Another genetic defect affects Angus cattle
By Jennifer Bremer
Angus breeders will have another management challenge for the coming breeding season, as another genetic defect has been identified among certain pedigrees.
On April 13, the American Angus Association released the identity of preliminary test results for Angus bulls to determine whether they are carriers or free of the mutation identified for severe hydrocephalus.
University of Illinois researcher Dr. Jon Beever and Dr. David Steffen of the University of Nebraska are now referring to the genetic defect as Neuropathic Hydrocephalus (NH).
With the increase in the number of abnormal calves being reported, Drs. Beever and Steffen have seen an increase in cases other than the previously discovered defects, which has led to the discovery of NH, as well as others.
"NH calves are born near-term and have 25 to 35 pound birth weights. The cranium is markedly enlarged--volleyball to basketball sized. The bones of the skull are malformed and appear as loosely organized bony plates that fall apart when the cavity is opened. The cranial cavity is filled with fluid and no recognizable brain tissue is evident. The spinal canal is also dilated and no observable spinal tissue is found," said Dr. Steffen.
NH is a recessive defect just as the previously identified disorder arthrogryposis multiplex (AM) is.
NH would be described in a similar way to how AM has been described by the Angus association. If an animal is homozygous for the normal variant (called an allele) they are referred to as NH-free (NHF), indicating that they have been tested for the causative mutation and been found "free" of the mutation.
If an individual is found to be heterozygous, or a "carrier" for the mutation--meaning they possess one normal allele and one mutant allele--they are referred to as an NH-carrier (NHC).
Although affected calves are rarely tested, they would be homozygous for the mutation and referred to as NH-affected (NHA). Using the known genetic information from two animals, a breeder can determine the chances of having an affected calf, according to Dr. Beever.
After the release of the list of highly used A.I. (artificial insemination) sires who were tested for AM, the same group of A.I. companies requested that the Angus association provide to the membership the identity of and preliminary test results for Angus bulls tested by Dr. Beever to determine whether they were carriers of the NH mutation or were free of it.
The two lists have many of the same bulls on them but, after the release of the AM carriers and non-carriers, a couple hundred more bulls were added for testing, according to Dr. Beever.
While some bulls may show up positive for both AM and NH, the two defects are not linked at all.
"The genes are on different chromosomes, so they segregate independently. This means that from specific matings you might expect a predicted number of carriers for one, carriers for both, or carriers for neither," he said.
Dr. Beever explained if the normal NH gene is "A" and the defective gene is "a," mating a carrier bull with the genotype Aa for the NH gene to a carrier cow also with the genotype Aa for the NH gene will result in three calves that look normal at birth, but two of the three will be carriers for NH (Aa). The fourth calf will be born with NH (aa). Thus, mating two carriers gives a breeder a 25 percent chance of having an NH calf.
Mating a carrier bull or cow to a non-carrier cow or bull will result in 50 percent of the calves being NH carriers (Aa). A non-carrier would have the genotype AA.
While there are other forms of hydrocephalus, this particular form of hydrocephalus only occurs in Angus-based cattle because the founder animal is Angus. Dr. Beever pointed out that other breeds have Angus genetics in them, so they may also be affected by NH.
"There are also other forms of hydrocephalus in other breeds, Shorthorn and Hereford specifically," he said. "However, it is important to remember that the genetic etiology is not going to be the same.
Since each genetic defect is a little different, the information he collects about each varies a well. "We estimate based on allele frequency and number of affected calves reported that there is a relatively high fetal mortality rate, potentially upwards of 70 percent," he explained. "This is consistent with the mouse data where 100 percent of homozygotes die before term."
Dr. Beever and the Angus association would like breeders to continue to cooperate by submitting samples of calves showing any forms of genetic defects.
While Dr. Beever has developed a test for NH, it is not available commercially as of press time, he expects it to be available in the next few weeks.
To further the research on NH and other conditions, breeders are urged to report any suspected genetic defected calves immediately when they are detected, so that samples can be obtained on calves for study. Calves can be reported to Don Laughlin (email@example.com or 816-383-5140) at the American Angus Association, who will make arrangements for them to be shipped to Dr. David Steffen at the University of Nebraska for examination. DNA samples of the affected calves should go to Dr. Jon Beever at the University of Illinois.
For current information on all genetic defects affecting Angus cattle, visit www.angus.org.
Jennifer Bremer can be reached by phone at 515-833-2120 or by e-mail at firstname.lastname@example.org