Equine Dwarfism 101

Discussion in 'Horse Breeding' started by Threnody, Oct 8, 2010.

  1. Threnody

    Threnody Senior Member

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    *Warning: This thread will have pictures of aborted foals*

    *Information on this thread has been gathered from articles on dwarfism from researchers and rescuers who have had experience with dwarfs. Information has also been collected by breeders who have had dwarfs born and their experiences with these animals and their physical development and health issues.

    What is Dwarfism?
    Dwarfism is a genetic disorder that causes abnormally smaller stature in an individual of a species

    [FONT=&quot]•[/FONT]Equine dwarfism is most prevalent in American Miniature Horses. It much more rarely occurs in Friesians.
    [FONT=&quot]–[/FONT]Some dwarfs have been purposely used for American Miniature Horse breeding in the past 50 years (estimated at least 60% of AMHs carry and/or express some form of dwarfism)
    [FONT=&quot]
    –[/FONT]European miniature horses have fewer incidents and certain bloodlines appear to lack the disorder
    Miniaturization vs Dwarfism
    [​IMG]
    [FONT=&quot]
    [/FONT]Miniaturization
    [FONT=&quot]
    –[/FONT]Naturally small size [FONT=&quot]
    –[/FONT]Gradually bred for over many generations from normal sized horses
    [FONT=&quot]•[/FONT]Attempts to keep miniatures in proportion
    [FONT=&quot]•[/FONT]Not a genetic defect
    [FONT=&quot]•[/FONT]Dwarfism
    [FONT=&quot]–[/FONT]Genetic defect that causes smaller stature and disproportionate bodies
    [FONT=&quot]–[/FONT]Causes health disorders
    [FONT=&quot]–[/FONT]Inheritable recessive trait
    [FONT=&quot]–[/FONT]Multiple mutations can cause various forms



    Genetics/Inheritance


    [FONT=&quot]•[/FONT]Breeding has shown that all known forms of dwarfism in horses are recessive
    [FONT=&quot]–[/FONT]Both parents pass on affected allele (Parents can look normal)
    [FONT=&quot]–[/FONT]Was once thought to be only inherited through the sire


    [FONT=&quot]•[/FONT]There may be co-dominance occurring in some cases where a carrier of dwarfism may express minimal traits


    [FONT=&quot]•[/FONT]No genetic tests have been developed yet to determine the presence or type of dwarfism an individual may carry or express. Research on Friesian dwarfism is being started.



    Types of Dwarfism


    [FONT=&quot]•[/FONT]Since the genetics have not been isolated, dwarfism is described by the type of phenotypic expression. Some different forms may be caused by the same gene but this is currently unknown
    [FONT=&quot]–[/FONT]Individuals express forms in varying levels of severity

    [FONT=&quot]•[/FONT]They have been assigned types, ranging from least to most severe in their expression and impact on health
    [FONT=&quot]–[/FONT]Type 1, Diastrophia
    [FONT=&quot]–[/FONT]Type 2, Achondroplasia
    [FONT=&quot]–[/FONT]Type 3, Brachiocephalia
    [FONT=&quot]–[/FONT]Type 4, Hypochondrogenesis
    [FONT=&quot]•[FONT=&quot]
    [/FONT][/FONT]
    Only Achondroplasia (Type 2) is known to be a separate gene because of its distinct difference from the other forms



    Diastrophia (Type 1)
    [​IMG]

    •Translates to “twisted limb”
    –A misnomer as not all Diastrophics have leg issues


    •Has been seen in miniature horses and Friesians
    –Friesian dwarfism has been traced to several possible carrier stallions. Inbreeding has been attributed to the rise in its occurrence.


    •Characterized by severe, twisted extremities, ligament deformities, cow hocks, pot belly, camped out weak hind end and a roach back
    –Main body compacted, constricting organs
    –Legs are longer than what is normally attributed to dwarfism
    –They often require surgery, special shoes and leg braces
    –Roached backs and bloated body cavities become more pronounced with age
    –Premature arthritis is common

    Diastrophia Continuum (Minis)
    [​IMG]

    Mild
    –Slight to moderate roached back
    –Weak hind end
    –Camped out behind
    –Appears like a horse with poor conformation

    Moderate
    –Appear Dwarfish
    –Roached back & short neck
    –Compacted body
    –Very weak hind end
    –Camped out behind
    –May have ligament issues

    Severe
    –Obviously dwarfish & shorter
    –Roached back & short neck
    –Compacted main body
    –Loose tendons
    –Very camped out behind
    –Joint & ligament problems

