Molecular surveillance uncovers human Dirofilaria asiatica microfilaremia in Sri Lanka
Two asymptomatic human infections in Sri Lanka have been confirmed as Dirofilaria asiatica microfilaremia, detected incidentally during night blood surveillance for lymphatic filariasis in 2025. The finding matters because it pushes the parasite out of the realm of veterinary concern alone and into routine public health surveillance, where human cases can be missed unless blood films are examined carefully and species identity is verified molecularly.
Both patients lived in Kalutara district in western Sri Lanka, an area where stray dogs are common and mosquito vectors from several genera are present. The setting is biologically plausible for zoonotic transmission. Canines in Sri Lanka have already been documented as reservoirs for D. asiatica, and the parasite has been reported in subcutaneous nodules and other tissues in animals. Human cases, however, are far less straightforward, because most Dirofilaria infections in people are typically amicrofilaremic and present as isolated inflammatory lesions rather than circulating larvae.
Here, the parasites were found in peripheral blood.
That single laboratory observation changes the clinical frame.
Two asymptomatic patients, two different clinical paths
Case 1 involved a 60-year-old mason whose night blood smear showed seven microfilariae in 60 µL of blood. Morphology was compatible with genus Dirofilaria. He was already receiving care for diabetes mellitus and hypertension, and longstanding bilateral lower-limb swelling was present without a clear explanation. A few inguinal lymph nodes were palpable, but abdominal ultrasonography and ophthalmologic screening were unremarkable. After venous blood collection for molecular testing, he received diethylcarbamazine, 6 mg/kg/day for 12 days, plus albendazole, 400 mg. Microfilaremia cleared by five weeks after treatment.
Case 2 involved a 14-year-old boy with three microfilariae per 60 µL night blood smear. He had an occasional dry cough without diurnal variation, and hematology showed eosinophilia of 568/µL, or 10.5%. Chest and abdominal imaging, as well as echocardiography, were unremarkable. He was treated with doxycycline, 100 mg twice daily for 5 days, plus albendazole, 400 mg twice daily for 5 days. Microfilaremia persisted at one month, falling to one larva per 60 µL, so a second course was given. Clearance was achieved by three months.
The two cases illustrate the range clinicians face when zoonotic filarial infection is uncovered by surveillance rather than by symptoms. One patient had comorbid illness and chronic swelling that complicated interpretation; the other had minimal respiratory symptoms and eosinophilia but no radiographic abnormality. Neither presented as an acute parasitic syndrome, which is exactly why incidental detection during filariasis programs remains so important.
Why morphology alone was not enough
Giemsa-stained thick blood smears showed microfilariae that fit Dirofilaria morphology, but species-level identification required molecular work. Genomic DNA was extracted from venous blood, and a mitochondrial cytochrome c oxidase subunit I gene fragment was amplified, purified, and sequenced bidirectionally. The resulting sequences were deposited in GenBank under accession numbers PZ359659 and PZ359660.
BLAST analysis showed near-complete query coverage and greater than 99% identity to reference D. asiatica sequences. Phylogenetic analysis using a maximum-likelihood tree in MEGA version 11 placed both isolates squarely within the D. asiatica clade, alongside reference sequences from GenBank. The phylogeny resolved major lineages corresponding to D. asiatica, D. repens, D. immitis, D. ursi, Onchocerca sp., and an unidentified Onchocercidae group.
The molecular result is the decisive part of the report. Morphology suggested the genus; sequencing closed the case.
Microfilaremia challenges the usual human dirofilariasis model
Human dirofilariasis is usually described as a dead-end infection in which immature worms fail to mature fully, in part because of host inflammatory responses against the parasite and its Wolbachia endosymbiont. Those responses can lead to granuloma formation and destruction of the worm before circulating microfilariae become established. The two Sri Lankan cases show that this pattern is not absolute. At least in some humans, D. asiatica can be found in blood without overt illness, and that raises practical questions about parasite survival, host susceptibility, and the intensity or timing of exposure.
One patient may have been relatively immunocompromised because of chronic metabolic disease, which could have permitted further parasite development. The younger patient may have experienced repeated exposure events. Neither explanation is proven, but both fit the biology of a zoonotic filarial infection that depends on host-parasite-vector interactions rather than a single linear pathway.
Treatment choices were also shaped by local availability. Diethylcarbamazine and albendazole were used in one patient, while doxycycline and albendazole were used in the other because ivermectin was not registered for human use in Sri Lanka at the time. The literature review accompanying the report found three regimens in use without a preferred standard: diethylcarbamazine/albendazole, doxycycline/albendazole targeting Wolbachia, and ivermectin with doxycycline. Even after microfilaremia cleared, the viability of adult worms remained uncertain because attempts to locate them were unsuccessful.
Surveillance implications extend beyond the two cases
These infections were found during night blood surveillance for lymphatic filariasis, a reminder that elimination programs can surface unexpected zoonotic infections when microscopy is paired with molecular confirmation. Sri Lanka already carries a heavy burden of human Dirofilaria infections relative to other parts of Asia, and the presence of infected dogs, stray animals, and mosquito vectors creates a durable transmission ecology.
The report supports routine attention to animal reservoirs and vector control, not just human case management. Antifilarial treatment of dogs and cats, mosquito suppression, and broader awareness among clinicians and public health workers are the concrete responses suggested by the findings. Follow-up blood screening for five years was considered because residual adult worms could not be excluded.
For parasitologists, the message is clear: species-level diagnostics matter when microfilariae appear in human blood. For clinicians, asymptomatic microfilaremia should not be dismissed as a laboratory oddity. It can represent a real zoonotic infection with uncertain duration, uncertain tissue burden, and a transmission system still active in the community.