Horse Genetic Testing: Which Diseases Can DNA Tests Detect?

By Dr Duncan Houston

A genetic test can prevent the birth of a foal with a fatal inherited disease. It can also help explain repeated muscle problems, change an anaesthetic plan or identify a horse needing closer eye examinations.

However, DNA results are commonly misinterpreted.

A carrier is not necessarily a sick horse. A susceptibility marker does not mean disease is inevitable. A normal result only excludes the specific variant tested, and some commercially marketed equine genetic panels have not been scientifically validated.

Quick Answer

Validated equine DNA tests are available for many inherited conditions, including HYPP, PSSM1, malignant hyperthermia, HERDA, GBED, MYHM, fragile foal syndrome, SCID, cerebellar abiotrophy and several other breed-associated diseases.

The correct tests depend on the horse’s breed, ancestry, clinical signs and intended breeding use. Use a recognised laboratory, confirm that the test has been scientifically validated, and have the result interpreted according to its inheritance pattern rather than treating every positive result as a diagnosis. (AAEP)

What Can Horse Genetic Testing Tell You?

Equine DNA testing can provide several very different types of information.

Disease-causing variant tests

These identify a specific variant that causes or substantially contributes to a recognised inherited disorder.

Examples include:

• The SCN4A variant causing HYPP

• The GYS1 variant causing PSSM1

• The RYR1 variant causing malignant hyperthermia

• The PPIB variant causing HERDA

• The GBE1 variant causing GBED

• The PLOD1 variant causing fragile foal syndrome

These tests are most useful when a causal relationship has been demonstrated through properly phenotyped horses, inheritance studies and peer-reviewed research. (AAEP)

Susceptibility or risk-marker tests

A risk marker identifies increased susceptibility rather than confirming that the horse has or will develop the disease.

The ocular squamous cell carcinoma test is an example. Horses carrying two copies of the relevant DDB2 risk variant have a greater risk of ocular SCC, but not every genetically high-risk horse will develop cancer. Environmental exposure, age and other biological factors still matter. (Veterinary Genetics Laboratory)

Carrier testing

Carrier testing is particularly important for autosomal recessive diseases.

A carrier usually has one normal copy and one disease-associated copy of the gene. The horse is generally clinically normal but can transmit the variant to approximately half of its offspring.

A carrier is therefore not the same as an affected horse. The concern arises when two carriers are bred together. (Veterinary Genetics Laboratory)

Parentage verification

Parentage testing compares a horse’s DNA profile with the proposed sire and dam. It can exclude incorrect parentage and support registry accuracy.

Parentage verification is not a health test, and it is not automatically a breed-identification test. UC Davis specifically distinguishes its parentage and identity testing from a horse breed test. (aqha)

Coat colour, pattern and gait tests

Some DNA tests identify traits rather than disease.

Examples include:

• Coat colour and white-pattern variants

• Grey copy number

• Leopard complex

• DMRT3-associated gait traits

• Parentage markers

Some appearance variants also have health implications. Horses with two copies of leopard complex, for example, develop congenital stationary night blindness and have increased risk of equine recurrent uveitis. (Veterinary Genetics Laboratory)

What Can Genetic Testing Not Tell You?

A genetic panel cannot reliably predict:

• How talented a horse will be

• How fast it will race

• How high it will jump

• Its future temperament

• Whether it will remain sound

• Whether it will develop every possible inherited disease

• Whether a current clinical problem is definitely caused by the detected variant

Performance, conformation, behaviour and many disease tendencies are influenced by multiple genes together with training, nutrition, management and environmental exposure. The AAEP distinguishes these complex inherited tendencies from single-gene genetic defects. (AAEP)

A normal panel does not mean the horse is genetically perfect. It means the laboratory did not detect the variants included in that panel.

How Should Genetic Results Be Interpreted?

