The Test for Spider Lamb Syndrome Gene in Sheep
Bert Moore, Wes Limesand and Paul Berg
Spider lamb syndrome is a heritable congenital abnormality caused by a recessive gene that
produces a range of skeletal deformities in lambs. The problem, which seemed to have been first
observed in the early 1970's, became a topic of serious concern to breeders in the early and middle
1980's. Many of the most popular bloodlines in the Suffolk and Hampshire breeds seemingly
carried this damaging gene. Sheep which had tracings in their pedigrees to suspected carriers of
this gene were said to possess "grey" pedigrees, whereas those sheep which had no apparent
incidence of carriers in their pedigrees were designated as "white pedigreed" sheep.
After the incidence of "spider" lambs in both Suffolk and Hampshire flocks at North Dakota State University, the decision was made to eliminate "grey" pedigreed sheep from these flocks. This could be done with a high degree of confidence because all sheep could be traced in their female lines to original purchases when these flocks were established.
Suffolks traced to many purchases made in western Canada in 1945 and some of the Hampshires
could be traced to purchases made as early as 1914. Very few flocks in the U.S. could trace their
lines of ancestry to their beginnings in this fashion.
Funded in part by a grant from the National Suffolk Sheep Association, NDSU helped establish
the mode of inheritance and the diagnostic procedures for determination of spider lamb syndrome.
An outgrowth of these procedures was the establishment of a "test flock" of known spider
producing ewes. These ewes were maintained and mated to rams which were introduced into the
flock. Occurrence of sixteen "normal" lambs from these matings gave a 99%+ probability that the
ram was free of the spider gene. Incidentally, one ram which by pedigree analysis was believed to
be "white" pedigree did sire spider lambs from these test ewes. This ram had also been mated to a
number of ewes in the main flock. All his progeny were slaughtered and not allowed to go out as
breeding stock. This was a fortunate occurrence, because otherwise the progeny might have
unknowingly spread the incidence of the spider gene.
Because of NDSU's active involvement in the spider syndrome problem and dedication to
production of genetically sound sheep, it was natural to continue association in efforts to identify
the spider gene by DNA analysis. Development of the DNA test was researched by Dr. Michael
Bishop, ABS Global, DeForest, Wisconsin; Dr. Jon Beever, University of Illinois; and Dr. Nola
Crockett, Utah State University. Substantial numbers of blood samples were supplied to these
researchers for analysis in developing the test. Samples from the main flock were used to form the
baseline of the non-carriers. Samples from the "test" flocks of known spider producers were
analyzed and characterized as the known carrier group.
Test matings of Southdown rams were also done with the NDSU "test" ewes. This provided
additional information which Dr. Michael Bishop used in his work on the test. Genotypes of
lambs would then be analyzed along with the genotypes of their parents.
Results of these matings were as follows:
Lamb No. Genotype
NN = Normal/Normal
NS = Carrier
SS = Spider Lamb
The lamb 7421 had all the visual symptoms of a spider lamb and this diagnosis was confirmed by
Dr. George A. Schamber of the NDSU Veterinary Diagnostic Laboratory.
The development of a DNA test was accomplished and released in June, 1997. Dr. Jon Beever
sampled 325 head of sheep which were exhibited at the North American International Livestock
Exposition of the Suffolk, Hampshire, Shropshire, Southdown and Oxford breeds. His analysis
reported that 28% of those sheep sampled were identified as "carriers" of the spider gene. This
illustrates that the incidence of the gene is still rather widespread and a source of concern to sheep
breeders. The development of this test, however, improves considerably upon the time and
expense required to maintain test flocks and make test matings to check for the gene's occurrence.
Also, the test makes it possible for many more breeders to have the confidence that they are using
breeding stock which is free of the spider gene.