelial cells of the respiratory tract and, accordingly, are administered prior to infection as a prophylaxis.

Herpes simplex and varicella-zoster infections can be treated with acyclovir, valacyclovir, and famciclovir. Cytomegalovirus infection can be treated with ganciclovir, foscarnet, and cidofovir. All of these drugs are converted into a chemical that interferes with the production of the viral genome. As a viral enzyme produces the genome for these viruses, the chemical does not interfere with the production of genetic material for the host cell.

A number of drugs that inhibit reverse transcriptase have been developed for treatment of HIV. The best known of these is Zidovudine (AZT). The other major target for antiviral HIV drugs is the viral protease, an enzyme that cleaves both viral structural proteins and enzymes apart after formation by the host cell. Because the virus is noninfective if these cleavages do not occur, drugs inhibiting the protease action are effective antivirals. As advances in this field happen quickly, the International AIDS Society/USA Panel provides periodic recommendations as to what drugs given in what combinations have proven to be most effective inn the treatment of AIDS.

Finally, genetically engineered interferon has been used with some success against hepatitis B and C and human papillovirus. However, the severe side effects of this protein, in particular nausea and vomiting, have hampered its usefulness.