Dangerous ducks

Published : Feb 15, 2008 00:00 IST

Ducks and geese are relatively immune to the virus but are major carriers causing its spread.

in ThiruvananthapuramAt the Ha

IT is the quintessential Asian scene: a farming family grows rice and also raises a few ducks and chickens; after the rice is harvested, the farmer releases the ducks and ducklings into the paddy field; they feed on the grains that have fallen off during harvesting and, in return, fertilise the fields with their droppings; and when they are herded back from the field when it is time for planting and rounded up, they mature to provide eggs.

Unfortunately, when a highly virulent form of bird flu, such as the one caused by the H5N1 virus, is around, ducks and chickens form a dangerous combination as backyard poultry. While the virus is extremely lethal to chickens, ducks and geese are relatively immune to it. When infected by the virus, only a small proportion of these water fowl will show signs of ill-health. Although the others may look healthy, they will in fact be shedding vast quantities of the virus in nasal secretions and droppings. Thus these water birds can easily pass on the infection to any poultry flock that is around. In addition, their virus-laden secretions and droppings can contaminate water in rice paddies, ponds and rivers, spreading the infection to other poultry and even to wild birds.

Moreover, rapid economic growth in Asian countries has led farming families to increasingly breed poultry not just for their own consumption but to meet the rising demand in cities and towns. In much of Asia, there is a strong preference for fresh poultry meat, and consumers usually go to markets where live birds are available, which are then killed, dressed and cleaned in front of them. Transporting potentially infected birds, often across considerable distances, and keeping different kinds of poultry in large numbers in live markets increase the chances of bird flu infection.

It is not coincidental that China, Vietnam, Thailand and Indonesia, which have seen widespread outbreaks of H5N1, are all countries that have large numbers of chickens being kept along with ducks. China alone is said to have 70 per cent of the worlds population of ducks.

Neighbouring Bangladesh, where outbreaks of H5N1 in poultry began in February 2007, is said to have one of the largest populations of ducks in Asia. Although ducks form only 10 per cent of the countrys poultry, it is nevertheless a major source of animal protein and income to many small farmers and rural people, according to Bangladeshi researchers. This is true of West Bengal, which, according to statistics from the Union governments Department of Animal Husbandry, Dairying and Fisheries, has the most egg-laying ducks in India.

In a journal paper published recently, some leading influenza experts, including Guan Yi of the University of Hong Kong and Robert Webster of St. Jude Childrens Research Hospital at Memphis in the United States, have observed that ducks can serve as the Trojan Horse of H5N1. Monthly statistics of H5N1 virus isolations from poultry in one Chinese province during 2004-2005 showed that isolation of the virus in ducks and geese preceded those in chickens and minor poultry (such as quail and pheasant), they have pointed out.

Researchers have discovered that in Thailand the geographical spread of H5N1 did not match areas with high densities of chickens. The scientists then used remote sensing images to identify areas that supported more than one crop of paddy. In a paper published last year, Marius Gilbert of the Universite Libre de Bruxelles in Brussels and others reported that the number of rice crop per year predicted by remote sensing was strongly correlated with free grazing duck density and, thereby, [highly pathogenic avian influenza] presence during the 2004/2005 epidemic wave. In Thailand, the registration and pre-movement testing of free-grazing ducks was believed to have played a role in containing the bird flu problem.

Preliminary studies suggested links between highly virulent bird flu and duck concentrations in Vietnam and Indonesia too, they added.

Hong Kong has become something of a role model for keeping bird flu at bay. All farms in the territory and in mainland China that supply poultry are required to vaccinate their birds properly and adhere to stringent biosecurity measures to prevent infection. Chicken farms cannot keep other birds or pigs, movement of birds and feed between farms is controlled, poultry can be transported to wholesale markets from only one farm, and chickens from wholesale and retail markets are not allowed in local farms. There are rest days for wholesale and retail markets when trading stops; all remaining live poultry in retail markets are slaughtered and the premises thoroughly cleaned and disinfected.

Live ducks and geese are not permitted to be sold in retail markets in Hong Kong. The territorys ultimate aim is to stop the sale of live chickens too in retail markets and go in for centralised slaughtering.

