Jaw Defects in Sheep

prognathism and brachygnathism

History

  • Jaw defects in sheep are present in almost all breeds of sheep
  • It has been occuring in sheep throughout the United States
  • Especially common in meat-type (Suffolk, Hampshire, Shropshire) but do occur in wool breeds
  • "Bulldog" or "Monkey Mouth" are both common terms for undershot jaw or Brachygnathism
  • "Parrot Mouth" is a common term for overshot jaw or prognathism
  • Sale of registered or purebred rams that either show or carry either defect has been the cause of defective mouthed sheep in farm flocks and commercial range bands


The Importance of Checking Teeth

  • Poor teeth are a common reason for farmers to cull adult sheep
  • Dental disorders can be associated with weight loss and poor weight gain
  • It is important to check the flocks teeth to: determine age up to four years, diagnose dental disorders, and to find abnormal incisors
  • One can easily check their flocks teeth by pulling apart the lips
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Recognizing jaw defects

  • In normal dentition, the incisors meet the dental pad within 1-3 mm of the anterior edge, allowing them to graze efficiently
  • In undershot mouths, the lower jaw grows longer than the upper jaw
  • In overshot mouths, the lower jaw is short causing the incisor teeth to register on the roof of the mouth or back of the dental pad
  • Both defects cause sheep to have difficulty grazing short forages
  • This can cause them to be unthrifty and often times have low body condition scores and are likely to be poor quality
  • It can also cause the molars to not line up correctly, which in turn interferes with mastication (chewing)

Examples of Jaw Defects

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Why We Should Care

  • As stated above, sheep with jaw defects will likely have other problems including: trouble grazing, trouble with mastication, poor conditioning, unthriftiness, and earlier wearing down of teeth
  • Overshot jaw can cause ulcers or sores in the mouth from hitting the roof of the mouth
  • Jaw defects are heritable and they can pass it on to their offspring, spreading the problem in the flock
  • Breeders that sell rams with jaw defects or that carry the gene will pass on the problem to other farms and/or commercial operations

The Genetics of it

  • Jaw defects are a inherited trait
  • They are due to recessive characters
  • They can be hidden traits (both rams and ewes may have normal mouths and still produce lambs with jaw defects)
  • According to Kerkmann "An oligogenic inheritance including a dominant and recessive locus responsible for major gene effects and possibly further modifying loci" could be a factor in jaw defects
  • In Kerkmanns study, using ewes with brachygnathia occlusion, they found that 60 lambs were born with varying degrees of brachygnathia and that extremely affected lambs showed palatine ulcers and slow growth

Genotype

  • Because jaw defects are polygenic traits (affected by many genes) it can't be tested for
  • It is hard to know when and how (the severity of the defect) it will show up
  • We know that it is heritable so animals that show the defect or have ancestors that show the defect are capable of passing it on, therefore, they should be culled
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Some Exceptions

  • Sometimes sheep may have a mild cases, in which it may not be heritable due to a slightly disproportionate growth in the jaw
  • Defects can also be caused by some sort of trauma, or (prognathism) can be linked with ewes having a low calcium diet
  • Sometimes overshot jaws may self correct, but most bites are "set" by a few months of age
  • If ewes or rams with mild cases produce offspring with jaw defects, they should be culled
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Eradication

  • Breeders should be taught how to tell the difference between mild and bad cases and know how to cull animals accordingly
  • Purebred breeders should cull entire strains or family lines where defective animals came from
  • Rams that are free from jaw defects along with their ancestors should be used for breeding
  • Breeders should select rams carefully from flocks and should not be used if they throw lambs with jaw defects
  • All rams and ewes that produce lambs with offspring or that currently have jaw defects should be culled
  • If breeders and commercial sheep ranchers follow these guidelines they should maintain a flock relatively free from jaw defects
always check the mouths on rams that you might potentially buy
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Opinion

Being a sheep breeder myself, and having encountered these defects, I think that it is very important that sheep producers and breeders are educated about jaw defects and the problems that it can cause in the flock. They should know how to look for and recognize these defects and cull animals that have them. Breeders should not sell rams if they have jaw defects or that have ancestors that do. It's hard to know for sure if a sheep has the genes for it because it could be hidden and it can't be tested for, but breeders should do the best they can to sell correctly structured animals to farmers and commercial producers

References

  1. A.L. Ridler, D.M. West. "Examination of teeth in sheep health management" Small Ruminant Research. Volume 92, issues 1-3. pg. 92-95. August 2010. http://www.sciencedirect.com/science/article/pii/S0921448810001094
  2. B.A. McGregor. "Incisor development, wear and loss in sheep and their impact on ewe production, longevity and economics: A review" Small Ruminant Research. Volume 95, issues 2-3. pg. 79-87. February 2011. http://www.sciencedirect.com/science/article/pii/S0921448810003135
  3. Dr. Lyle G. McNeal. "Bulldog Jaw and Parrot Mouth Defects in Sheep" The Navajo Sheep Project. 1998. http://navajosheepproject.com/images/pdf/general/parrotmouth.pdf
  4. National Animal Disease Information Service http://www.nadis.org.uk/bulletins/tooth-problems-in-sheep.aspx
  5. Kerkmann, A. Kuiper, H. Ganter, M. Distl, O. "Review and results from Matings of East Friesian milk sheep affected with brachygnathia inferior" BERLINER UND MUNCHENER TIERARZTLICHE WOCHENSCHRIFT. Volume 121. pg. 292-305. July 2008 http://oregonstate.summon.serialssolutions.com/document/show?id=FETCHMERGED-LOGICAL-p867-81f237b8d4eaf37bbfe6db7746e12ded31993c20d9de125afe02b4f9c016d2c91&s.cmd=addFacetValueFilters%28SubjectTerms%2Csheep%29&s.q=jaw+defects+in+sheep