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Sitting ducks
IN 2004, Pakistan’s animal husbandry commissioner, ministry of food and agriculture, confirmed an outbreak of avian influenza in southern Sindh, including Karachi. He acknowledged that between four and six million chickens had been affected, having a total load of 2.5 million cases of avian influenza, besides 1.2 million deaths. Consequently, at least half a million birds had to be culled.
He, however, claimed that no case had been reported in which humans were infected. Despite his assurances, public consumption of chicken meat and eggs plummeted. The market crashed soon afterwards and the poultry industry suffered a loss running into millions.
A paediatrician belonging to the National Institute of Child Health reported at the time that cases of respiratory disease among children were on the rise, possibly due to consumption of chicken meat and eggs. However, despite several clinical tests, a definitive link between the disease and children’s diet could not be established.
The truth is, there exists no facility in the country to confirm or rule out cases of avian influenza, also known as bird flu. It is still not known if the National Health Institute, Islamabad, plans to establish any such facility in line with the proposals put forward by World Health Organization (WHO), which has warned of a possible pandemic this year. This is also despite a grant worth $390,000, which has already been provided to the Pakistan Agriculture Research Council, Islamabad, by UN’s Food and Agriculture Organization to monitor and control avian influenza among chickens. The poultry industry, which suffered huge losses during the 2004 crisis, is still waiting anxiously for corrective steps by the authorities.
The global scenario
The crisis in southern Sindh had come hot on the heels of epidemics in China, Cambodia, Indonesia, Japan, Laos, South Korea, Thailand and Vietnam in which 100 million birds died or were ultimately destroyed. In addition, there were 12 confirmed human cases of avian influenza in Thailand and 23 in Vietnam, resulting in 23 deaths.
Following a brief hiatus, avian influenza has lately made a comeback. With new outbreaks of avian influenza H5N1 in the countries mentioned and 55 human cases in Asia, the situation has necessitated a public health alert by WHO. Experts agree that another human pandemic may be inevitable, even imminent, unless outbreaks among chickens are controlled.
In 1997, avian influenza, also known as bird flu, caused severe respiratory problems in 18 residents of Hong Kong, six of whom died. In recent years the H7N7 virus caused mild illness in 83 people in Holland. One of the sick persons, a veterinarian, actually died.
In avian influenza, humans usually get infected after close contact with sick birds. However, the case in Hong Kong was unique because it revealed the first human-to-human transmission of acute respiratory illness.
How influenza spreads
Let’s turn, briefly, to the possible reasons for the spread of influenza. The disease is caused by a zoonotic virus belonging to the Orthomyxovirideae family of RNA viruses consisting of four genera — influenza virus A, virus B, virus C and Thogotovirus.
Influenza A viruses are further divided into two types based on proteins called hemagglutinin (H) or nuraminidase (N) that are attached to their surfaces. There are 15 different hemagglutinin and 9 different nuraminidase subtypes, all of which are found in influenza A viruses afflicting wild birds, ducks, chickens, pigs, whales, horses and seals.
Most influenza A viruses have low pathogenicity and cause a mild respiratory disease in birds. However, in the last few years some highly pathogenic influenza A viruses, namely H5 and H7, have caused widespread fatalities among chickens in many Asian countries.
Avian influenza viruses of low pathogenicity mutate into highly pathogenic viruses after circulation for some time in birds, especially ducks. Not long ago, low pathogenic avian influenza viruses H5N2 and H7N1 mutated into highly pathogenic forms in the US. The mortality rate was as high as 90 per cent and the resulting destruction of 17 million birds cost nearly $65 million. A similar situation was experienced in Mexico where a highly pathogenic epidemic of H5N2 caused heavy mortality as it continued unabated for more than 3 years. In 2003 due to an outbreak in Holland some 30 million birds had to be culled. Similarly, in Belgium 2.7 million and in Germany 400,000 birds had to be destroyed, respectively, to contain the situation.
Link between birds and humans
Influenza A viruses have a species-specific lineage of viral gene, but sometimes these viruses cross over from one species to the other, causing illnesses among humans. Since 1957, avian influenza viruses have been responsible for worldwide outbreaks of “Asian Flu” and “Hong Kong Flu”, claiming thousands of lives.
The first pandemic of “Spanish Flu”, caused by subtype H1N1, killed 500,000 human beings in the world. Since 1997, many Asian countries have been facing a serious situation due to the avian influenza subtypes H5, H7 and H9, resulting in mortality and culling among chickens and sickness and fatalities in humans.
In addition to Vietnam, Thailand and Cambodia, cases have been reported in recent years from Canada and the US due to the viruses H7N3, H7N2, H1N1 and H5N2.
Last year the Pakistani poultry industry was hit by the strains H7 and H9 and not H5, which has been cited in the more dangerous cases reported from other Asian countries. Bird flu has so far not been officially reported from India, Bangladesh, Mongolia, Singapore, Nepal, Myanmar and Sri Lanka.
Because avian influenza epidemics have become endemic to some Asian countries, human infections are likely to surface frequently in the future as well, posing a serious public health threat. This is particularly worrisome at a time when poultry production is mounting and international trade in chicken meat is increasing.
It should be kept in mind that birds recovering from influenza continue to shed live virus for a long period of time. This has implications for the other birds as well as humans who might come in contact with them. The virus may also be transmitted through an intermediate host like pigs and ducks. A more serious problem is the ability of the virus to mutate through different assortments.
Frequency of pandemics
Based on historical events, human influenza pandemics are likely to occur two or three times every century. Presently, frequent outbreaks among birds are likely to increase the possibility of human infection. If humans are simultaneously infected with the human and avian strains, then there is every likelihood that, following the emergence of a novel subtype with sufficient human genes, the disease would easily become transmissible among humans. This may trigger an influenza pandemic.
Influenza viruses also have the ability to mutate through antigenic ‘shift’ and ‘drift’. Influenza viruses possess eight separate gene segments, which allow viruses from different species to mix and, thus, create a new virus. If a pig is simultaneously infected with the human as well as avian influenza viruses, the two could interact to produce a new virus having most of the human virus genes but a hemagglutinin or nuraminidase from chickens. This virus could then transmit between people. Since at the moment people do not have any immunity from this new type of virus, transmission among people could easily bring about a pandemic.
Similarly, a re-assortment of viruses may take place if a person gets infected with both the avian and human influenza viruses, giving rise to a new virus having a hemagglutinin from birds and the other gene from humans. Among the 15 subtypes of avian influenza A, H5N1 has the ability to mutate the fastest because of its ability to acquire genes from the viruses of other species, thus causing serious diseases among humans as experienced in Vietnam. In short, it is apprehended that if the human and avian influenza viruses continue to infect human populations a ‘mixing vessel’ could emerge which may propel new viruses that in turn could cause epidemics.
The WHO has started to monitor the avian influenza situation carefully so that measures could be suggested to avert possible outbreaks among humans.
The destruction of infected birds initially helped contain the outbreak in recent years, but the disease has reappeared. Vaccination of birds is now being tried out in some countries, including Pakistan and Italy.
Vaccination of humans for protection against the disease has not been permitted due to a large variation of virus strains that do not possess cross-immunity. In April of 2004 WHO made the prototype seed strain for H5N1, but so far only two manufacturing companies in the US have started testing vaccines. Anti-Flu or anti-viral treatment is recommended on an on-going basis during suspected infection. The WHO and Centres for Disease Control (CDC) have asked stakeholders to join hands in efforts aimed at checking influenza in birds as well as human beings to avoid a potential disaster.
The writer, a leading virologist, works as a poultry pathologist
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