A cross sectional serological survey of arboviral infections in humans was conducted on the three islands of the Union of Comoros, Indian Ocean, in order to test a previously suggested contrasted exposure of the three neighboring islands to arthropod-borne epidemics. Four hundred human sera were collected on Ngazidja (Grande Comore), Mwali (Mohéli) and Ndzouani (Anjouan), and were tested by ELISA for IgM and/or IgG antibodies to Dengue (DENV), Chikungunya (CHIKV), Rift Valley fever (RVFV), West Nile (WNV), Tick borne encephalitis (TBEV) and Yellow fever (YFV) viruses and for neutralizing antibodies to DENV serotypes 1–4. Very few sera were positive for IgM antibodies to the tested viruses indicating that the sero-survey was performed during an inter epidemic phase for the investigated arbovirus infections, except for RVF which showed evidence of recent infections on all three islands. IgG reactivity with at least one arbovirus was observed in almost 85% of tested sera, with seropositivity rates increasing with age, indicative of an intense and long lasting exposure of the Comorian population to arboviral risk. Interestingly, the positivity rates for IgG antibodies to DENV and CHIKV were significantly higher on Ngazidja, confirming the previously suggested prominent exposure of this island to these arboviruses, while serological traces of WNV infection were detected most frequently on Mwali suggesting some transmission specificities associated with this island only. The study provides the first evidence for circulation of RVFV in human populations from the Union of Comoros and further suggests that the virus is currently circulating on the three islands in an inconspicuous manner. This study supports contrasted exposure of the islands of the Comoros archipelago to arboviral infections. The observation is discussed in terms of ecological factors that may affect the abundance and distribution of vector populations on the three islands as well as concurring anthropogenic factors that may impact arbovirus transmission in this diverse island ecosystem.

In the Union of Comoros, interventions for combating malaria have contributed to a spectacular decrease in the prevalence of the disease. We studied the current distribution of Plasmodium species on the island of Grande Comore using nested PCR. The rapid diagnostic tests (RDTs) currently used in the Comoros are able to identify Plasmodium falciparum but no other Plasmodium species. In this study, we tested 211 RDTs (158 positive and 53 negative). Among the 158 positive RDTs, 22 were positive for HRP2, 3 were positive only for pLDH, and 133 were positive for HRP2 and pLDH. DNA was extracted from a proximal part of the nitrocellulose membrane of RDTs. A total of 159 samples were positive by nested PCR. Of those, 156 (98.11%) were positive for P. falciparum, 2 (1.25%) were positive for P. vivaxI, and 1 (0.62%) was positive for P. malariae. None of the samples were positive for P. ovale. Our results show that P. falciparum is still the most dominant species on the island of Grande Comore, but P. vivax and P. malariae are present at a low prevalence.

Our study indicates a significant decline in malaria morbidity and mortality in the islands of Moheli, Anjouan and Grand Comoro from 2010 to 2014. This considerable reduction is attributed to a combination of malaria prevention and control interventions implemented during the study period.

Between May and June 2001, efficacy of chloroquine was assessed in 5 sentinel sites in the 3 Comoro islands. Among the 183 children, age range between 6 and 59 months, followed up for 14 days, clinical failure rates ranged between 31.2 and 73.1% and the total failure rate (clinical and parasitological) between 50 and 88.5%. Failures were mainly early treatment failures. The Ministry of health, during a consensus meeting decided to change the first line drug and to gather baseline information on the efficacy and the tolerance of the combination artemether-lumefantrine. Between June and September 2004, among the 164 children, age range between 6 and 59 months included, the success rate of the combination was 99.4% in the 3 sites with a follow-up of 28 days. No serious drug related adverse event was reported.

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