Modelling the effects of immigration on the re-introduction of onchocerciasis

BACKGROUND: Onchocerciasis is a filarial neglected tropical disease targeted by the World Health Organization for elimination (interruption) of transmission (EOT), principally by mass drug administration (MDA) with ivermectin. Variable effectiveness and success of MDA, among other factors, has led to a markedly heterogeneous contemporary spatial landscape of endemicity and transmission, with some foci having achieved or nearing EOT, while in others, transmission persists despite decades of MDA or has only recently been identified. Communities reaching EOT or free from infection are thus vulnerable to re-introduction of infection imported by immigrants from areas with ongoing transmission. METHODS: We use the stochastic, individual-based EPIONCHO-IBM transmission model to quantify the risk of transmission persistence resulting from importation events and characterise the dynamics of ensuing onchocerciasis outbreaks in terms of microfilarial prevalence (in all ages) and anti-Ov16 seroprevalence (in children aged 5-9 years) in infection-free communities with local populations of black fly vectors. RESULTS: We show how the vulnerability of infection-free communities depends on their population size, the local annual biting rate (ABR, number bites/person/year) and the magnitude of importation events, defined by the number of immigrants arriving in the community and their worm burden. We show that small communities with modest ABRs are particularly vulnerable to transmission persistence following importation, with risk exacerbated by the magnitude of infection importation. We illustrate that onchocerciasis outbreak dynamics can be protracted, with seroprevalence in children often taking substantially longer than the currently recommended 3-5 years of post-treatment surveillance (PTS) to exceed 5%. CONCLUSIONS: Our findings highlight the vulnerability of infection-free communities to introduction/re-introduction of infection and suggest that proposed PTS durations may need to be extended and complemented with additional surveillance activities and migration studies to detect and respond robustly to nascent outbreaks and sustain elimination.