Zoonotic Dirofilaria sp. "hongkongensis" in subcutaneous nodules from dogs and cats, Hong Kong SAR

Abstract

Background: Dirofilaria sp. "hongkongensis" is a putative Dirofilaria species, initially identified in subcutaneous nodules in humans in Hong Kong and in other South and Southeast Asian regions. While it differs genetically from the better-known zoonotic species, Dirofilaria repens and Dirofilaria immitis, information on the lesions caused by Dirofilaria sp. "hongkongensis" in the hosts as well as on its biology is scarce. This study documents for the first time the presence of this filarioid nematode in subcutaneous nodules in dogs and cats in Hong Kong, where it was originally described in human patients, therefore providing evidence for the zoonotic nature of this parasite.

Methods: Records of Veterinary Diagnostic Laboratory of City University of Hong Kong were searched between 2019 and 2024 for histological reports of possible filarioid-associated lesions. Tissue samples were collected by excisional surgical biopsy and processed with routine paraffin techniques. Selected slides were stained using various staining techniques [i.e., hematoxylin and eosin, periodic acid-Schiff (PAS), Grocott methenamine silver (GMS) or Ziehl-Neelsen (ZN) and Gram stain]. DNA from formalin-fixed paraffin-embedded tissue were extracted, submitted to conventional polymerase chain reaction (cPCR) and sequencing (i.e., cox1 and 12S rRNA genes) and phylogenetic analyzed.

Results: A total of five subcutaneous nodules from four cats and one from a dog with histopathology suggestive of filariosis were selected. The presence of Dirofilaria sp. "hongkongensis" was morphologically and molecularly confirmed in one dog and one cat. Both histopathological presentation and phylogenetic analysis enabled classification of this species close to D. repens and within the subgenus Nochtiella. In the remaining three cases, one showed histological evidence of aberrant nematode migration, while non-parasitic causes were identified in the other two.

Conclusions: This study provides the first evidence of Dirofilaria sp. "hongkongensis" in subcutaneous nodules in cats and dogs. The histology of clinical lesions of this filarioid species herein described is closely related to those caused by D. repens. Overall, this species should be considered in differential diagnoses of subcutaneous lesions in both animals and humans in the region.

Keywords: Dirofilaria sp. “hongkongensis”; Cats; Dogs; Subcutaneous dirofilariosis; Zoonoses.

Fig. 1

Histologic sections of nodules from case 1 and 5 (hematoxylin and eosin staining). Mass at the right scrotum of a cat with tangential sections of nematode, embedded in the core of inflammatory cells. Scale bar 200 µm. (A). At higher magnification the nematode cuticle is evenly spaced with longitudinal ridges (thick arrow), lateral cords (*), internal ridges (thin arrow), tall coelomyarian-polymyarian musculature (+), ovary (black circle), and uteri (arrow heads) (B). Mass at the mammary gland of a dog with multiple cross sections of nematode. Scale bar 200 µm. (C). At higher magnification, longitudinal ridges (arrow), lateral cords (*), internal ridges (thin arrow), tall coelomyarian-polymyarian musculature (+), and reproductive tract (arrow head) is evident. Scale bar 50 µm. (D). Mammary mass in a cat. Scale bar 200 µm. (E) At higher magnification parasite fragments show structures resembling the possible reproductive tract of a nematode (arrows). (F). Scale bar 50 µm

Fig. 2

Clades of Dirofilaria species were based on 12S rRNA gene (A) (accession numbers PQ032750–PQ032751) and cox1 gene (B) (accession numbers PQ327004–PQ327005). The phylogenetic trees were inferred using Bayesian inference. The sequences are aligned for the respective genes and included information on the host of collection, geographical provenience, and accession number. Sequences from the present study are highlighted in bold. Ascaris lumbricoides was used as outgroup and numbers at nodes correspond to the posterior probability support values