Poultry production has undergone rapid changes during the past decades due to the introduction of modern intensive production methods, new breeds and improved biosecurity and preventive health measures. Moreover, these intensive production methods place high demands on proper health, hygiene and management and require only a small, but very skilled labour force. In developing countries, however, adoption of this type of production has been limited due to the high inputs as listed above. The progress in industrial poultry production methods has thus had little effect on subsistence poultry production in the rural and peri-urban areas. In these areas access to poultry meat and eggs depends on village-level poultry production. Although poultry production is considered as secondary to other agricultural production systems it has an important role in supplying villagers with additional income and high quality protein. This system provides valuable protein through a low input system, now representing 30% or more of the protein consumed (FAO, 1998).
Almost all families in developing countries keep a chicken flock with an average size of about 10 adult chickens, varying from 5 to 50 animals. The majority of these animals are kept in free-range scavenging systems, where the birds scavenge around the house during daytime. Primitive housing of the birds during the night, however, often takes place. Supplementary feed consists mainly of household wastes, insects, larvae and seeds (Minga et al., 1989; Kabatange et al., 1990; Aini, 1990; Pandey, 1992).
Mortalities observed are in the range of 80 - 90% within the first year after hatching (Matthewman, 1977; Wilson et al. 1987). For the same reasons the owners never include chicks when they refer to the flock size. The mortality is believed to be caused by mismanagement, lack of supplementary feeding, predators and diseases (Aini, 1990; Pandey, 1992). Little research has been published on rural poultry health, despite the fact that up to 80% of the poultry population in Africa and Asia is kept by the households as free-range chickens (Minga et al. 1989; Aini, 1990).
Although solid data have not been published, Newcastle Disease (ND) is regarded as the principle factor limiting rural family poultry production in all African and Asian countries. ND may kill up to 80% of household poultry in Africa (Minga et al. 1989; Aini, 1990; Bell, 1992), but is not expected to account for the high early mortality rate according to the authors. In addition, detailed epidemiology of the disease in the village situation is largely unknown (Yongolo, 1997). Furthermore, recent studies have shown that other diseases are present in scavenging poultry communities (Bell et al. 1990; Cumming, 1991; Bell, 1992; Chrysostome et al., 1995; Permin et al., 1999). Since most of our knowledge relies on seroprevalence studies, solid longitudinal studies on causes of mortality are strongly needed to improve our knowledge on the prevalence and significance of the single diseases under village conditions. The following data therefore mainly reflect experience obtained under backyard conditions in developed countries.
Approximately 80% of the world poultry population is kept as free-range poultry (Minga, 1989; FAO, 1998). The free-range poultry production system has also been designated as the 'low input - low out' system (Pandey, 1992). Mortality in this system is in the range of 80 - 90% within the first year after hatching (Matthewman, 1977; Wilson et al., 1987) and is believed to be caused by mismanagement, lack of fresh water and supplementary feed, predators and diseases (Aini, 1990; Pandey, 1992). Of these, diseases are believed to be the main limiting factor to the production of indigenous chickens (Aini, 1990). Among causes of early mortality nutritional diseases might be expected to dominate due to shortage of supplementary feed before and after hatch. In addition, the quality of hatching eggs might be questioned under the climatic conditions present in these countries. Avitaminoses and lack of protein weaken the chicks and make them vulnerable to other diseases and predators. Diseases are also easily contracted under free-range conditions due to the scavenging habits of these birds (Soulsby, 1982; Pandey, 1992). With an unconfined type of management, disease control is very difficult to carry out and is therefore rarely practised by the owners.
As mentioned earlier, Newcastle Disease is believed to be the most important disease in free-range systems (Minga, 1989; Aini, 1990; Bell, 1992). During outbreaks of the disease up to 80% of the population may die. This, however, is dependant on different factors including the virulence of the strain causing the outbreak (Alexander, 1997). A recent study in Nicaragua (Kyvsgaard, 1999) has, however, shown that in ND-immunised birds mortality is still high. The majority of the mortality is found in chicks and growers up to 3-4 month of age. In these groups up to 52.5% of the animals died due to causes other than ND. Similar studies in Mali by Wilson et al. (1987) have shown that chick mortality is in the range of 60% within the first 3 months after hatching.
A study in Morocco (Bouzoubaa et al., 1992) has revealed that up to 58% of the village chickens had antibodies against Salmonella gallinarum and S. pullorum. Similar findings were reported in Nigeria by (Adesiyun et al. 1984). Chryosostome et al. (1995) also reported that 10% of the village chickens had antibodies against S. pullorum and that 62% had antibodies against Mycoplasma gallisepticum. Furthermore, 65% of the animals had antibodies against ND. In Mauritania, Bell et al. (1990) found that 17.5% of the birds had antibodies against S. pullorum and that up to 46.2% of the birds had antibodies against Gumboro disease. In the same animals 7.5% had antibodies against ND.
In Tanzania, Permin et al. (1999) examined 600 live chickens and found the presence of a range of diseases. All animals were parasitised with one or more (up to 14 species) species of endoparasites. In total 29 different species were detected in the study. Furthermore, 65.7% of the animals were parasitised with Cnemidocopt-es mutans, Dermanyss-us gallinae and/or Echidnopha-ga gallinacea.
The animals were also infected with a range of haemoparasites, the most common being Plasmodium juxtanucleare and Aegyptinella spp. Antibodies against Newcastle disease was seen in 7.3%, against Salmonella enteriditis in 2.0%, against Salmonella gallinarum/pull-orum in 52.7%, against Infectious Laryngotracheitis in 58.3% and against Gumboro disease in 42.3%. Similar studies have, to the knowledge of the authors, not been carried out in Asian countries. The significance of all these diseases, however, remains to be investigated. In addition, it should be noted here that a general trend for these studies is that they have only looked for antibodies against selected diseases.
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