Dermatosparaxis is an inherited connective tissue disorder that results in the deposition of abnormal collagen in the skin (Fjølstad and Helle, 1974). Collagen is like the scaffolding of the skin, and without it, the skin loses its strength, and becomes fragile.
The disease is predominantly seen in the White Dorper breed of sheep; however, it has also been reported in Merino and Border Leicester-Southdown sheep. Similar conditions occur in cattle, cats and humans (Ehlers-Danlos Syndrome type VIIC).
A farmer who owns around 30 White Dorper ewes from Girilambone first contacted the CWLHPA in September 2011. Over the previous month he noticed two unmarked lambs, only a few weeks old, with wounds. He put this down to predation. These lambs subsequently died or were euthanased. He then yarded the sheep for lamb marking and noticed one lamb with a skin tear under its front leg. On release from the lamb marking cradle, the skin tore away from its hind leg in his hand. He monitored the lamb, but it became fly struck and was euthanased.
This farmer was new to sheep production and had in the past purchased one White Dorper ram from a backyard breeder and was attempting to breed his own replacement rams and ewes.
I visited the property in October 2011 after the farmer had yarded the mob and had noticed another affected lamb.
At the time of the visit, there were three rams (one older, purchased ram, and two younger home-bred rams) running with approximately 30 ewes, all with lambs at foot.
The affected lamb was approximately 3 months old, was in moderate body condition and was bright and walking. There was a large torn skin flap under the ventral abdomen and degloving injuries were present on both front legs. These lesions can be seen in Figures 1a & b.
When attempting to catch this lamb, it hit a fence, creating the skin wounds on its head shown in Figures 2a & b.
The lamb was euthanased for welfare reasons. On examination, the skin peeled off the animal with no dissection. There was significant subcutaneous oedema under and surrounding the more chronic ventral abdomen and leg wounds. A full necropsy was not conducted. No skin was submitted for histopathological examination.
Three spots of blood were collected from the affected lamb post mortem, and from one replacement ram lamb by making a very small incision in the lamb's ear. Drops of blood were placed on a special DNA card supplied by EMAI laboratory shown in Figure 3. It was decided by the farmer not to test the three sires already in use, but to cull them and start again with fresh rams.
The affected lamb that was euthanased was found to be homozygous for the mutation responsible for ovine dermatosparaxis in White Dorper sheep (Zhou et al. 2011). The replacement ram was found not to be a carrier of the mutation. This ram was to be kept for future breeding in the flock. In December 2011 the farmer tested another replacement ram using this method and it was found to be a carrier for the mutation. This ram was subsequently culled.
Dermatosparaxis is inherited as an autosomal recessive condition (Hanset 1971, O'Hara et al. 1970, Fjølstad and Helle, 1974). It is caused by mutations in the pro-collagen-N-proteinase gene (ADAMTS-2) which results in defective processing of type 1 pro-collagen to collagen, accumulation of molecules that retain the amino-terminal pro-peptide, and assembly of ribbon like fibrils that do not give proper tensile strength to connective tissues (Fernandes et al. 2001). Put simply, affected animals do not have mature collagen molecules in their skin.
Clinical signs are usually seen in lambs that are less than a few months old. There is no treatment and attempts to stitch the skin wounds back together fail as there is no collagen to support the healing process. The severity of the condition results in death or euthanasia.
It seems that the incidence of dermatosparaxis cases in this particular flock were increased due to inbreeding - the owners had purchased one ram (who was probably a carrier of the mutation responsible for ovine dermatosparaxis) and then bred replacement rams from him. They were also retaining ewe lambs to increase their breeding stock numbers.
Recommendations were made to the farmer to purchase new rams, and ensure they are tested negative for the dermatosparaxis gene prior to breeding. It was also recommended that he sell all ewe lambs as fats in future and instead buy new replacement ewes from a novel source to increase breeding numbers. This will ensure that the new rams will not end up being mated to their daughters and the genetic gene pool of the flock will not be reduced.
In NZ cases examined (Vaastra et al. 2011), histology of the affected skin revealed diffuse hyalinisation of dermal collagen compared with unaffected lambs, protein-filled peri-adnexal clefts and areas of deep dermal and subcutaneous granulation tissue consistent with previous separation of skin from the subcutis.
Dermatosparaxis was first reported in South Africa in the late 1980's (Van Halderen and Green, 1988), and was first diagnosed in NSW at EMAI in 2005 (Brendon O'Rourke, pers comm.).
Samples from this original affected lamb and from related individuals were used to identify the causal mutation by staff at the Genetics Laboratory (EMAI), which has since been reported by Zhou et al. (2011). A diagnostic screening test was subsequently offered and has provided farmers with an avenue to manage this deleterious condition since 2006. In the past, the testing procedure was carried out on hair follicles, but it was found that the quality of the hair sample was subject to seasonal variations which often led to failed tests. In 2011, a system using blood cards was developed by the Genetics Laboratory at EMAI, Menangle, which provides more reliable DNA quality. EMAI is currently the only provider of this testing method in the world.
To date, the genetics laboratory at EMAI Menangle reported that approximately 15.3% of submitted samples from NZ, South Africa and Australia have been identified as carrying the defective gene (Brendon O'Rourke, pers comm.). As the reasons for testing are unknown, this may not represent the allele frequency in the White Dorper population. Because of the presence of the gene in the Australian population and the severity of the disease the Dorper Breed Society of Australia recommends screening of breeding stock for the presence of the defective gene.
The method of sample collection is so simple that sheep studs and commercial producers can sample breeding stock themselves. Each test costs $58.75. However, if samples are submitted through the Dorper Breed Society there is a discounted rate of $34.15 (Naomi Porter, pers comm.).
Dr Brendon O'Rourke & Naomi Porter, Molecular Diagnostic and Genetics Unit, State Veterinary Diagnostic Laboratory, Elizabeth Macarthur Agricultural Institute, Menangle NSW.