    Diastrophia Phenotypes (Friesians)
    [​IMG]



    Achondroplasia (Type 2)
    [​IMG]

    [FONT=&quot]•[/FONT]Translates to “short extremities”
    [FONT=&quot]–[/FONT]The upper leg growth is inhibited
    [FONT=&quot]–[/FONT]Contracted tendons common


    [FONT=&quot]•[/FONT]Head and body are normal but limbs are shortened and ears are very small


    [FONT=&quot]•[/FONT]A more common form of dwarfism
    [FONT=&quot]–[/FONT]Gene is estimated to exist in more than 25% to around 50% of miniature horses


    [FONT=&quot]•[/FONT]These dwarfs can live fairly normal lives and often do not suffer as severe health problems from their disorder (Compared to other dwarf types).
    [FONT=&quot]–[/FONT]Premature arthritis is the most common affliction

    Achondroplasia Continuum
    [​IMG]

    [FONT=&quot]•[/FONT]Mild
    [FONT=&quot]–[/FONT]Legs are much shorter than what is desirable
    [FONT=&quot]–[/FONT]Front upper leg & cannons about even (upper leg normally longer)
    [FONT=&quot]–[/FONT]Back appears long
    [FONT=&quot]–[/FONT]Move and function fairly normally

    [FONT=&quot]•[/FONT]Moderate
    [FONT=&quot]–[/FONT]Appear Dwarfish
    [FONT=&quot]–[/FONT]Upper legs are noticeably shortened
    [FONT=&quot]–[/FONT]Back appears very long
    [FONT=&quot]–[/FONT]Ears are small
    [FONT=&quot]–[/FONT]May have difficulty moving in early development

    [FONT=&quot]•[/FONT]Severe
    [FONT=&quot]–[/FONT]Obviously dwarfish
    [FONT=&quot]–[/FONT]Upper legs severely shortened
    [FONT=&quot]–[/FONT]Back appears very long
    [FONT=&quot]–[/FONT]Ears are small
    [FONT=&quot]–[/FONT]May have contracted or loose tendons,
    [FONT=&quot]–[/FONT]May have difficulty moving and straightening out legs




    Brachiocephalia (Type 3)
    [​IMG]

    [FONT=&quot]•[/FONT]It can cause multiple defects to varying extents. Not all individuals will express the same traits or to the same extent.
    [FONT=&quot]
    •[/FONT]Diminutive structure
    [FONT=&quot]•[/FONT]Domed large head
    [FONT=&quot]•[/FONT]Short neck
    [FONT=&quot]•[/FONT]Protruding roached back
    [FONT=&quot]•[/FONT]Bulging eyes
    [FONT=&quot]•[/FONT]Misaligned jaws
    [FONT=&quot]•[/FONT]Protruding tongue
    [FONT=&quot]•[/FONT]Ligament disorders “twisted limbs”
    [FONT=&quot]•[/FONT]Obstructed nasal passages
    [FONT=&quot]•[/FONT]Pot belly
    [FONT=&quot]•[/FONT]Internal organs are large for the chest cavity

    [FONT=&quot]•[/FONT]Their legs often need braces, special shoes and surgery to walk
    [FONT=&quot]•[/FONT]Roached backs and bloated body cavities can become more pronounced with age

    [FONT=&quot]•[/FONT]Severely affected individuals do not live long and even high functioning individuals have shorter lifespans due to heart and organ failure

    Brachiocephalia Continuum
    [​IMG]

    [FONT=&quot]•[/FONT]Mild
    [FONT=&quot]–[/FONT]Looks like a stout poorly conformed horse
    [FONT=&quot]–[/FONT]Convex profile, large eyes
    [FONT=&quot]–[/FONT]Short neck
    [FONT=&quot]–[/FONT]Has an underbite
    [FONT=&quot]–[/FONT]Compact body
    [FONT=&quot]–[/FONT]May have some hoof or leg issues

    [FONT=&quot]•[/FONT]Moderate
    [FONT=&quot]–[/FONT]Appears dwarfish
    [FONT=&quot]–[/FONT]Convex profile, bulging eyes
    [FONT=&quot]–[/FONT]Very short neck
    [FONT=&quot]–[/FONT]Has an underbite
    [FONT=&quot]–[/FONT]Compressed body cavity
    [FONT=&quot]–[/FONT]Hoof and leg issues

