Feline autoimmune lymphoproliferative syndrome, described in the British Shorthair (FALPS), caused by a variant in the FASLG gene. It presents with lymphadenopathy, splenomegaly, anaemia and lymphocytosis of double-negative lymphocytes (CD4-/CD8-). The mention of other breeds (e.g. British Longhair) is not supported by the available literature: limited data.

British Shorthair. Allele frequency 0.14 in 32 cats from New Zealand; the variant was not detected in 510 non-British Shorthair cats. British Longhair: limited data.

- Persistent generalised lymphadenopathy
- Splenomegaly
- Lymphocytosis with double-negative T lymphocytes (CD3+ TCRαβ+ CD4− CD8−) in peripheral blood
- Autoimmune haemolytic anaemia and/or thrombocytopenia in some cases
- Lymphoma in a proportion of affected animals
- Hypergammaglobulinaemia

ALPS was characterised in humans in the 1990s and was associated with defects in the FAS/FASL pathway of lymphocyte apoptosis; in humans it is defined by the expansion of double-negative (DN) CD4−/CD8− T lymphocytes. In cats, the syndrome has been described in limited veterinary reports concerning British Shorthair, with involvement of specific families. The molecular characterisation in cats is partial and has been associated with a variant in the FASLG gene, analogous to the human mechanism. Published series are scarce and the disease is considered rare. Limited data on the exact chronology of the veterinary publications and the true prevalence in the breed.

- Test FASLG in British Shorthair breeding animals, especially in lines with previous cases.
- Do not mate two carriers: 25% homozygous affected.
- A carrier only with a free mate; test the offspring intended for breeding.
- Avoid inbreeding, the main factor in the expression of rare recessives in pedigree cats.
- In the presence of lymphadenopathy or splenomegaly in kittens of ~6 weeks, suspect ALPS: immunophenotype of double-negative lymphocytes and molecular study.

Differential diagnosis with lymphoma, feline leukaemia virus (FeLV) and other causes of lymphadenopathy and lymphocytosis. The immunophenotypic finding characteristic of ALPS is the expansion of double-negative T lymphocytes (CD4−/CD8−) —not double-positive—, so immunophenotyping by flow cytometry with CD3, CD4, CD8 and TCRαβ markers is the key tool. Lymph node histopathology is useful to confirm and exclude lymphoma. The disease should be considered a rare and possibly underdiagnosed entity; consult a feline internal medicine service when suspected.

1. Aberdein D et al. 2017, frecuencia de una variante del gen FAS ligand asociada al síndrome linfoproliferativo autoinmune felino en British shorthair de Nueva Zelanda (PMID 28814155)
2. OMIA:002064 Síndrome linfoproliferativo autoinmune felino (FASLG)