Evaluation of innate immunity to newcastle disease in selected free-range local chicken ecotypes in Tanzania
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Date
2020
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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.
Description
PhD Thesis
Keywords
Innate immunity, Newcastle disease, Free-range local chicken, Tanzania