Type of result	What it usually means	What it does not mean
N/N or clear	No copies of the particular tested variant were detected	The horse is free from every genetic disorder
Carrier of a recessive variant	The horse is usually clinically normal but can pass the variant to offspring	The horse necessarily has the disease
Two copies of a recessive variant	The horse is expected to be affected, although penetrance may vary in selected disorders	Every recessive condition has identical severity
One copy of a dominant variant	The horse may be affected or susceptible and can pass the variant to approximately half its offspring	Clinical signs must already be present
Codominant variant with variable penetrance	One or two copies increase susceptibility, but some horses never develop recognised signs	A positive test alone establishes the cause of every symptom
Risk marker	The horse has an increased or reduced statistical risk	The horse definitely will or will not develop the disease

These distinctions are demonstrated by conditions such as recessive fragile foal syndrome, dominant HYPP, variably penetrant MYHM and the DDB2 ocular SCC risk marker. (Veterinary Genetics Laboratory)

What Does Autosomal Recessive Mean?

An autosomal recessive disease generally requires two copies of the disease-associated variant.

When two carriers are bred:

• There is a 25 percent chance of an affected foal

• A 50 percent chance of a carrier foal

• A 25 percent chance of a clear foal

When a carrier is bred to a genetically clear horse, no affected foals are expected from that particular mating, although approximately half may inherit carrier status. (Veterinary Genetics Laboratory)

This is why identifying a carrier does not automatically mean the horse must be removed from breeding.

For uncommon breeds with limited genetic diversity, carefully breeding a valuable carrier to a clear partner can avoid affected foals while preserving desirable bloodlines. UC Davis specifically supports carrier-to-clear breeding strategies for disorders including hoof wall separation disease, Friesian dwarfism and junctional epidermolysis bullosa. (Veterinary Genetics Laboratory)

What Does Autosomal Dominant Mean?

For an autosomal dominant disorder, one copy of the variant may be enough to produce susceptibility or disease.

A horse with one copy can usually transmit the variant to approximately 50 percent of its offspring.

Examples include:

• HYPP

• PSSM1

• Malignant hyperthermia

Clinical severity may still vary. An HYPP-positive horse may have frequent attacks, rare attacks or no recognised episode for years, while another horse with the same genotype may experience weakness, collapse or respiratory compromise. (Veterinary Genetics Laboratory)

Which Equine Diseases Have Validated Genetic Tests?

The following table contains selected examples rather than every test currently available.

Breed or ancestry	Important validated tests to consider
Quarter Horse, Paint and related stock breeds	HYPP, PSSM1, MH, GBED, HERDA, MYHM and, where relevant, EJSCA
Arabian and Arabian-related horses	SCID, cerebellar abiotrophy, lavender foal syndrome and one recognised form of OAAM
Warmblood and sport-horse populations	Fragile foal syndrome type I
Thoroughbreds	FFS type I and equine familial isolated hypoparathyroidism where relevant
Friesians	Dwarfism, hydrocephalus and distichiasis
Connemara Ponies	Hoof wall separation disease
Fell and Dales Ponies	Foal immunodeficiency syndrome
Belgians and some related draft breeds	JEB1, Belgian hydrocephalus and ocular SCC susceptibility
American Saddlebreds and related breeds	JEB2
Appaloosas and leopard-complex breeds	Leopard complex-associated congenital stationary night blindness and uveitis risk
New Forest Ponies	Congenital myotonia
Paint, Quarter Horse and other frame-overo populations	Lethal white overo syndrome
Selected Haflinger, Belgian and related populations	Ocular SCC susceptibility

The AAEP maintains a broader list of validated single-gene tests and notes that equine genetic information is developing rapidly. Test choice should therefore be reviewed against the current laboratory and breed-registry recommendations before breeding. (AAEP)

Which Genetic Tests Are Important in Quarter Horses?

Quarter Horses appear frequently in genetic-testing discussions partly because the breed is large, extensively studied and supported by an organised registry testing programme. Several disease variants have also been amplified within influential bloodlines.

