Congenital Contractural Arachnodactyly (CA)
Also known as
Fawn calf syndrome
This is an autosomal recessive genetic disorder in cattle. This means that two copies of an abnormal gene must be present (one from each parent) in order for the disease or trait to show up in the offspring. The carriers of the defective gene are normal. Without genetic testing, these carrier animals are indistinguishable from animals that are free of the gene.
This genetic defect is the result of a mutation in an individual animal several decades ago in the Angus breed. It was first identified in Australia in 1998 but has since been reported in many countries and traced back to a mutation in the family line of a popular sire in the U.S. Abnormal calves did not begin to show up until carrier animals (some of the descendants of that sire) were mated to each other, doubling up the recessive gene, producing offspring with this defect.
A few years after the first defective calves were reported, a University of Illinois research team identified the mutated gene causing this defect, and in 2010 developed a DNA test for this gene. By July 2010, the researchers had screened 1,246 AI sires and identified 39 bulls as carriers. Further testing over the years revealed many more.
This genetic defect involves the connective tissue of muscles and joints, leading to contracture of the calf’s upper limbs and laxity of the joints of the lower limbs. It is most obvious in the hind limbs, with abnormal angles, and weak pasterns. Due to these contractures, the calf has an abnormal crouched posture, resembling an elk or deer fawn, with the spine arched. The feet are often placed more to the rear than normal, with hocks pulled up and back.
Severely affected calves have abnormalities that include a long, arched back, with very short underline and long leg bones (elbows and stifles practically touching, beneath the belly), and long, weak pasterns. In some instances the limb malformations may cause dystocia. Although many of the severely affected calves are unable to stand without help, some of them do survive.
They need help to stand and suckle at birth and even if they are raised by their mothers or on a bottle they often reach a point as they get older when it is difficult (and painful) for them to get around because of their abnormal joints.
In their first days of life, fawn calves are generally walking on their pasterns (dewclaws flat down on the ground). There is reduced range of movement (due to contractures) in the upper limb joints, particularly the hip, stifle and hock, but increased looseness of the lower limb joints, particularly the pasterns.
Affected calves are often taller and more slender (with smaller, finer leg bones), than their unaffected siblings.
One diagnostic test is to place the newborn calf flat on the ground on its side and check the range of motion of the hind leg joints. Inability to passively extend the hip, stifle and hock joints to the normal extent by pulling downwards on the foot of a newborn calf--while held on its side on the ground--is a sign of this defect.
This was a challenging disease to figure out because most stockmen just assumed it was an odd birth defect and didn’t realize it was genetic. For a long time, the Angus breed was accused of hiding it, simply because this defect wasn’t being reported and researchers didn’t know much about it.
Even if a breed association is conscientious, if people aren’t reporting it, there is nothing to investigate—and many breeders weren’t reporting it because they didn’t know the disease existed. Some cases are subtle enough that the calves seemed to grow out if it, and at first some cattle producers didn’t recognize it as a serious problem.
Affected calves are born alive, have normal birthweight, but are visibly abnormal. Most of them can get up and walk, suckle and survive, but their upper limb joints (particularly hip, stifle and hock) have a reduced range of motion.
Mildly affected calves may appear normal by the time they reach four to six months of age, but most of them perform poorly and are relatively tall and slender with compromised feet and leg conformation.
As weanlings and yearlings, these animals appear lighter framed and lighter muscled, particularly in the hindquarters and are tall, slender animals with poor foot conformation. Mild cases may have more normal appearance as mature adults, but may suffer early onset of degenerative arthritis, particularly in the stifle joints.
- Newborn calf with arched back
- Crouched posture
- Abnormal leg angles
- Long weak pasterns often flat on the ground
Congenital contractural arachnodactyly is a heritable disease of newborn Angus and Angus-derived cattle caused by an error in the DNA genetic code transmitted from parents to their progeny.
There is a DNA test that can determine the carrier status of an animal. If all purebred breeding stock are tested, and no carriers mated with other carriers, no “fawn” calves will be produced. In a commercial herd, if no untested Angus bulls (or untested crossbred or composite bulls with Angus breeding) are used, there will be no fawn calves produced even if some of the cows are carriers, since this genetic defect must be doubled up (one from each parent) for it to show up in offspring.
Recessive genetic defects are most likely to show up in animals that are the result of inbreeding and linebreeding (unless the parents were tested to determine they are free of the recessive gene) since there is more chance for doubling up an unwanted trait.
An easy way to remember how it works: Mating a normal animal with a carrier animal will result in offspring that have a 50% chance of being normal and a 50% chance of being a carrier. Mating two carrier animals results in a 25% chance of a normal calf, a 25% chance of a fawn calf and a 50 percent chance of a carrier calf.
There is no treatment for this genetic disease.