Equine Dwarfism 101, version 2 (Updated 2014) *Warning: This thread will have links to multiple pictures of aborted/stillborn foals* This is an updated thread from the original Equine Dwarfism 101. http://myhorseforum.com/threads/361669/ The original version of this thread was posted when solid scientific research was sparse on the genetic details of equine dwarfism. The old thread is very outdated and has information that has been revealed as false due to new information brought about by new research and scientific methods. Due to the visual presence the first thread had, this was made to allow the most recent advancements on the subject to be shared and available to those it may help. The information on this thread is based on results of scientific research, and the experiences of breeders and dwarf owners. What is Dwarfism? Dwarfism is a genetic disorder that causes abnormally smaller stature in an individual of a species. This can refer to overall reduced stature, or reduced stature in specific body parts. •Equine dwarfism is most prevalent in American Miniature Horses. It more rarely occurs in Friesians, Welsh, and donkeys. Miniaturization vs Dwarfism •Miniaturization –Naturally small size –Gradually bred for over many generations from normal sized horses •Attempts to keep miniatures in proportion •Not a genetic defect •Dwarfism –Genetic defect that causes reduced growth of body parts and disproportionate bodies in equines –Causes health disorders –Inheritable recessive trait –Multiple mutations can cause various forms Dwarfism Inheritance •Breeding and research has shown that all known forms of dwarfism in equines are recessive. Some are lethal. –It takes 2 dwarf genes, one from the stallion and one from the mare, to have a dwarf foal –Parents can be normal or be dwarves themselves –Was once thought to be only inherited through the sire, this is not the case as both a mare and stallion can pass on the genes for dwarfism Types of Dwarfism Chondroplasia-like dwarfism (ACAN Dwarfism) •These dwarf types were formerly called Diastrophia, Bracchiocephalia, and Hypochondrogenesis. These are no longer accepted names for them. •Caused on the ACAN locus by 4 known mutations (more mutations likely exist but have not been isolated yet) –All 4 mutations can combine and express together as compound heterozygotes in various pair combinations (D1/D2 ect.) –Certain mutations, and combinations of these mutations, are lethal and result in aborted foals. Half of all combinations are lethal, the other half are viable. Physical Affects: •There are multiple physical affects from this form of dwarfism –Cartilage and bone growth are inhibited causing compacted bodies limbs and craniofacial (head/face) deformities. –Legs can be deformed with ligament, bone, and joint malformations. They often require special shoes, leg braces, or even surgery. –Underbites and tooth crowding can occur due to inhibited jaw and facial growth. –Premature arthritis is common •Overall compacted appearance. –Roach backs and muscle atrophy are common. –The ribcage is compacted and the organs often continue to grow as normal, restricting inside the body cavity. –Roached backs and bloated body cavities become more pronounced with age –Not all ACAN dwarves will express in the same manner or to the same extent due to various gene combination possibilities Identified mutations of Chondroplasia-Like Equine Dwarfism: Images of tested animals found here: http://uknowledge.uky.edu/cgi/viewcontent.cgi?article=1012&context=gluck_etds D1 - Lethal. When combined with itself or any other dwarfism mutation the fetus is non-viable and aborted. Only when D1 is paired with a non-dwarf gene will it result in a viable carrier. D2 - Viable, except when combined with D1. D3 - Viable, except when combined with D1. Although no homozygous ( D3/D3 ) horses were found in the research samples, this pairing is considered possibly viable. D4 - Lethal when combined with itself or D1, viable with all other combinations. Creates more severe dwarfism traits than D2 or D3. The neck often appears almost non existent in this form than the other viable ones with a much more compacted body. Although no homozygous ( D4/D4 ) horses were found in the research samples, this pairing is considered possibly lethal. (See ACAN gene combination chart in Breeding and Dwarfism section) –These gene discoveries may explain, in part, why miniature horses have a noticeably