Selecting for a Disease Resistant Flock

Ariena Shepherd (MMedvet Smallstock)

Tugela Veterinary clinic

036 448 2613


Sheep have a relative short generation interval and therefore genetic progress is relatively quick. They also have a wide genetic variability both between and within breeds which again helps with genetic selection. Selecting for disease resistance can be done on many levels. The development of resistance to anthelmintics brought about the focus on breeding disease resistance flocks.

Suddenly sheep farmers (and veterinarians) had to change the way they treat disaeases .

Also in South Africa and especially KZN, husbandry practices had to change due to external factors like stock theft and predators as well as consumer preferences. This is also happening in other countries e.g. Australia has been mulesing their sheep for decades but will have to stop due to consumer pressure and thus had to find alternative solutions.

Many countries are currently involved in the sheep genomics project which started in 2002. They have managed to map the sheep genome and are busy identifying genetics sequences for diseases, production parameter and other aspects of breeding.

These genetic sequences are called single nucleotide polymorphism (SNP) arrays or SNP chips. These chips can select for multiple genes at the same time but cost currently prevent commercial use.

The main areas of selection are internal parasites, external parasites, feet problems, mastitis, eye problems and other diseases.

As with all genetic selection practices it is important to monitor both the genetic progress of the selection criteria as well as other parameters to guard against involuntary selection of negative traits. Often arguments are made that production will be negatively affected if the selection criteria are aimed more at disease resistance rather than more production parameter and disease treatment.

However most disease resistant selection is far more cost effective than treatment of disease e.g. mastitis. Higher milk producing animals are more prone to mastitis but treatment of a case and the concurrent production loss far outweigh the possible loss in production.


Internal parasites

FAMACHA, faecal egg counts (FEC) and dag scoring (DS) are easy on farm tools. They can be used to select for both resilience and resistance. FEC are most useful in young animals. In ram selection doing individual FEC about 3 times between 3 and 10 months will identify both resistant and resilient animals. Combined with monthly FAMACHA, DS and body condition scores (BCS) suitable rams can be selected. In ewes the cost of doing individual FEC limits it use. FAMACHA, DS and BCS are easy to do and will help to identify poor doers. It is best to do this monthly but again it is not always possible.

An easy time to do it is at weaning and about 6 weeks later. Individual FEC are most useful if done pre-lambing and when weaning lambs. Score averages are used to cull animals. As resistance increases cull scores can be adjusted. Care must be taken with FEC e.g. with Haemonchus counts may be very high where as with Teladorsagia they may be very low.

Implementing this takes about 3-5 years. The level depends on the farmer. Some of our flocks are now at one anthelmintic treatment annually while others are still doing 2 or 3. It can be done gradually or fast, depending of the farmer. We found that there was a production penalty during the first 2 to 3 years. We were struggling to wean more than 110 to 120%.

However this did not last and now farmers are weaning more than 150%. It is important to make sure that the anthelmintics used are effective. If you are only dosing for survival then animals will die if the remedy is not effective. Good record keeping is also necessary.

You must keep track of the remedy used i.e. which group number(s), which animals and the date treated. Good dosing practices are also necessary. Equipment must be good, dosages accurate and given correctly. Production penalties can be limited and even negated by feeding extra protein.



Generally have no effect. No scientific papers exist that show that tapeworms affect production in normal healthy lambs or ewes. These worms are easy to see and therefore farmers always want to treat for them but it is not cost effective. Lambs will only have a problem if they are stressed i.e. not enough milk, food, other disease. We advised to treat small, struggling lambs and then cull them. We generally also cull the mother as not enough milk is the most common cause of poor doers.


External parasites

Fly strike can lead to severe economic losses. Factors which affect susceptibility are body wrinkles around the breech area, breech soiling, inflammatory response to the larvae, skin exudates and odours and other concurrent skin diseases especially dermatophilosus. Selection for a smooth breech area seems most effective at about yearling age. Culling of all animals with skin conditions that have to be treated more than once also reduce the incidence of fly strike. All fly strike cases must also be culled if possible. If the incidence is very high then at least cull all severely affected animals.


Other external parasites

Body louse, mange, scab and keds can all lead to severe economic losses especially for wool sheep. Selecting against these is more difficult as infections are not always present. However the same principles apply. Culling all animals that show severe disease or which do not respond to a single treatment must preferably be culled.



Most cases of lameness in sheep can be attributed to footrot, foot abscess or interdigital dermatitis. Selection is not aimed at any specific disease but rather lameness in general. All animals which need more than one treatment in a season (or annually) or which have evidence of chronic lesions must be culled.

Sometimes this will mean a large number of animals. It is best to do no feet trimming. A large survey study done in the UK has shown that feet trimming are actually leading to more lameness cases. This study has also shown that isolating affected cases (for more than 3 weeks) and only breeding from non infected animals lead to lower incidence of lameness.



Most research has been done in milking sheep but the principles are the same. Selecting for lower somatic cells counts are used in milking sheep but this is generally not useful for meat/wool producing types.

Udder conformation and teat placement can used but again is more difficult in non milking sheep. In meat/wool breeds the easiest is just to cull all positive cases. This is done by culling all ewes that need treatment as well as culling all ewes with udder lesions about 6 weeks after weaning. If this is done consistently the incidence is lowered significantly.


Eye problems

Selecting for pigmented eyelids is an easy way to reduce eye problems. If a flock has many ewes that are unpigmented it is very important to make sure all rams are pigmented. As new ewes are selected no pigmented ewes can be culled.


Other diseases

SNP chips are being developed for other diseases. Scrapie is currently the only disease for which a chip is commercially available. The use of this has greatly reduced the incidence during the last 10 years.



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