Nature of viral diseases

Viral diseases are manifested in acute, sub-acute or chronic forms, as frank clinical cases or as latent infections, some of which are fatal. These diseases occur in epidemic or endemic forms.  Viruses spread either directly by contact or indirectly through vectors and fomites such as through attendants, infected clothes, insects, feed and water troughs or by droplet infection. In Marek's disease of poultry and infectious bovine rhinotracheitis and foot-and-mouth disease in bovines, there is true 'carrier' status where the animals harbor the virus even after recovery from the disease. They act as sources for the spread of the disease under favorable conditions. Activation of a latent viral infection occurs when the resistance of the host is decreased on account of physiological stress including pregnancy and parturition, prolonged treatment with immunosuppressive drugs or due to insidious diseases such as mycotoxicosis.

The epidemiology deals with the occurrence of virus in a population with reference to its distribution and the factors which determine the observed distribution. The epidemiological methods, including descriptive, analytical and experimental, provide data which when subjected to statistical interpretations form the basis for formulation of control strategies. Serological surveys employing modern techniques, viz. enzyme- linked immunosorbent assay (ELISA), radioimmunoassay, fluorescent antibody, western blotting etc. have proved useful in diagnosing virus diseases. Nucleic acid based techniques viz. nucleic acid hybridization, polymerase chain reaction and nucleotide sequence analysis have also been used for confirmatory diagnosis and for epidemiological interpretations.Virus infections, in general, are not influenced by chemotherapeutic agents or antibiotics, except for the diseases caused by large-sized viruses of Herpes and Pox group. The use of specific antisera has been in vogue for the treatment of certain virus diseases, viz. rinderpest, swine fever in animals and measles, rabies and hepatitis in human beings.

Whereas a durable immunity following recovery from a bacterial disease is uncommon, one attack of a virus disease leaves behind a solid and lasting immunity, e.g. Yellow fever and small pox in human beings and rinderpest in cattle. In some diseases, however, recovery results only in a transient immunity as in common cold and influenza in human beings and foot-and-mouth disease in cattle. The resistance whether solid, partial or transient is due to the presence of antibodies circulating in the blood stream or as a result of cell-mediated immunity or both The second infection is prevented because the circulating antibodies attach themselves to the virus particles and thereby render them non-infective to susceptible host cells, if any. After vaccination against or after recovery from a virus infection, the virus is present in all the cells of a particular type for which it has predilection so that they are not any more susceptible to the entry of the same or a related virus; the recurrence of the disease is thus prevented. This is termed as the 'interference phenomenon' mediated by the production of interferon. Examples of such resistance are the immunity following vaccination against rinderpest with modified attenuated rinderpest virus, or against fowl-pox with modified attenuated fowl-pox or pigeon-pox virus.

The role of specific antibodies in affording protection to the infected host against particular virus(es) is well studied. It is now understood that the host's immune response is determined by a delicate balance of the circulating antibody mediated reactions such as virus neutralization and activation of complement system, cell mediated reactions (mainly thymus-dependent) resulting in the production of a battery of biologically active substances called lymphokines, which act on cell mitosis, cell metabolism, cell motility and cell function and other mechanisms such as phagocytosis, inflammation and blood coagulation, besides several other physiological changes, which include certain hormones, altered body temperatures, malnutrition, concurrent infections, stress, non-specific humoral inhibitors and age. Innate genetic resistance of the host is also an important factor in the natural immunity against specific virus diseases.