higher miscarriage rate, and lower live foal rate, compared to other horse breeds. Affected Breeds: American Miniature Horse, Shetland, Welsh. Any grade horses with dwarfism-carrying breeds in their background are also at risk. Skeletal Atavism Dwarfism (Abbreviated "SA" dwarfism for this thread) –These dwarves were formerly called Achondroplasic. This is no longer an accepted term for them. –Caused on a separate locus from Chondroplasia-like dwarfism. –Overall "dachshund' appearance with long back and short legs –Upper leg and pelvic growth is inhibited. This causes short legs with a normal length back causing the back to appear long. –Ears are short. What is atavism and how does it relate to this form of dwarfism? –The term atavism is, a reappearance of a trait or character that was seen in all earlier evolutionary specimens of a particular species, but has not been seen in recent ancestors. –Ancestral horses had rotatable limbs similar to dogs with the radius and ulna existing as separate bones. These are the two bones in humans (located between the wrist and elbow) that allow the arm to twist open a door knob. The tibia and fibula are comparable bones in the hind legs. These used to be separate in equids such as Hyracotherium and Mesohippus. As horses evolved increasingly specialized towards running and larger size the ulna and fibula fused to their respective bone pair to better support the body while in motion. – In Skeletal Atavisim horses the radii and fibulas in their respective front and back legs are not fused. This is where the term Atavism comes from in the name since these horses display this trait. This mutation prevents full normal development of these bones. Affected Breeds: American Miniature Horse, Shetland, Welsh. Any grade horses with dwarfism-carrying breeds in their background are also at risk. Osteochondrodysplasia Dwarfism (Friesian dwarfism) –These dwarves were formerly called Diastrophic. This is no longer an accepted term for them. –Dwarf Friesians' have legs that are 25% shorter and they weigh half as much as a normal Friesian. Their ribcage is compacted and the formation of bone and cartilage are disrupted in the limbs. Back appears long, lax hind ligaments are common. Corrective shoes and boots can aid in walking. –Hooves are often severely contracted for a horse their size –This form of dwarfism has some parallels to Skeletal Atavism in appearance with shortened upper limbs and an unaffected head. –The causative mutation has not been isolated yet, but the chromosome on which it occurs has been located. Further research is needed to identify the mutation. However, the inbred nature of the Friesian gene-pool has hindered fine-mapping the causative gene. –Friesian dwarves have harder physical challenges due to their size. There is more weight and mass acting on their bodies and organs than in miniature horses. This allows the possibility of increased problems in their limbs while raising their risk of colic and organ failures. Donkey Dwarfism – Donkey dwarfism is not as common as the other types previously described. There is limited information available on the subject because of this. – Dwarf donkeys are identified through their disproportionate bodies. Donkey registries look at the head to leg length ratio to help determine if a donkey is disproportionate. If they have a more extreme head to leg length ratio, they are considered more than likely to be dwarfs. – Dwarf donkeys have shown up in two types. One that resembles ACAN dwarfism with short compacted bodies, leg anomalies, and domed heads with underbites and facial deformities. And another resembles skeletal atavism with shortened upper limbs. Given these physical expressions, it is likely that they occur on the same gene locations as dwarfism found in miniature horses. The short leg dwarfism is the most common with the one resembling ACAN dwarfism appearing much more rarely. Dwarf donkeys often have short necks, high croups, short legs, large, misaligned jaws, and pot bellies. – This website has silhouettes of donkeys expressing dwarfism: http://www.lovelongears.com/sidebyside.html http://www.lovelongears.com/silhouettes.html http://www.lovelongears.com/dwarfmeasure1.html Affected Breeds: Only Sicilian (miniature) donkeys are known to have dwarfism occur. Grade donkeys of mixed backgrounds are also at risk. Treatment and Health Concerns for Dwarf Horses Facial: Dental abnormalities