Unlocking the identity of a case of feline subcutaneous dirofilariasis in Hong Kong

Background Dirofilaria asiatica is a recently recognised zoonotic filaria of dogs in Asia, but uncertainty remains regarding its taxonomy as a single species or species-complex and the role of non-canid reservoir hosts. Methods A single male filarial nematode was surgically removed from a subcutaneous nodule of a domestic cat in Hong Kong presenting with a microfilaremia of 36,907 microfilariae/mL. Adult worms and circulating microfilariae were examined morphologically and morphometrically. Phylogenetic inference was performed using mitochondrial genome sequences obtained from microfilariae. Results Qualitative adult male morphology was congruent with published descriptions of Dirofilaria minor (Sandground, 1932) from leopard cats ( Felis bengalensis ) in Vietnam, Dirofilaria repens (Bauche & Bernard, 1911) from domestic dogs in Vietnam, and Dirofilaria asiatica from dogs in Sri Lanka. Quantitative morphometric indices of the adult worm, particularly male spicule length and ratio, aligned most closely with D. minor (Sandground, 1932). In contrast, morphometric measurements of methylene blue-stained microfilariae overlapped entirely with those reported for D. repens (Bauche & Bernard, 1911) from dogs in Vietnam. Formalin fixation precluded molecular characterisation of the adult nematode; however, mitochondrial genome sequencing of microfilariae placed the Hong Kong feline isolate within a well-supported clade comprising D. asiatica from dogs in Sri Lanka and Candidatus Dirofilaria hongkongensis from a human infection in India, and distinct from Dirofilaria sp. “Thailand II” from cats in Thailand and D. repens from Europe. Conclusions These findings suggest that D. minor (Sandground, 1932) and D. repens (Bauche & Bernard, 1911) may represent synonyms of D. asiatica , and that domestic and potentially wild felids can harbour patent infections and may act as epidemiologically relevant reservoirs in Asia. Morphometric characters of D. asiatica exhibit potential geographic and host-associated variability, limiting their diagnostic utility when used without complementary molecular approaches.