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The term myxo refers to the observation that these viruses interact with mucins. Influenza A virus causes worldwide epidemics (pandemics) of influenza, influenza B virus causes major outbreaks of influenza, and influenza C virus causes mild respiratory tract infections but does not cause outbreaks of influenza. Influenza B virus does not cause pandemics, and the major outbreaks caused by this virus do not occur as often as those caused by influenza A virus. Influenza viruses, especially influenza A virus, show changes in the antigenicity of their hemagglutinin and neuraminidase proteins; this property contributes to their capacity to cause devastating worldwide epidemics (pandemics). There are two types of antigenic changes: (1) antigenic shift, which is a major change based on the reassortment of segments of the genome RNA; and (2) antigenic drift, which is a minor change based on mutations in the genome RNA. The virus is transmitted by airborne respiratory droplets. The ability of influenza A virus to cause epidemics is dependent on antigenic changes in the hemagglutinin and neuraminidase. After the virus has been inhaled, the neuraminidase degrades the protective mucus layer, allowing the virus to gain access to the cells of the upper and lower respiratory tract. Although most diagnoses of influenza are made on clinical grounds, laboratory tests are available. The test most commonly used is an enzyme-linked immunosorbent assay (ELISA). Oseltamivir (Tamiflu) and zanamivir (Relenza) are used for both the treatment and prevention of influenza. The main mode of prevention is the vaccine, which contains both influenza A and B viruses.
Avian Influenza Virus Infection in Humans
The spread of the H5N1 strain from person to person occurs rarely but remains a major concern because it could increase dramatically if reassortment with the human-adapted strains occurs. The ability of the H5N1 strain to infect chickens (and other birds) more effectively than humans is due to the presence of a certain type of viral receptor throughout the mucosa of the chicken respiratory tract. The virulence of the H5N1 strain is significantly greater than the H1N1 and H3N2 strains that have been causing disease in humans for many years. The H5N1 strain is sensitive to the neuraminidase inhibitors, oseltamivir (Tamiflu) and zanamivir (Relenza), but not to amantadine and rimantadine. Tamiflu is the drug of choice for both treatment and prevention. Prior to this time, the H7N9 strain affected only birds, especially chickens. In April 2009, a novel swine origin strain of influenza A (H1N1) virus (swine origin influenza virus, S-OIV) caused an outbreak of human influenza. The key point is that most people worldwide do not have protective antibodies against the swine hemagglutinin of S-OIV even though they may have antibodies against the seasonal strain of H1N1 virus acquired either by immunization or by exposure to the virus itself.
The paramyxovirus family contains four important human pathogens: measles virus, mumps virus, respiratory syncytial virus, and parainfluenza viruses. The genome RNA and nucleocapsid of measles virus are those of a typical paramyxovirus. Measles virus is transmitted via respiratory droplets produced by coughing and sneezing both during the prodromal period and for a few days after the rash appears. The attack rate is one of the highest of viral diseases; most children contract the clinical disease on exposure. Lifelong immunity occurs in individuals who have had the disease. Although IgG antibody may play a role in neutralizing the virus during the viremic stage, cell mediated immunity is more important. Measles in a pregnant woman leads to an increased risk of stillbirth rather than congenital abnormalities. Measles virus infection of the fetus is more severe than rubella virus infection, so the former typically causes fetal death, whereas the latter causes congenital abnormalities. Most diagnoses are made on clinical grounds, but the virus can be isolated in cell culture. There is no antiviral therapy available. Prevention rests on immunization with the live, attenuated vaccine. The vaccine is effective and causes few side effects.
This virus causes mumps, a disease characterized by parotid gland swelling. It occurs primarily in childhood. The genome RNA and nucleocapsid are those of a typical paramyxovirus. Mumps virus is transmitted via respiratory droplets. Mumps occurs worldwide, with a peak incidence in the winter. Lifelong immunity occurs in persons who have had the disease. There is a popular misconception that unilateral mumps can be followed by mumps on the other side. Mumps occurs only once. After an incubation period of 18 to 21 days, a prodromal stage of fever, malaise, and anorexia is followed by tender swelling of the parotid glands, either unilateral or bilateral. The diagnosis of mumps is usually made clinically, but laboratory tests are available for confirmation. The virus can be isolated in cell culture from saliva, spinal fluid, or urine. PCR assay can also be used. There is no antiviral therapy for mumps. Prevention consists of immunization with the live, attenuated vaccine. The vaccine is effective and long-lasting (at least 10 years) and causes few side effects. Two immunizations are recommended, one at 15 months and a booster dose at 4 to 6 years, usually in combination with measles and rubella vaccines.
Respiratory Synctial Virus
Respiratory syncytial virus (RSV) is the most important cause of pneumonia and bronchiolitis in infants. The genome RNA and nucleocapsid are those of a typical paramyxovirus. Transmission occurs via respiratory droplets and by direct contact of contaminated hands with the nose or mouth. RSV causes outbreaks of respiratory infections every winter, in contrast to many other “cold” viruses, which renter the community every few years. RSV infection in infants is more severe and more often involves the lower respiratory tract than in older children and adults. The infection is localized to the respiratory tract; viremia does not occur. The severe disease in infants may have an immunopathogenic mechanism. In infants, RSV is an important cause of lower respiratory tract diseases such as bronchiolitis and pneumonia. An enzyme immunoassay (“rapid antigen test”) that detects the presence of RSV antigens in respiratory secretions is commonly used. Aerosolized ribavirin (Virazole) is recommended for severely ill hospitalized infants, but there is uncertainty regarding its effectiveness. There is no vaccine. Previous attempts to protect with a killed vaccine resulted in an increase in severity of symptoms. Passive immunization with a monoclonal antibody directed against the fusion protein of RSV (palivizumab, Synagis) can be used for prophylaxis in premature or immunocompromised infants.