INFLUENZA VIRUSES

Structure & Property	Transmission	Pathogenesis	Clinical findings	Laboratory diagnosis	Treatment	
Family:
- only members of orthomyxovirus family
- ‘myxo’: interact with mucins

Size: 80-100 nm

Genome: segmented single-stranded RNA, negative polarity

Nucleocapsid: 
- helical, surrounded by protein capsomeres.
- 8 discrete RNA segments
- group-specific antigen that distinguishes influenza A, B, and C

Envelope:
- lipoprotein
- covered with spikes:
- hemagglutinin: virus attachment to host cells
- neuraminidase: cleaves neuraminic acid to release virus from infected cell	Transmitted by airborne respiratory droplets

Ability of influenza A to cause epidemics is dependent on antigenic changes in hemagglutinin and neuraminidase

Antigenic changes:
- antigenic shifts: recombination of RNA segments of two antigenic types (major).
- antigenic drifts: mutations (minor)

Occurs primarily in the winter months, when it and secondary bacterial pneumonia cause a significant number of deaths, esp. in old people.	Primary site of multiplication: superficial epithelium of lower respiratory tract.

Inhalation of virus.

Degradation of mucus layer by neuraminidase

Virus gain access to cells of upper and lower respiratory tract

Infection limited to this area; viremia is rare.

Systemic symptoms such as myalgias, are due to cytokines circulating in the blood.

Necrosis of superficial layers of respiratory epithelium.

Immunity:
- mainly IgA in respiratory tract
- IgG
- cytotoxic T cells	Incubation period: 24-48 hours.

Fever, myalgias, headache, and cough develop suddenly.

Symptoms resolve in 4-7 days

Secondary bacteria infections after damage of bronchial mucosa:
- streptococcus pneumoniae
- haemophilus influenzae
- staphylococcus aureus

Reye’s syndrome:
- encephalopathy & liver degeneration
- associated with influenza B, chicken-pox and aspirin intake.	Antibody staining:
- virus grown in cell culture from throat washings
- identified by fluorescent-antibody staining of infected cells
- by using antisera to influenza A and B.
- takes several days

Serology:
- rise in antibody titer of at least 4-fold in paired serum samples taken early in illness
- either the hemagglutination inhibition or complement fixation test is used to assay the antibody titer.	Amantadine:
- effective against influenza A not B
- used to prevent influenza in a confined, elderly, unimmunized population.
- effective in treatment if given early in infection.

Inactivated vaccine (grown in chick embryos) prophylaxis for high risk groups during winter – problems of timely preparation, short-lived immunity, incomplete protection.

Guillain-Barre syndrome is a complication of vaccine.

Minimize spread by respiratory route.