Critical issues in diagnosing acute

Critical issues in diagnosing acute H5N1 virus infection are to
determine which patient to test (on the basis of epidemiological
and clinical findings), to collect proper clinical specimens, and
to use appropriate testing methods. The World Health Organization
(WHO) issued definitions for the classification of suspect,
probable, and confirmed cases of H5N1 virus infection,
for epidemiologic and reporting purposes [25]. Risk factors for
H5N1 virus infection are direct physical contact with or close
exposure (i.e., !1 meter) to sick or dead poultry in the week
before illness onset [26–28] and visiting a live poultry market
[28–30]. However, for some cases, exposure to H5N1 virus was
not identified [31]. In a small number of cases in clusters,
limited, nonsustained human-to-human transmission of H5N1
virus likely occurred [32–34]. Therefore, H5N1 virus infection
should be considered in a person with febrile, acute respiratory
illness in countries where highly pathogenic H5N1 poultry outbreaks
have occurred, for whom there is a recent history of
direct or close exposure to sick or dead poultry, who has visited
a live poultry market, or who has had close contact (within 1–
2 meters) with an individual with H5N1 virus infection. Clinical
diagnosis during early H5N1 illness is challenging because of
the nonspecific signs and symptoms and rarity of H5N1 disease.
No cases of H5N1 virus infection have been identified in travelers
to date, and seasonal influenza A virus infection was diagnosed
in 25 (42%) of 59 returned US travelers with suspected
H5N1 [35]. The Centers for Disease Control and Prevention
(CDC) has issued guidelines for testing suspected cases of H5N1