AQHA’s current standard health panel includes:

1. Hyperkalemic periodic paralysis

2. Polysaccharide storage myopathy type 1

3. Malignant hyperthermia

4. Glycogen branching enzyme deficiency

5. Hereditary equine regional dermal asthenia

6. Myosin-heavy chain myopathy

AQHA requires breeding stallions to have DNA and a genetic health panel recorded before resulting foals can be registered. Registry requirements can change, so current rules should always be confirmed directly before breeding. (aqha)

Hyperkalemic periodic paralysis

HYPP is caused by a dominant muscle sodium-channel variant.

Affected or susceptible horses may experience:

• Muscle trembling

• Weakness

• Prolapse of the third eyelid

• Loud or abnormal breathing

• Collapse

• Paralysis

• Sudden death during a severe episode

Diet, stress and changes in exercise can influence clinical expression. One copy is sufficient to create susceptibility, while horses with two copies are often more severely affected. (Veterinary Genetics Laboratory)

Polysaccharide storage myopathy type 1

PSSM1 is caused by a dominant GYS1 variant that alters muscle glycogen storage.

Possible clinical signs include:

• Muscle stiffness

• Pain after exercise

• Sweating

• Reluctance to move

• Weakness

• Recurrent tying-up

• Poor performance

The variant occurs in Quarter Horse-related breeds, several draft breeds and a range of other horse populations. Clinical severity depends on genotype, breed, diet, exercise and other modifying factors. (College of Veterinary Medicine)

Malignant hyperthermia

Malignant hyperthermia is associated with an RYR1 variant.

Potential triggers include:

• Certain inhalational anaesthetics

• Succinylcholine

• Severe stress or excitement

• Illness in some affected horses

An episode can cause rapid hyperthermia, rigid muscles, abnormal heart rhythm, sweating and life-threatening metabolic collapse. A positive result should be clearly recorded in the horse’s veterinary file and disclosed to the anaesthesia team before any procedure. (Veterinary Genetics Laboratory)

Glycogen branching enzyme deficiency

GBED is an autosomal recessive and usually fatal disorder.

Affected foals cannot properly store glycogen in several organs and may be aborted, stillborn or born weak. Carrier horses remain clinically normal, but two carriers have a 25 percent chance of producing an affected foal. (Veterinary Genetics Laboratory)

Hereditary equine regional dermal asthenia

HERDA is a recessive connective-tissue disorder that causes fragile, hyperextensible skin and severe wounds, particularly across the back.

Signs may not become obvious until the young horse begins training and saddle pressure causes the skin layers to separate. Carriers are clinically normal and should be bred only to clear partners. (Veterinary Genetics Laboratory)

Myosin-heavy chain myopathy

MYHM is associated with a variant in MYH1 and can produce two clinical presentations:

• Immune-mediated myositis, involving rapid muscle loss

• Non-exertional rhabdomyolysis, involving severe muscle pain and damage without exercise

The inheritance is codominant with variable penetrance. Horses with one or two copies are susceptible, but not every positive horse develops recognised disease. Horses with two copies may experience more severe clinical disease. (Veterinary Genetics Laboratory)

Equine juvenile spinocerebellar ataxia

EJSCA is a newer recessive neurological disorder identified in Quarter Horse foals.

Affected foals generally become incoordinate between one and four weeks of age, deteriorate rapidly and may become unable to stand. A validated test is available separately, although EJSCA is not listed among AQHA’s current standard six-panel diseases. (Veterinary Genetics Laboratory)

Should MYHM-Positive Horses Avoid Vaccination?

A positive MYHM result should lead to an individual veterinary vaccination plan, not the blanket abandonment of disease protection.

Infection and vaccination have both been reported as possible immune triggers, but many affected horses have no obvious preceding trigger. Infectious respiratory disease itself may also carry substantial risk.

