Every year animal diseases lead to millions of rand worth of production losses. Since control is becoming increasingly more expensive and challenging, scientists are looking at the possibility of identifying and increasing the number of animals that are genetically (naturally) resistant to diseases.
Renewed interest in genetic resistance has been sparked due to the cost of treating livestock diseases, the building of resistance against medications, the fact that few new antibiotics have been developed over the past three decades and consumer resistance against the deposition of drugs in meat.
The estimated cost of livestock disease control in developed countries comprises up to 20% of producers’ turnover, while it can be as high as 35 to 50% in developing nations. The actual cost is probably even higher considering the indirect attributable expenses relating to diseases and their control measures.
Some diseases can be transferred to people and other species, an aspect that is often not considered when putting a cost to a particular animal disease. Certain species can also become hosts for the organisms causing the diseases. Scientists are encouraged by this to explore more possibilities for control, which includes identifying animals with natural (genetic) resistance against disease and to increase their numbers.
New research is backed by the fact that significant additional genetic variation exists between and within livestock breeds for their survival, longevity and resistance against tropical environmental factors, which include diseases, parasites, high temperatures, the sun’s rays and nutritional stress. Some of these traits are highly hereditary.
Research done on mice showed that if one selects for certain genetic traits, other ones, including resistance against other diseases and barriers, can be affected and even significantly weakened. To counteract these degenerative genetic relationships, it may be necessary to select for the different traits in the paternal and maternal line, so that they can complement rather than counteract each other when the animals are mated, thus effecting optimal genetic resistance against disease.
Intensive farming systems
Dr Michael Bradfield of Breedplan SA says the transition to intensive farming systems has made natural (genetic) resistance against diseases even more imperative. Producers are demanding a solution, because consumers are questioning whether the continuous administering of drugs is a proper practice for mankind as a whole.
“It is a well-known fact that in an area where specific diseases and parasites causing them occur, animals do not become sick. Different species contract different diseases, and the increase in game numbers, along with various other factors such as the genetic composition, nutrition, condition, stress, management and immune status of the animal, is playing an increasingly important role.
“A combination of these factors is the reason why the animal did not become sick. Many diseases are transferred by ticks and although the affected animals did carry ticks, it could well be that they that did not become sick since they were not bitten by an infected tick. Therefore, to merely label an animal that did not become sick in any given season as resistant against a particular disease, will usually not be accurate.
“By artificially infecting animals with the disease to determine which ones do not become sick or are less severely affected by the disease, may be more accurate, but is an expensive and time-consuming exercise – because the animals have to be monitored closely and the infection closely observed.
“Before such an infection process can take place, it is necessary to provide ethical justification for the process. The immunological pathology of various veterinary measures must also be taken into account to ensure that other diseases than those for which the test is intended, are not spread in the process,” he explains.
Michael says a subclinical condition exists where the animals in fact contracted the disease, but did not exhibit any signs of disease. Certain diseases can appear to be others, such as bronchitis which is diagnosed as pneumonia, while they are, in reality, two different diseases. He says the greatest challenge is collecting the phenotype data to determine and discern the influence of genetics and the environment.
Research has shown that the hereditability of resistance against diseases is between 20 and 30%, which allows for progress through selection based on genetic traits. The challenge is to collect and process the information.
Therefore, selection can be done directly in the herd using phenotype data. The animals can be exposed to the relevant diseases in a controlled manner, or through observation of the natural resistance of animals’ progeny and the identification of bulls/rams which transfer resistance against diseases.
Somatic cell counts
Michael says indirect selection is possible by, for example, recording somatic cell counts in the milk of cows in order to indirectly identify resistance against mastitis. In small stock, manure egg counts can be used to determine the parasite infection of animals kept under the same conditions, and to identify animals with naturally fewer parasites.
Genomics brings hope to the industry, as it will allow for a simple solution that will make it possible to determine resistance in an animal by means of blood or hair samples. Research is usually conducted on mice, as their numbers can be increased easily and rapidly, and they carry diseases. The aim of this research is to create models by means of which it is easy to understand diseases in other species.
Many of the genes that cause or prevent diseases have been identified in mice. The challenge lies in the fact that diseases and resistance are not located in a single gene. Therefore, if one selects for resistance against a specific disease, it will also affect other genes.
A number of genes have been identified in cattle and sheep, such as the Nramp1 gene which is related to brucellosis and tuberculosis, as well as a large gene system called the major histocompatibility complex (MHC) which relates to the immune system. The challenge here is that many genes are associated with MHC and a large number of genetic markers are involved.
“We are increasingly realising that many genes are involved in disease resistance, making genomic selection possible, but not at all easy. However, considering the progress made in genetics over the past 30 years with regard to all species, it is only a matter of time before a solution is found,” Michael says. -Andries Gouws, Stockfarm