Avian Flu: Do You Need to Be Prepared?

Avian influenza primarily affects wild birds and domesticated poultry. Humans acquire avian influenza viruses chiefly through direct contact of the mucous membranes with secretions or excreta from infected birds or contaminated poultry products. The upper respiratory tract and conjunctivae appear to be the main portals of entry.

Influenza pandemics occur when new virus subtypes emerge and become readily transmissible among humans. On average, pandemics occur about 3 or 4 times per century. Avian influenza is not a pandemic influenza. It is not easily transmitted among humans, and it has not been found in the United States.¹ Therefore, at present, the risk to persons in this country is considered low.

However, the spread of avian influenza A subtype H5N1 worldwide and the outbreak in Southeast Asia are warning signs. Since influenza viruses have the ability to change, there is concern that the H5N1 virus could mutate and become more easily transmitted among humans.

WHY WORRY?
More than 200 laboratory-confirmed cases of human infection with H5N1 viruses have been reported since 2003.² Mortality has been about 55%.² Another alarming observation is that most cases have occurred in previously healthy children and young adults. Most cases have been associated with direct or close contact with infected poultry or contaminated surfaces.

So far, human-to-human transmission of avian influenza viruses has been inefficient. In the 1997 outbreak in Hong Kong, only one household contact of an infected patient and 3.7% of involved health care workers were subsequently seropositive.^3,4 However, the changing epidemiology and pathogenicity of avian influenza viruses underscore the potential for a severe epidemic.⁵

Several avian influenza A virus subtypes, such as H5N1, H9N2, H7N3, and H7N7, have been shown to cross the species barrier from wild migratory birds or poultry to humans.⁶ Since its emergence in 1997, the H5N1 virus (Figure) has undergone multiple genetic reassortment events with avian viruses and could mutate further, potentially becoming more efficient with respect to transmission.^6,7

PRESENTATION AND DIAGNOSIS
There are no pathognomonic symptoms or signs of avian influenza. Patients often present with the typical influenza symptoms, such as fever, cough, sore throat, and myalgia; conjunctivitis is fairly common as well. The patient's history is particularly important--recent travel to an area where avian influenza is endemic and a history of contact with poultry are major diagnostic clues.

Symptoms seem to occur 2 to 4 days after exposure to infected birds.⁸ Compared with typical influenza illness, avian influenza has a more aggressive course, with progression to pneumonia and severe respiratory failure within a few days of the onset of symptoms.^8-10 Most patients who have died of avian influenza had multiorgan failure and acute respiratory distress syndrome.⁵ Leukopenia on presentation also appears to be a significant prognostic indicator.^8,9

The diagnosis of avian influenza can be confirmed by viral culture or reverse transcriptase polymerase chain reaction (PCR) assay of throat swabs or nasopharyngeal aspirates. Reverse transcriptase PCR may be the most promising method of rapid testing.⁵

PREVENTION AND TREATMENT
The US Department of Agriculture has implemented measures to help prevent an outbreak of avian influenza. These measures include surveillance of bird populations, restrictions on bird importation, and regulation of agricultural practices.¹¹

There is no commercially available vaccine to prevent avian influenza. However, vaccines are being developed and clinical trials are under way.

In vitro data and animal studies suggest that oseltamivir (Tamiflu) and zanamivir (Relenza) might be effective in treating avian influenza in humans.^1,5,12,13 The World Health Organization preferentially recommends oseltamivir.^14,15 However, the development of resistance to oseltamivir has been reported in Vietnamese patients who were infected with H5N1.¹⁶ Amantadine and rimantadine are not considered to be the antivirals of choice, partly because of the emergence of resistance.⁵