Gastrointestinal parasites can play havoc in the digestive tract of all animal species, causing a lack of appetite, diarrhoea and poor growth, leading to economic losses for the farm. Selective breeding for parasite resistance in combination with other integrated control methods is considered an alternative means of parasite control.

Gastrointestinal parasites negatively affect all livestock species’ health, welfare, production and the quality of food resources, and despite all the traditional disease control systems, they continue to hamper the livestock industry.

Approaches such as the control of disease vectors and appropriate management methods help to reduce these adverse effects. However, there are often constraints on the sustainability of such disease control strategies, like the environmental and food safety-related impacts of chemical treatments, the affordability and accessibility of treatments to poorer livestock producers, and the evolution of parasite resistance to the treatments.

Considering the economic, health and welfare impacts of parasites on the livestock industry, prevention has more advantages than curing parasitic infections. Different environmental and host factors, some metabolic diseases and host immune status can affect an animal’s resistance to gastrointestinal parasites. Therefore, the genetics of disease resistance involving immune and non-immune mechanisms is an approach to this issue.

What is disease resistance?
Disease resistance is defined as the inherent capacity of a previously unexposed animal to resist disease when challenged by pathogens or parasites and is considered as the host’s ability to moderate the pathogen or parasite lifecycle, and its resistance to the disease consequence of infection.

Natural resistance is heritable and transmissible from parent to offspring. Therefore, increasing the overall level of genetic resistance of a population through the use of selective breeding programmes could improve animal health management systems.

Advantages of genetic disease resistance
The benefits of incorporating genetic elements in disease management strategies include the permanence of genetic change once it has been established, the consistency of the effect, the absence of the need for purchased inputs once the effect is established, the effectiveness of other methods is prolonged as the likelihood of resistance emerging is reduced.
On top of this there is the possibility of broad spectrum effects and increasing resistance to more than one disease, as well as having less impact on the evolution of macro-parasites, such as helminths, compared to other strategies (such as chemotherapy or vaccination), plus it adds to the diversity of disease management strategies.

Application of genetic management strategies
Applying different strategies to the genetic management of diseases depends on the nature of the problem and the resources available. These approaches include choosing the appropriate breed for the production environment, crossbreeding to introduce genes into well-adapted breeds for the required purpose, and selection for individuals with high levels of disease resistance.

There are breeding programmes focused on selecting commercial animals for enhanced resistance to some diseases, such as parasite infections and some forms of mycotoxin poisoning. Finding genetic markers associated with resistance to infection potentially allows selection for increased resistance in the absence of infection.

Marker-assisted selection
Marker-assisted selection is a process in which a morphological, biochemical, or DNA/RNA-based marker is used for indirect selection of a trait of interest, such as disease resistance. The marker used for selection is associated at high frequency with the gene of interest due to genetic linkage. There are selectable markers which eliminate certain genotypes from the population and screenable markers which cause certain genotypes to be readily identifiable, at which point the experimenter must score or evaluate the population and act to retain the preferred genotypes.

Depending on the trait of interest, marker-assisted selection may be cheaper and faster than conventional phenotypic assays. Multiple markers can be evaluated using the same DNA sample, and once DNA has been extracted and purified, it may be used for multiple markers for the same or different traits, thus reducing the time and cost per marker.

Identifying genetic markers
Genetic markers can be identified by a range of molecular techniques, such as microsatellites or single nucleotide polymorphism detection. Linked markers are molecular markers located very close to major genes of interest. There are several known phenotypic and genetic markers for gastrointestinal parasite resistance in naturally infected animals that could potentially assist response to selection.

The Major Histocompatibility Complex (MHC) involves a series of highly polymorphic genes which are responsible for the initiation of the immune response when an animal is challenged by pathogens or parasites. The MHC is divided into 3 regions: class 1, class 2 and class 3.

Faecal Egg Count (FEC) is used as an indicator trait to determine resistance to gastrointestinal parasites. The heritability of the FEC varies considerably depending on both the parasite species and animal breed. Immune response evaluation is another indicator trait that can be used.

Selective breeding
Selective breeding to take advantage of within-breed variation in disease resistance is an important disease control strategy. Selection for parasite resistance alone can result in negative traits, such as lower live-weight gains. Therefore, it is essential to apply appropriate selection policies and to understand the genetic architecture of underlying resistance to predict genetic risk or selective breeding.

The application of selective breeding for parasite resistance in combination with other integrated control methods is considered an alternative means of parasite control in the long run. However, disease resistance varies between species, between breeds and between individuals within breeds, and identification of the phenotype for disease resistance is challenging, because in a population containing both healthy and sick animals, not all healthy animals may be disease resistant.