Mortality rates adjusted for unobserved deaths and associations with Newcastle disease virus serology among unvaccinated village chickens in Myanmar

Abstract

Village chickens are an important livestock for many rural families in Myanmar and other developing countries. Village chickens are kept under free-ranging conditions, with confinement only at night. Therefore, it is likely that some deaths are not observed by farmers. We conducted a longitudinal study from November 2003 until May 2004 to describe temporal patterns of mortality of village chickens in 10 villages in Myanmar. Field veterinarians first identified the numbers of birds in all chicken-owning households in each village. We then selected 307 households randomly with stratification by flock size. Each study household was then visited once monthly at which time questionnaires were completed recording current flock structure and numbers of hatchings, mortalities, sales and birds consumed since the previous visit. In addition, sera were collected from a sample of adult birds and growers. Depending on month and age group of chicken, from 71 to 231 (out of 290–307) households had discrepancies in the counts of birds. For chicks, at least one-quarter of the households had unobserved losses of at least 5 chicks per household (maximum 66 chicks); unobserved losses were less for growers and adult chickens. The median month-specific, village-specific mortality rates per 1000 bird-days at risk (counting missing birds as deaths) ranged from 0.8 to 1.7 for adults, from 0.4 to 4.7 for growers and from 8.0 to 16.5 for chicks.

Across all birds, the prevalence of protective titres against Newcastle disease virus was 79% (95% confidence interval 74, 84); higher prevalences of protective titres were associated with reduced mortality rates in the following months.

Introduction

In Myanmar, village chickens are a source of both income and food for rural households. Village chickens are kept under free-ranging conditions, with confinement only at night (Henning et al., 2007). Newcastle disease (ND) is considered to be the major constraint to raising village chickens in developing countries of Asia and Africa (Aini, 1990, Permin and Bisgaard, 2000) and is the most important poultry disease in Myanmar villages (Henning et al., 2007). However, the antibody seroprevalence to ND has not previously been evaluated longitudinally in this country. Deaths of village chickens can also occur due to predation, exposure to extreme weather conditions (for example, heat stress or drowning in the raining season) and other diseases (Henning et al., 2007). Older birds can be lost due to theft (Henning et al., 2007). Mortality rates (MRs) expressed as incidence rates are rarely reported for village-chicken populations because data from repeated visits, including precise numbers of withdrawals and additions, are seldom collected in rural villages. Because chickens scavenge and move without restriction within villages, it is also likely that some deaths are not observed by farmers, particularly at times of the year when other agricultural activities are of greater importance than chicken keeping (Henning et al., 2007). However, these discrepancies between reported and unobserved mortalities have never been comprehensively quantified. If the number of unobserved mortalities among village chickens could be estimated, the MRs expressed as incidence rates could be adjusted accordingly. We had four objectives: to estimate the antibody seroprevalence of protective titres against ND virus (NDV); to report bird-count discrepancies and the age-group-specific MRs that considered “missing” birds as deaths; to describe cause-specific MRs; and to describe the monthly MRs in relation to the seroprevalence of protective titres in the preceding months.