Newcastle disease (ND)
FAO estimates a population of approximately 1.38 billion chickens in Africa of which more than 70% are in villages playing a vital role in the livelihood strategies of many poor rural households. With their economic asset value estimated at US$ 5.75 billion, village chickens provide meat, eggs, food for special festivals and petty cash for school fees and medial expenses. However, one major constraint to production of village chickens in many developing countries is ND whose outbreaks are unpredictable and discourage poultry keepers from paying proper attention to the husbandry and welfare of their chickens.
The incubation period of ND varies from 4 to 5 days (range 2 to 15 days) depending on the strain of the infecting virus. The ND virus may persist in undispersed chicken faeces for more than six months but under village conditions the virus is unlikely to survive outside a host for more than one month. In commercial poultry production, and also under experimental conditions with compost from chicken faeces, the virus has been found to survive for 16-21 days. The clinical signs of ND vary considerably depending on the virulence and tropism of the virus involved; the species of bird, the age of host, the immune status of the host and environmental conditions.
ND is present everywhere on the African continent with the exception of Egypt, Morocco and Tunisia which have not reported the disease occurrence in the past four years. Libya reported suspected disease outbreak in 2005.
In countries where there are circulating strains of ND virus, chickens may experience mortalities of up to 100% in unprotected flocks. Village poultry keepers tend to accept losses due to ND as near inevitable, which acts as a powerful disincentive to improve other aspects of the husbandry of their birds. Observations from Mozambique indicate that controlling ND had the potential to increase incomes derived from poultry by 42%. Adoption of additional simple and low-cost husbandry measures increased this to 82% and after one year of successful campaign, the average chicken flock had increased from 5 to 13 birds thus increasing off-take numbers for increased consumption of chicken products or sales to increase household purchasing power, and because women were involved throughout, increased decision-making power for women.
A preliminary estimate generated by a model developed by ILRI suggests that in Africa, more than 589 million chickens are at risk of ND. The model predicts that if an ND vaccine was used in 10% of the birds at risk, this would potentially prevent more than 6 million chicken deaths.
Current strategies for control using vaccination
Vaccination is the only effective way of controlling ND. Thermostable ND vaccines, together with inactivated ND vaccines have been used successfully in the backyard sector in many African countries. An advantage of the inactivated ND vaccines, (though more expensive), is that they can be found commercially, and incorporated into programs aimed at supporting backyard poultry owners. Another advantage of is that the number of applications per year are fewer. Many vaccines (both live and inactivated) produced by multinational companies are freeze-dried into multi-dose vials, often containing 1,000 or 2,500 doses and, because of high number of doses they are mainly of benefit to commercial poultry producers whose chickens are kept in large, single-age or confined flocks. The live vaccines must be kept under "cold chain" from manufacture until administration to the chickens.
In contrast, village chickens are raised in small, multi-age, free-range flocks and large multi-dose vials of vaccine are therefore inappropriate. Further, it is difficult to maintain "cold chain" under village conditions. In many rural areas in Africa, these vaccines are there not even available. In this regard, the future long-term control of ND in village chickens in Africa lies in supporting the production and utilization of thermostable I-2 vaccines.
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