Mansonella perstans is a vector-borne human filarial nematode, transmitted by tiny blood-sucking flies (biting midges). It is widespread in many parts of Sub-Saharan Africa and also occurs in parts of Central and South America. Despite the commonness of this parasite very few studies have been carried out on its epidemiology and on the morbidity resulting from it, and only few thorough drug trials have been conducted to look for effective and suitable drugs and drug regimens for treatment and control. Here, we review currently available knowledge on M. perstans infections in Africa, including documented aspects of biology, vectors, transmission, diagnosis, epidemiology, morbidity and treatment. It is concluded that there is an urgent need for more research on this widespread but greatly neglected infection in order to properly assess its public health significance and as a background for identifying and recommending optimal means and strategies for treatment and control.

Surveys for Mansonella perstans infection and potentially related clinical manifestations were undertaken in two endemic communities in Mukono and Luwero districts of Uganda where no other human filarial infections are transmitted. A sensitive and accurate counting chamber method was used for quantifying microfilaraemia in 100microl of finger-prick blood. Among 575 and 991 examined individuals aged >or=1 year in the two communities, the overall microfilariae (mf) prevalence was significantly higher in Mukono (76.5%) than in Luwero (57.7%). As early as age 1-4 years, 40.6% and 20.5% of the children were mf-positive. Prevalences increased rapidly with increasing age to reach 89.2% and 81.4% in the 15-19 years age group and then remained high in subsequent age groups. The geometric mean mf intensity among mf-positive individuals was slightly higher in the Mukono community (32.4mf/100microl) than in the Luwero community (29.9mf/100microl), and this parameter increased with age in both communities. No obvious associations were observed between various clinical parameters and M. perstans microfilaraemia in any of the study communities. The observed patterns of microfilaraemia and the lack of obvious visible clinical manifestations suggest that the host's regulatory responses are downregulated in M. perstans infections. [ClinicalTrials.gov identifier: NCT00215280].

The safety and efficacy of a single dose of ivermectin alone (150-200 microg/kg bodyweight), albendazole alone (400 mg) or the combination of these two drugs was assessed, in Uganda, in three groups of individuals infected with Mansonella perstans (with 15, 13 and 15 subjects in each group, respectively). No side-effects were observed or reported during the first 7 days post-treatment and every subject remained microfilaraemic during the 12 months of follow-up. In the subjects given ivermectin alone or albendazole alone, the microfilarial intensities consistently remained close to their pre-treatment levels. In the subjects given both drugs, however, the microfilarial intensities decreased slightly after treatment and at 1 and 3 months post-treatment (but not at 6, 9 or 12 months) they were significantly lower than in the two other groups combined. The three single-dose drug regimens investigated were thus well tolerated but had disappointingly low efficacies in the treatment of M. perstans microfilaraemias.

The Mansonella perstans microfilarial (mf) periodicity in peripheral blood was analysed in a group of 32 healthy individuals from an endemic community in Uganda. The majority of individuals had maximum mf intensity during the first and minimum during the last 12 h of the day. Mean mf ratios (i.e. time-specific counts in relation to mean count for the individual) indicated a weak but significant diurnal periodicity with peak intensity around 0800 h. Trigonometric analysis of the ratios, assuming a harmonic wave pattern of periodicity, gave a periodicity index of 11.1 and a peak time of 0645 h. The higher mf intensities in the early morning appeared to coincide with the preferred biting hour of the vectors. The observed weak pattern of mf periodicity indicates that the sampling time of blood specimens for diagnosis will have only minor effect on diagnostic sensitivity.

The effect of a single dose of ivermectin alone (150-200microg/kg body weight) or in combination with albendazole (total of 400mg) in Mansonella perstans infection was assessed in a randomised, double-blind field trial in two endemic communities in Mukono and Luwero districts of Uganda. No side effects were observed or reported during the first 7 days after treatment. The effect on microfilaraemia was analysed among individuals with >or=20 microfilariae (mf) per 100mul of blood at baseline, who took the treatment and who attended follow-up examinations at 6 months and 12 months after treatment (48 and 46 in Mukono and 48 and 40 in Luwero for the ivermectin and combination treatment, respectively). In both communities, the combination treatment appeared slightly more effective than ivermectin alone, but the difference was not statistically significant. Both drug regimens were more effective in Luwero than in Mukono, probably owing to different diets in the two areas. However, in general both treatment regimens in both communities had limited effect on microfilarial intensities, and only one individual (given combination treatment in Luwero) was mf-negative at 6 months and 12 months after treatment. [ClinicalTrials.gov identifier: NCT00215280].