Flu-like illness with respiratory symptoms; GI symptoms are uncommon in adults, but occur in up to 25% of children in school outbreaks; [CCDM]
Influenza A (H1N1), A (H3N2), and influenza B viruses currently circulate among humans worldwide. The main two types of influenza are seasonal and zoonotic. The best way to prevent influenza is annual flu vaccination. Avian flu carried by wild birds, especially water fowl, does not commonly infect humans. From 1997 through March 2016, about 650 humans were infected with avian influenza A (H5N1) virus. The H5N1 strain causes 53% mortality in humans and is acquired from close contact with sick or dead poultry. This strain is considered endemic among poultry in 6 countries: Bangladesh, China, Egypt, India (West Bengal), Indonesia, and Vietnam. [CDC Travel, p. 206-14] Swine flu viruses do not usually infect humans. "In 2012, a multistate outbreak of influenza caused by a swine-origin (variant) influenza A (H3N2) virus occurred in the United States, with 307 recorded cases in 11 Midwestern and Middle Atlantic states." [PPID, p. 167] See information on Avian Flu and Swine Flu. [www.cdc.gov/flu] Influenza is spread by all routes (droplet, droplet nuclei, and contact). [CCDM, p. 309]
Arthralgia is common. Other symptoms are dry cough, sore throat, rhinorrhea, congestion, and slightly enlarged, tender cervical lymph nodes. Fever usually lasts for 3 days, sometimes 4-8 days. Children tend to have higher fevers and more prominent cervical lymphadenopathy. Some older patients present with high fever and confusion without respiratory symptoms. Primary influenza viral pneumonia is associated with rheumatic heart disease/mitral stenosis. Myocarditis and pericarditis are rare complications. [PPID, p. 2011-3] Mild conjunctivitis may occur. [Merck Manual, p. 1406] Complications of influenza include viral pneumonia, bacterial pneumonia, encephalitis, Reye's syndrome (after aspirin use), and rhabdomyolysis (myoglobinuria) followed by acute renal failure. [ID, p. 1990-1] Biphasic fever is uncommon. Other complications are sepsis, thrombocytopenia, disseminated intravascular coagulation, aseptic meningitis, myelitis, and Guillain-Barre syndrome. Dyspnea, lung infiltrates, and bloody sputum are findings in primary influenza viral pneumonia. [Cecil, p. 2098] Other complications are transverse myelitis and toxic shock syndrome. Sputum production is scanty in primary influenza viral pneumonia. [Harrison ID, p. 838]
BIRD FLU (H5N1):
The first outbreak of H5N1 (Bird) flu occurred in Hong Kong in 1997. "The current situation of widespread outbreaks of highly pathogenic avian influenza A (H5N1) virus infection among poultry is of great concern because the A (H5N1) virus is now endemic in poultry in some countries, causes high rates of death among infected poultry, and causes high case fatality in humans (60% of reported human cases have been fatal)." Patients present with severe pneumonia (fever, cough, and dyspnea). An atypical presentation is fever, diarrhea, seizures, coma and no pneumonia. Common laboratory abnormalities are leukopenia, thrombocytopenia, and elevated aminotransferases. [CCDM, p. 313-22] Poor prognostic signs in H5N1 are leukopenia, thrombocytopenia, and elevated liver enzymes. [PPID, p. 2007] "An outbreak of human infections with a new avian influenza A (H7N9) virus was first reported in China by the World Health Organization on April 1, 2013." [http://www.cdc.gov/flu/avianflu/h7n9-virus.htm] Conjunctivitis is a characteristic symptom of avian H7 infection. [Cecil, p. 2097]
"Effectiveness of influenza vaccines varies from year to year and depends on the age and health status of the person getting the vaccine and the similarity of 'match' between the viruses or virus in the vaccine and those in circulation. . . Oseltamivir or zanamivir are recommended currently for chemoprophylaxis or treatment of influenza. . . . TIV [trivalent inactivated vaccine] can be administered to exposed, unvaccinated HCP at the same time as chemoprophylaxis, but LAIV [live attenuated influenza vaccine] should be avoided because the antiviral medication will prevent viral replication needed to stimulate a vaccine response. . . In many instances of HCP exposure, watchful waiting and early initiation of treatment if symptoms appear is preferred rather than use of antiviral chemoprophylaxis immediately after exposure." [ACIP, 2011]
For updated text and symptoms of infectious diseases, see iddx.com.