Cracking down on Newcastle Disease
Newcastle disease, or pseudo-fowl pest, is to chickens what rinderpest is to cattle. It is a highly infectious viral disease that causes very high mortality (up to 100% in severe epidemics) in poultry and wild birds around the world. However unlike rinderpest, which has been eradicated from many areas where it was prevalent, Newcastle disease remains endemic in many regions and continues to severely limit poultry production in a number of developing countries. It is particularly devastating for small village farmers who usually have limited means of protecting their flocks. Newcastle Disease not only affects domestic fowls, but can also affect turkeys, pheasants, pigeons, quail and guinea fowl to varying degrees. Ducks and geese are also susceptible to infection, but these species rarely succumb to the disease.
Newcastle disease remains the major impediment to village poultry production. The disease is highly transmissible: birds are normally infected through direct contact with diseased or carrier birds but the virus can also be carried on contaminated objects such as chicken or egg crates, feed, vehicles, dust and clothing. The virus is usually inactivated by direct sunlight but in cool weather, the virus can survive in faeces and contaminated housing for up to 21 days. The virus can also persist in poultry products (meat and eggs) and can be carried by migrating wild birds. Eradication of Newcastle disease is therefore unlikely and there are few poultry species which are resistant to the disease. Continual vaccination programmes currently offers the only sustainable prospect for control.
Vaccines and virulence
Conventional vaccines are unsuitable for sustainable backyard poultry production as they are expensive, only available in large quantities and are affected by heat. Some also have to be injected, which is not feasible in multi-age village flocks. However new vaccines, developed through projects sponsored by the Australian Centre for International Agricultural Research (ACIAR), are thermostable, can be produced locally and can be delivered by eye drop or in the drinking water. These vaccines, known as strain NDV4-HR and strain I-2, have proved effective in trials under laboratory conditions and in villages, particularly in Malaysia, where Newcastle Disease has been largely controlled through the use of vaccines supplied in commercial food. Although only in use for a few years, the local production of the I-2 vaccine is most advanced in Vietnam and Mozambique where it is used for ND control in village chickens. In both Vietnam and Mozambique, the I-2 vaccine is routinely applied via eyedrop and often by the farmers themselves. The test is to see whether local production is economically sustainable in the long term. Apart from the thermostability of these vaccines, they are relatively cheap, safe to both chicken and handler (i.e. overdosing causes no ill effect) and can spread horizontally (from vaccinated to non-vaccinated birds).
Newcastle disease is caused by a number of strains of virus that belong to the Rubulavirus genus of the family Paramyxoviridae and virulence of the disease depends on the particular strain of the virus. Of the highly virulent strains, which are particularly common in south-east Asia, some grow in the gut (viscerotropic strains), while others grow mainly in the central nervous system (neurotropic strains). The most common indication of a serious outbreak of a neurotropic strain of the disease is seen in a nervous symptom exhibited in infected birds where the neck twists right back and the chickens simply fall over and die. Less virulent strains, such as those that are endemic in Australia, affect only the respiratory symptom, with varying degrees of severity. Symptoms may include loss of appetite, a dramatic drop in egg production, increased respiration, coughing, gasping and even rapid death without any exhibition of other symptoms.
However, mild strains of the disease have been known to mutate and become increasingly virulent as was recently discovered in the outbreaks of Newcastle Disease in New South Wales, Australia in 1998-9. Rapid diagnostic tests developed by the CSIRO's Australian Animal Health Laboratory (AAHL) demonstrated that the genetic 'fingerprint' of the disease involved in the outbreaks was related to an existing, milder strain already known to be present in Australia and was not as a result of disease brought in from overseas. Genetic fingerprinting of the outbreak strains also indicate that they are significantly different to the avirulent vaccine strains NDV4-HR and I-2. Serious disease causing strains of Newcastle Disease had not occurred in Australia since 1932 and there are now concerns that other mild strains present in the country could also mutate and become virulent.
In developing countries, diagnostic tests for the disease are not readily available so successful control will be dependent on an effective prevention program that has strong links with agricultural extension networks. In Bangladesh, the DANIDA poultry project (see Moving up the Pecking Order) has employed the use of poultry workers: rural people selected from local communities and trained to administer the eye-drop vaccine to all chickens in a particular village on a regular basis. NGOs have helped to distribute the vaccine to poultry workers and, although the total coverage within Bangladesh is not high, there has been very effective coverage of vaccine administration in the pilot project village areas. Similar systems involving community vaccinators administering locally produced I-2 vaccine are giving encouraging results in Mozambique. This approach was initiated by VetAID (a British NGO) and taken up by a number of NGOs, including World Vision.
Newcastle Disease is by no means the only disease to affect backyard poultry but it is certainly the most important disease in many areas and, until Newcastle Disease is controlled, farmers are reluctant (and it is generally not cost-effective) to control other poultry diseases. Work in Asia has shown that once Newcastle Disease can be controlled, farmers become more interested in investing in controlling other diseases. The potential benefits of controlling Newcastle Disease with respect to household security and poverty alleviation is demonstrated by the current efforts by the scientific community to understand the impact of this disease on village poultry,and the research and trials to provide a suitable vaccine for use in rural areas. However, Newcastle Disease is unlikely to be effectively controlled unless farmers are provided with ways and means to understand the best way for tackling the disease and this can only be achieved through extension services, which are appropriate to the needs of village poultry owners (see Bridging the Gap).Further information:
Newcastle Disease in Village Chickens: A Field Manual by Robyn Alders and Peter Spradbrow (1998) which can be accessed at ACIAR/University of Queensland www.vsap.uq.edu.au/ruralpoultry