Emergence of a novel zoonotic brugian filarial infection during post-validation surveillance for lymphatic filariasis in Sri Lanka.

Lymphatic filariasis (LF) affects approximately 120 million people globally and is caused by Wuchereria bancrofti, Brugia malayi and Brugia timori. Sri Lanka was historically endemic for both bancroftian and brugian filariasis, with B. malayi presumed eliminated by the late 1960s. The country achieved validation of LF elimination as a public health problem in 2016. However, sporadic Brugia infections have been detected in microfilaria surveys from the mid-2000s onwards; the sub-periodic periodicity of which is suggestive of a zoonotic origin and warrants further research. Night blood survey (NBS) surveillance was conducted by the Anti-Filariasis Campaign, Ministry of Health, Sri Lanka, from 2019 to 2023. Brugia-positive human samples and archived canine samples were analysed using a next-generation sequencing metabarcoding platform targeting mitochondrial and nuclear loci. Bayesian and neighbour-joining phylogenetic analyses and minimum-spanning network approaches were used to characterise lineage relationships. Among 1,855,165 individuals screened, 52 were identified with Brugia-like microfilariae. Metabarcoding generated 1.2 million high-quality reads with both mitochondrial cox1 and ribosomal DNA markers confirming a single Brugia lineage circulating in humans, mainly in children. Sequences of the c ox1 gene showed 99.4-100 percent nucleotide identity to the Brugia Sri Lanka (SL) genotype previously detected in dogs in Sri Lanka and Tamil Nadu (India). rDNA sequences showed low similarity to B. malayi or Brugia pahangi, indicating that the parasite represents a distinct Brugia taxon lacking a reference rDNA sequence. The findings provide evidence that human brugian filariasis in Sri Lanka is caused by a previously unrecognised zoonotic Brugia species maintained in dogs. This has important clinical and public health implications given that human infections have occurred in children despite LF being declared eliminated as a public health problem. Crucially, the incorrect classification of circulating filarioid species may lead to inappropriate clinical and programmatic responses. For example, the newly identified canine reservoir of this Brugia genotype means that human-only mass drug administration might not be sufficient to interrupt this species' transmission. These results underscore the need for integrated One Health surveillance to prevent re-establishment of LF and safeguard regional elimination goals. No specific funding was received for the present study. This study was supported by a consulting funding (PRJ_002971).