    [FONT=&quot]•[/FONT]Severe
    [FONT=&quot]–[/FONT]Obviously dwarfish
    [FONT=&quot]–[/FONT]Bulging eyes
    [FONT=&quot]–[/FONT]Appears to have no neck
    [FONT=&quot]–[/FONT]Twisted legs, ligament issues
    [FONT=&quot]–[/FONT]Has an underbite, tongue out
    [FONT=&quot]–[/FONT]Bloated body cavity
    [FONT=&quot]–[/FONT]Very dished face, domed head



    *Warning: Here are the pictures of aborted foals*

































    Hypochondrogenesis (Type 4)
    [​IMG]

    [FONT=&quot]•[/FONT]Horses with the most severe form of dwarfism are not carried to term:
    [FONT=&quot]–[/FONT]Most type 4 foals are likely reabsorbed in the womb and to not make it past the embryonic stage, others are aborted as fetuses
    [FONT=&quot]–[/FONT]They express extremely small limbs and the most exaggerated dwarfism traits.
    [FONT=&quot]–[/FONT]Bones are not well formed or fused properly for their stage in development


    [FONT=&quot]•[/FONT]Hypochondrogenic foals are likely caused by more than one form of dwarfism being homozygously inherited and expressed in an individual



    Breeds That Carry Dwarfism

    [FONT=&quot]•[/FONT]American Miniature Horse (most common)
    [FONT=&quot]•[/FONT]Falabella (Rare)
    [FONT=&quot]•[/FONT]American Shetland (Less common)
    [FONT=&quot]•[/FONT]UK Shetland (Rarer dwarfism rates than US bloodlines)
    [FONT=&quot]•[/FONT]Friesian – (T1 only, very rare)
    [FONT=&quot]•[/FONT]Corolla Spanish Mustang aka Banker Horses. Feral horses from North Carolina's Outer Banks (recently recorded due to inbreeding & limited diversity, inconclusive type)
    Miniature Donkeys (T3 in expression, Rare and likely separate mutation from horse form)



    Common Dwarfism Disorders

    [FONT=&quot]•[/FONT]Contracted tendons
    [FONT=&quot]–[/FONT]In some cases a dwarf horse will have a contracted leg tendon causing the hoof to rock backwards.
    [FONT=&quot]
    [/FONT]
    [FONT=&quot]•[/FONT]Locking stifles
    [FONT=&quot]–[/FONT]An inability to flex the patella.
    [FONT=&quot]
    [/FONT]
    [FONT=&quot]•[/FONT]Shoulder dysplasia
    [FONT=&quot]–[/FONT]Severe rotator cup dislocation may require reconstructive surgery.
    [FONT=&quot]
    [/FONT]
    [FONT=&quot]•[/FONT]Restricted breathing
    [FONT=&quot]–[/FONT]Many Brachiocephalic dwarfs have "mashed-in" faces, often causing restricted breathing. These types of dwarfs can suffocate , especially when suffering from respiratory ailments, or when their permanent teeth grow-in, further constricting their airways.
    [FONT=&quot]
    [/FONT]
    [FONT=&quot]•[/FONT]Arthritis
    [FONT=&quot]–[/FONT]Many dwarf horses suffer from equine arthritis.
    [FONT=&quot]
    [/FONT]
    [FONT=&quot]•[/FONT]Neurological Issues (Retardation, Depression)
    [FONT=&quot]–[/FONT]Depression likely caused from pain and restricted ability to move, eat, breath ect.




    Identifying Dwarf Foals

    [FONT=&quot]• [/FONT]If a foal is likely a dwarf if:
    [FONT=&quot]– [/FONT]Born significantly smaller than what is normal (Mini: 18lbs 21inches) even if they initially appear normal
    [FONT=&quot]– [/FONT]Matures to be much smaller than both parents
    [FONT=&quot]– [/FONT]Has noticeable facial, ligament and/or leg issues
    [FONT=&quot]– [/FONT]Born fairly normal but develope dwarfish traits as they mature (often between 2 months to 2 years)
    [FONT=&quot]• [/FONT]Pot belly, short neck roach back, bulging forehead and eyes



    Dwarf Foals – Diastrophia (Type 1)
    -Minis

    [​IMG]

    -Friesians
    [​IMG]

    •Type 2 foals are born with compacted bodies usually with normal length legs
    –This makes their legs appear very long at birth and their heads appear large
    –Hind legs are often camped out and are back at the stifle (Weak hind end)
    –Often have fairly normal front end in comparison
    –Main body often appears normal at birth but later develops a roached back and pot belly due to compacted growing organs
    –Most common ligament disorder in Type 1 causes them to be down on the hind fetlocks
    –Special braces and shoes can help correct tendons by the time maturity is reached
    –Less severely malformed Diastrophics are harder to identify



