Orthomyxoviruses (influenza A, B, and C
viruses) are enveloped viruses that contain single-stranded RNA in a
helical array of nucleoprotein. Inserted in the lipid envelop are two
glycoproteins- the hemagglutinin (HA) and the neuraminidase (NA).
Influenza viruses have the unusual
capacity of changing the antigenic identities of their HA and NA
polypeptides, thus creating numerous antigenic variants. The host of
different types of virus give their names to the disease, as in, for
example, human, swine, and avian influenza.
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Pulmonary Infection
;
Avian Influenza (Bird Flu)
Influenza viruses are highly contagious
and afflict people of all ages. They are transmitted by aerosols
generated by coughing and sneezing. Influenza A virus, the most common
cause of viral pneumonia in adults, infects animals and man and
produces pandemics.
Several types of
pneumonia associated with the influenza virus infection have been
reported: 1) influenza complicated by secondary bacterial pneumonia,
2) primary influenza virus pneumonia, 3) combined influenza virus and
bacterial pneumonia. Secondary bacterial pneumonia often produces a
syndrome that is clinically distinguishable from that of primary viral
pneumonia.
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Influenza B virus is apparently
restricted to man, causes epidemics and is associated with Reye’s
syndrome in children and pneumonitis and croup in infants. Influenza C
virus causes sporadic respiratory infections, but not epidemic
influenza. Influenza has significant mortality and morbidity, and may
have long-term sequelae.
Patients with viral influenza during
the third trimester of pregnancy, the aged, and persons with valvular
heart disease or chronic bronchopulmonary disease all have increased
susceptibility to bacterial superinfection.
Bronchopneumonia ;
Cardiac Valve Disease
Superinfection usually occurs 1 to 5
days after the onset of the vial illness, while the patient appears to
be getting well.
After an incubation period of 18 to 72
hrs, the onset is characteristically abrupt, with fever, chills,
generalized malaise, myalgias and headache. As the fever and systemic
symptoms subside, the respiratory symptoms become prominent.
The histopathologic features include a necrotizing bronchitis and diffuse
hemorrhagic necrotizing pneumonitis with
pulmonary edema. Ciliated
epithelial cells are destroyed and goblet cells and mucous glands
disrupted. Bronchioles become thickened, distended, and infiltrated
with mononuclear cells. There is often severe inflammatory edema, and
the fluid exudates in the alveolar spaces has a hyaline appearance.
Chronic bronchitis
In influenza
significant
life-threatening pathological conditions that could be considered the
cause of death included diffuse alveolar damage , extensive secondary
pneumonia , extensive intraalveolar hemorrhage , viral pneumonitis ,
myocarditis
, and meningo- encephalitis.
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