UC Davis currently advises discussing the following precautions for N/My or My/My horses:

• Avoiding strangles vaccination

• Using intranasal formulations where appropriate

• Limiting intramuscular vaccination to those considered necessary for the horse’s geographic and exposure risk

• Spacing vaccinations rather than administering many at once

These decisions should be made with the treating veterinarian. Do not simply leave a high-risk horse unprotected against tetanus, encephalitis or other serious disease after reading a DNA report. (Veterinary Genetics Laboratory)

What Genetic Tests Are Available for Arabian Horses?

The UC Davis Arabian health panel includes four autosomal recessive diseases.

Severe combined immunodeficiency

SCID foals are usually normal at birth but develop severe recurrent infections, fever, respiratory disease or diarrhoea during the first weeks of life because they cannot mount an effective immune response. The condition is fatal. (Veterinary Genetics Laboratory)

Cerebellar abiotrophy

Affected foals may develop:

• Head tremors

• Exaggerated limb movements

• Wide-based stance

• Severe incoordination

• Falling

• Difficulty rising

Carriers remain neurologically normal. (Veterinary Genetics Laboratory)

Lavender foal syndrome

Affected foals have a dilute lavender, pink or silver appearance together with profound neurological abnormalities such as seizures, paddling and inability to stand. The condition is fatal. (Veterinary Genetics Laboratory)

Occipitoatlantoaxial malformation

OAAM causes abnormal development around the skull and upper cervical vertebrae, potentially compressing the spinal cord.

An important limitation is that the available Arabian test detects one recognised OAAM-associated variant. A normal result does not exclude every possible genetic or developmental cause of OAAM. (Veterinary Genetics Laboratory)

What Is Fragile Foal Syndrome?

Fragile foal syndrome type I, previously called Warmblood fragile foal syndrome, is a recessive connective-tissue disorder caused by a PLOD1 variant.

Affected foals may be born with:

• Extremely thin and fragile skin

• Large skin tears

• Hypermobile joints

• Haematomas

• Prematurity

• Subcutaneous air

• Severe mucosal injury

The condition is lethal. Carriers are clinically normal, so testing both intended parents is the only reliable way to avoid carrier-to-carrier matings. The variant has been detected in multiple Warmblood populations and at lower frequencies in Thoroughbreds and several other breeds. (Veterinary Genetics Laboratory)

What Does the Ocular SCC Genetic Test Mean?

Ocular squamous cell carcinoma is a common equine eye tumour, particularly around the limbus and third eyelid.

A DDB2 variant has been associated with increased ocular SCC risk in Haflingers, Belgians and some related populations.

The result should be interpreted as follows:

• N/N: lower genetic risk from this marker

• N/R: one copy, not considered at the same elevated risk as R/R horses

• R/R: increased risk compared with N/N and N/R horses

An R/R result is not a cancer diagnosis. It indicates that routine veterinary eye examinations, careful monitoring and UV-protective fly masks are particularly sensible. Any new pink, white, raised, ulcerated or rapidly growing ocular lesion still requires examination regardless of genotype. (Veterinary Genetics Laboratory)

Is There a Genetic Test for PSSM Type 2?

There is currently no scientifically validated DNA test that identifies the underlying cause of PSSM2.

PSSM1 has a validated test for the GYS1 variant. The term PSSM2 has historically been used for horses with compatible clinical signs and muscle-biopsy changes that do not carry the PSSM1 variant, but the underlying genetic cause remains unidentified. (College of Veterinary Medicine)

Commercial tests have marketed variants labelled P2, P3 and P4 as indicators of PSSM2 or myofibrillar myopathy. A peer-reviewed comparison found no significant association between those commercial variants and biopsy-confirmed disease in Warmbloods or Arabians. The researchers did not recommend using those results for diagnosis, breeding decisions or pre-purchase examinations. (PubMed)

A horse with recurrent muscle stiffness still deserves investigation. Depending on the presentation, this may include:

• Clinical history

• CK and AST measurements

• Controlled exercise testing

• Electrolyte and nutritional assessment

• Testing for validated variants such as PSSM1, MYHM, HYPP or MH

• Muscle biopsy in selected cases

A normal PSSM1 result does not mean the horse has “PSSM2”, and an unvalidated commercial variant does not provide that diagnosis.

