Evaluation of innate immunity to newcastle disease in selected free-range local chicken ecotypes in Tanzania

Abstract

Newcastle disease (ND) is a major problem in poultry husbandry, causing tremendous losses to farmers in Tanzania. The problem is even bigger in the extensive free-range management system practices for free-range local chickens (FRLC) in rural areas of the country. Practical control of ND in FRLC is mainly by vaccination. The extensive husbandry system exposes FRLC to high risk of ND infection due to interaction with other bird species and contaminated environment. This study focused on the evaluation of ND in FRLC flocks to improve their productivity through selection of ND resistant FRLC ecotypes in Tanzania. A flock of 389 FRLC (324 females and 65 males) of three Tanzanian ecotypes namely; Ching’wekwe, Morogoro-medium and Kuchi were sampled from the coastal, central, northern and lake zones of Tanzania to establish a breeding parent stock for experiments. Each chicken was identified using numbered wing tag to maintain identity between parents and progenies. The chickens were first characterized phenotypically and genotypically. From the parents, morphometric parameters were taken, i.e. body length (BL), neck length (NL), chest girth (CG), shank length (SL) and shank girth (SG) were measured in centimetres (cm) using a tailor’s measuring tape. Body weights (BW) were measured in grams (gm) using a 0.01gm sensitive electronic weighing scale. These measurements were analysed using one-way analysis of variance (ANOVA) and compared among the three ecotypes. Differences in traits between ecotypes were considered significant at p≤0.05. Results from the morphometric analyses showed that the chickens are three distinctly three ecotypes. A total of 1,399 progeny chicks (477 Ching’wekwe, 315 Kuchi, and 607 Morogoromedium) were produced from the parent stocks in five rounds of incubation and hatching for use in the determination of immune response traits during infection with ND and also for determination of the population structure using single nucleotide polymorphism (SNPs) genotypes. Blood samples were collected on FTA cards (Sigma-Aldrich, St. Louis, MO, United States) at three weeks of age from the chicks and genotyped for 600K SNP panel with reference to Galgal 5 reference genome. The FRLC population structure was determined through admixture analysis using the SNP genotypes. Results showed that the FRLC ecotypes are two populations instead of three ecotypes as indicated by the phenotypic morphometric traits. One population is composed of Morogoro-medium and Ching’wekwe (population 1) and the other population is composed of the Kuchi (population 2). Immune response comparision between the populations was done in two appraches; - first, the chickens were infected with LaSota strain of Newcastle disease virus (NDV) at 28 days of age. Tears were collected at 2 and 6dpi days post infection. Blood samples were also collected for serum at 10 dpi. Viral loads at 2 and 6 dpi iwere determined after a quantitative real time polymerase chain reaction (RT PCR) and viral clearance rates were determined. Anti-NDV antibodies levels at 10dpi were determined from the serum samples. Results showed that chickens in population 1 had significantly higher viral loads at 2dpi than at 6dpi compared to chickens in population 2. They also had lower viral clearance rate (VCR) than chickens in population 2. The results further showed that population 2 birds had significantly higher sero-conversion rate than chickens in population 1. There was weak but positive correlation between antibody response and the VCR (0.08) for both population 1 and 2. In the second approach of the experiment, the chicks used in the first part of the experiment were challenged with virulent field strains of NDV on day 34. Viral loads at 2 and 6 days post infection (dpi), anti-NDV antibody titers, growth rate before and after infection with LaSota strain of NDV and growth rate after infection with virulent field stains of NDV were recorded. Correlation analyses among the traits before challenge with LaSota and after challenge with field strains of NDV were also evaluated. The results showed that population 1 chickens (Kuchi ecotype) had higher mean value measurements for all morphometric traits compared to population 2 chickens (Morogoro-medium and Ching’wekwe) indicating that the linear body measurements can be used for phenotypic selection of the chickens. However, there were individual variations; some individuals had extreme values that overlapped between the chicken populations. The anti-NDV antibody response was also weakly and negatively correlated to lesion scores after exposure of the chickens to virulent strains of NDV. Since results indicated that the chickens are admixed populations with large individual variations, selection for ND resistance chickens is important and requires the use of genetic tools as the canonical selection methods instead of the customary phenotypic methodologies that are being used for selection FRLC in rural areas.