NoteThe importance of animal influenza for human disease
Introduction
Influenza viruses continue to evolve, and new antigenic variants (drift strains) emerge constantly, giving rise to yearly epidemics. In addition, strains to which most humans have no immunity appear suddenly, and the resulting pandemics vary from serious to catastrophic. Influenza viruses are unique among respiratory tract viruses in that, they undergo continuous genetic variation. Both surface antigens of the influenza A viruses undergo two types of variation: drift and shift [1]. Antigenic drift involves minor changes in the haemagglutinin (HA) and neuraminidase (NA), whereas antigenic shift involves major changes in these molecules resulting from replacement of the gene segment.
Section snippets
Reservoirs of influenza A viruses
Aquatic birds are the reservoirs of all 15 subtypes of influenza A viruses. In wild ducks, influenza viruses replicate preferentially in the cells lining the intestinal tract, cause no disease signs, and are excreted in high concentration in the feces (up to 108.7, 50% egg infectious doses/g) [2]. Avian influenza viruses have been isolated from freshly deposited fecal material and from unconcentrated lake water, which indicates that waterfowl have a very efficient way to transmit viruses, i.e.
Evolutionary pathways for influenza viruses
Studies on the ecology of influenza viruses have led to the hypothesis that all mammalian influenza viruses derive from the avian influenza reservoir. Support for this theory comes from phylogenetic analyses of nucleic acid sequences of influenza A viruses from a variety of hosts, geographic regions, and virus subtypes. Analyses of the nucleoprotein (NP) gene show that avian influenza viruses have evolved into five host-specific lineages: ancient equine, which has not been isolated in over 15
Influenza A viruses in lower animals and birds are the source of genetic information for the emergence and re-emergence of new influenza A virus in humans
Over the past two and a half centuries, 10–20 human influenza pandemics have swept the globe. The most devastating pandemic, the so-called Spanish flu of 1918–1919, caused more than 20 million deaths and affected more than 200 million people. This pandemic probably obtained its gene segments from the avian reservoir and pigs may have been involved in interspecies transmission.
Since the first human influenza virus was isolated in 1933, new subtypes of human type A influenza viruses have
Is the pig the intermediate host?
Current human influenza viruses are believed to have arisen by genetic re-assortment between previous human influenza viruses and non-human viruses (Fig. 1). Where did the re-assortment between genes of humans and avian influenza viruses occur? Swine have been considered a logical intermediate for the re-assortment of influenza viruses, for they can serve as hosts for viruses from either birds or humans [12]. Additionally, pigs have receptors for both avian and human influenza viruses [13] and
The H5N1 avian influenza incident
One of the best examples of the importance of influenza viruses in lower animals and birds to human disease was the bird flu incident in Hong Kong, Special Administrative Region of China in 1997. In late March and early May 1997, an H5N1 influenza virus caused high mortality on three chicken farms in the New Territories, Hong Kong, China. Approximately 75% mortality occurred on the three farms, with a loss of over 6500 chickens. Also in early May, a descendant of the H5N1 virus contracted by a
Live bird markets and the epidemiology of influenza
The chicken/Pennsylvania (H5N2) influenza outbreak in 1983 to 1984 demonstrated that live bird markets play an important part in the spread of influenza viruses in avian species. In 1997, Senne et al., [23] described live bird markets as the “missing link in the epidemiology of avian influenza”, for H5N2 viruses related to chicken/Pennsylvania/83 had been isolated from live bird markets until 1986, several years after they had been eradicated from poultry in Pennsylvania. These H5N2 viruses
Challenge for the 21st century
The World Health Organization can be justly proud of the international surveillance system for influenza in humans that has been improved and strengthened over the past 50 years. A further challenge is to utilize the information about the emergence of human pandemic influenza viruses from lower animals and birds to design a surveillance system for influenza in lower animals and birds that is workable and serves both agricultural and human needs. The absence of non-pathogenic H5N1 viruses that
Acknowledgements
These studies were supported by Public Health Research Grants AI29680 and AI95357 from the National Institute of Allergy and Infectious Diseases, by Cancer Center Support CORE Grant CA-21765, and the American Lebanese Syrian Associated Charities. I thank Enid M. Bedford for manuscript preparation.
References (23)
Intestinal influenza: replication and characterization of influenza viruses in ducks
Virology
(1978)Genetic relatedness of haemagglutinins of the H1 subtype of influenza A viruses isolated from swine and birds
Virology
(1983)Origin and molecular changes associated with emergence of a highly pathogenic H5N2 influenza virus in Mexico
Virology
(1995)Clinical features and rapid viral diagnosis of human disease associated with avian influenza A H5N1 virus
Lancet
(1998)Different haemagglutinin cleavage site variants of H7N7 in an influenza outbreak in chickens in Leipzig, Germany
Virology
(1996)- Murphy BR, Webster RG. Orthomyxoviruses. In: Fields BN, et al., editors. Fields Virology. New York: Raven, 1998. p....
Epizootiology of avian influenza—simultaneous monitoring of sentinel ducks and turkeys in Minnesota
Avian Dis.
(1983)Mass morality of harbor seals: pneumonia associated with influenza A virus
Science
(1982)Characterization of two influenza A viruses from a pilot whale
J. Virol.
(1986)Evolution of the nucleoprotein gene of influenza A virus
J. Virol.
(1990)