One of the constraints limiting increased chicken production is poor health. Newcastle disease (ND) is regarded as the principal factor limiting rural chicken production in Africa and can typically kill up to 80% of household poultry.
Birds that survive ND have high antibody levels and are resistant for a while. However, the level of antibodies decline with time resulting in reduced protection as antibodies wane and eventually become susceptible again. This cycle is repetitive in the life of the birds.
In intensive small scale chicken flocks, Gumboro or Infectious Bursal Disease (IBD) is very important. This disease results in up to 50% mortality in a susceptible flock.
IBD also has an immuno-suppressive effect, resulting in poor response to vaccination and increased susceptibility towards other pathogens. Intestinal and ecto-parasites are also of importance in traditional chicken production and can cause high mortality and as well as compromised production.
Other pathogens that have been observed to cause major losses in chicken include Salmonella, Mycoplasma, infectious laryngotracheitis, E. coli and fowl pox virus.
These myriad pathogenic challenges prevalent in tropics overwhelm the immunity of chicken resulting in reduced efficiency of the memory cells as some of these diseases infect birds later in life. This leads to delayed and inefficient response hence making the birds more susceptible.
Chicken exhibit variation in resistance and tolerance. This variation can be exploited and technology transferred to resource poor farmers.
Three different ways or strategies can be used to genetically improve disease resistance: First, selecting and breeding the birds that have shown ability to survive infections; for example the simple or empirical method is to ensure that all candidates for becoming parents are exposed to pathogens.
Those that survive have proven resistance, the genetic element of which will be transmitted to the next generation. This is how indigenous chickens have been naturally selected for adaptation to locally present pathogens.
Another approach available to commercial breeding companies is to make an offspring test. An offspring test involves producing a number of offspring from each breeding candidate and placing them in an infected environment.
The rate of mortality due to infectious diseases in the offspring groups is a direct measure of breeding value for disease resistance in the parents, and can be used to inform the selection of birds for breeding. For example, there has been use of an offspring test to obtain lines resistant to Marek’s disease after four generations.
There has been use of Sheep Red Cell as a noble antigen to select divergently for low and high responders after 30 generations. This method is not practiced due to animal welfare concerns.
The second method is selection of parents based on offspring upon measuring immune proteins. For example, the most extensive breeding experiments for immune traits have been performed in cattle and chicken where genetic parameters have been estimated.
The third method is using molecular methods to choose breeding animals with genes or markers for genes that are known to give a better protection against infection.
This involves detecting direct markers or genes responsible for the expression of the trait of interest or using genome wide association studies to identify QTL conferring resistance.
In light of this, InCIP has started an elaborate on-station experiment in Kenya to gather genetic variation of certain immune traits.
This study being conducted by Mr. Joel Khobondo; a PhD student of Animal Breeding and Genomics at Egerton University, Department of Animal Sciences intends to quantify genetic variation in birds and identify genes that influence disease resistance.
The information will then be used in selection and breeding for disease resistance in indigenous chicken to meet the low input management systems. The study proposal for this work has been accepted and approved by the graduate school of Egerton University and sampling of animals for DNA extraction is underway.