Introduction:Many of the inherited and acquired defects are congenital or present at birth. They occur sporadically and rarely contribute to major losses of lambs and kids. There are more than 40 known or suspected inherited or acquired defects in sheep and goats. Many are lethal, while others allow the animal to live but have some type of physical deformity. Some common defects are entropion (inverted eyelids), cleft palate, parrot mouth (undershot jaw), cryptorchidism (one or both testicles retained in the abdomen), hernias, abdominal impaction, supernumerary teats (extra teats), spider lamb disease, arthrogryposis, cyclopia (ingesting Veratrum californicum - skunk cabbage), atresia ani (no opening to the anus), border disease, and prolapses.
Causes of Inherited Defects: Chromosomes inherited from parents determine an animalís genetic make-up. There are many genes in each chromosome. Genetic abnormalities occur when genes are missing, in excess, mutated, or in the wrong location (translocation). Often, the genes that cause the defects are recessive, meaning two must be present to cause an abnormality. In these cases, both parents must be carriers of the gene for a newborn to be abnormal. When two carriers of a particular recessive genetic defect are bred together, typically only one of every four offspring will be abnormal, two will be carriers, and one will be normal.
Causes of Acquired Defects: Acquired defects are usually caused by something that is ingested by the mother during pregnancy or something that infects the mother during pregnancy. If these organisms or substances reach the fetus at the appropriate time during gestation, defects and abortions can result.
Common Inherited Defects
Cryptorchidism:A male that has one or both testicles retained in the abdomen or not fully descended into the scrotum is considered a cryptorchid. Cryptorchidism presents itself in one of two forms: 1) unilateral cryptorchidism - normal descent of only one testicle, 2) bilateral cryptorchidism - retention of both testicles. Unilateral cryptorchid animals are usually capable of breeding, whereas bilateral cryptorchids are generally sterile. The condition usually is inherited as a simple recessive trait. There seems to be some association between this condition and the polled characteristic found in some fine-wool rams. Purebred breeders should make every effort to eliminate this condition. In spite of the fact that bilateral cryptorchid animals are often sterile, both bilateral and unilateral cryptorchids should be castrated. Unilateral cryptorchids should never be used in a breeding program.
Entropion (Inverted Eyelid): Entropion is widespread among most breeds of sheep and goats. This trait can be passed on through genetics, so any animal with this problem should be culled. See page C202 for additional details on entropion.
Jaw Defects: Jaw defects are present in almost all breeds of sheep and many breeds of goats. These conditions are associated with failure of the lower incisor teeth to properly meet the dental pad. A jaw is undershot if the incisor teeth extend forward past the dental pad; it is overshot if the teeth hit in back of the dental pad (this condition is known as parrot mouth). Cull animals (sire and dam) with either of these genetic defects.
Underbite (undershot, reverse scissors bite, prognathism): In this condition, the lower jaw is longer than the upper jaw. If the upper and lower jaw meet each other edge to edge, the bite is referred to as an even or level bite. If the lamb or kid has either an overbite or an underbite, it will not be able to properly nurse, get enough nutrition, eat from the creep feeder, or graze.Overbite (overshot, parrot mouth, overjet, mandibular brachygnathism): With this condition, the lower jaw is abnormally short. In the newborn, there is a gap between the dental pad and the lower incisors when the mouth is closed. Some animals that are born with an overbite might self-correct if the bite is no larger than the head of a wooden match. In most breeds of sheep and goats, the bites are "set" by the time the newborn is a few months old. An overshot bite will rarely improve after the animal reaches maturity.
Rectal Prolapse: Rectal prolapse is a serious defect most commonly associated with the meat-type sheep. It is believed that this weakness is an inherited defect. This condition is sometimes corrected by surgery, but affected animals often continue to prolapse after surgery. It is recommended that these animals be culled from the breeding flock/herd. See page C900 for additional details.
Spider Lamb Syndrome (SLS): SLS or ovine hereditary chondrodysplasia is a genetic disorder causing skeletal deformities in young lambs. These defects commonly include abnormally long and bent limbs, twisted spines, shallow bodies, flattened rib cages, and long necks. The syndrome is inherited as a genetic recessive disorder, meaning that affected lambs must inherit the mutation from both their parents. Because of this inheritance pattern, the identification of genetic carriers of SLS has been difficult without the use of progeny testing. The presence of SLS in several breeds has prompted breeders to divide pedigrees into two categories: "gray-pedigreed" animals having ancestors that have produced spider lambs, and "white-pedigreed" animals having ancestors that have never produced affected lambs.
There is a DNA test that can be used to help identify carriers as well as animals that are free of SLS. Since its development, the DNA test has been validated in thousands of animals. To date, it has been 100% accurate in the proper identification of animals that are genetically free or carriers of this defect. The test has been successfully used in Suffolks, Hampshires, Southdowns, and Oxfords.
Common Acquired Defects
Akabane Virus:When the mother (ewe or doe) becomes infected with this virus during her pregnancy, the fetus can abort or die very soon after birth. These lambs and kids have central nervous system problems (hydrocephalus, hydranencephaly, microencephaly) and arthrogryposis (where the joints are held in different degrees of flexion). The virus is not found in the United States and there is no treatment for this infection.
Border Disease ("hairy shaker"): This problem is caused by a virus that infects both sheep and goats. It causes abortions, stillbirths, and weak newborns. The hairy shaker terminology comes from the fact that affected lambs and kids will tremor or shake, and affected lambs will have an unusual hairy appearance. Adult animals become infected by inhaling or ingesting the virus. The fetus becomes infected while it is in the mother. There is no effective treatment for this disease, so all animals that abort or are born with these signs should be isolated and potentially culled. Some reports state that a cattle bovine viral diarrhea (BVD) vaccine can be used to curtail abortion problems caused by this virus in sheep flocks.
Summary: Consult a veterinarian if animals suspected of having an inherited or acquired defect are found. Investigate all symptoms and possible causes before concluding the problem is inherited or acquired. If a genetic defect is found, a careful review of the breeding records should be conducted to identify animals, matings, or familial groups that may be associated with the problem. The detection of carrier animals makes it easier to eliminate genetic disorders. If possible, contact the breed association and give them a full report of the findings. Progressive breed associations are working to reduce the frequency of genetic abnormalities within their breed.
The practice of inbreeding within the flock/herd enhances the occurrence of genetic defects. Crossbreeding to a different breed is an alternative that will help reduce the chances of genetic defects.