Vietnam, too, which was one of the first countries to be severely affected by H5N1, has been trying to restructure its poultry industry in order to reduce vulnerability to the virus. The government has banned the keeping of poultry in urban areas and has also tried to stop live poultry markets in big cities.

When a group of foreign journalists, including this correspondent, visited Vietnam in 2006, government officials spoke of wanting to prevent ducks being kept as backyard poultry and of gradually shifting poultry production from backyards to large farms where biosecurity could be better enforced.

According to the United Nations Food and Agriculture Organisation (FAO), China has achieved considerable success in controlling H5N1 outbreaks, particularly in the wealthier coastal provinces where restructuring of the poultry industry has reduced the risk of dissemination of the disease.

In their fight against H5N1, both Vietnam and China have gone in for nationwide poultry vaccination to protect domestic birds, including ducks. As one Vietnamese official put it, If outbreaks are occurring literally everywhere, eradication is not an option. Chinas programmme, with potentially over 14 billion poultry to be vaccinated twice annually, is the largest in the world, says the FAO.

Quite apart from being costly to implement, such mass vaccination campaigns pose considerable logistical problems when large numbers of backyard poultry are involved. Vaccinators must visit a large number of places and, with village flocks likely to have birds of different ages, they must be trained to deal with even small birds without harming them.

In Vietnam, vaccination teams went to every commune and village, Bui Quang Anh, Director-General of the countrys Department of Animal Health, told the visiting journalists. People could bring their birds to one place to have them vaccinated or else the teams went door-to-door injecting the birds, he said.

At a poultry

But mass vaccination is not a magic bullet and, if not properly carried out, can actually aggravate the problem. Guan Yi and others point out in their journal paper that a vaccination programme of backyard poultry was unlikely to be very effective unless its coverage was comprehensive. They also warned that such a vaccination programme could be potentially detrimental since the vaccine-giver may be carrying the virus from backyard to backyard. Moreover, partial immunity would only fuel endemicity of the virus. Vaccination of chickens and minor poultry should be done only if a good vaccination programme for ducks and geese was already in place, they added.

Research published in Nature by scientists at the Universities of Edinburgh and Warwick in Britain suggests that incomplete vaccination of poultry flocks could increase the spread of deadly strains of bird flu. Properly vaccinated poultry become less susceptible to the disease. Since vaccinated birds shed much less virus even when infected, the chances of the disease spreading was reduced.

But, using mathematical models, Nicholas Savill and his colleagues found that if flocks were not adequately covered by vaccination, outbreaks could become more difficult to detect, thereby increasing the risk of a silent spread of the virus to other flocks. If vaccination is to be used it needs to be done extremely well or it could make the problem worse, rather than better, commented Savill in a press statement.

Although their mathematical models had studied how the virus might spread among caged birds, theoretically the results should hold for backyard poultry as well, he said in an e-mail to this correspondent.

In their paper, Guan Yi and his colleagues used a simplified mathematical model of coupled epidemics between aquatic birds (mainly ducks and geese) and chickens to estimate transmission within and between these two poultry populations. The study indicated that vaccinating ducks and geese, with no vaccination of chickens, might be sufficient to interrupt a self-sustaining outbreak. However, they caution that this did not mean that all transmission would be completely eliminated in the absence of additional measures.

Vaccination can help to bring down levels of infection, thus reducing the risk of transmission to humans and other poultry, and can substantially reduce the socio-economic cost of control, say the World Organisation for Animal Health (OIE) and the FAO in a document outlining a global strategy for prevention and control of bird flu.

When carried out in accordance with OIE/FAO guidelines and in combination with other disease control measures, including enhanced biosecurity, culling of infected flocks with compensation, poultry movement control and management of markets, vaccination has a powerful impact in reducing disease incidence and virus load in the environment as had been demonstrated in Vietnam, the two organisations noted.

Vaccinating a rooster

But vaccination must also be supported by post-vaccination monitoring to ensure that adequate flock protection was being achieved and to determine whether the virus was circulating in inadequately vaccinated flocks, they added.

In Vietnam, no outbreaks of H5N1 were reported for about a year after nationwide poultry vaccination was instituted in the latter half of 2005. Then from December 2006 onwards, several provinces began reporting cases of the disease.

Many of these outbreaks have occurred in duck flocks and are thought to indicate a failure to vaccinate such flocks.

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