A 56-year-old man from Tahiti had lived for several years with a firm, painless swelling on the left side of his scrotum. By the time he sought care in mainland France, it measured 18 centimeters, roughly 7 inches. When surgeons operated, they found remnants of a filarial worm and widespread dead tissue inside the fluid-filled mass.
The diagnosis was lymphatic filariasis, a mosquito-borne parasitic disease. The case appears in IJID Regions, published online June 18 and included in the journal's September 2026 volume. The authors, from Nancy Regional University Hospital Center in France and Université Laval in Quebec, describe the case as rare for French Polynesia because of the swelling's size.
Clues in the Blood Pointed to a Parasite
The man had no prior medical history and traveled abroad for the first time in January 2024 to mainland France. He came to the Nancy hospital that July, reporting discomfort but no pain. He denied unexplained fevers or leg swelling.
Blood tests showed eosinophilia, a high count of a type of white blood cell often associated with parasitic infections. Antibody testing for filariasis was positive. A single blood sample drawn at 10 p.m., when filarial larvae typically circulate, showed none under the microscope.
CT and MRI scans revealed a partly fluid mass measuring 18 by 11 centimeters, with very high protein content. There were no enlarged lymph nodes and no signs of tuberculosis or blood disorders.
Doctors started the antibiotic doxycycline. After four weeks, because of the discomfort the mass was causing, surgeons operated in October 2024. Given its size, adhesions, and contamination risk, they removed the left testicle and reconstructed the scrotum.
Following World Health Organization recommendations, he continued doxycycline for three months and then received a single dose of diethylcarbamazine, an antiparasitic drug. Pathology showed residual filarial worms and extensive necrosis in the inflamed fluid, but no spread into the testicle. His skin healed within a month, and there was no recurrence over 12 months of follow-up.
A Mosquito-Borne Disease That Does Its Damage Slowly
Lymphatic filariasis is caused by thread-like worms, most often Wuchereria bancrofti, which accounts for about 90% of infections. Infected mosquitoes deposit larvae on the skin during a bite. The larvae migrate to the lymphatic system and mature into adult worms, which release microscopic offspring called microfilariae into the blood.
According to the CDC, most infected people never develop symptoms. Others develop lymphedema, which is swelling of the legs, arms, breasts, or genitals, or hydrocele, a fluid buildup in the scrotum, sometimes years after infection. The World Health Organization estimates that 51 million people were infected as of 2018, a 74% decline since its global elimination program began in 2000, and that 657 million people in 39 countries lived in areas that still required preventive treatment in 2023.
Transmission concerns persist in French Polynesia. A 2025 study in PLOS Neglected Tropical Diseases used molecular testing of mosquitoes to look for possible transmission hotspots on the island of Huahine.
The negative blood smear did not rule out the disease. The authors explain that many patients with symptoms are no longer actively infected by the time they seek care, because the immune system may already have cleared the adult worms. The same shift toward inflammation that helps eliminate the parasite also drives chronic tissue damage. In this man, most of the worms appeared to have died, which the authors say accounts for the extensive necrosis.
Doctors Checked Carefully for Cancer
The authors stress that every scrotal mass needs microscopic examination, even when filariasis is the likely cause. They cite reports of tumors arising in filarial scrotums, most often leiomyosarcoma, a cancer of smooth muscle tissue, including one giant leiomyosarcoma case. They propose that chronic inflammation and blocked lymph flow might create an environment that favors cancer.
That link is not established. The International Agency for Research on Cancer does not classify lymphatic filariasis as a cancer-causing infection. In this case, pathology found no malignancy, though the team planned more than five years of follow-up.
A Tropical Diagnosis Can Surface Far from the Tropics
The authors argue that clinicians outside endemic areas will increasingly encounter parasitic diseases because of travel, migration, and climate-driven shifts in where disease-carrying insects live. They also note that people without lifelong exposure may mount stronger inflammatory reactions to a first infection.
For U.S. readers, the risk remains low. The CDC says lymphatic filariasis rarely occurs in casual travelers because infection takes repeated bites from infectious mosquitoes. Travelers most likely to be infected are those who stay in affected areas for long periods, such as missionaries, Peace Corps volunteers, and field researchers.
Prevention centers on avoiding mosquito bites. The CDC recommends EPA-registered insect repellents, long sleeves and pants, and screened windows and doors, and it advises extra caution between dusk and dawn, when the mosquitoes that carry the parasite usually bite. Health care providers can obtain diethylcarbamazine from the CDC after lab results confirm infection, but the drug does not reverse chronic damage such as hydrocele or elephantiasis.
This is a single case report, and antibody tests generally cannot tell a current infection from a past one. Anyone with a persistent scrotal swelling, particularly after time in tropical regions, should be evaluated by a clinician.
Key Questions Answered
What happened?
A 56-year-old man from Tahiti with a years-old, 18-centimeter scrotal swelling was diagnosed in France with lymphatic filariasis after surgeons found filarial worms in the mass.
What causes lymphatic filariasis?
Thread-like parasitic worms, most often Wuchereria bancrofti, spread through repeated bites from infectious mosquitoes.
Why was his blood test negative for larvae?
Most of the worms had likely died by the time he sought care. A negative smear does not rule out past infection.
Why did surgeons remove his testicle?
The mass was large, stuck to surrounding tissue, and carried a contamination risk. Pathology found no disease inside the testicle itself.
Can U.S. travelers catch this?
It is rare in casual travelers. The CDC says infection requires repeated mosquito bites, usually during long stays in affected areas.
How can people lower their risk?
Avoid mosquito bites with EPA-registered repellents, protective clothing, and screened windows and doors, especially from dusk to dawn.
Does filariasis cause cancer?
That has not been established. Some tumors in affected tissue have been reported, but the disease is not classified as a cancer-causing infection.
