To Breed or Not to Breed: Selecting for Parasite Resistance

Reviewing Bishop, 2012 (Possibilities to breed for resistance to nematode parasite infections in small ruminants in tropical production systems)

TZ field work 005
Herd of goats in northern Tanzania

Parasite trouble in paradise

In tropical ecosystems livestock production is a critical source of food security and livelihood, particularly in low and middle income countries. Diseases that negatively affect livestock health and production can also have a serious impact on human health, through loss of income, food, and milk supply. Parasitic disease is considered the most important disease of livestock in developing countries, especially in sheep and goat production systems. Of particular concern is the parasite of sheep and goats known as Haemonchus contortus (or the barber pole worm). This parasite feeds on blood in the stomach of sheep and goats causing anemia and sometimes death.

Current practices aimed at controlling these parasites are centered on regularly administering drugs (anthelmintics), however, their widespread use has lead to a growing number of parasites that are resistant to drugs. This expanding army of drug resistant worms is now a global problem–one with no ready solution. What is clear is that alternative and supplementary methods need to be identified and used to combat parasites, as we can no longer rely on drugs to do so. Today’s blog post discusses the addition of selective breeding to the arsenal of parasite control measures.

Breed advantage

It is well known that among sheep and goats, certain breeds are more resistant to parasites than others. In general, breeds of livestock that have been developed in a tropical climate are better suited for living with the rigors of such an ecosystem, including the local parasite fauna. This makes sense when you think about it; when it comes to parasite resistance in sub-Saharan Africa for instance, who would you wager would be more successful…..Red Maasai sheep or Mary’s little lamb?

Mary's
Red massai Mary’s Little Lamb versus
Red Maasai sheep

Unfortunately, it is not as simple as merely selecting the right breed. There is something called within breed variation that needs to be accounted for. In any population there exists a natural variability of heritable traits. Simply put, heritable traits are characteristics passed down through generations, such as eye color in humans. In livestock, resistance to parasitism has been shown to be a heritable trait. Within any herd, you can expect some genetic differences between individuals in their ability to cope with parasites. By only selecting animals for breeding that have strong genetic advantages in parasite resistance, it is possible to create future generations that are better adapted to parasites than preceding ones. It all may sound straight forward, but the trouble is, how do you choose which animals have genetic advantage?

Choosing the champions

There are several genetic markers that can be used to identify resistance to parasitism, but these methods are highly technical and are not of practical use in tropical production systems- which leaves us to choose animals based on physical characteristics that can be more easily observed (phenotypes), but that may not be as precise with regards to the genetic resistance of an animal.

There are several methods by which a farmer could choose the best candidates for breeding parasite resistance. One of the most common methods used is measuring fecal egg counts (FEC). This test is performed by taking a stool sample from each individual animal and counting the number of parasite eggs present in a proportion of the sample. Monitoring FEC can be time consuming, but may give an indication of the level of parasitism in each animal, though the FEC technique has several drawbacks.

FEC
View of parasite eggs from a sheep through a microscope

First, the number of eggs shed by an animal may vary substantially from day to day, and even within one fecal sample, leaving a lot to be desired in terms of accuracy. Second, although FEC may give an indication as to an animal’s resistance toward becoming infected, it does not measure tolerance.Tolerance is the ability of an animal to do well despite parasitism, and is debatably a better trait than resistance. For instance, animals with a high FEC may be doing just as well (or perhaps even better) in terms of production than its herd mates with lower FEC.

There are several strategies that exist for selecting breeding champions based on tolerance to parasitism. Animals that require drug treatments less often can be considered tolerant to parasites (different that resistant), and measuring the growth rate of individuals can allow selection of animals that achieve the best production results regardless of their parasite burden.

FAMACHA
Examining a goat’s inner eyelid for evidence of anemia

 

Similarly,in the case of Haemonchus contortus (the blood sucking worm of sheep and goats), infected livestock can be evaluated for anemia by examining their inner eyelid color. Animals that have pale inner eyelids are more affected by the blood sucking parasite; therefore, non-anemic animals can be selected for breeding programs.

 

 

In truth, it is likely a combination or resistance and tolerance that best approximates the genetic advantage of some individuals. The strategy employed to identify individuals for a selective breeding program will most likely depend on which method is most practical for the individual farmer.

Summary

Parasitism is a pervasive problem in tropical livestock systems, and has particularly harsh health and economic impacts in the developing world.  The rising tide of drug resistance demands that alternative and supplementary methods of controlling parasites be identified and implemented.

The existing genetic diversity in parasite resistance among livestock allows for the opportunity to enhance parasite control through selective breeding. Using appropriate local breeds that are well adapted to tropical ecosystems is an important first step. Genetic variation in resistance also exists between individuals of the same breed. Identifying animals that excel at resisting or tolerating parasitism for targeted breeding programs can lead to future generations that are more resistant, and potentially reduce the amount of drugs needed for parasite control.

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