are common in ACAN dwarfs. Undershot jaws and crowded teeth often occur in ACAN dwarves. This page chronicles a facial surgery performed on an ACAN mini named Dobby. The surgery was performed to allow him to breath better. It also has information about his hoof, dental, and leg care. http://www.boundarywatersfarm.com/Dobby's Page.htm Dental abnormalities are rarer in SA and Friesian dwarfs due to their heads mainly being unaffected by their form of the disorder. Legs and Farriery: Premature and severe arthritis are common in all forms. Dwarfs of any type with severe leg issues may require corrective surgery or special shoes/braces to walk comfortably. Some dwarfs legs will continue to worsen over time, and leg joints may fuse. Internal Organs: Colic is a constant risk for dwarfs due to certain types having restricted body cavities and normal sized internal organs. Colic is more common in ACAN and friesian dwarfs. Skeletal Atavism horses are likely at less risk of internal issues due to their main bodies being normal sized. But there are no statistics on this. Mental Health: Some dwarf horses have mental retardation due to their disorder. The malformation of the cranium can affect mental development. Some dwarfs may also suffer from depression due to their condition and constant pain. Life Expectancies: Viable dwarfs can survive anywhere from a few hours to a few years. Much of this depends on severity of dwarfism and care. Severe dwarfs appear to average a few years or less. Mildly expressed dwarfs could live into double digits. Dwarfs, in general, have shorter life expectancies than non-dwarf horses. Due to multiple physical abnormalities, these alone greatly increase risk of premature mortality. For many dwarfs euthanasia is often performed once quality of life is affected, or quality of long term care cannot be provided. Dwarfs possessing D1/D( ) or D4/D4 are naturally aborted early. These horses never survive outside the womb. Other dwarfs can be aborted early even if their gene combination is viable. Others are stillborn for either unknown reasons, or complications during birth. They are at higher risk for these problems. Dwarf friesians often live much shorter lives than non-dwarves or even miniature horse dwarves. Many dwarf frisians only survive a few years. For perspective, the longest-lived dwarf friesian, Silhouette, survived to be 14 years old. The second longest-lived dwarf friesian was less than half that age and died at 6. Many others are humanely euthanized due to their condition right after birth. Identifying Dwarf Foals • A foal is likely a dwarf if: ACAN Dwarfism – Born significantly smaller than what is normal (Mini: 18lbs 21inches) even if they initially appear normal – Matures to be much smaller than both parents – Has noticeable facial, ligament and/or leg issues – Born fairly normal but develop dwarfish traits as they mature (often between 2 months to 2 years) • Pot belly, short neck roach back, bulging forehead and eyes Skeletal Atavism Dwarfism • Foals can be identified by their severely shortened upper legs at birth –May be over at the knee, and camped out behind, until later in maturity •Legs in “four corners” and not balanced underneath –May need special shoes or braces •Head fairly normal, ears very small Friesian Dwarfism Foals are born smaller than normal –Hind legs are often camped out and are back at the stifle (Weak hind end) –Main body often appears normal at birth but later develops a roached back and pot belly due to compacted growing organs –Loose ligaments are common. May be loose to the point that the fetlocks touch the ground –Special braces and shoes can needed to correct tendons by the time maturity is reached –Weigh half as much as other foals in the same age range Donkey Dwarfism –Dwarf donkeys will generally be smaller. –As they mature their proportions will not even out. Head-to-leg ratio should be taken to determine disproportion status. Equine Dwarfism History and Gene Flow Dwarfs descend from the 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. 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 attempt to remove the disorder from the gene pool as much as possible. 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. 