Inherited hydrocephalus
An unusually large head in a foal may reflect hydrocephalus: fluid accumulates within the developing brain’s cavities. A severe inherited form in Friesian horses is associated with a B3GALNT2 variant. It can also make delivery difficult for the mare. DNA testing is especially useful for identifying unaffected carriers before a mating.
Recessive inheritance
How a buildup of fluid in the brain causes hydrocephalus
The brain contains cavities called ventricles through which cerebrospinal fluid circulates. Fluid is normally produced and reabsorbed in balance. A persistent disturbance can enlarge these cavities and compress surrounding tissue. In a developing fetus, the skull can expand abnormally as the process continues.
In the Friesians studied, hydrocephalus develops before birth. An anatomical study of four stillborn foals found narrowing of the jugular foramen, an opening at the base of the skull. The researchers proposed that this narrow passage makes it harder for blood to drain through the veins, which in turn reduces reabsorption of fluid around the brain. That explanation comes from observations of these four cases; it is not an established explanation for every type of hydrocephalus.
Which change in B3GALNT2 is linked to inherited hydrocephalus?
Genetic research published in 2015 identified the c.1423C>T variant in B3GALNT2. It introduces a premature stop instruction during protein production. The resulting protein is shortened and its normal function is compromised. Such a change is called a nonsense variant; this specific change is what the DNA test looks for.
All sixteen available affected cases in the validation study had two copies. The mothers of affected foals each had one, matching the carrier status expected from the family history. These findings made genetic screening possible. They provide strong evidence for this inherited form, although the developmental steps connecting the molecular defect to the anatomical abnormalities are more complex than a single blocked pipe.
Why hydrocephalus in a foal can make birth dangerous for the mare
Marked enlargement of the skull, often with a domed appearance, is the most obvious feature. Affected pregnancies can end in fetal loss or stillbirth. Liveborn foals may have severe neurological problems. Neither the timing of loss nor every aspect of the foal’s appearance is identical in all cases.
An enlarged head can obstruct delivery and cause dystocia, meaning difficult foaling. The consequences therefore extend beyond the foal to the mare. A delivery that is not progressing normally needs prompt veterinary attention. Knowing genetic results can help the investigation, but those results are not instructions for managing an obstructed delivery without professional assistance.
Which breeds have documented cases of the B3GALNT2 hydrocephalus variant?
The B3GALNT2-related form was characterized in Friesians, and testing is offered for Friesians and their crosses. Population history and the use of related breeding animals help explain how a variant can circulate. They do not establish that every horse of that breed is a carrier or affected.
A 2019 report also associated this B3GALNT2 variant with hydrocephalus in a Belgian draft horse. Other cases of hydrocephalus can have different underlying causes. Similar skull enlargement does not prove the same genetic mechanism. Carrier frequencies from a particular Friesian sample should not be presented as current figures for every country, family or generation. An individual horse’s identified test result answers the individual question.
How the vet investigates the cause of hydrocephalus
The veterinarian considers the foal’s head, the pregnancy and delivery, and the parents’ information. Imaging or examination after death can help define the abnormalities. Genetic testing then investigates a specific explanation, rather than every possible cause of an abnormally formed skull.
Two copies support the diagnosis of this B3GALNT2-related form. A negative result in a malformed foal does not end every possible investigation. Following pregnancy loss or stillbirth, appropriate samples and identification can be discussed with the veterinarian and laboratory. Parental results add information but cannot automatically substitute for investigating the foal itself.
A horse without hydrocephalus can carry the inherited B3GALNT2 variant
Reports may use N/N, N/H and H/H to describe zero, one and two copies. A one-copy horse is an unaffected carrier of this recessive form. Appearance, performance and previous unaffected foals do not identify or rule out carrier status. Both breeding animals’ results are relevant even when neither has ever shown a problem.
N means the tested variant was not detected on that copy; it does not mean the gene is absent. A favorable result is neither a complete developmental assessment nor a guarantee against every inherited condition. A report used for breeding should reliably identify the horse and the actual test performed. Sampling requirements come from the laboratory carrying out the analysis.
