Case Study: Dirofilaria

Daisy Jones, a Resident in Clinical Pathology at the Veterinary Pathology Group (VPG), has seen many interesting cases during her residency, including a recent case where Dirofilaria immitis was considered highly probable.

Daisy’s study on this interesting case can be found below:

Presentation: A 9 year old Labrador retriever presented to a veterinary centre for investigation of a dermal mass on the right thorax. The mass was reported to be unchanged for 2 years.

Cytology: Cellularity was high, and preservation was good. Within a faint basophilic background were moderate amounts of blood, moderate to high numbers of inflammatory cells, moderate numbers of squamous epithelial cells and small amounts of keratin. Inflammatory cells were predominated by non-degenerate neutrophils and macrophages. Macrophages were often activated and occasionally multinucleated (Image 1). Squamous cells contained an oval central nucleus and variable amounts of angular mid to deeply basophilic cytoplasm. Two long slender organisms were observed, which had internal structure (nematodes) (Image 2).

Interpretation and comment: The overall cytological pattern was compatible with moderate neutrophilic and macrophagic inflammation, likely in response to a ruptured keratin containing lesion given the presence of squamous epithelial cells and keratin.

Interestingly, two long slender organism were present which were compatible with the presence of nematodes. Differentials included Dirofilaria immitis (due to blood contamination, commonly referred to as heartworm) and Dirofilaria repens (as they can migrate through the skin).

Blood film review and serological testing were advised to further investigate. Serological testing for heartworm (D. immitis) was performed which was positive.

Conclusion: D. immitis infection due to blood contamination was considered highly probable. Concurrent infection with D. repens could not excluded as PCR was not performed on the submitted slides. Cross-reactivity of D. repens for the antigen test for D. immitis is reported, but this is typically after heat treatment which was not performed in this case, and therefore was considered unlikely (but not entirely excluded).

Note: The travel history for this case was unknown. As Dirofilaria is not endemic in the UK, it was presumed that the patient had been exported from, or travelled to, a foreign country.

Fig 1: Moderate neutrophilic and macrophagic inflammation with small amounts of keratin and occasional nucleated squamous cells
Image 2: Nematode with moderate amounts of blood, low numbers of neutrophils and a small amount of keratin