Influenza

INFLUENZA

Synonyms

  • In animals: Bird flu, Fowl plague, Swine flu, Equine flu 
  • In humans: ‘Pandrikaicchal’ (Tamil), ‘Paravaikaicchal or Kozhikaicchal’ (Tamil)

Type of zoonosis

  • Direct anthropozoonosis

Definition

  • It is an acute contagious viral disease of birds and mammals, caused by Influenza virus type A, clinically characterized by high fever, chills, headache, myalgia, malaise, coryza, pharyngitis and cough with full recovery within two weeks, although viral or secondary bacterial pneumonia may develop, especially in the elderly people. 

Etiology

  • Influenza type A belongs to the family Orthomyxoviridae, and RNA virus.
  • Nomenclature of the virus: 1. Virus type; 2. Host of origin; 3. Geographic origin; 4. Strain number; 5. Year of isolation and 6. Antigenic description of subtype. Example: A/duck/Ukraine/1/63 (H3N8). 
  • There are three types of influenza virus:  Influenza virus type A, B and C.
  • Influenza virus type A and B can cause disease in humans, but type A is most common.
  • The surface antigens (‘H’ - Haemagglutinin and ‘N’ - Neuraminidase) present on the influenza virus may produce several combinations and cause human infection.
  • Sixteen forms of haemagglutinin and nine forms of neuraminidase have been identified. 
  • Haemagglutinin surface antigen is immunogenic and confers immunity.
  • The serotypes that have been confirmed in known humans pandemic deaths are:
    • H1N1: Spanish flu (1918) and Swine flu (2009), Bird flu (2010)
    • H2N2: Asian flu (1957)
    • H3N2: Hong Kong flu (1968)
    • H5N1: Bird flu (2004) - highly pathogenic avian influenza virus of type A of subtype H5N1 (HPAI-A H5N1)

Reservoir and incidence

  • Humans, wild and domestic birds, horses and pigs are reservoirs of influenza viruses, which appear to be species specific.
  • Convalescent carriers act as reservoir hosts between epidemics.
  • Swine and water fowls act as mixing vessel for several influenza viruses.
  • Antigenic shift is probably required before animal virus becomes epidemic in humans. Sporadic human infections with swine and avian strains have been reported.
  • Human cases of influenza have occurred from contamination by aerosols from infected ferrets and vice versa.
  • New influenza viruses are constantly evolving by mutation or by reassortment.
  • Mutations can cause small changes in the haemagglutinin and neuraminidase antigens on the surface of the virus, is called antigenic drift, which slowly creates an increasing variety of strains until one evolves that can infect people who are immune to the pre-existing strains.
  • When influenza viruses reassort, they acquire completely new antigens. For example by reassortment between avian strains and human strains, is called antigenic shift.
  • If a human influenza virus is produced that has entirely new antigens, everybody will be susceptible, and the novel influenza will spread uncontrollably, causing a pandemic. 

Transmission

  • By inhalation of droplets produced by coughing and sneezing, especially in crowded, and enclosed spaces.
  • Influenza virus shedding (the time during which a person might be infectious to another person) begins the day before symptoms appear and virus is then released for between 5 to 7 days.
  • The amount of virus shed appears to correlate with fever, with higher amounts of virus shed when temperatures are highest.
  • Influenza can be spread in three main ways:
  • By direct transmission (when an infected person sneezes mucus directly into the eyes, nose or mouth of another person).
  • Air-borne route (when someone inhales the aerosols produced by an infected person coughing, sneezing or spitting). The droplets that are small enough for people to inhale are 0.5 to 5 micrometer in diameter and inhaling just one droplet might be enough to cause an infection.
  • Through hand-to-eye, hand-to-nose or hand-to-mouth transmission, either from contaminated surfaces or from direct personal contact such as a hand-shake.
  • The survival of the influenza virus in the environment is influenced by the levels of humidity and sunlight (UV radiation). 
  • As the influenza virus can persist outside of the body, it can also be transmitted by contaminated surfaces such as door knobs, light switches and other household items. The length of time the virus will persist on a surface varies, with the virus surviving for one to two days on hard, non-porous surfaces such as plastic or metal, for about fifteen minutes from dry paper tissues, and only five minutes on skin. Avian influenza viruses can survive indefinitely when frozen.
  • They are inactivated by heating to 56°C for a minimum of 60 minutes, as well as by acids (at pH <2).

Disease in animals

  • Sudden onset of fever followed by anorexia, coughing, respiratory distress and mucoid nasal discharge, with rapid recovery.
  • Consolidation in the lung.
  • Copious mucopurulent exudate fills the bronchioles.

Disease in humans

  • Symptoms of influenza can start quite suddenly one to two days after infection. Usually the first symptoms are chills or a chilly sensation, but fever is also common early in the infection, with body temperatures ranging from 38-39°C, headache, myalgia, malaise, coryza, pharyngitis and cough, and recovery within two weeks.
  • Sometimes, viral or secondary bacterial pneumonia may develop, especially in the elderly people.

Diagnosis

  • Virus isolation and identification
  • Haemagglutination and haemagglutination inhibition test

Treatment

  • Complete rest.
  • Drink plenty of water, but avoid taking alcohol and tobacco.
  • If necessary, take antipyretic (paracetamol), and analgesics and cough syrups.
  • Antibiotics have no effect on the infection, unless secondary bacterial infection sets, such as bacterial pneumonia. 
  • The two classes of antiviral drugs used against influenza are neuraminidase inhibitors and M2 protein inhibitors (Adamantane derivatives). Neuraminidase inhibitors are currently preferred for flu virus infections since they are less toxic and more effective.
  • The World Health Organization recommended that persons in high risk groups, including pregnant women, children under two years of age, and persons with respiratory problems, begin taking antivirals as soon as they start experiencing flu symptoms. Antivirals include oseltamivir (Tamiflu) and zanamivir (Relenza).
  • Ribavirin aerosol may help to reduce the severity of illness in patients.

Prevention and control

  • Strict biosecurity measures to be followed during suspected flu outbreaks, includes disinfection, sanitations, restriction on movement of vehicles between farms, mass culling of birds, etc.
  • Personal hygiene is very important.
  • Wearing face mask by the human population.
  • Thorough cooking of foods and food products of animal origin.
  • Strict vigilance on bird santuaries.
  • Avoidace of contact between domestic animals and wild duck population.
  • Import restrictions on animals and animal products.
  • Quarantine of all imports and exports, including human beings.
  • Restriction on movement of human population.
  • Chemoprophylaxis with amantadine will markedly reduce the attack rate among exposed individuals if begun immediately and continued for 10 days.
  • Prohibit imports of live poultry and poultry meat from countries where fowl plague occurs.
Last modified: Friday, 16 September 2011, 5:40 AM