The story begins with a sick cat. The animal presented with a subcutaneous nodule, a lump under the skin, and a blood test revealed an extraordinarily high level of circulating microfilariae, the larval stage of filarial worms, at 36,907 microfilariae per millilitre of blood. Surgeons removed a single adult male nematode from the nodule, and that worm, together with the larvae swimming in the cat’s bloodstream, became the basis for a combined morphological and genetic investigation. The case is notable because Dirofilaria infections are usually associated with dogs, and the role of cats or other non-canid animals as reservoir hosts for these parasites has remained uncertain.

Filarial nematodes of the genus Dirofilaria are best known through Dirofilaria immitis, the canine heartworm, but several other species infect the tissue under the skin rather than the heart. Dirofilaria repens, long recognised in dogs across Europe, Asia and Africa, is a well-documented zoonosis, meaning it can also infect humans, typically producing migrating nodules under the skin. More recently, researchers described Dirofilaria asiatica, a newly recognised zoonotic filaria of dogs in Asia, first characterised from specimens collected in Sri Lanka. However, uncertainty has surrounded D. asiatica from the start: scientists have debated whether it truly represents a single species or a species complex, and whether animals other than dogs might harbour it.

To identify the Hong Kong worm, the team took a two-pronged approach. First, they examined the adult male worm morphologically and morphometrically, measuring the anatomical features that parasitologists use to distinguish closely related Dirofilaria species. Second, they turned to genetics, sequencing the whole mitochondrial genome from the microfilariae found in the cat’s blood, and obtaining partial sequences of two mitochondrial markers, the cytochrome c oxidase subunit 1 gene, known as cox1, and the 12S mitochondrial ribosomal RNA gene, from the adult worm itself. Mitochondrial genomes are workhorses of parasite taxonomy because they evolve quickly enough to distinguish closely related species while being easy to recover from small amounts of material.

The qualitative morphology of the adult male proved frustratingly ambiguous. Its features were congruent with the original descriptions of three different species: Dirofilaria minor, described by Sandground in 1932 from leopard cats, Felis bengalensis, in Vietnam; D. repens, described from dogs in Italy in 1911; and D. asiatica, described from dogs in Sri Lanka. These species are so similar in their gross anatomy that a single specimen cannot be assigned to one of them by appearance alone. The quantitative measurements, however, offered a sharper clue. The length and ratio of the spicules, the needle-like mating structures of male nematodes, and the number of pre-anal papillae, small sensory structures arranged in front of the anus, aligned most closely with D. minor.

The microfilariae told a complementary story. When stained with methylene blue and measured, the larvae from the cat’s blood overlapped morphometrically with values reported for both D. repens and D. asiatica, again failing to single out one species definitively. This overlap illustrates a persistent problem in filarial diagnostics: the larval stages of these parasites are morphologically conservative, and even careful measurements of stained specimens can straddle the boundaries between species. It is precisely this kind of ambiguity that has fuelled the debate over whether the Asian parasites represent one species, several species, or a species complex in the making.

Genetics broke the tie. Phylogenetic analysis of the whole mitochondrial genome recovered from the microfilariae, together with analyses of the 12S rRNA and cox1 fragments from the adult worm, placed the Hong Kong parasite within a well-supported clade that included D. asiatica from dogs in Sri Lanka and a parasite known in the literature as Dirofilaria sp. “hongkongensis”, recovered from a human infection in India. The genetic evidence was therefore clear: the worm in the cat was D. asiatica, regardless of what its anatomy suggested. The convergence of molecular and morphological data on different answers is itself informative, revealing that the current species boundaries in this group do not map neatly onto either morphology or genetics alone.

Here lies the twist that gives the study its wider significance. The adult worm was genetically D. asiatica, yet morphologically indistinguishable from the original description of D. minor, a species named more than ninety years ago from a wild felid in Vietnam. The authors argue that the parasite long referred to as Dirofilaria sp. “hongkongensis”, and recently formally described as D. asiatica, may in fact be D. minor. Under the rules of zoological nomenclature, the oldest available name takes priority, so if the hypothesis is confirmed, D. asiatica would become a junior synonym of D. minor and the older name would prevail. Resolving the question requires additional specimens from Vietnam, the type locality of D. minor, so that morphology and genetics can be compared directly against the species as originally defined.

Beyond the taxonomic debate, the case carries practical implications for medicine and public health in Asia. D. asiatica is recognised as a zoonotic parasite, and the discovery of a genetically confirmed infection in a cat with an enormous microfilaremia demonstrates that domestic felids can sustain patent infections, producing larval stages in their blood that mosquitoes can pick up and transmit. Cats, in other words, may serve as reservoir hosts capable of maintaining and spreading the parasite in urban environments. The earlier detection of the same lineage in a human patient in India underscores that the parasite already crosses species barriers into people, usually manifesting as subcutaneous nodules that can be mistaken for tumours or other conditions.

The Hong Kong study also highlights the value of combining classical parasitology with modern genomics. Morphological examination of a single worm produced measurements pointing to a ninety-year-old name, while mitochondrial genome sequencing anchored the specimen firmly within a clade of Asian parasites linked to both dog and human infections. Neither approach alone could have resolved the identity of the parasite, and neither alone could have exposed the mismatch between the two lines of evidence. As the authors note, further detailed morphological and genetic studies using additional specimens, particularly from Vietnam, are needed to confirm whether D. asiatica and D. minor are one and the same. Until then, a worm from a Hong Kong cat stands as a reminder that some of the most consequential discoveries in parasitology begin with a routine clinical case, and that the names scientists give to parasites may conceal as much as they reveal about their origins, hosts and potential to cause human disease.

Subject of Research: Taxonomic identification of a zoonotic Dirofilaria nematode infecting a domestic cat in Hong Kong

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

Article References: Frias, L., Zhou, Y., Almendros, A., Li, R., & Traub, R. J. (2026). Unlocking the identity of a case of feline subcutaneous dirofilariasis in Hong Kong. Parasites & Vectors. https://doi.org/10.1186/s13071-026-07677-7

Image Credits: AI Generated

DOI: 10.1186/s13071-026-07677-7

Keywords: Dirofilaria asiatica, Dirofilaria minor, filarial nematodes, cats, zoonosis, Hong Kong, mitochondrial genome, phylogeny, morphology, parasite taxonomy, microfilariae, reservoir hosts