Lack of Evidence for Human-to-Human Transmission of Avian Influenza A (H9N2) Viruses in Hong Kong, China 1999

Timothy M. Uyeki, Centers for Disease Control and Prevention, Atlanta, Georgia, USA; Yu-Hoi Chong, Department of Health, Hong Kong Special Administrative Region of China; Jacqueline M. Katz, Centers for Disease Control and Prevention, Atlanta, Georgia, USA; Wilina Lim, Yuk-Yin Ho, Department of Health, Hong Kong Special Administrative Region of China; Sophia S. Wang, Thomas H.F. Tsang, Centers for Disease Control and Prevention, Atlanta, Georgia, USA; Winnie Wan-Yee Au, Shuk-Chi Chan, Department of Health, Hong Kong Special Administrative Region of China; Thomas Rowe, Jean Hu-Primmer, Jensa C. Bell, William W. Thompson, Carolyn Buxton Bridges, Nancy J. Cox, Centers for Disease Control and Prevention, Atlanta, Georgia, USA; Kwok-Hang Mak, Department of Health, Hong Kong Special Administrative Region of China; Keiji Fukuda, Centers for Disease Control and Prevention, Atlanta, Georgia, USA

Disclosures

Emerging Infectious Diseases. 2002;8(2) 

In This Article

Abstract and Introduction

In April 1999, isolation of avian influenza A (H9N2) viruses from humans was confirmed for the first time. H9N2 viruses were isolated from nasopharyngeal aspirate specimens collected from two children who were hospitalized with uncomplicated, febrile, upper respiratory tract illnesses in Hong Kong during March 1999. Novel influenza viruses have the potential to initiate global pandemics if they are sufficiently transmissible among humans. We conducted four retrospective cohort studies of persons exposed to these two H9N2 patients to assess whether human-to-human transmission of avian H9N2 viruses had occurred. No serologic evidence of H9N2 infection was found in family members or health-care workers who had close contact with the H9N2-infected children, suggesting that these H9N2 viruses were not easily transmitted from person to person.

In April 1999, two World Health Organization reference laboratories independently confirmed the isolation of avian influenza A (H9N2) viruses for the first time in humans[1]. H9N2 viruses were isolated from nasopharyngeal aspirate specimens collected from two young children who were hospitalized in Hong Kong during March 1999[2]. The children were not related, were hospitalized at different facilities, did not have any known contact with or link to each other, and had not traveled outside Hong Kong[2]. Both children had uncomplicated, febrile, upper respiratory tract illnesses and fully recovered (Table 1)[2]. Evidence for five additional human illnesses attributed to H9N2 in Guangdong Province, China, during 1998 has been reported[3]. Detection of antibody to H9N2 has been reported from persons in northern and southern China[3,4] and poultry workers in Hong Kong[5], suggesting that additional unrecognized human H9N2 infections have occurred.

H9N2 viruses have been prevalent in domestic poultry (chickens, ducks, geese, quail, and pigeons) throughout Asia since the early 1990s and were also isolated from swine in Hong Kong in 1998[6]. H9N2 viruses circulating in Asia have been classified into three antigenically and phylogenetically distinct sublineages[7]. Two of these Asian H9N2 virus sublineages, influenza A/Quail/Hong Kong/G1/97 (G1-like lineage) and influenza A/Chicken/Hong Kong/G9/97 (G9-like lineage), were isolated from poultry in Hong Kong[6]. The two Hong Kong children were infected by G1-like viruses, influenza A/Hong Kong/1073/99 and A/Hong Kong/1074/99[8]. The H9N2 viruses that have been isolated from poultry in Hong Kong are not highly pathogenic in chickens[8], whereas antigenic analysis of the H9N2 viruses isolated from humans in southern China suggested that they were more closely related to the G9-like viruses[9]. However, the G1-like viruses contain internal genes that are highly homologous to those of highly pathogenic influenza A (H5N1) viruses isolated from chickens and humans in Hong Kong in 1997[7].

The first and only documented human outbreak of highly pathogenic avian influenza A (H5N1) virus infections resulted in 18 hospitalizations and six deaths among Hong Kong residents during 1997[10,11,12]. A case-control study identified recent exposure to live poultry as an important risk factor for H5N1 infection[13], and cohort studies suggested that human-to-human transmission of H5N1 virus was limited[14,15]. The poor transmissibility of these H5N1 viruses among humans and the elimination of approximately 1.5 million chickens appear to have been key factors that stopped this outbreak[12].

Avian populations, including domestic poultry and waterfowl, are the natural reservoir for all 15 known Influenza A virus (FLUAV)(2) hemagglutinin (HA) subtypes, including H5 and H9 viruses[16]. Viruses with novel HA can emerge when animal and human FLUAV genes undergo reassortment in the same host or when viruses from an animal host, such as swine or poultry, directly infect susceptible persons who lack protective immunity against the novel HA[17,18]. In addition to ability to infect humans, the transmissibility of a novel Influenzavirus is a key factor influencing whether the novel virus can cause an influenza pandemic[19]. The emergence of novel influenza A (H1N1), A (H2N2), and A (H3N2) viruses led to three influenza pandemics during the 20th century[19].

The identification of two children who had acute infection with novel H9N2 virus strains provided the first opportunity to investigate their transmissibility and pandemic potential among humans. We report the results of four retrospective cohort studies designed to detect serologic evidence of H9N2 virus infection among family members and health-care workers (HCWs) exposed to the two H9N2 patients, as well as unexposed controls.

2. This usage reflects the taxonomic style published in September 2000 by the International Committee on Taxonomy of Viruses (available at http://www.ncbi.nlm.nih.gov/ICTV/)

Comments

3090D553-9492-4563-8681-AD288FA52ACE
Comments on Medscape are moderated and should be professional in tone and on topic. You must declare any conflicts of interest related to your comments and responses. Please see our Commenting Guide for further information. We reserve the right to remove posts at our sole discretion.
Post as:

processing....