The prevalence of MDR1/ABCB1 c.1930_1931delTC variant is rare in US domestic cats

Objective: To determine the prevalence of the 2-bp deletion genetic variant (ABCB1 c.1930_1931delTC) in the feline multidrug resistance ABCB1 (previously MDR1) gene, which has been associated with sensitivity to P-glycoprotein (P-gp) substrate drugs in cats with homozygous mutations, utilizing a large population screen of genomic DNA samples from purebred and mixed-breed domestic cats in the US. Methods: This cross-sectional epidemiological study utilized conveniently available, unbiased, residual whole blood samples that were obtained from 12,000 US domestic cats and submitted to Idexx Laboratories between March and October 2025. Variant detection was performed with a validated dual-probe real-time PCR assay to detect 3 potential genotypes: wild type, homozygous mutant for variant allele, or heterozygous. Results: A majority of the cats tested were wild type (> 98%), while 1.5% (181 cats) had the heterozygous genotype. Only 0.025% (3 cats) were homozygous for the ABCB1 c.1930_1931delTC variant. The variant allele frequency was 0.8%. Conclusions: This survey of domestic pet cats in the US demonstrated that the prevalence of the homozygous ABCB1 c.1930_1931delTC variant is rare. Clinical Relevance: The low prevalence of homozygous mutations in cats in this large-scale survey provides veterinarians confidence in prescribing products that are P-gp substrates, such as cyclosporine, loperamide, emodepside, and macrocyclic lactones (ivermectin, eprinomectin, selamectin, milbemycin oxime, and moxidectin). If concerns arise for individual cats, veterinarians can consider ABCB1 genotyping prior to administering P-gp substrates. While these results provide information on the prevalence of ABCB1 c.1930_1931delTC, future research on other known polymorphisms will help us fully understand ABCB1/MDR1 mutations in cats.