| dc.description.abstract | This study aimed at investigating the influence of genetic and non-genetic factors
on immune traits to inform on possibilities of genetic improvement of disease
resistance traits in local chicken of Kenya. Immune traits such as natural and specific antibodies are considered suitable indicators of an individual's health status
and consequently, used as indicator traits of disease resistance. In this study, natural antibodies binding to Keyhole Limpet Hemocyanin (KLH-NAbs) was used
to measure general disease resistance. Specific antibodies binding to Newcastle
disease virus (NDV-IgG) post vaccination was used to measure specific disease
resistance. Titers of KLH-NAbs isotypes (KLH-IgM, KLH-IgG and KLH-IgA) and
NDV-IgG were measured in 1,540 chickens of different ages ranging from 12 to
56weeks. A general linear model was fitted to determine the effect of sex, generation, population type, phylogenetic cluster, line, genotype and age on the antibody traits. A multivariate animal mixed model was fitted to estimate heritability
and genetic correlations among the antibody traits. The model constituted of
non-genetic factors found to have a significant influence on the antibody traits as
fixed effects, and animal and residual effects as random variables. Overall mean
(±SE) concentration levels for KLH-IgM, KLH-IgG, KLH-IgA and NDV-IgG were
10.33±0.04, 9.08±0.02, 6.00±0.02 and 10.12±0.03, respectively. Sex, generation
and age (linear covariate) significantly (p<0.05) influenced variation across all
the antibody traits. Genotype effects (p<0.05) were present in all antibody traits,
apart from KLH-IgA. Interaction between generation and line was significant
(p<0.05) in KLH-IgM and NDV-IgG while nesting phylogenetic cluster within
population significantly (p<0.05) influenced all antibody traits, apart from
KLH-IgA. Heritability estimates for KLH-IgM, KLH-IgG, KLH-IgA and NDVIgG were 0.28±0.08, 0.14±0.06, 0.07±0.04 and 0.31±0.06, respectively. There
were positive genetic correlations (0.40–0.61) among the KLH-NAbs while negative genetic correlations (−0.26 to −0.98) were observed between the KLH-NAbs
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| MIYUMO et al. 107
1 | INTRODUCTION
Indigenous chicken (IC) (Gallus gallus domesticus) plays
significant roles in nutrition, food security and economic
growth in many rural households in most countries in the
tropical regions (Alders & Pym, 2009). In Kenya, IC account for about 80% of the total chicken population and
is kept by over 75% of the rural households (Magothe
et al., 2012). Their popularity, particularly among rural
households, is attributed to their ability to produce under
low-input systems and adapt to local environmental conditions (Olwande et al., 2010). Despite their adaptive ability, IC is predominantly raised under scavenging systems
that are constrained in terms of, among other challenges,
diseases that limit optimal utilization and expansion of
the sub-sector (Lamont, 2010). For instance, depending on
the season, disease prevalence and mortality rates, reduction in productivity and product condemnation of about
20%–100% are experienced at the farm level and these
cumulatively translate to 10%–15% of annual economic
losses (Okeno et al., 2011; Rist et al., 2015). With respect
to climate change effects, environmental conditions are
expected to favour pathogens and parasite proliferation
resulting in increased disease occurrence.
Among the various poultry diseases, Newcastle disease
(NCD) which is endemic among chicken in the tropics is
currently considered of importance because of the massive
production and economic losses it causes in the industry
(Alders et al., 2018). While bio-security measures combined with vaccinations have proved useful in controlling
NCD, the effects are, in some cases, temporary and/or
highly influenced by the environment (Zanella, 2016).
Control by use of anti-microbial drugs, on the other hand,
is beneficial but often misused leading to product safety
concerns (Lamont, 2010). Furthermore, the reliance on
free-range scavenging system among smallholder farmers'
increases transmission rates of NCD between and within
flocks (Lwelamira, 2012). These factors emphasize the
importance of considering alternative measures to maintain or enhance disease resistance in chicken flocks.
****Selective breeding for disease resistance, as a control measure, offers an opportunity to enhance adaptability of IC, especially, under scavenging systems where
they are exposed to a myriad of disease pathogens (Cheng
et al., 2013; Lwelamira et al., 2009). Besides, disease resistance is ranked a trait of economic importance among
IC farmers in Kenya (Okeno et al., 2012), an indication
that the trait should be considered in breeding goals.
Disease resistance is generally defined as the ability to
prevent infection when exposed to a pathogen or control
a pathogen's life cycle (Zanella, 2016). The trait, however,
is not often absolute because of the complex biological
networks and host-pathogen interactions that control disease resistance, and the high sensitivity to environmental
stressors (Cheng et al., 2013). On the other hand, disease
resistance measurement requires that animals are challenged with pathogens and this severely violates animal
welfare, presents biosecurity risks and is economically
costly (Zanella, 2016). To circumvent these limitations,
health traits related to the immune function are suitable
indicator traits for indirect improvement of disease resistance (Cheng et al., 2013). Success of their utilization
is, however, dependent on whether the traits are related
to disease resistance, easy and cheap to measure and of
utmost importance, heritable. Natural antibodies (NAbs)
binding keyhole limpet hemocyanin (KLH-NAbs) and
specific antibodies (SpAbs) binding NCD virus (NDV-IgG)
have been used extensively to measure general disease resistance and specific resistance against NCD, respectively
(Lwelamira, 2012; Sun et al., 2013).
Considering the intrinsic nature of innate humoral
immunity, KLH-NAb titres is expected to vary among
individuals (Mangino et al., 2017). Previous studies on
chicken found that part of this variation was due to additive genetic effects and further estimated moderate to low
(0.44–0.07) heritability for KLH-NAb isotypes (Berghof
and NDV-IgG. Results from this study indicate that non-genetic effects due to
biological and environmental factors influence natural and specific antibodies
and should be accounted for to reduce bias and improve accuracy when evaluating the traits. Subsequently, the moderate heritability estimates in KLH-IgM and
NDV-IgG suggest selection possibilities for genetic improvement of general and
specific immunity, respectively, and consequently disease resistance. However,
the negative correlations between KLH-NAbs and NDV-IgG indicate the need to
consider a suitable approach that can optimally combine both traits in a multiple
trait selection strategies. | en_US |