    Dwarf Foals – Achondroplasia (Type 2)

    [​IMG]

    [FONT=&quot]•[/FONT]Type 1 foals can be identified by their severely shortened upper legs at birth
    [FONT=&quot]–[/FONT]May be over at the knee, and camped out behind, until later in maturity
    [FONT=&quot]•[/FONT]Legs in “four corners” and not balanced underneath
    [FONT=&quot]–[/FONT]May need special shoes or braces
    [FONT=&quot]•[/FONT]Head fairly normal, ears very small



    Dwarf Foals – Braciocephalia (Type 3)
    [​IMG]

    [FONT=&quot]•[/FONT]Overall born extremely small and compacted
    [FONT=&quot]–[/FONT]Often has leg anomalies that need surgery and corrective shoes
    [FONT=&quot]–[/FONT]Head is very domed at birth
    [FONT=&quot]–[/FONT]Body cavity bloats and gains a roached back as organs continue to grow with age
    [FONT=&quot]–[/FONT]Severe T3s often put down at birth to prevent unneeded suffering



    Dwarfism Carriers
    [​IMG]
    Former show mare and Achondroplasic (T2) carrier

    [FONT=&quot]•[/FONT]Because dwarfism is recessive, carriers will often appear normal
    [FONT=&quot]–[/FONT]Because dwarfism in in over 50% of miniature horses, a majority of breeders will eventually have dwarf offspring appear from phenotypically normal carrier stock
    [FONT=&quot]–[/FONT]Dwarfism effects all miniature horse bloodlines, from pets to show quality champions

    [FONT=&quot]•[/FONT]There is possibly some minimal expression of dwarfism in carriers
    [FONT=&quot]–[/FONT]However, in American Miniature Horses, a dished face and large eyes are bred for
    [FONT=&quot]–[/FONT] This makes phenotypic identification of carriers an unreliable practice

    Dwarfs who are Carriers
    [FONT=&quot]•[/FONT]Dwarfs who are expressing one form, but carrying another
    [FONT=&quot]–[/FONT]Express the dwarfism they inherited 2 alleles of, but show fainter traits of a form they likely carry 1allele of
    [FONT=&quot]–[/FONT]These dwarfs likely have more severely expressed malformations due to multiple genetic issues



    Breeding and Dwarfism

    [FONT=&quot]•[/FONT]It is considered good practice to geld stallions and retire mares from breeding if they produce a dwarf offspring
    [FONT=&quot]–[/FONT]Since dwarfism is recessive both parents are carriers

    [FONT=&quot]•[/FONT]Dwarf horses should not be bred-because they will only create more carriers or more dwarf offspring
    [FONT=&quot]–[/FONT]Dwarf stallions should be gelded
    [FONT=&quot]–[/FONT]Dwarf mares should be spayed and/or kept away from stallions
    [FONT=&quot]
    •[/FONT]Dwarf pregnancies not only carry on the dwarf gene, but can physically be dangerous for a dwarf dam and her possible offspring. Constricted body cavities prevent proper room for both internal organs and a foal.

    Irresponsible breeding can result in high mortality rates, foals needing expensive surgery or needing to be humanely put down




    [​IMG]
    It is known that all forms of dwarfism are recessive. It is hypothesized that each form has their own mutation, this has been supported through breedings. (There are no official dwarfism genetic abbreviations, these are my own)


    [​IMG]
    Normal horses will produce normal horses. Dwarfs of the same type will only produce more dwarfs.



    [​IMG]
    Dwarf x Non Dwarf: Dwarfs bred to Non-carriers will always produce phenotypically normal offspring. However, all of these offspring are guaranteed carriers who can further spread the allele.

    Carrier x Carrier: The problem with carriers is that when they are bred to other carriers, they have a 25% chance of producing a dwarf, and a 50% chance of creating more carriers like themselves. There is only a 1 in 4 chance of having a normal non carrier foal, so the dwarfism allele is spread within their offspring 75% of the time.

    [​IMG]

    Dwarf carriers of different types can never produce a dwarf when bred together. However they will create more carriers and 25% of their offspring will be double carriers.


