This Southdown ewe has tested Gaucher free along with the lambs’ sire so her lambs are Gaucher free by pedigree.
What is Gaucher disease?
Despite its name, Gaucher disease is not an infectious disease but genetic disorder that has been found in humans, dogs, mice and some Southdown sheep. Overall the expression of this disorder in Australian Southdowns has been low.
Gaucher disease is an autosomal recessive disorder. When a lamb has two copies of the mutated gene (allele), they are born unable to stand, often have a leathery skin and will die from hypothermia and hypoglycaemia. If a sheep carries just one copy of the mutated gene they will be a carrier and able to pass a copy of the mutated gene onto their progeny. The third alternative is that the sheep carries no mutated genes for Gaucher disease (Gaucher Free) and cannot pass the mutated gene onto their lambs.
There is a simple DNA test available to identify the Gaucher status of individual sheep.
Because the Gaucher trait is recessive, the genetic variant can spread unnoticed through a flock starting from just one carrier sheep, with more and more sheep becoming carriers through the generations. Eventually a carrier ewe will start to be joined with a carrier ram resulting in a one in four chance of an unviable lamb.
This scenario could occur where Gaucher status is not monitored in purebred Southdown flocks, flocks based on the Southdown breed and composite breed flocks based on Southdowns. It is not known if the disorder could be more widespread and found in other breeds of sheep.
At this stage there are no known consequences and there are not expected to be consequences for prime lamb producers using Southdowns as terminal sires.
The origin of Gaucher disease in sheep is unknown. As with all genetic disorders and the vagaries that accompany them, it has taken time to come to an understanding and conclusion about how the genetics of Gaucher disease in sheep work.
In 2010, Gaucher disease in sheep was officially identified by scientists after examining lambs presented by an Australian Southdown breeder. Since we don’t know when and where the mutation first appeared and Southdowns are one of the oldest breeds of sheep in the world and consequently a foundation breed for many other breeds, it is hard to know how widespread Gaucher disease is through the sheep world.
Australia has been at the forefront of Gaucher disease research for sheep. Testing was done in two prominent Australian Southdown sheep studs during 2012 to find lambs with the Gaucher disease mutation to assist with research of the disorder in humans. Two years later, Southdown Australia took the initiative to let the Southdown breeder community know about the research being done. This culminated in Senior Veterinary Officer Robert Suter writing a letter explaining Gaucher disease, which was sent out to all registered Southdown breeders. Testing of Southdown sheep continued for years through the original laboratory, Sahmri, until it was agreed that a more affordable test option should be developed. Southdown Australia then asked Lincoln University in New Zealand to develop a commercially viable test. This became officially available in 2017.
A relatively simple DNA test for the Gaucher status of a sheep is available through the Lincoln University Gene-Marker Laboratory in New Zealand. The test involves putting a drop of blood from the sheep onto a special collection paper and posting it to the laboratory along with a with a current New Zealand Ministry for Primary Industries import permit (see full instructions). The current import permit can be downloaded from the laboratory website.
Southdown Australia has always encouraged testing for Gaucher and strongly recommends that breeders test registered stud rams as a minimum. Test collection cards are available from the Secretary of Southdown Australia (send a message) as well as the Laboratory.
Eradication of Gaucher disease in sheep is likely to be possible through widespread DNA testing and identification of carrier sheep. The following strategies are suggested.
1. Test and cull Test all sheep (rams and ewes) and cull carrier sheep. For this option, the flock after culling will be Gaucher free. This strategy is simple, effective and quick but may involve culling sheep that breeder would prefer to keep.
In flocks where Gaucher pedigrees are easily tracked (see suggestions below), it may be possible after testing the oldest sheep to identify progeny that are Gaucher free by pedigree and don't need testing.
2. Progressive testing Test and use non carrier rams as the first step and then work through testing the ewes in the flock. If ewes are found to be carriers, the breeder can choose to cull the ewes. Alternatively, the breeder may choose to use the ewes and test their lambs to identify non-carrier lambs to replace the ewes over time.
After testing, track Gaucher status of sheep in pedigrees with the aim to get to the stage of running flocks known to be Gaucher free by pedigree. A lamb bred from a non carrier ram and non carrier ewe will also be a non carrier. There are some suggestions below for ways that sheep that are Gaucher free by pedigree could be identified and tracked.
3. Check status of sheep coming into flock The Gaucher status of any new sheep brought into the flock will need to be checked to ensure that no additional Gaucher carriers are introduced. If there is any doubt about the Gaucher status of a sheep, test the sheep to confirm its Gaucher status.
Some strategies for tracking Gaucher pedigrees
After testing, Gaucher status for carrier and non-carrier sheep could be recorded by the breeder in a database that tracks pedigrees. This might be as simple as adding N for non-carrier and C for carrier at the end of the name or in the ID code for the sheep.
Sheep that have tested negative or are known to be negative by pedigree could be tagged with a coloured tag. At lambing time, if only non-carrier sires are used, it would be possible to look at the tag on the ewe and note that her lambs that are negative by pedigree. Depending on the Gaucher status of the flock, the breeder may also choose to tag the known carrier sheep.
Once a flock is known to be Gaucher free, there should be no need to track the Gaucher status of individual sheep. Of course, the Gaucher status of sheep coming into the flock would need to be checked to avoid reintroducing the Gaucher gene.
Recording Southdowns known to be Gaucher free on website
In order to share information more widely, Southdown Australia is collating lists of Southdown rams and ewes that are known to be non-carriers through testing or by pedigree where the ancestors have been tested. This allows breeders to report and trace Gaucher Free status of sheep by testing and pedigree as they work to achieve Gaucher Free flocks. Southdown Australia hopes that other countries and other breeds will feel encouraged to have the same transparency and foresight.
Owners of registered Southdown flocks can submit lists of sheep in their flock that are known to be Gaucher free through testing or by pedigree. The information will be published on the Southdown Australia website.