Despite decades of a solid leprosy control program, including active case finding and follow-up on therapeutic outcome, the Comoros islands of Anjouan and Moheli continue to be hyperendemic for leprosy, with leprosy case notifications far exceeding those for tuberculosis, while the third island, Grande Comore, presents few cases. The high proportion (31% in 2015) of disease in children indicates that recent transmission is a major driver of the persistent endemicity, and that present control measures are not sufficient. The low proportion (2.6% average in last 10 years) of grade II disabilities in newly diagnosed cases indicates that case detection is early.

The main objective of the present proposal is to identify which persons would most benefit from prophylactic treatment. The secondary objective is to unravel human, bacterial and environmental risk factors for transmission of and progression to leprosy disease, with the ultimate goal to reduce the leprosy incidence.. The program has remaining expertise to re-establish laboratory confirmation of leprosy patients, allowing to optimize and validate molecular genotyping techniques to complement conventional epidemiological investigations in a 3-year prospective cohort of leprosy patients and their close contacts, aiming to identify transmission links. A third objective is to document diagnostic delays in more detail

As the leprosy control programme has initiated a pilot study on rifampicin prophylaxis in four villages on Anjouan in 2015, a prospective cohort study will permit measuring the leprosy incidence in close contacts as well as those in neighboring houses, who did or did not receive rifampicin prophylaxis.

The expected outcome of this project will be to identify risk factors for leprosy transmission. Specifically, we expect to identify those contacts at highest risk of developing leprosy disease, who would most benefit from rifampicin prophylaxis or other preventive measures.

Inclusion Criteria:

• in the prospective cohort study: All newly diagnosed leprosy patients (any age) on the island of Anjouan who provide written informed consent.
• In the in-depth interviews: Patients who are part of a cluster and provide written informed consent. Additional written informed consent will be asked for questions in which the identity of other member(s) of a cluster needs to be revealed.

Exclusion Criteria:

• in the prospective cohort study: None.
• In the in-depth interviews: Patients belonging to clusters not extending beyond the same first or second circle

The Comoros islands, located between Madagascar and Mozambique, continue to be hyperendemic for leprosy. Periodic active case finding across the entire island of Anjouan is in place, resulting in early case detection, and every patient identified is being treated with highly effective multi drug therapy. Nevertheless, the high proportion of leprosy in children (31% were <15y old in 2015) in combination with a low relapse rate (<2%) indicate transmission continues unabated. The majority of the patients in Anjouan are paucibacillary.

Data sharing will be considered once the datasets are locked at time of publication.