Junctional epidermolysis bullosa
In a foal with junctional epidermolysis bullosa, ordinary pressure can cause serious wounds because the skin is not securely attached to underlying tissue. Two genetic forms are tested in horses, JEB1 and JEB2. Their clinical signs overlap, but they involve different genes and require separate interpretation.
Recessive inheritance
Why junctional epidermolysis bullosa makes a foal’s skin so fragile
Skin layers must remain connected as a newborn stands, lies down and nurses. A protein called laminin-332, previously known as laminin-5, helps provide that attachment. When it cannot form correctly, friction and pressure can separate layers, producing blisters that develop into open lesions.
Mucous membranes, including the lining of the mouth, can also be involved. This is therefore more than sensitive skin or an accidental scrape: tissue attachment is fundamentally impaired. Ordinary activities of newborn life can reveal that fragility even without an unusual injury.
JEB1 and JEB2 are two genetic forms of junctional epidermolysis bullosa
JEB1 is the form associated with a LAMC2 variant identified in draft horses. That variant is an insertion: DNA has been added to the gene’s sequence, preventing normal production of one laminin component. JEB2 is a different form described in American Saddlebreds. It involves a LAMA3 deletion, meaning a stretch of DNA is missing. That change disrupts a different component of the same protein.
These names do not describe early and late stages, or mild and severe versions of one test result. Each test looks for a different DNA change. Published findings explain these specific forms; they do not establish that every case of fragile skin in a horse must involve one of these two variants.
What signs of junctional epidermolysis bullosa appear at birth or soon afterward?
Affected foals are born alive. Blisters and areas of damaged skin may already be visible or soon appear at pressure points. Lesions spread and may develop secondary infections. Ulcers inside the mouth can also make feeding painful.
In severe cases, the hoof wall can detach, including loss of the hoof. Premature eruption of the front teeth has also been reported. The combination and extent of lesions vary, but the genetic forms discussed here have a very poor prognosis. A small initial lesion cannot reliably predict how the condition will develop.
Which breeds have documented cases of junctional epidermolysis bullosa JEB1 or JEB2?
JEB1 is documented in Belgian Draft Horses and related populations, including Breton and Comtois horses. JEB2 is described in American Saddlebreds. These associations help select testing without implying that every horse of a listed breed is a carrier.
Bloodlines, crosses and individual findings matter. A carrier frequency from one country or tested group does not automatically describe another horse. Different ancestry is also not proof that a horse is free of every inherited skin disorder. Population information should guide investigation rather than replace an individual result.
Why junctional epidermolysis bullosa JEB1 and JEB2 need separate genetic tests
A JEB1 report concerns LAMC2; a JEB2 report concerns LAMA3. For each variant, zero copies means it was not detected, one identifies an unaffected carrier of that form, and two identify the affected form. Laboratories may use different letters in their reports.
Testing negative for JEB1 does not answer the JEB2 question. Even negative results for both do not investigate every possible cause of skin lesions. Check the horse’s identity, sample information and exact test name so that results for different variants are not treated as equivalent evidence.
How a vet investigates junctional epidermolysis bullosa in a foal
For a newborn with wounds, the veterinarian assesses when they appeared, their distribution, pain and complications. An isolated injury or another skin condition cannot be diagnosed from photographic resemblance. The foal’s ancestry contributes to the history without establishing a diagnosis.
DNA testing helps identify a known form. A skin biopsy may also show where tissue layers are separating and support diagnosis. Investigations depend on the foal’s condition: waiting for a laboratory report cannot take the place of caring for a painful newborn.
Why wound care cannot cure junctional epidermolysis bullosa JEB1 or JEB2
There is no established cure for these equine forms. Lesions, pain and infections progress, and affected foals die or are euthanized within their first days. Treating an individual wound does not restore the genetically impaired attachment throughout the tissues.
The veterinarian supports decisions about care and welfare in the individual case. This severe outlook applies to affected foals with two copies in the relevant gene. It should not be applied to an adult with one copy, which does not develop that recessive form simply because it is a carrier.
Testing both parents to assess the foal’s risk of junctional epidermolysis bullosa JEB1 or JEB2
JEB1 testing concerns the LAMC2 variant; JEB2 testing concerns the LAMA3 variant. Consider a mare and stallion that each carry one copy of the same variant in the same gene. At each conception, their foal has a 25% chance of inheriting that variant from both parents and having the corresponding disease, a 50% chance of inheriting it from only one parent and being an unaffected carrier, and a 25% chance of inheriting it from neither. Make this calculation separately for JEB1 and JEB2: inheriting a LAMC2 variant from one parent and a LAMA3 variant from the other is not inheriting two copies of one variant.
If the mare carries the tested JEB1 variant and the stallion is confirmed by testing not to carry it, the foal cannot inherit that variant from both parents and will not have JEB1. It still has a 50% chance of inheriting the variant from its mother and being a carrier. The same reasoning applies to JEB2 using the separate LAMA3 test. These probabilities apply to every pregnancy, without a guaranteed pattern across four births. Both breeding animals’ complete results help avoid matings that can produce affected foals while also considering genetic diversity.
Understanding the genetic test for junctional epidermolysis bullosa JEB1
Junctional epidermolysis bullosa JEB1 variant not detected
The LAMC2 variant cannot be passed on. This result does not answer the separate JEB2 test.
One JEB1 variant copy: carrier without junctional epidermolysis bullosa JEB1
The horse is unaffected by this form and has a 50% chance of passing on its variant copy at each conception.
Two JEB1 variant copies: affected by junctional epidermolysis bullosa JEB1
The foal has two copies of the LAMC2 variant and is affected by this severe form.
These categories cover only the tested JEB1 variant in LAMC2. N indicates absence of that variant, not absence of the gene.
Understanding the genetic test for junctional epidermolysis bullosa JEB2
Copy counting applies here to a different gene. These copies are not interchangeable with JEB1 copies.
Junctional epidermolysis bullosa JEB2 variant not detected
The LAMA3 deletion was not detected. This gives no result for the LAMC2 variant.
Two JEB2 variant copies: affected by junctional epidermolysis bullosa JEB2
The foal has two copies of the LAMA3 deletion and is affected by this severe form.
One JEB1 copy plus one JEB2 copy is not the two-copy result described in either table.
Questions about the condition: Junctional epidermolysis bullosa
Is JEB2 a more advanced stage of junctional epidermolysis bullosa JEB1?
No. They are separate genetic forms involving LAMA3 and LAMC2. Their lesions may look similar, but testing targets different changes.
Scientific sources about the condition: Junctional epidermolysis bullosa
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.
- Junctional Epidermolysis Bullosa (JEB1) in Belgians
UC Davis — Veterinary Genetics Laboratory · Test documentation
- Junctional Epidermolysis Bullosa (JEB2) in Saddlebreds
UC Davis — Veterinary Genetics Laboratory · Test documentation
- Junctional Epidermolysis Bullosa: clinical signs, diagnosis and prognosis
UC Davis — Center for Equine Health · University resource
- Junctional Epidermolysis Bullosa: Understanding and Preventing a Deadly Disease in Foals
Oklahoma State University Extension · University resource
- Spirito et al. (2002) — Absence of laminin 5 causes hereditary junctional mechanobullous disease in the Belgian horse
Journal of Investigative Dermatology — INSERM and collaborators · Research
- Graves et al. (2009) — Partial deletion of LAMA3 in hereditary junctional epidermolysis bullosa of the American Saddlebred
Animal Genetics — University of Kentucky · Research