ORTHOMYXOVIRUSES Influenza Viruses

Respiratory illnesses are responsible for more than half of all acute illnesses each year in the united states of America. The orthomyxoviridae which are the influenza viruses are the major determinant of morbidity and mortality caused by respiratory disease, and outbreaks of infection sometimes occur in worldwide epidemics. Influenza has been responsible for millions of death worldwide. Mutability and high frequency of genetic reassorment and resultant antigenic changes in the viral surface glycoproteins make influenza viruses formidable challenges for control efforts. Influenza type A is antigenically highly variable which is responsible for most epidemic influenza cases. Influenza type B may show antigenic changes and may also leads to epidemics. Influenza type C is antigenically stable and can leads to only mild illness in immunocompetent human being.
Properties of Orthomyxoviruses
Three immunologic types of influenza viruses are known, designated A, B and C. Antigenic changes often happen within the influenza type A group of influenza viruses and to a lesser degree in the type B group, whereas type C appears to be antigenically stable. Influenza A strains are also known for aquatic birds, seals, horses. Some of the strains isolated from animals are antigenically similar to strains circulating in the human population.

Influenza Virus Infections in Humans
Influenza virus is a communicable disease that can be communicated from one person to another person through air borne droplets or also through contact with contaminated hands or surfaces. A few cells of respiratory epithelium are infected if deposited virus particles avoid removal by the cough reflex and escape neutralizations by pre-existing specific IgA antibodies or inactivation by nonspecific inhibitors in the mucous secretions. Progeny virions are soon produced and spread to adjacents cells, where the replicative cycle is repeated. Viral NA lowers the viscosity of the mucous film in the respiratory tract, laying bate cellular surface receptors and promoting the spread of virus-containing fluid to lower portions of the tract. Within a short time, many cells in the respiratory tract are infected and eventually killed.
The incubation period from exposure to virus and the onset of illness varies from a day to four days , depending upon the size of the viral dose and the immune status of the host. Viral shedding starts the day preceeding onset symptoms, peak within 24 hours, remains elevated for 1 to 2 days, and then declines over the next 5 days . infectious virus is very rarely recovered from blood.

Interferon is detectable in respiratory secretions about a day after viral shedding begins. Influenza viruses are very sensitive to the antiviral effects of interferon, and it is believed that the interferon response contributes to host recovery from infection. Specific antibody and cell-mediated responses cannot be detected for another 1 to 2 weeks.
Influenza infections cause cellular destruction and desquamation of superficial mucosa of the respiratory tract but do not affect the basal layer of epithelium. Complete reparation of cellular damage probably takes up to 1 month. Viral damage probably takes up to a month. Viral damage to the respiratory tract epithelium lowers its resistance to secondary bacterial invaders, especially staphylococci, streptococci, and Haemophilis Influenzae.
Edema and mononuclear infilterations in response to cell death and desquamation due to viral replication probably account for local symptoms. The prominent systemic symptoms associated with influenza probably reflect the production of cytokines.
Influenza attacks maily the upper respirarory tract. It poses a serious risk for the elderly, the very young, and people with underlying medical conditions such as lung, kidney, or heart problems, diabetes or cancer.
Symptoms
Symptoms of classic influenza usually appear abruptly and include chills, headache, and dry cough, followed closely by high fever, generalized muscular aches, malaise, and anorexia. The fever usually lasts 3 to 5 days, as do the systemic symptoms, respiratory symptoms typically last another 3 to 4 days. The cough and weakness may persist for 2 to 4 weeks after major symptoms subside.
Mild or asymptomatic infections may occur. These symptoms may be induced by any strain of influenza A or B. Influenza C seldom caused the influenza syndrome, instead it leads to a common cold illness. Coryza cough as a symptom may last for several weeks.
Clinical symptoms of influenza in children are similar to those in adults, although, children may have higher fever and a higher incidence of gastrointestinal manifestations such as vomiting. Febrile convulsions can occur. Influenza A viruses are an important cause of croup in children under a year of age, which may be severe. Finally, otitis media may develop.
When influenza appears in epidemic form, clinical findings are consistent enough that the disease can be diagnosed, sporadic cases cannot be diagnosed on clinical grounds, as disease manifestations cannot be distinguished from those caused by other respiratory tract pathogens. However, those agents rarely cause severe viral pneumonia, which is a complication of influenza A virus infection.

Serious complications usually occur only in the elderly and debilitated, especially those with underlying chronic disease. Pregnancy appears to be a risk factor for lethal pulmonary complications in some epidemics. The lethal impact of an influenza epidemic I reflected in the excess deaths due to pneumonia and cardiopulmonary disease.