Snowy Owl
Retired in 2010, In Memoriam
Update on avian influenza
The H5N1 avian influenza virus has evolved into several clades , which have remained largely separate geographically (each having its own "territory"), although the clade from China has recently circulated in other Southeast Asian countries.
The spread of the virus appears to be principally related to the movement of poultry and poultry products, although recent outbreaks in sub-Saharan Africa, Egypt and Europe may indicate introduction of the virus by wild birds.
Migratory birds may spread the virus to new geographic regions, but their importance as an ecologic reservoir is uncertain.
The risk of the introduction of avian influenza into North America by birds migrating through Alaska is still considered low, but is under constant evaluation.
Update on human infections
The number of cases of humans infected by the avian influenza virus is proportionately low d espite widespread exposure to infected poultry in many countries.
Since May 2005, the number has nonetheless increased, in part because of the spread of the virus across Eurasia and to Africa .
Surveillance for human cases of H5N1 influenza has focused on patients with severe illness, leaving doubt as to whether milder cases are brought to the attention of the authorities.
However, limited epidemiological studies conducted since 2003 involving villagers, poultry workers and health care workers in affected countries suggest that asymptomatic or mild human H5N1 infection is rare.
The median age of patients with H5N1 infection is 18.
The overall case fatality (number of deaths in relation to the number of infected humans in the world) is 61%. It is highest among young people between the ages of 10 and 19, and lowest among adults 50 years of age or older.
The explanation for the apparently lower frequency of infection and lethal illness among older adults is uncertain. Pre-existing immunity and lower exposure to infected poultry are the most frequently cited possibilities, but have yet to be confirmed.
To date, no cases of H5N1 illness have been identified among short-term travellers visiting affected countries. The risk of contamination is real, however, and it is essential to continue advising travellers.
Update on transmission to humans
Most infected individuals have acquired H5N1 influenza from poultry: when slaughtering, defeathering or preparing poultry for cooking; holding dead poultry; handling fighting cocks or ducks that appear to be well; and consuming raw or undercooked poultry or poultry products
In one quarter of patients with H5N1 influenza, the exact source of exposure is unclear. For some patients, the only identified risk factor was visiting a live-poultry market.
Plausible transmission routes include contact with contaminated fomites or with fertilizer containing poultry feces, self-inoculation of the respiratory tract and inhalation of aerosolized infectious excreta.
Since environment-to-human (rather than infected-poultry-to-human) transmission remains possible,any ingestion of virus-contaminated products or swimming or bathi ng in virus-contaminated water may pose an important risk.
On the other hand, drinking potable water and eating properly cooked foods are not considered to be risk factors.
Clusters of human H5N1 illness (affecting at least two patients) have been identified in several countries. Most clusters have involved two or three persons, and the largest affected eight. More than 90% of case clusters have occurred among blood-related family members, suggesting possible genetic susceptibility, although one statistical model indicated that these clusters may have occurred because of chance alone.
Most persons in case clusters probably acquired infection from common-source exposures to poultry, but limited, nonsustained human-to-human transmission has probably occurred during very close, unprotected contact with a severely ill patient. Respiratory secretions and all bodily fluids should be considered potentially infectious.
The H5N1 virus can also infect multiple mammalian hosts, including domestic cats and dogs. However, to date, none of these has been implicated in transmission to humans, but that risk cannot be ignored.
Update on the clinical features of human infection
After exposure to infected poultry, the incubation period generally appears to be 7 days or less, and in many cases is 2 to 5 days.
The median time from the onset of the first symptoms to death, if it occurs, remained unchanged (at 9 to 10 days) from 2003 through 2006. However, observed differences from that standpoint are difficult to interpret because of major variations in medical practices and the time from the onset of illness to treatment among affected countries.
Currently, illness due to the avian influenza H5N1 virus typically manifests as severe pneumonia that often progresses rapidly to the acute respiratory distress syndrome.
Febrile upper respiratory illnesses without pneumonia in children have been reported more frequently since 2005. Early consultation and antiviral therapy may have altered the clinical course of these illnesses.
H5N1 infection can begin in the gastrointestinal tract, in humans as in other mammals. For several patients, diarrheal disease preceded respiratory symptoms. Ingestion of virus-contaminated products or swimming or bathing in virus-contaminated water was probably involved in those cases.
The nonspecific clinical presentation of avian influenza (H5N1) disease has prompted WHO and national health authorities to recommend that health care staff include H5N1 infection in the differential diagnosis for patients who present with epidemiological risk factors and unusual courses of illness, especially rapidly progressing pneumonia.
Source: The New England Journal of Medicine , Volume 358:261-273, January 17, 2008, Number 3.