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. Gene Flow of ACAN Dwarfism and Skeletal Atavism In miniature horses of the past, small size was valued over conformation. In the 60's and 70's dwarf horses were winning in the show ring because of their diminutive size caused by their disorder. Dwarf stallions such as Bond Tiny Tim (ACAN) and Del Terras Lord of the Isles (Skeletal Atavism) spread their dwarfism genes to all their offspring (There were other historically prolific dwarf stallions besides these two. Bond and Del Terra's are better known examples.) Bond Tiny Tim a 1970 stallion with ACAN dwarfism (Shown as a foal and adult in Identifying Dwarf Foals section) with two of his ACAN dwarf offspring. Del Terra's Lord of the Isles a 1972 miniature horse stallion with skeletal atavism dwarfism. Because dwarf stallions were so highly favored for a time, the breed has suffered from the disorder being spread through the effect of Popular Sire Syndrome. A gene flow effect where prolific males greatly spread a trait or disorder within a gene-pool. (The quarter horse stallion Impressive who spread HYPP is another example of a popular sire effect) All descendants of dwarf sires at least carry dwarfism, but not all of their grand-foals do. Although lines that have used dwarves directly should be cautioned, it is good to note that not all animals decedent from these lines will carry the disorder. Regardless, all miniature horse lines are capable of carrying dwarfism regardless of pedigree. It has been estimated that over half of all miniature horses carry some form of dwarfism. Gene Flow of Osteochondrodysplasia (Friesian Dwarfism) The friesian breed underwent a severe bottleneck (being reduced to a small number of animals). All modern day friesians are descendant from a handful of individuals. It has been theorized that the 3 stallions all modern friesians are descendant from likely carried dwarfism. In the past, dwarf mares have been used for breeding as recently as the 1990's. This spread the disorder since dwarf mares passed on the gene to every one of their foals. Dwarf stallions were banned registry during this time. (See this link for a 4 year old dwarf mare with foal at side http://www.fenwayfoundation.com/pdf/Boerma et al. EVE 2012.pdf ) Currently dwarf friesians are not allowed breeding rights within the registry. Stallions can have their registration revoked for producing too many dwarf offspring. Although dwarfism is not encouraged, there is little to be done to eradicate it without a gene test. The population of the friesian breed is limited. There is concern that the already small gene-pool will be made much smaller if all dwarfism carriers are removed. This is a valid concern. If a test is created, dwarfism should be managed like HERDA in Quarter Horses and SCID in Arabians. These are recessive disorders, like dwarfism, that are managed by never breeding carrier to carrier. This will allow carriers to contribute genetically without having affected offspring only if breeding is done responsibly. However, the Friesian breed cannot begin to implement such a policy without a genetic test. Another possibility to reduce dwarfism and increase genetic viability in the Friesian breed is to allow an outcross/breed-back program. Screened and carefully selected non-friesian horses would be introduced to create crosses. These crosses would then be bred back to friesians to increase genetic variance and increase the amount of dwarfism-free individuals while reestablishing and maintaining 'type' of the breed. (This method has been successfully used in preservation of the kladruber breed's genetic viability by combining bloodlines) Due to the strict rules regarding the friesian breed, this is unlikely to be considered as an option. Friesians are at risk of spreading dwarfism to other horses. The popularity of friesian crosses, 'warmbloods' and/or 'sporthorses' has widened the spread of the breed's dwarfism gene across other areas of the equine gene pool. These friesian crosses are being bred together or back to full-blooded friesians. Dwarf offspring can and are occurring from these combinations. There are multiple cases of mix-breed dwarfs with friesian blood on both sides. Breeding and Dwarfism When breeding for smaller size, dwarfism will inevitably be selected. Dwarf breeds of dog, cattle, and others have shown that when breeding smaller size, mutations for dwarfism are more likely to occur. As hormones and genes combine to create smaller animals, these are more likely to allow mutation. This is similar to how selecting for more docile personalities in animals