Is There a Genetic Test for Equine Asthma?

No validated single-gene clinical test is currently available for equine asthma.

Research supports a hereditary component in some families, but exposure to hay dust, mould, bedding, stabling and other environmental factors strongly affects whether disease is expressed. Genetic studies have identified candidate regions and quantitative trait loci, but not a single causal variant suitable for routine diagnosis. (PubMed)

Equine asthma should therefore be diagnosed through clinical assessment, respiratory examination and appropriate airway testing rather than a consumer DNA panel.

Why Do Quarter Horses Seem to Have So Many Genetic Diseases?

There are several reasons.

Quarter Horses are numerous, genetically well documented and supported by a breed association that has invested heavily in health testing. This improves detection and transparency.

Some variants also became more common because influential breeding animals produced very large numbers of descendants. HYPP, for example, is associated with descendants of the stallion Impressive, and AQHA has specific testing and registration rules relating to that lineage. (aqha)

Finding more genetic disease in a breed does not necessarily mean the breed is being managed poorly.

In many ways, open testing is evidence that the breed is confronting inherited disease rather than hiding it.

Other breeds almost certainly have genetic disorders whose causative variants have not yet been discovered. Recent whole-genome research estimated that disease-causing variants have been identified for fewer than 20 percent of suspected equine genetic diseases. (PubMed)

When Should a Horse Be Genetically Tested?

Before Breeding

Testing is most valuable before the mating is selected, not after the mare is already pregnant.

Test when:

• The breed registry recommends or requires it

• Either parent belongs to an at-risk breed

• A disease has previously occurred in the family

• The horse has mixed ancestry involving an affected breed

• Frozen semen or imported genetics are being used

• The intended mating could involve two recessive carriers

• A dominant disease variant is present in the pedigree

The results from both parents should be considered together.

Before Buying a Horse

Genetic testing may be useful during a purchase when:

• The horse belongs to an affected breed

• The official result is unavailable

• The horse has unexplained muscle, skin or neurological signs

• The horse may be bred

• Anaesthetic risk is relevant

• Registry records are incomplete

• The seller provides only an unofficial screenshot

• A result could materially change management or value

Genetic testing does not replace a pre-purchase examination.

For a high-value purchase, have the sample collected under verified identity, ideally by the buyer’s veterinarian or through the relevant registry process.

When a Horse Has Clinical Signs

Testing can support diagnosis when a horse has signs consistent with:

• HYPP

• PSSM1

• MYHM

• Malignant hyperthermia

• HERDA

• A breed-associated neurological disorder

• A recognised inherited bleeding or immune disorder

A positive result must still be matched to the clinical presentation. A horse can carry a disease-associated variant while its current symptoms are caused by something else.

Before Anaesthesia

Testing or reviewing existing results is particularly important when a Quarter Horse, Paint, Appaloosa or related horse may carry malignant hyperthermia.

The anaesthesia team should know about:

• MH status

• HYPP status

• Previous anaesthetic reactions

• Previous episodes of unexplained hyperthermia or rhabdomyolysis

• Concurrent PSSM1

The combination of PSSM1 and MH may produce a more severe muscle-disease phenotype. (PubMed)

How Do You Collect a Horse DNA Sample?

Many equine genetic tests can be performed using mane or tail hairs with intact roots.

