Rare Mosquito-Borne Worm Infection Can Present as a Suspicious Breast Nodule
A solitary breast nodule does not always point to malignancy. In a new case report and systematic review, breast dirofilariasis emerged as an uncommon but clinically important mimic of breast neoplasia, most often in adult women living in or traveling through endemic regions.
The report centers on a woman in her 50s in Israel whose routine breast imaging in 2010 identified a new 1.4-cm mixed-echogenic lesion in the left breast. Mammography had shown no new abnormality compared with the prior year, but ultrasound raised concern. Two radiologists did not agree on the level of suspicion: one classified the lesion as BI-RADS 3, while the other assigned BI-RADS 4. Because of the discordant assessment and the patient’s preference for a definitive diagnosis, the lesion was sampled.
The laboratory workup did not suggest systemic parasitic disease. Complete blood count, differential, and inflammatory markers stayed within reference limits, and there was no peripheral eosinophilia. Physical examination was unrevealing. The decisive finding came from a core-needle biopsy, which showed a parasitic worm. Histologic review of the specimen, later forwarded to the Centers for Disease Control and Prevention, identified a Dirofilaria species with features most consistent with D. repens. A thick multilayered cuticle, prominent musculature, and paired reproductive tubes supported that interpretation, although D. tenuis could not be ruled out on morphology alone.
The lesion was removed completely, and excision proved both diagnostic and curative. No additional treatment was given, and follow-up imaging showed no recurrence.
A Small Case Becomes a 40-Patient Review
What began as a single breast lesion quickly expanded into a broader review of the literature. Investigators searched PubMed/MEDLINE, Embase, and Scopus from inception through May 11, 2026, then added hand searching of reference lists. They included peer-reviewed case reports and case series that described histopathologically or molecularly confirmed Dirofilaria infection with the breast as the primary site. The final dataset contained 40 breast dirofilariasis cases reported between 1959 and 2025.
The demographic pattern was strikingly narrow. Most patients were female, 38 of 40, and all were adults. The geographic distribution also clustered in known endemic settings. More than half of the cases, 21 of 40, came from the Mediterranean rim, and 6 came from India. Many patients had histories consistent with exposure in endemic regions rather than illness acquired in a single obvious local setting.
Only a handful of cases reached species-level certainty by modern methods. In 37 of 40 cases, species assignment rested on morphology alone, which makes those cases probable rather than definitive. The most common species was D. repens, identified in 31 cases. Molecular confirmation was rare, occurring in only 2 cases across the review.
Why the Breast Is an Unusual Site
Human dirofilariasis is a mosquito-borne zoonosis that usually infects carnivores and occasionally spills over into humans. In people, D. immitis is more often linked to pulmonary disease, while D. repens, D. tenuis, and D. ursi are more often associated with subcutaneous infection. The breast is not the classic target organ. Head and neck lesions are more common overall, and ocular or periocular involvement accounts for a substantial share of reported human dirofilariasis cases. Breast involvement remains rare enough that it can be overlooked when imaging shows a discrete mass.
That rarity creates a practical diagnostic problem. A parasitic nodule can resemble a tumor on ultrasound and trigger the same pathway as a malignant lesion. In this case, the indeterminate imaging findings and the BI-RADS 4 interpretation were enough to justify tissue diagnosis. The pathology result then redirected the entire clinical course.
A single biopsy can settle the diagnosis, but only if the lesion is sampled.
Travel, Local Exposure, and the Limits of Species Identification
The patient lived in an urban area in central Israel, but her travel history created several possible exposure windows. She had visited Southeast Asia, Central Europe, and North America during the two years before diagnosis. Central Europe includes recognized D. repens-endemic areas, and infection there could not be excluded. Local transmission remained plausible as well, since infected dogs have been documented in Israel and human cases have been reported from the country and the broader Eastern Mediterranean region. The exact source of infection could not be determined.
A domestic cat was present in the patient’s household, but that did not establish a likely route of infection. Cats are generally considered poor or dead-end hosts for D. repens, and transmission still requires a mosquito vector. The animal was not tested, and available assays are neither sensitive nor species-specific in cats.
The review also exposes a recurring diagnostic constraint. Morphology alone often guides species identification in published cases, but the approach leaves room for uncertainty, especially when D. repens and D. tenuis can appear similar. Without molecular confirmation, species-level attribution should be treated carefully. Only two breast cases in the review were molecularly confirmed, which limits how firmly clinicians can link geography, vector ecology, and parasite species in many older reports.
What Clinicians Should Do With a Solitary Breast Nodule
The clinical message is direct. In patients who live in or travel through endemic regions, breast dirofilariasis belongs in the differential diagnosis of an isolated breast nodule, even when routine laboratory testing is normal and eosinophilia is absent. The absence of peripheral eosinophilia does not rule out the diagnosis. Imaging may look suspicious enough to warrant biopsy, and histopathology remains central to confirmation.
Excision is usually sufficient. In the reviewed cases, surgery resolved the lesion in all patients. No recurrences were described after removal. The finding matters because it can spare patients from unnecessary concern about cancer once the lesion is identified, while still respecting the need to treat a suspicious mass as potentially malignant until pathology proves otherwise.
For clinicians, the main lesson is not that breast parasites are common. They are not. The point is that a rare zoonotic infection can enter the same diagnostic pathway as a breast tumor and only become visible when pathology is reviewed with the right question in mind. In regions where Dirofilaria circulates in animals and mosquitoes, that question belongs on the differential before a benign-appearing or indeterminate breast lesion is dismissed or, conversely, assumed to be cancer without tissue confirmation.