    [​IMG]

    Double carriers of the same type can produce phenotypically normal foals (who are mostly carriers), dwarfs (who are mostly dwarfs carrying another form and likely at risk for more health issues than non carrier dwarfs) and Type 4s. Only 1out of every 16 foals has a chance of not inheriting any dwarfism alleles. This combination greatly spreads dwarfism alleles into the population. This illustrates how a pair who can produce more that one type dwarf will greatly spread the disorder. This pair would have a higher chance of miscarriages even of the non hypochondrogenetic dwarf foals.


    [​IMG]

    Dwarfs should never be bred. There is a high risk for complications and death of the mare and/or foal. If a pair of dwarfs of different types (who are also not carrying other forms) were bred together they would never produce a dwarf offspring. However ALL foals from such a pair would be guaranteed double carriers.

    *Continued and completed on post #8*

     
    Last edited: Oct 19, 2010
  2. Seein'Spots

    Seein'Spots Senior Member+

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    Very sad....and to think some people are purposely breeding their dwarfs.:no:
     
  3. Limespark

    Limespark Senior Member

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    Does dwarfism ONLY occur in miniatures and Fresians?? No other breeds? Because if so, that's pretty crazy...
     
  4. emali06

    emali06 Senior Member

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    okay I know this is a horrible picture to judge by but this is my friends mothers miniature that she bought (she also bought his in foal Dam) Do you think he looks like a dwarf of some sort? I was there when she bought them and the Stallion that the previous owner had really looked a like a dwarf from what I can remember. He just seemed off in some way.

    [​IMG]
     
  5. ACCphotography

    ACCphotography Senior Member

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    Probably and not really. Since it's recessive it generally shows itself due to a very limited gene pool which is just not a problem in many breeds.
     
  6. roro

    roro Senior Member+

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    ------------
     
    Last edited: Oct 9, 2010
    1 person likes this.
  7. Caterina

    Caterina Senior Member

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    She may have stated that, however she cant leave us for now almost 12 hours (I have a feeling it will be more) and expect no one to post.
    Perhaps if she had posted all her info all in one sitting, it would have worked out alright.

    OP should ask this thread to be closed and once she has all her info to post at once then she should post a new thread and then ask it to be stickied.

    People are going to post replies.... its the nature of a forum....
     
  8. Threnody

    Threnody Senior Member

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    It's ok, life happened in between. I'm just editing onto the original post.


    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    Edit 10/13/10:
    Can't add more pictures to the OP so Ive added on here so all post are still on the first page.


    Friesian Dwarfism Charts

    [​IMG]
    Since there is only Diastrophic dwarfism in friesians, there is no threat of normal looking individuals carrying more than one form of dwarfism. Dwarf parents are guaranteed carriers.


    Dwarfism Family Tree

    [​IMG]

    Dwarfs show up because of inbreeding. This is why some state that dwarfism is caused by inbreeding. Dwarfism is revealed through inbreeding, the mutated alleles cause dwarfism.



    Origins of Equine Dwarfism

    Dwarfs descend from the original individuals who mutated dwarfism on both sides. No founders of any dwarfism mutations are known. The original founders of the dwarfism mutations would have been carriers and appear normal because of its recessive inheritance.

    Because dwarfism is recessive, it can hide in a family for numerous generations and even indefinitely if no inbreeding occurs. However, because of the nature of artificial selection, inbreeding will eventually happen even if it happens centuries later.

    An example of a recessive disorder in a family tree is HERDA. The Poco Bueno (1944) line is the founding line of HERDA. No HERDA effected individuals showed up until 1971. This was because inbreeding of the Poco Bueno lines carrying the recessive mutation did not occur until 1971. Dwarfism is similar in that affected individuals did not show up until later generations.

    Since most dwarfs were either not recorded or died at birth, there are no complete records that date back far enough to identify founders of any dwarfism mutations. Gene tests need to be developed to help reduce and eventually remove the disorder from the gene pool entirely.


    Genetic Research on Equine Dwarfism


    All all genetic research on equine dwarfism is currently being done by John Eberth at the University of Kentucky. He owns a miniature horse farm, so the breed most effected by this disorder is a large part of his life. Research is slow and currently underfunded. Mr. Eberth has spent tens of thousands of his own money to fund this research.

    This is a link to the AMHA website describing how to send donations and equine DNA samples for research.
    http://www.amha.org/index.asp?KeyName=142


    Thank you for reading ^_^
     
    Last edited: Oct 20, 2010
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  9. ACCphotography

    ACCphotography Senior Member

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    Good idea Threnody.
     
  10. Threnody

    Threnody Senior Member

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    Thanks ^_^

    Sorry for the long wait. More will be added at a later time, but diagrams and breeding charts need to be drawn for that.
     

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