UC Davis advises collecting approximately 20 to 40 rooted hairs. Tail hair is preferred for young foals because the roots are generally easier to obtain. The sample is secured according to the laboratory’s submission instructions and kept dry. (Veterinary Genetics Laboratory)

Important points include:

1. Confirm the horse’s identity before sampling.

2. Pull hairs rather than cutting them.

3. Check that roots are attached.

4. Keep samples from different horses completely separate.

5. Use clean, clearly labelled paperwork or envelopes.

6. Follow that laboratory’s exact packaging instructions.

7. Record the horse’s microchip or registration number.

8. Use veterinary or registry-controlled collection when identity may later be disputed.

Some laboratories accept whole blood or other samples. Do not assume the collection and shipping requirements are identical between laboratories.

How Do You Choose a Reliable Genetic Test?

Before ordering, ask:

Has the variant been scientifically validated?

There should be credible evidence connecting the variant with disease in properly diagnosed affected horses and suitable controls.

A computer prediction that a variant might alter a protein is not enough by itself. Healthy horses carry many variants that look potentially harmful during sequence analysis but have no recognised clinical effect. (College of Veterinary Medicine)

Has the work been peer reviewed?

A company testimonial or internal white paper does not carry the same weight as peer-reviewed research that can be independently assessed.

Is the test appropriate for this breed?

A variant validated in one breed may not explain similar-looking disease in another.

Testing every horse for every available variant wastes money and may create confusing results of uncertain relevance.

Is the result described accurately?

Be cautious when a company labels:

• A risk marker as a diagnosis

• A common variant as definitively disease-causing

• A performance-associated marker as a guarantee of talent

• An unvalidated association as a breeding defect

Is the laboratory recognised?

The AAEP recommends using licensed laboratories and checking breed-association requirements before relying on a test for breeding or registration. (AAEP)

How Serious Is a Positive Result?

Low Risk

The horse:

• Is clear for the tested variant

• Has no corresponding clinical signs

• Is a healthy carrier of a recessive condition and will not be bred

• Has been bred only to a confirmed clear partner

What to do: Keep the official report and continue ordinary health care. Remember that a clear result applies only to the tested variant.

Moderate Risk

The horse:

• Is a recessive carrier being considered for breeding

• Has a susceptibility marker such as R/R ocular SCC

• Has one copy of a variably penetrant dominant or codominant variant

• Is clinically normal but needs management adjustments

What to do: Discuss breeding, monitoring, vaccination, anaesthesia or management implications with your veterinarian. This is usually a planning issue rather than an emergency.

High Risk

The horse:

• Has a genotype associated with disease and compatible clinical signs

• Is homozygous for a potentially serious dominant or codominant variant

• Is part of a proposed carrier-to-carrier mating

• Has a positive MH result before surgery

• Has recurrent collapse, muscle damage or neurological signs

• Is a foal suspected of having a lethal recessive disorder

What to do: Arrange a veterinary examination and condition-specific management promptly. Do not breed the horse until the result has been properly interpreted.

Critical

The horse has:

• Muscle tremors with weakness or collapse

• Loud breathing or respiratory distress during a suspected HYPP episode

• Extreme temperature and muscle rigidity during or after anaesthesia

• Severe muscle pain with dark urine

• Rapid muscle loss over several days

• Seizures

• Severe incoordination

• Inability to stand

• A newborn foal with extensive skin tearing

• Collapse or rapidly worsening weakness

What to do: Treat this as an emergency and contact an equine veterinarian immediately.

When Is This an Emergency?

A genetic result itself is rarely an emergency. The clinical syndrome may be.

Seek immediate veterinary help for:

• Suspected HYPP with breathing difficulty, profound weakness or collapse

• Suspected malignant hyperthermia during anaesthesia

• Body temperature above 40°C with rigid muscles and rapid heart rate

• Non-exertional rhabdomyolysis with dark urine, severe muscle pain or recumbency

• Rapid and profound muscle wasting

• A foal developing progressive ataxia

• Seizures or inability to stand

• Severe respiratory infection in a young Arabian foal suspected of SCID

• A newborn with fragile skin, open wounds or severely hypermobile joints

• Any foal that cannot nurse, stand or maintain normal posture

These presentations can deteriorate rapidly and cannot be managed by waiting for a routine genetic-test result. (Veterinary Genetics Laboratory)

What Should You Do After Receiving a Genetic Result?