increases the chance of white patterns mutating. Careful genetic research and responsible breeding reduce the spread of dwarfism disorders within a population. It is considered good practice to geld stallions and retire mares from breeding if they produce a dwarf offspring –Since dwarfism is recessive both parents are carriers •Dwarf equines should not be bred-because they will only create more carriers or more dwarf offspring –Dwarf stallions should be gelded –Dwarf mares should be spayed and/or kept away from stallions •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. –Even non-dwarf mares can potentially sustain trauma from birthing a dwarf foal, resulting in injury and/or death. This is due to some dwarf foals having odd proportions that can cause complications during birth. Irresponsible breeding can result in high mortality rates, foals needing expensive surgery or needing to be humanely put down 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 dwarfism. 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. Former show mare. Has produced a foal with skeletal atavism. –Although there is no surefire way to visually determine dwarfism carriers from non-carriers, some have noticed that miniature horses who are Skeletal Atavism carriers tend to have more 'horse-like' heads. Others have noticed smaller ears are also present in their carrier stock. Although a correlation, horses with these head and ear types are not guaranteed carriers. Nor are those lacking these traits guaranteed to be non-carriers. *Correlation does not equal causation.* More information on the structure of carriers must be obtained to draw any clear conclusions, though determining possibilities is a first step. ACAN dwarfism combination chart. There are many genetic combinations of this type that are possible. Dwarfism Type Combinations Chondroplasia-Like Dwarfism (ACAN) and Skeletal Atavism combinations –Skeletal Atavism horses have been found carrying the ACAN mutation D2 (D2/N Sa/Sa) They could likely carry other heterozygous ACAN mutations and still be viable. –Horses who carry one copy of ACAN and one copy of Skeletal Atavism have been found by breeders and appear normal. Non-dwarf mini stallion. Known ACAN and Skeletal Atavism carrier ( D/N , Sa/N ). Has sired both types of foals. –Horses homozygous for both ACAN Dwarfism and Skeletal Atavism (D/D Sa/Sa ) are unknown. It is possible that this combination is non-viable. Miniature Horse and Donkey Dwarfism. (Dwarf Mules) If the genes for miniature horse dwarfism and donkey dwarfism are on the same gene locus, they can interact together to create mini mules with dwarfism. It is a rare occurrence since mini mules aren't often bred for, and dwarfism occurs more rarely in the donkey parents of mini mule combinations. A mini mule expressing skeletal atavism. This means that skeletal atavism in minis and at least one form of dwarfism in donkeys are on the same locus and can interact. If donkeys also posses a dwarfism mutation on ACAN, it may also create ACAN dwarf mini mules. Limits of current knowledge •Results of genetic research are fairly recent. At the time of this being written, a public test for any form of known equine dwarfism are not yet available. The specific mutations for Skeletal Atavism and Friesian Dwarfism are not isolated yet, but are closer to being discovered. •There are not many visual examples of tested ACAN animals currently available to the public. The range of expression is unknown for the viable forms and their respective mutations. •ACAN dwarfism study sample sizes are small. 200 American Miniature Horse Association (AMHA) horses were sampled and and 44 American Miniature Horse Registry (AMHR) horses were used in the study on ACAN dwarfism. The 44 horses tested in the AMHR group is small. AMHA group had all 4 ACAN dwarfism mutations, while the AMHR group only possessed D2. AMHR horses initially appear to have less prevalent dwarfism (in mutation numbers and overall percentage) than AMHA, but there need to be more studies done for definitive statistics. Information on research and how to help Research on ACAN dwarfism is being performed by Dr. John Eberth at the University of Kentucky -Information on how to send samples or donations are at the bottom of this presentation. http://www.amha.org/pdf/memb/DPresentation.pdf Research on Donkey Dwarfism is being performed by Dr. Gus Cothran of Texas A&M -Information on how to send