Virus Diseases

widespread diseases in nature caused by viruses and found in humans, animals, birds, fish, insects, plants, Protozoa, and even in bacteria. Virus diseases in man include smallpox and chicken pox, measles, herpes, influenza, German measles, mumps, poliomyelitis, viral hepatitis, endemic encephalitis (transmitted by ticks, mosquitoes, and other insects), trachoma, and yellow fever. In the 1950’s and 1960’s the viruses of the following diseases were discovered: parainfluenza, the severe infectious common cold, adenoviral infections, and respiratory syncytial virus infections. These viruses cause acute inflammation of the upper respiratory tract, acute bronchitis, pharyngoconjunctival fever, and epidemic keratoconjunctivitis. Many other diseases of the respiratory tract are caused by certain kinds of enteroviruses, which are also the causative agents of aseptic serous meningitis, of some gastrointestinal diseases, and so on. A human can contract rabies, foot-and-mouth dis-ease, endemic encephalitis, viral hemorrhagic fever, and vesicular stomatitis from animals and ornithosis or psittacosis from birds.

After a virus enters an organism complicated processes begin to take place in that organism leading to the development of a symptomatic acute infection or an asymptomatic infection, or a chronic infectious disease. Thus, the mechanisms of development and the clinical symptoms of virus diseases are extremely varied and depend on the nature of the virus, its virulence, its mode of penetration into the organism, on “tissue tropisms” (there are dermotropic viruses, which multiply in the skin; pneumotropic viruses, which multiply in the lungs; and neurotropic viruses, which multiply in the nervous system), and on the penetration of the natural defense barriers of the organism (such as the skin, the mucous membranes, and cellular defense mechanisms). The incubation period of virus diseases is from two or three days (influenza, certain kinds of encephalitis, and others) to 30 days and longer (rabies, epidemic hepatitis, and others). Infection can take place through the air, through food, milk, water, or various objects, and through the bite of bloodsucking arthropoda (mosquitoes, sand flies, and ticks).

It is typical for antiviral immune mechanisms that although the virus is phagocytized by a leukocyte, it is not destroyed in the leukocyte; of great importance in immunity against viruses are special cellular and humoral antibodies, whose production in the organism depends on the functional activity of the reticuloendothelial system as a reaction to the entry of a pathogenic agent. One of the factors in the natural defense of cells against viruses is the production by cells of interferon—a substance that makes the cell resistant to the virus. This defensive mechanism, however, is only of limited duration.

Treatment, on the whole, is symptomatic. In the case of certain virus diseases (encephalitis carried by ticks) treatment consists in the use of a specific antiserum containing antibodies to the pathogen, a specific vaccine (the treatment of persistent forms of trachoma, herpetic keratoconjunctivitis), or interferon (in the case of viral conjunctivitis). Antibiotics do not have any effect on viruses and their use during viral diseases is advisable only for the prevention of bacterial complications. Prophylaxis consists of early recognition of the disease and hospitalization of the patient, destruction of the diseased animals that were the source of infection or carriers, and the use of specific vaccinations with live or attenuated vaccine. The majority of virus diseases are subject to obligatory registration and statistical recording. There are international quarantine conventions on smallpox and yellow fever.