1. Confirm the Horse’s Identity

Check that the report matches:

• Registered name

• Registration number

• Microchip

• Sex

• Sample date

• Parentage where relevant

A perfectly accurate laboratory result attached to the wrong horse is still useless.

2. Determine What Kind of Test It Was

Ask whether the result represents:

• A causal disease variant

• A recessive carrier state

• A dominant susceptibility

• A risk marker

• A coat-colour trait

• Parentage

• An unvalidated commercial association

Do not act until this distinction is clear.

3. Review the Inheritance Pattern

For breeding stock, test the intended partner rather than interpreting one horse in isolation.

The key question is not merely, “Does this horse carry something?”

It is, “Could this specific mating produce an affected foal?”

4. Assess the Horse Clinically

A positive genetic result does not replace:

• Physical examination

• Bloodwork

• Muscle enzymes

• Neurological examination

• Muscle biopsy

• Eye examination

• Anaesthetic assessment

• Other appropriate diagnostics

Treat the horse, not just the laboratory report.

5. Update the Medical Record

Relevant results should be recorded where they can be found before:

• Vaccination

• Sedation

• Anaesthesia

• Surgery

• Emergency treatment

• Breeding

• Sale

6. Discuss Management Changes

Depending on the result, changes may involve:

• Diet and exercise for PSSM1

• Potassium management for HYPP

• Anaesthetic planning for MH

• Vaccine planning for MYHM

• Routine ophthalmology for SCC risk

• Breeding selection for recessive diseases

7. Keep the Official Report

Do not rely only on a social media post, sales advertisement or handwritten note.

Keep the original laboratory report and any registry-recorded result.

Common Genetic-Testing Mistakes

Treating Every Positive Result as Disease

A recessive carrier is usually healthy. A risk marker only changes probability. A variably penetrant variant may never produce recognised clinical disease.

Assuming N/N Means Genetically Perfect

It only means the tested variant was not detected.

The horse may carry another recognised variant, an undiscovered variant or a complex inherited tendency that is not covered by the panel.

Buying an Unvalidated PSSM2 Panel

Commercial P2, P3 and P4 results have not reliably distinguished biopsy-confirmed PSSM2 or MFM horses from controls and should not be used as stand-alone diagnoses or breeding exclusions. (PubMed)

Testing the Wrong Breed Panel

A large panel is not automatically a better panel.

Testing should be guided by breed, ancestry, clinical signs and registry requirements.

Removing Every Recessive Carrier From Breeding

This may unnecessarily reduce genetic diversity.

For many recessive disorders, a carrier can be bred safely to a clear partner while clear offspring are gradually retained for future generations. (Veterinary Genetics Laboratory)

Changing Vaccinations Without Veterinary Advice

A positive MYHM result requires risk-based planning. It does not justify leaving a horse unprotected against every infectious disease.

Believing a Genetic Result Predicts Performance

No routine DNA panel can guarantee speed, jumping ability, trainability or future soundness.

Accepting an Unverified Screenshot

For breeding or purchase decisions, obtain the official laboratory report and verify that the sample belonged to the horse being sold.

How Can Inherited Disease Be Prevented?

Test Before Selecting the Mating

Do not wait until the mare is pregnant.

Testing both parents allows the risk to be calculated before conception.

Avoid Recessive Carrier-to-Carrier Matings

This prevents the 25 percent risk of an affected foal while allowing valuable carriers to remain in carefully managed breeding programmes.

Use Dominant Variants More Cautiously

A horse carrying a dominant disease variant may transmit it even when currently asymptomatic.