samples or donations are at the bottom of this page. http://www.anes-miniatures.fr/wp-content/uploads/2011/01/A-TINY-CAUTIONARY-TALE.htm If you own a dwarf: Please share and contact researchers. Part of the reason why research has been slow in these fields is that dwarfism is a taboo subject amongst equine breeders. The more the subject of dwarfism is shared, the less stigma discussing the subject will have. This will allow more information about it to be gained. If you are a member of an affected breed registry: Ask for further research sponsorship into dwarfism. If a gene for dwarfism has been isolated in your breed, ask that testing be encouraged within the registry. This will allow the breed population to work towards reducing the occurrence of this disorder and the number of foals affected. There will be a greater response to controlling a breed's dwarfism if there is a greater demand by its members to do so. Sources and External Links: Chondrodysplasia-Like Dwarfism in the Miniature Horse, John Eberth 2013 http://uknowledge.uky.edu/cgi/viewcontent.cgi?article=1012&context=gluck_etds Dwarfism in the Miniature Horse, John Eberth (Older powerpoint) http://www.amha.org/pdf/memb/DPresentation.pdf Skeletal Atavism in a Miniature Horse, Reid Tyson, DVM, John P. Graham, MVB, MSC, Patrick T. Colahan, DVM, Clifford R. Berry, DVM onlinelibrary.wiley.com/doi/10.1111/j.1740-8261.2004.04060.x/abstract Genome-wide SNP association-based localization of a dwarfism gene in Friesian dwarf horses. Orr N, Back W, Gu J, Leegwater P, Govindarjan P, Conroy J, DUcro B, Van Arendonk JA, MacHugh DE, Ennis S, Hill EW, Brama PA. http://www.ncbi.nlm.nih.gov/pubmed/21070269 Dwarfism in the Friesian Warmblood and Friesian horse, an article written by the Australian Warmblood Friesian Association. Includes details on dwarfism history in the friesian breed. http://www.friesianhorses.com.au/healthdwarfism.htm Clinical Commentary, The Friesian horse breed: A clinical challenge to the equine veterinarian?, S.Boerma, W. Back†‡ and M.M. Solet van Oldruitenborgh-Oosterbaan*† http://www.fenwayfoundation.com/pdf/Boerma et al. EVE 2012.pdf The Drost Project. A veterinary website with information and images on various livestock reproductive issues. Has images of a severe ACAN dwarf mini foal with rotated malformed legs, and a stillborn dwarf who may have been some form of D1. These images located under Equine Reproduction > Teratology > Congenital Anomalies. http://www.drostproject.org/en_equrep/guide.html Tifrons the dwarf friesian filly. Website and owner's account of Tifron's life and details on managing her condition, mobility, and comfort. http://www.dragonflyacres.ca/tifrons.htm Little Magic Shoes. A website run by Jill Jensen that chronicles experiences with aiding dwarf minis' mobile needs. Jill owns and has cared for many various types of dwarf minis. http://www.littlemagicshoes.com/ Dobby an ACAN dwarf mini. This page has photographs and shares details on his care, dental procedures, special farriery needs, and a facial surgery performed to allow him to breath better. This page shows the dedication and extreme care required for owning severely affected dwarfs. http://www.boundarywatersfarm.com/Dobby's Page.htm JCP Miniatures. A website about Julia's dwarf horses and experiences with them. Helped obtain visual example of non-dwarf ACAN and SA carrying mini. Many images and examples were provided by her to further educate others on this equine disorder. http://jcpminiatures.weebly.com/equine-dwarfism.html A Tiny Cautionary Tale. Information on miniature horse and mini donkey dwarfism. Contains information to help with donkey dwarfism research. http://www.anes-miniatures.fr/wp-content/uploads/2011/01/A-TINY-CAUTIONARY-TALE.htm Love Long Ears. A miniature donkey website with silhouettes of visually identified dwarf donkeys. Also contains pages on measuring for disproportion and dwarfism identification. http://www.lovelongears.com/sidebyside.html http://www.lovelongears.com/silhouettes.html http://www.lovelongears.com/dwarfmeasure1.html Case Story Romeo. A story about a dwarf donkey named Romeo and the identification of his disorder. http://www.lovelongears.com/dwarf_romeo.html Sincere thanks to Ellen Dahlstet for sending information on donkey dwarfism research. Special thanks to all of the researchers and dwarf owners and who have helped bring about their experiences with these equines to the forefront.