Avian Influenza Infection In Poultry Handlers Biology Essay


Is there a problem. During the past decade, outbreaks of infectious disease have been a major cause for concern, particularly in the developing countries where control is restricted by inadequate infrastructural and financial resources. It was noted that many of these emerging diseases are zoonoses (Frederick, 2008). That is, infectious diseases which are transmitted between humans and animals. Over 30 new human pathogens have been detected in the last three decades, 75% of which have originated from animals and have zoonotic potential (WHO 2010a). Although, diseases of humans caused by bacteria, virus, parasitic and other unconventional agents of animal origin have been in existance for centuries now, it is becoming obvious that nearly all the recent disease episodes that have caught public attention in the past decade have involved zoonotic infectious agents (Council for Agricultural Science and Technology, 2005; Womack, 2005). Humans have long relied on animals, both as a major source of protein and as companions or pets. As a result, the risk of zoonotic disease transmission has been increased and has become a major public health concern especially when considering the fact that it is now possible to travel arround the world within the incubation period of most infectious agents.

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What is it? An unprecedented increase in the incidence of avian influenza virus that is highly pathogenic has succeeded in crossing the boarders of many countries causing large outbreaks in poultry populations and simultaneously crossing the species barrier to cause many human fatalities (World Health Organisation, 2005). There are four instances in the 20th century were antigenically different strains transmissible in humans has occurred, 1918 (H1N1), 1957 (H2N2), 1968 (H3N2) and 1977 (H1N1), each resulting in a pandemic with the new virus completely replacing the previous virus in the human population during the 1957 and 1968 pandemic (Capua and Alexander, 2002). There are quite a number of influenza viruses currently circulating avian population; however H5N1 virus is presently presenting the greatest concern for public health (World Health Organisation, 2010b). This virus, which was first isolated during a highly pathogenic poultry disease outbreak in Hong Kong in 1997, reappeared in February 2003 when it was suspected in three family members of whom one died and was then confirmed to have actually re-emerged in December 2003 following laboratory confirmation of samples from a human case in Republic of Korea (Wong and Yuen, 2006). Since then the H5N1 virus has continued to spread across different countries, particular south Asian countries, causing disease in both human and poultry populations. This particular virus has now succeeded in crossing the species barrier because as at 5th July, 2010, there was 500 laboratory confirmed cases including 296 deaths (World Health Organisation, 2010c), with many more suspected, unreported and unidentified cases/death yet to be discovered. An implication for public health which is of utmost concern is the assumption that if given enough opportunities, the H5N1 virus will develop the characteristics it needs to start a new influenza pandemic. According to the World Health Organisation (2006) this virus has met all prerequisites for the start of a new pandemic except one, that is, the ability to spread efficiently and sustainably from person to person. Hence, the author decided to study avian influenza infection in poultry handlers in an attempt to contribute to effort made towards preventing the occurrence of a pandemic.

1.2 Preliminary Literature Review

The purpose of a literature review is to explore an identified topic or problem in order to gain some insight of it. This study will begin with a review of the literature on avian influenza infections, as established by theoretical and empirical research. The review will help to acquaint the reader with the findings of other studies that are closely related to the research topic. The pool of research which exists on avian influenza is an evidence of its economic importance for the poultry industry and most importantly, its public health significant. Hence, an analysis of the issues relating to the occurrence of avian influenza infection in humans is necessary and must precede the design of strategies for tackling this public health problem, particular among poultry handlers.

In this section, the author will review a representative selection of literature on avian influenza, from different perspectives. The aim of the review is to give an insight into the issues influencing the occurrence of the infection in poultry handlers and to identify gaps in the literature that needs to be filled, in an attempt to tackle this public health problem.

1.2.1 Human/Avian Influenza Link

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Influenza viruses belonging to group A are one of the most important and widely researched genera of Orthomyxoviridae, a group of single stranded minus-sense RNA viruses with segmented genome. The genome of this viruses have eight RNA segments which encode 11 viral proteins; include the polymerase proteins (PB1, PB2, PA, PB1-F2), nucleocapsid protein, hemagglutinin, neuraminidase, matrix proteins (M1, M2), and non-structural proteins (NS1, NS2) (Webster et al., 1992; Wong and Yeun, 2006). Of these, Hemagglutinin (H) and neuraminidase (N) are the major antigenic determinants and are used for the subtype classification of influenza A viruses. Viruses belonging to this group have the ability to undergo antigenic variations, either by antigenic drift or antigenic shift. While antigenic drift involve minor alterations in the antigenicity of hemagglutinin or neuraminidase by mutation, antigenic shift involves genetic reassortment of the eight gene segments of these viruses and can result in the appearance of a new combination of hemagglutinin and neuraminidase for which the human population has little or no immunity (Webster and Laver, 1975; Wong and Yeun, 2006). The most likely explanation that have been provided for the appearance of new pandemic strains of influenza A viruses in humans is that they were derived from avian influenza viruses either after reassortment with the currently circulating human strain or by direct transfer (Webster et al. 1992). That is, infection of human host with different strains of influenza A viruses, leads to the development of pandemic of influenza viruses with efficient transmission of the virus from human to human {1918 (H1N1), 1957 (H2N2), 1968 (H3N2) and 1977 (H1N1)}. However it is believed that a pandemic virus can also be generated by mere mutation of an avian influenza virus such as the A/H5N1 that is believed to have adapted to the human host, causing disease in the absence of genetic reassortment (Wong and Yeun, 2006). Theoretically, there are 2raise to power 8 (=256) different possible combination of RNA segments that can occur following a dual infection with different strains of influenza A viruses, however in practice only a few combinations is believed to possess the correct gene assemblage required for viability (Webster et al. 1992). Capua and Alexander (2002) reported that research carried out during the late 1970s and 1980s demonstrated that the 1969 pandemic virus 'H3N2' differed from the 1957 pandemic virus 'H2N2', in the substitution of two genes that are thought to have originated from avian species influenza viruses. The findings of the various researches carried out during this period led Fang et al. (1981) to suggestion that dual infections with viruses of human and avian origin resulted in reassortment of genes and subsequently antigenic shift occurred. Volunteer experiments also showed that transitory infections resulted when humans were infected with some viruses of avian origin; indicating that the specificity of viral receptors and their lower optimal replication temperatures cannot be regarded as a complete barriers for host range spread, and also that the possibility for reassortment of an avian virus with a current human strain exist (Beare and Webster, 1991).

