Pathology of Influenza Virus Infection (Orthomyxoviruses)

Dr Sampurna Roy MD

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. 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.

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.

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.

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 meningoencephalitis.

Images

 

Risk factors for the development of influenza viral pneumonia include: - Lack of previous exposure to influenza virus with related surface glycoproteins. (Individuals who have not been previously exposed to an antigenically related influenza virus lack the protection of the lung against viral infection conferred by specific IgG, which reaches the alveolar lining fluid by transudation from the serum) - Age greater than 65 years, - Children younger than 5 years. - Pulmonary disease, (chronic obstructive pulmonary disease, asthma, and pulmonary fibrosis, which involve remodeling of airways or distal lung parenchyma and thus reduce pulmonary defense against infectious pathogens) - Cardiovascular disease, and pregnancy. (There are no clear explanations for the increased risk of influenza viral pneumonia from cardiovascular disease or pregnancy. It has been speculated that pulmonary hypertension secondary to cardiovascular disease or from the increased blood volume in pregnancy may predispose the lung to pulmonary oedema when the alveolar septa are damaged by the virus) -People with diabetes (type 1, type 2, or gestational), even when well-managed, are at higher risk of developing serious flu complications, which can result in hospitalization and sometimes even death. Pneumonia, bronchitis, sinus infections and ear infections are examples of flu-related complications.