A blood test on a sick cat in Hong Kong returned 36,907 microfilariae per milliliter—larval threadworms circulating at a density most clinicians would expect in a heavily infected dog, not a felid.

Behind that number sits a taxonomic reshuffle. Researchers at City University of Hong Kong, working with a colleague at Murdoch University in Australia, report in Parasites & Vectors that the single adult nematode pulled from the cat's subcutaneous nodule is genetically Dirofilaria asiatica.

Morphologically, however, the same worm matches a 1932 description of Dirofilaria minor from a leopard cat in Vietnam. The mismatch may force a name change with a ninety-year head start.

What did the case show?

The cat presented with a subcutaneous nodule and underwent surgery to remove one adult male worm. The team then sequenced the whole mitochondrial genome from the microfilariae in the blood, plus partial cox1 and 12S rRNA sequences from the adult. Phylogenetic analysis placed the parasite in a well-supported clade with D. asiatica from Sri Lankan dogs and a lineage recovered from a human patient in India, Dirofilaria sp. "hongkongensis."

Why is the taxonomy disputed?

Three Asian filariae look almost identical under the microscope:

• D. minor — Sandground, 1932, leopard cat, Vietnam
• D. repens — Italian dog, 1911, well-documented zoonosis across Europe, Asia and Africa
• D. asiatica — Sri Lankan dog, recent description

The Hong Kong specimen's spicule length, spicule ratio, and pre-anal papilla count aligned most closely with D. minor. Stained microfilariae straddled measurements reported for both D. repens and D. asiatica. The authors conclude that morphology alone cannot resolve the group, and that cryptic species boundaries have likely been missed by classical taxonomy.

Could cats spread the parasite to people?

D. asiatica is a recognised zoonosis, and the Hong Kong cat's patent microfilaremia—more than 36,000 larvae per milliliter—demonstrates that domestic felids can sustain infections dense enough for mosquitoes to pick up. That makes cats candidate reservoir hosts in urban Asia, alongside the dogs in which the parasite was first characterised. Human cases, including the Indian patient carrying the "hongkongensis" lineage, typically present as migrating subcutaneous nodules that mimic tumours or cysts.

Will the name actually change?

Under the International Code of Zoological Nomenclature, the oldest available name takes priority. If future studies confirm the genetic and morphological overlap, D. asiatica would become a junior synonym of D. minor, and the 1932 name would prevail. Confirming the synonymy requires new specimens from Vietnam, the type locality of D. minor, so that topotype material can be sequenced and measured against the Hong Kong worm.

What should clinicians take away?

The case is a reminder that a routine subcutaneous nodule in a cat can carry a zoonosis. A blood microfilaria test, not fine-needle cytology alone, flagged the infection. The authors' combined workflow—classical morphometrics plus whole mitochondrial genome sequencing—also offers a template for future filaria puzzles. Neither method alone could have named this parasite, and neither alone would have exposed the contradiction hiding inside the existing taxonomy.

The team calls for additional morphological and genetic studies, particularly from Vietnam, to settle whether D. asiatica and D. minor are one species. Until those specimens are in hand, a single worm from a Hong Kong cat has already redrawn the map of Asian filarial nematodes.