Based on the ability to cause disease, Influenza A viruses infecting poultry have been divided into two groups; the very virulent viruses that cause highly pathogenic avian influenza (HPAI) resulting to mortality as high as 100% and those that have low pathogenicity resulting in much milder disease. For these viruses to become infectious and cause disease, they require cleavage of the HA0 precursor of their main functional HA glycoprotein by host proteases; while the low virulence viruses are limited to cleavage (of the two basic amino acids at their HA0 cleavage site) by trypsin and trypsin-like enzymes which are only found in certain sites in the host, the high virulent viruses (which possessed multiple basic amino acids) appear to be cleavage by a ubiquitous protease(s) which is yet to be fully identified (Stieneke-Grober et al., 1992). However it has been demonstrated that mammals, including humans, have furin-like proteases capable of cleaving the multiple basic amino acids motifs of high virulent avian influenza A viruses (Stieneke-Grober et al., 1992) but it was not until 1997 that avian influenza viruses with multiple basic amino acids at their H0 cleavage site was first confirmed to cause systemic infections, in contrast to all the other viruses known to have infected humans (H1, including the 1918 pandemic virus, H2 and H3 subtypes) which all had structural pattern at the cleavage site indicating that they would be cleaved only by trypsin-like enzymes (Capua and Alexander, 2002).

1.2.2 Epidemiology of Avian Influenza Infections in Humans

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In 1997, influenza virus of H5N1 subtype was isolated from a child who died in Hong Kong and then 7 months later, 18 other people of whom 6 eventually died, were also confirmed to have been infected with the same virus type before it was brought under control (Shortridge et al., 1998). It was later realized that the viruses isolated from human cases in 1997 were the identical with the viruses isolated following an outbreak of highly pathogenic infection that affected chickens in Hong Kong during the same year and that all the viral isolate from both the human cases and poultry outbreaks had multiple basic amino acids at their HA0 cleavage site (Capua and Alexander, 2002). This was the first sign that avian influenza viruses can effectively infect human, directly from avian species, in a large scale. After these events, H5N1 viruses continued to cause outbreak in poultry without any report of human infection. Human infections with H5N1viruses resurfaced again in 2003 in Hon Kong when it was suspected in 3 family members, among whom 1 person died of severe respiratory disease (Wong and Yuen, 2006). Since 2003 until date the A/H5N1 influenza viruses have continued to cross species barrier to cause human infections which is reported to have reached 500 laboratory-confirmed human cases including 296 deaths as at 5th July 2010 (World Health Organisation, 2010c). Another notable feature of the H5N1 virus is the geographic extent of its occurrence. Although, china is regarded as the epicentre for both human and avian influenza viruses, the virus has been stretching its borders into some European, African, and Northern and Western Asian countries. This expanding geographical distribution of the virus puts an increasingly larger population of people at risk of becoming infected(Chen, 2005).

A list of the major events relating to H5N1 avian influenza outbreaks in poultry populations and human cases/death between 1996 and 2010 in various parts of the world can the accessed from the World Health Organisation's web site (World Health Organisation 2010b).


Transmission of avian influenza viruses to human occur by inhalation of infectious droplets, by direct contact, by indirect contact (with fomite), or by self-inoculation of the virus onto the upper respiratory tract or conjunctival mucosa (Bridges and Kuehnert, 2003). Although, relative efficiency of these modes of transmission has not been established, most available evidences are consistent with bird-to-human, possibly environment-to-human, and limited non-sustained human-to-human transmission (World Health Organisation 2005). Human-human transmission of avian influenza viruses is believed to occur occasionally. This rare possibility was reported by Bridges et al. (2000) who carried out a study involving health care workers, in which he found that 3.7% of health-care workers who managed avian influenza H5N1 patients subsequently became seropositive for avian influenza virus. It was also reported that during the 2003 outbreak of H7N7 infection in the Netherlands, there was evidence to show that person-to-person transmission occurred because H7N7 infections developed in three household contacts of the initial outbreak cases (Capua and Yuen, 2006). Despite suggestion for human-to-human transmission of the virus, the efficiency of such transmission is very low (Bridges et al., 2000). Most evidences available to date indicates that the principal sources of infection with H5N1 virus are sick or dead birds (Mount et al., 1999; Bridges et al., 2002), especially occurring when people practice risky behaviors such as slaughtering, defeathering, butchering, and preparation of infected birds for consumption (World Health Organisation, 2006).