Clinical Significance Of Interferons And Laboratory Relevance In Establishing Diagnosis In Viral Infections

Interferons are proteid substances produced by the host cells in the early phase of infection. Other stimuli also may stimulate the production of interferons and hence this response is not strictly specific. Interferon protects the cell against the virus by interfering with the translation of viral messenger and thereby inhibiting viral multip;ication. The interferons are not viral specific, but they show species specificity. Only interferons produced in human or primate cells have protective effect against human infections. Viruses show variation in their susceptibility to interferons. Interferons exert their protective role even before humoral antibodies develop.

Laboratory Methods To Establish The Diagnosis Of Viral Infections

Isolation of the organism: Viruses can be isolated by inoculating material removed from the subject at the optimum time, into suitable cell systems like tissue culture media, chick embryos or experimental animals. Specimens include throat swabs, sputum, rectal swabs, blood, urine, CSF or vesicle fluid. Since the virus is present in the host in the early part of illness, specimens have to be collected early to isolated the organism. They have to be transported in sterile screw-capped bottles containing Hank’s balanced salt solution. Such specimens can be stored at -70⁰C without affecting the viability of the virus.

Demonstration of viral inclusions, viruses and viral antigens: Earlier, in this century (21^st), viral infection was diagnosed by the demonstration of inclusion bodies under light microscopy, eg, rabies and smallpox. At present, the viruses can be identified by electron microscopy, eg, Rota virus in feces.

The presence of viruses or viral antigens in tissues can be demonstrated by immunofluorescence, immunodiffusion, immune-electron microscopy, radio-immunoassay, countercurrent immunoelectrophoresis and enzyme0linked immunosorbent assay (ELISA) techniques. These methods are being employed increasingly when viral isolation is difficult or impossible.

Antibodies against viruses: These can be demonstrated in the serum of the patient. Paired samples of sera are collected, first sample after an interval of 12 to 14 days. Humoral antibodies can be demonstrated by techniques such as virus neutralization, complement fixation, hemagglutination inhibition, indirect hemagglutination, immunofluoresence, radioimmunoassasy and ELISA.

The diagnosis of a viral infection can be confirmed by isolating the virus and demonstrating a four-fold increase in the specific antibody titers in paired samples of serum.

At present, protection against many viral infections is possible by specific immunization. Live vaccines or inactivated vaccines have been used for this purpose. Live vaccines are used in the cases of small pox, poliomyelitis (Sabin’s oral vaccine), yellow fever, measles, influenza, mumps and rubella. Killed virus vaccines are used for prevention of rabies (Semple’s vaccine and duck egg vaccine), poliomyelitis (Salk vaccine) and Japanese encephalitis.

Passive immunization against diseases such as measles, mumps and infective hepatitis can be conferred by the used of human gammaglobulin or convalescent sera, early during the incubation period. Hyper-immune serum, which gives better protection, is used for the prevention of rabies.

Chemoprophylaxis is available against a few viruses, eg, Oral N-methylisatin-thiosemicabazone (methisazone) against smallpox and amantadine hydrochloride for influenza.