The welfare implications, penetrance, severity and breed-registry rules should all be considered before breeding.

Record and Disclose Results

Transparent records allow:

• Better mating decisions

• Safer anaesthesia

• Earlier diagnosis

• More informed purchases

• Better research

• Improved breed health

Retain Clear Offspring Strategically

When a valuable recessive carrier is bred to a clear partner, test the offspring and preferentially retain clear individuals for future breeding.

This can reduce variant frequency gradually without discarding an entire family line in one generation.

Review New Tests Critically

Equine genetics evolves quickly. A test that did not exist several years ago may now be validated, while another heavily marketed test may still lack adequate evidence.

Will a Horse With a Positive Result Be Okay?

Often, yes.

A carrier of a recessive disorder is generally healthy and may never need any medical treatment related to that variant.

A horse with a dominant or variably penetrant variant may remain asymptomatic, develop manageable disease or experience a severe episode depending on the condition and environmental triggers.

Some disorders, including HYPP and PSSM1, can often be managed successfully in selected horses. Other genotypes, particularly lethal recessive foal diseases, carry a grave prognosis when two disease copies are inherited.

The result is not the prognosis by itself.

The prognosis depends on:

• The specific condition

• Number of variant copies

• Penetrance

• Clinical signs

• Age

• Environment

• Management

• Concurrent disease

• Speed of veterinary intervention

Frequently Asked Questions

Can a genetically affected horse look completely normal?

Yes. Some dominant and codominant conditions have variable penetrance, meaning a horse can carry the disease-associated genotype without currently showing recognised signs. Risk may still emerge after stress, illness, exercise, anaesthesia or another trigger.

Can a healthy carrier be bred?

A carrier of an autosomal recessive disorder can often be bred to a confirmed clear horse without producing affected foals. The offspring should still be tested because approximately half may be carriers.

Does a clear genetic panel mean the horse will remain healthy?

No. It only excludes the variants included in that panel. Most lameness, colic, respiratory disease and performance problems cannot be ruled out through a genetic panel.

Is there a reliable DNA test for PSSM2?

No validated causal DNA test is currently available for PSSM2. Commercial P-variant results should not be used by themselves to diagnose muscle disease, reject a purchase or exclude a horse from breeding. (PubMed)

Should a MYHM-positive horse stop receiving vaccines?

Not automatically. The horse needs a veterinarian-designed plan that balances possible immune triggers against the danger of infectious disease. UC Davis recommends specific precautions, including avoiding strangles vaccination and spacing necessary vaccines, but decisions should be individualised. (Veterinary Genetics Laboratory)

Can a DNA test identify my horse’s breed?

Parentage and identity testing can compare a horse with known relatives, but routine DNA profiling does not necessarily identify an unknown horse’s breed. UC Davis specifically states that its horse parentage testing is not a breed test. (Veterinary Genetics Laboratory)

The Real Takeaway

Genetic testing is one of the most powerful tools available to modern horse owners and breeders, but only when the result is interpreted properly.

A validated test can:

• Prevent a fatal carrier-to-carrier mating

• Identify susceptibility before clinical disease develops

• Change anaesthetic or vaccination planning

• Support diagnosis

• Improve transparency during purchase and breeding

It cannot predict every disease or determine a horse’s future talent.

The most important distinctions are simple:

• Carrier does not always mean affected

• Risk does not mean certainty

• Clear does not mean free from every inherited disease

• Commercially available does not always mean scientifically validated

Test the right horse for the right variant, use a recognised laboratory and interpret the result alongside the horse’s clinical condition, ancestry and intended breeding plan.

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Unsure which genetic tests are appropriate for your horse or how to interpret a carrier, affected or risk-marker result? ASK A VET™ can help you organise the laboratory report, breed history, clinical signs and intended mating so you can prepare the right questions for your veterinarian, genetic laboratory or breed registry.