Foal immunodeficiency syndrome
A foal with FIS may appear normal at birth, then become weak, anemic and highly vulnerable to infections. Foal immunodeficiency syndrome affects red blood cell production as well as an essential part of the immune system. This fatal inherited disease is documented particularly in Fell and Dales ponies.
Recessive inheritance
How foal immunodeficiency syndrome weakens immune defenses and reduces red blood cells
Red blood cells carry oxygen around the body. A shortage causes anemia, contributing to the foal’s weakness. FIS also involves a deficiency of B lymphocytes, cells essential for making antibodies. The foal is therefore poorly equipped to respond to infections as it needs to develop its own immune protection.
The 2011 study identified a variant, meaning a change in the DNA sequence, in SLC5A3. That variant is associated with FIS. The gene helps transport a small molecule called myo-inositol and helps cells adapt to changes in their surroundings. Researchers do not yet fully understand how the altered protein produces anemia and immune problems. The test can nevertheless identify the variant in a horse’s DNA; using that information does not require every step of the disease to have been explained.
Why foal immunodeficiency syndrome can go unnoticed at birth
Colostrum, the mare’s first milk, supplies maternal antibodies that can offer temporary protection. As that protection declines, the affected foal’s own defenses do not take over normally. Signs often become apparent between two and eight weeks, with differences depending on infections encountered and the circumstances.
Weakness, poor appetite, slow growth, diarrhea or coughing may develop. Gums and other mucous membranes may look pale because of anemia. No single sign identifies FIS. A foal becoming weaker needs prompt veterinary assessment even if it grew well or seemed vigorous during its first days. A normal birth does not rule out this inherited disease.
FIS and SCID are different diseases that weaken defenses against infection
Both conditions can cause severe susceptibility to infections after an apparently normal birth. They nevertheless involve different defects. The SCID described in Arabians involves PRKDC and affects B and T lymphocytes. FIS involves SLC5A3 and is characterized particularly by progressive anemia and B-lymphocyte deficiency.
A negative SCID result is therefore not a FIS result, and the reverse is also true. The exact condition tested for matters, including within a panel containing several tests. Tests cannot be substituted merely because both conditions affect immunity. The foal’s signs and ancestry help frame the investigation, while each test answers its own specific genetic question.
Which ponies and horses can carry the foal immunodeficiency syndrome variant?
FIS was previously called Fell Pony syndrome. The disease and variant were subsequently documented in Dales ponies. The University of Kentucky also mentions Gypsy horses, making crosses and ancestry relevant rather than the name on an animal’s registration alone.
Published frequencies describe particular groups and dates. They do not automatically represent current populations or every family. An adult without signs can carry one copy, and previously producing an unaffected foal does not prove non-carrier status. Individual results provide the information needed to assess a mating. A list of documented populations is not evidence that the variant is absent from every other population.
How a vet diagnoses foal immunodeficiency syndrome
The vet assesses age, growth, infections and blood tests. Anemia and abnormal immune cell findings can guide the investigation, but weakness and infections have other possible causes. Inadequate transfer of antibodies through colostrum, for example, is a different problem from the inherited defect in FIS.
Testing for the associated variant can confirm the diagnosis in a compatible case. It does not show which pathogens are present, how severe the anemia is or what the foal needs immediately. Those questions require clinical assessment. Waiting for a DNA report should not replace care for a sick young animal; genetic investigation and management of the current illness proceed together.
The foal immunodeficiency syndrome test counts copies of the disease-associated variant
The horse has two versions of the tested SLC5A3 region, one inherited from its mother and one from its father. The FIS test reports how many of them contain the disease-associated variant. Zero means that variant was not detected. One means the horse can pass the variant on without having this form of FIS. Two means the foal has FIS, even if signs are not yet visible when the sample is taken.
A negative result does not assess the whole immune system or rule out anemia from other causes. The laboratory’s key explains its notation. Check both the animal’s identity and the exact test name and follow the laboratory’s sampling instructions. Absence of symptoms is not interchangeable with a negative result, and an untested parent cannot simply be treated as a confirmed non-carrier.
What care is possible for a foal with foal immunodeficiency syndrome?
There is no effective treatment that corrects the FIS described here. Veterinary care can address infections and the foal’s needs, but an initial response does not permanently restore the deficient functions. The disease progresses and its outlook is grave.
Clinical resources generally describe death or euthanasia during the first months, often before four months of age. That range is not a forecast of a date for an individual foal. The vet reassesses comfort, anemia, infections and realistic care options. This severe prognosis does not apply to a one-copy carrier, which does not have this recessive condition, nor should temporary improvement be mistaken for a genetic cure.
How testing both parents helps prevent foal immunodeficiency syndrome
The mare and stallion can each carry one copy of the SLC5A3 variant associated with FIS without having FIS. 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.
If one parent carries the SLC5A3 variant associated with FIS 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 FIS, 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 genetic test results for foal immunodeficiency syndrome
Foal immunodeficiency syndrome variant not detected
The tested FIS variant was not detected and cannot be passed on by this horse. Other causes of anemia and infections remain possible.
One FIS variant copy: carrier without foal immunodeficiency syndrome
The horse does not have this recessive condition but has a 50% chance of passing on the variant at each conception.
Two FIS variant copies: affected by foal immunodeficiency syndrome
The foal has two copies of the variant associated with this fatal syndrome, even if it still appears normal early in life.
These results concern the SLC5A3 variant tested for FIS. They do not, on their own, establish the result for the PRKDC variant associated with SCID.
Questions about the condition: Foal immunodeficiency syndrome
Can the mare’s first milk cure foal immunodeficiency syndrome?
No. Its antibodies can provide temporary protection but do not correct the inherited defect or the progressive anemia associated with the disease.
Does an adult carrying the foal immunodeficiency syndrome variant have weakened immune defenses?
Having one copy of the SLC5A3 variant associated with FIS does not cause this condition. The result means the horse can pass the variant on, rather than that its immune defenses are defective. It does not describe every aspect of its health.
Scientific sources about the condition: Foal immunodeficiency syndrome
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.
- Fox-Clipsham et al. (2011) — Identification of a mutation associated with fatal Foal Immunodeficiency Syndrome in the Fell and Dales pony
PLOS Genetics · Animal Health Trust / University of Liverpool · Research
- Foal Immunodeficiency Syndrome
UC Davis Center for Equine Health · University resource
- Foal Immunodeficiency Syndrome: genetic test
UC Davis Veterinary Genetics Laboratory · Test documentation
- Disease Mutation & Trait Testing — Foal Immunodeficiency Syndrome
University of Kentucky Gluck Equine Research Center · University resource
- Severe Combined Immunodeficiency: a distinct inherited disorder
UC Davis Veterinary Genetics Laboratory · Test documentation