THE ORGANISM

• Belongs to the Orthomyxoviridae family: 80-120nm, enveloped single- stranded RNA virus with hemagglutinin (H) or neuraminidase (N) activity
• 3 antigen types: Influenza types A and B are responsible for epidemics and type C causes mild respiratory illness.
• Antigenic variation of the external glycoproteins (H and N), causes small (drift) and major (shift) antigenic changes.
• Person-to person transmission: indirectly by droplets and directly through contact with respiratory secretions. (1,4)

EPIDEMIOLOGY(1)

Influenza is an acute, viral illness of the respiratory tract. The attack rate is highest in the young and mortality highest in the elderly. It causes more than 20,000 deaths in the U.S. and more than 100,000 hospitalizations each year (3).

• During epidemics (October to April in the Northern Hemisphere) different strains within a subtype or different subtypes of influenza are circulating.
• Pandemics: antigenic shifts leading to rapid worldwide transmission with high attack rates in all age groups, high levels of mortality particularly in healthy young adults. Pandemics have occurred in 1889, 1918, 1957, and 1968 (2).
• New strains emerge each year as a result of antigenic variation.

UNCOMPLICATED INFLUENZA

• The incubation period is 1-5 days.
• Systemic symptoms include abrupt onset of fever, chills, headaches, myalgia, and malaise.
• Respiratory symptoms include dry cough, pharyngeal pain, nasal obstruction/ discharge.
• Illness typically resolves after 1-2 weeks, however fatigue may continue for several weeks past illness. Virus can be recovered from secretions for 10 days.

INFLUENZA IN CHILDREN

Cause of severe systemic infections:

• Croup(associated with Influenza A virus) is more severe, but less frequent than that associated with RSV and Parainfluenza type 1 or 3.
• Onset: fulminate acute respiratory obstruction with sore throat, fever.
• Primary Influenza Pneumonia: occurs among young healthy adults or individuals with cardiovascular or pulmonary diseases. Although uncommon, it has a high fatality rate. The symptoms of fever, cough and purulent sputum persist for sveral days.

INFLUENZA in elderly

Can be the cause of severe disease, especially in patients with chronic cardiac, pulmonary, metabolic and other diseases.

• Primary Pneumonia: progression of ARDS with fever, cough, dyspnea, cyanosis.
• Secondary Bacterial Pneumonia: starts as a classic flu and recrudescence of fever, cough and sputum production with typical Chest X-ray findings and culture results.
• Exacerbation of chronic pulmonary disease such as asthma or chronic obstructive pulmonary disease (COPD).

INFLUENZA in the immunocompromised host

Populations at risk: HIV patients, bone marrow or other transplant recipients, patients with leukemia and cancer. These individuals have an increased severity of illness and shed the virus for an increased duration.

DIAGNOSIS

• Virus isolation and detection: by inoculation onto cell cultures of the specimen from nasal, throat swab, sputum or washes. It takes more than 3 days to detect (+) cultures.
• Rapid Test: such as immunoflourescence (IF), ELISA or PCR are available for Influenza A.
• Serologic Tests: are sensitive and specific, although cannot affect clinical decisions (acute sera drawn during illness and convalescents 2-3 weeks later).

TREATMENT

• Antiviral drugs (Amantadine and Rimantadine) are used for treatment and prophylaxis of Influenza A and reduce the effects of infection.
• Neuraminidase inhibitors (Zanamivir and Oseltamivir) are agents with activity against Influenza A and B. Both agents are approved for treatment, only Oseltamivir is approved for prophylaxis. These agents will reduce the severity and duration of symptoms as well as preventing disease (4, 9, 10, 11).



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NOSOCOMIAL TRANSMISSION

The source of infection can be a healthcare worker, a patient or a visitor. The risk of influenza infection is high among unvaccinated, immunocompromized, chronically ill, HIV, transplant, the very young, and elderly patients.

INFLUENZA VACCINATION

Due to the current vaccine shortage, recommendations for vaccination are available from the CDC at CDC.GOV

REFERENCES

1. CDC. Recommendations of the Advisory Committee on Immunization Practice. Prevention and control of influenza. MMWR 2004 53(RR6).
2. Cox NJ, Subbarao K. Global Epidemiology of influenza: past and present. Annu.Rev.Med 2000; 51: 407-21.
3. Neuzil KM, Mellen BG, Wright PF, Mitchel EF, Griffin MR . The effect of influenza on hospitalizations, outpatient visits, and courses of antibiotics in children. N Engl. J Med. 2000;342:225-31.
4. Cox NJ, Subbarao K. Influenza. Lancet 1999.354(9186): 1277-82.
5. James JM, Zeiger RS, Lester MR, et al. Safe administration of influenza vaccine to patients with egg allergy. J Pediatric 1998;133: 624-8.
6. Demicheli V, Jefferson T, Rivetti D, Deeks J. Prevention and early treatment of influenza in healthy adults. Vaccine 2000;18: 957-1030.
7. Walker FJ, Singleton JA, Lu PJ, Strikas RA. Influenza vaccination of health care workers in the United States, 1989-1997 Infect Control Hosp Epidemiol 2000; 21: 113.
8. CDC. Recommendations of the Advisory Committee on Immunization Practice. Prevention and Control of Influenza. MMWR 1992; 41: 1-17.
9. Hayden FG, Gubareva LV, Monto AS, et al. Inhaled zanamivir for the prevention of influenza in families N Engl. J Med. 2000; 343: 1282-1289.
10. Treanor JT, Hayden FG, Vrooman PS, et al. Efficacy and safety of oral neuraminidase inhibitor oseltamivir in treating acute influenze. JAMA 2000; 283(8): 1016-1024.
11. Hayden FG, Treanor JT, Fritz RS, et al. Use of oral neuraminidase inhibitor oseltamivir in experimental human influenza: randomized controlled trials for prevention and treatment. JAMA 1999; 282: 1240-1246.