Why inherited hydrocephalus linked to B3GALNT2 threatens the foal’s life
Clinical resources describe no established cure for this severe inherited form. Outcomes include fetal loss, stillbirth and liveborn foals with problems that warrant euthanasia. That outlook should not be extended to every type of hydrocephalus in other animals or with other causes. The scope here is the equine inherited disorder investigated.
Veterinary decisions consider the foal’s suffering, the mare’s safety and realistic welfare outcomes. An adult with just one copy is not expected to develop this congenital recessive form later in life. Its result chiefly matters for transmission. Investigating a case in a family can therefore provide information that helps prevent future affected conceptions.
Testing both parents to prevent inherited hydrocephalus in their foal
The mare and stallion can each carry one copy of the B3GALNT2 variant associated with this hydrocephalus without having this inherited form of hydrocephalus. Their foal develops this condition when it inherits that variant from both parents. This is autosomal recessive inheritance: male and female foals can be affected. If both parents carry one copy of this variant, each conception has a 25% chance of a foal inheriting it from both parents and being affected, a 50% chance of inheriting it from only one parent and being an unaffected carrier, and a 25% chance of inheriting it from neither parent. These probabilities apply to every pregnancy; four births need not follow that exact distribution. These proportions concern conceptions, including pregnancies that do not result in a liveborn foal.
If one parent carries the B3GALNT2 variant associated with this hydrocephalus and the other is confirmed by testing not to carry it, their foal cannot inherit that variant from both parents. This mating cannot produce a foal affected by this inherited form of hydrocephalus, although offspring can be carriers and pass the variant on. Knowing both breeding animals’ results makes it possible to avoid matings that can produce affected foals while retaining bloodlines and genetic diversity. Reports must identify the horses correctly and remain available for decisions in later generations. Avoiding this particular condition does not guarantee the absence of every other health problem.
Understanding the test for the B3GALNT2 variant linked to inherited hydrocephalus
One B3GALNT2 hydrocephalus variant copy: unaffected carrier — N/H
The horse has a 50% chance of passing on the variant at each conception, even if it appears completely healthy.
Two B3GALNT2 variant copies: inherited hydrocephalus — H/H
The foal has two copies of the variant associated with this severe hydrocephalus. Clinical assessment and welfare decisions require veterinary involvement.
Laboratory notation can differ. These results concern B3GALNT2 c.1423C>T, rather than every cause of hydrocephalus.
Questions about the condition: Inherited hydrocephalus
Does a mare carrying the B3GALNT2 variant have hydrocephalus herself?
No. One copy does not cause this recessive form, but it can be passed on. The stallion’s result also determines the foal’s genetic risk.
Is a large head enough to diagnose inherited hydrocephalus in a foal?
No. Examination and further investigations characterize the abnormality. The B3GALNT2 test answers one defined genetic question.
Does having one foal without hydrocephalus rule out inherited hydrocephalus in future foals?
No. Two carriers may produce foals without two copies before an affected pregnancy occurs. The probabilities apply anew at every conception.
Scientific sources about the condition: Inherited hydrocephalus
This article brings together the research and the university and laboratory resources listed below. Numbers after each section link to the references used. Sources consulted in November 2026.
- Friesian Horse Hydrocephalus: DNA test and interpretation
UC Davis Veterinary Genetics Laboratory · Test documentation
- Hydrocephalus: signs, diagnosis and prognosis
UC Davis Center for Equine Health · University resource
- Ducro et al. (2015) — A nonsense mutation in B3GALNT2 is concordant with hydrocephalus in Friesian horses
BMC Genomics · Wageningen / Utrecht · Research
- Sipma et al. (2013) — Phenotypic Characteristics of Hydrocephalus in Stillborn Friesian Foals
Veterinary Pathology · Utrecht / Ghent / Pretoria · Research
- Kolb & Klein (2019) — Congenital hydrocephalus in a Belgian draft horse associated with a nonsense mutation in B3GALNT2
Canadian Veterinary Journal · University of Calgary / Lodi Veterinary Care · Research