Dwarfism (Condrodysplasia)

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Description:

Dwarfism, or chondrodysplasia, is an inherited defect reported in many breeds of cattle. The severity of presentation is highly variable, but in general calves are born with a shortened stature, and deformity of the head, spine, and limbs.


Etiology:

Chondrodysplasia is a heritable defect of cattle, and has been described in multiple breeds. Unfortunately no one gene is responsible for the condition, but the end result is a failure of proper cartilage development, which causes stunting of the skeletal system. For example, Dexter cattle, which have become famous for the incidence of “bulldog” calves, have been identified to carry a single mutation in the ACAN gene. This gene is recessive, with incomplete penetrance, meaning that depending on how many copies of the gene a calf inherits, a the defect may vary in severity. A calf that inherits two copies of the mutation will develop the severe lethal phenotype, but a calf that inherits only one copy of the mutation may show a range of severity, and is actually the desired “miniature cattle” phenotype.

Angus cattle showed an increased incidence of “long-nosed dwarf” calves in 2002, and the American Angus Association was quick to respond. Heritability studies were able to identify the responsible mutation in the GMP dependant, type II protein kinase (PRKG2) gene, which is responsible for chondrocyte maturation in the normal animal. Since then, a genetic test was developed, and a policy was instituted to forbid carriers of the mutation to be registered with the breed association.


In contrast to Dexter and Angus cattle, which carry a point mutation, the mode of inheritance is more complicated in other breeds (e.g. Herefords). The development of dwarf calves may be the result of multiple genes, causing a challenge in regards to removing the trait from a breeding herd. To further confuse matters, reports of Manganese deficiency and bovine viral diarrhea virus have also been implicated in the development of dwarf cattle.


Symptoms:

Dwarfism can be seen in a wide variety of presentations depending on the severity of the disease. Improper cartilage formation results in a bone that is disorganized and a skeletal system that is shorter than normal. The head, limbs, and vertebrae are often malformed. Common concurrent lesions include cleft palate, abdominal hernia, and hydrocephalus. Dexter cattle that inherit two copies of the ACAN mutation will abort during the third trimester, however the heterozygous individuals will show the desired miniature cattle phenotype.

 

Prevention:

As this condition is genetically inherited, known carrier sires and dams should be removed from the breeding stock, with the exception of herds intentionally breeding for heterozygous dwarfs (Dexters). If a more conservative approach is desired, carrier dams for a recessive trait may be kept, and bred only to bulls that are known to be free of the gene, as two copies of the gene, from both the sire and dam, are required for development of the disease. Carrier animals should always be reported to the breed association. Currently dwarf animals or known carriers of the defect are permitted to be registered with the American Angus Association. A genetic test is available for both Dexter and Angus cattle to identify carriers of the ACAN and PRKG2 mutations (www.metamorphixine.com)


Treatment:

No treatment exists, and severely affected animals may be euthanized on a case-by-case basis.
 


 

Sources: 

Cavanagh JA, Tammen I, Windsor PA, Bateman JF, Savarirayan R, NIchoals FW, Raadsma HW. Bulldog dwarfism in Dexter cattle in caused by mutations in ACAN. Mamm Genome. 2007;18(11):808-14.


Gregory, PW. Evidence for the same dwarf gene in Hereford, Aberdeen-Angus, and Certain other breeds of cattle. J Hered (1956) 47 (3): 107-111


Leipold HW, Hiranga T, Dennis SM. Congenital defects of the bovine musculoskeletal system and joints. Veterinary Clinics of North America: Food Animal Practice. 1993;9(1):95-97.
 

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