Influenza belong to the viral group Orthomyxoviradae and are RNA viruses.  There are three antigenic types, A, B, and C.  A and B are responsible for epidemics and C usually manifests as an URI.   Two surface proteins, neuraminidase and hemagglutinin determine the serotype and these proteins are further subdivided into antigenic types.  Following infection, immune response to the surface proteins correlate with protection from disease.

Influenza A may undergo major changes of the neuroaminidase and hemagglutinin, "(shift") that allows for reinfection after previous disease.  This may be related to acquisition of genetic material from animal or avian influenza viruses.  This usually occurs every 10 years. Minor changes ("drifts") may occur in type A and B yearly and result in reinfection with a type of influenza that the patient has had previously

Influenza viruses attack the respiratory epithelium and there is resultant loss of ciliary action and sloughing of superficial layers.  Because of these changes, there is an increase risk of secondary bacterial infection.

Epidemiology
1. Greatest incidence in the winter months and is highly contagious
2. Incubation period 24-72 hours and may shed virus for average of 7 days.
3. Spread by large droplet, contact, and virus may live on fomites. Hospitalized patients must be on droplet and contact isolation and strict hand washing must be enforced.
4. Children generally shed viruses longer and are the major sources of spread of infection in the general population
5. Infants are very susceptible because of low antibody titers acquired from mothers and it falls in the first three months

Clinical Manifestations
1. May have only URI symptoms and some patients will manifest as croup, pneumonia, and bronchiolitis
2. Pharyngitis, dry cough, conjunctivitis
3. fever, myalgias, headache, myositis,
4. May be indistinguishable from RSV, parainfluenza, and adenovirus infection.

Diagnosis
1. Culture- may take 2-3 days to grow
2. ELISA from nasal aspirate
3. Titers- compare acute and convalescent serum
4. DFA

Complications
1. Acute otitis media
2. Secondary bacterial pneumonias
3. myositis and myoglobinuria
4. Myocarditis, toxic shock, Reyes syndrome when patient takes aspirin.

Treatment
1. Fluids, non aspirin containing antipyretics and pain relievers,
2. Amantidine- will reduce symptoms if given within 48 hours of symptoms.  Recommended for patients at an increased risk for complications and is not approved for patients less than 1 year old.  Has been shown to only be effective against influenza A.
3. Oseltamavir-For treatment in children > 1 year of age with Influenza A and B.  This is a Neuramiinidase inhibitor.  Must be started within 48 hours of onset for best results. 
4. 2011 RECOMMENDATIONS-Oseltamir for treatment or chemoprophylaxis in children less than 1 year of age with suspected or confirmed influenza, becasue of the high risk for complications. 

Preventin
1. An inactivated influenza vaccine is available that is changed annually.  It should be administered from October- November to allow the development of protection prior to the "flu" season.

   a. not approved for infants less than 6 months old.  Should be given to all children 6-24 months.
   b. must be given in two doses, 1 month apart, in children who have not been immunized before.
   c. recommended for chronically ill children, cardiopulmonary disease, diabetics, neuromuscular disorders, metabolic diseases, renal disease, hemoglobinopathies, immunocompromised children and children on chronic aspirin therapy.

2. FluMist- Intranasal live attenuated vaccine for children > 5 years of age.

References
1. 2003 Red Book
2. Ottolini MG and Cheng TL. Influenza Update. Pediatrics in Review. 1999; 20:33.
3. Bonner A. Impact of the Rapid Diagnosis of Influenza on Physician Decison Making and Patient Management in the Pediatric Emergency Department: Results of a Randomized, Prospective, Controlled Trial. Pediatrics August 2003
4. Gerberding J, et al. An 18-Year-Old Man with Respiratory Symptoms and Shock.  NEJM  March 18 2004;350:1236-47
5. Treanor J. Influenza Vaccine-Outmanuvering Antigenic Shift.  NEJM January 15  2004:350;218
6. American Academy of Pediatrics. Recommendations for Influenza Immunizations of Children. Pediatrics May 2004
7. Moscona A. Drug Therapy: Neuraminadase Inhibitors for Influenza. NEJM Vol 353 No. 13 September 29, 2005.
8. Bhat N. Influenza Associated Deaths among children in the United States 2003-2004.  NEJM Dec 15, 2005
9. Poehling K et al. The Underrecognized Burden of Influenza in Young Children.  NEJM July 6, 2006
10. Belshe R.B. Live Attenuated versus Inactivated Influenza Vaccine in Infants and Young Children NEJM Feb 15,2007
11. Editorial Inactivated and Live Attenuated Influenza Vaccines in Young Children-How do they Compare?  NEJM Feb 15, 2007
12. Committee on Infectious Disease.  Prevention of Influenza: Recommendations for Influenza Immunization in Children 2008-2009.  Pediatrics November 2008
13. Glezen W.P. Prevention and Treatment of Seasonal Influenza. NEJM Dec 11, 2008
14. ACIP 2011 guidelines for Anti-viral use in Influenza