Loiasis: the individual factors associated with the presence of microfilaraemia

No microfilariae are detectable in a significant percentage of those infected with the filarial worm Loa loa. While the probability of an infected individual becoming microfilaraemic is known to increase with age, the mechanisms underlying this trend are not well understood. Epidemiological data from an endemic village in central Cameroon were therefore explored, in an attempt to determine if, after taking into account any history of filaricidal treatment, the presence of Loa microfilaraemia in an individual was related to his/her gender, age, and/or exposure to the human-infective larvae of the parasite. An index of exposure, based on the monthly transmission potentials of the Chrysops in each of the main types of vegetation in a village and on the activity schedule of each inhabitant of the village, was developed. The results of the data analysis confirm that the acquisition of microfilaraemia is gender-dependent (males generally being more likely to be microfilaraemic than females), and indicate that, in males, a high level of exposure to infective larvae determines the shift from amicrofilaraemic to microfilaraemic status. They also indicate that filaricidal treatments have a long-lasting suppressive effect on Loa microfilaraemia, an observation that may have important implications for any strategy to limit the risk of Loa-associated encephalopathy following ivermectin treatment.

Microfilarial distribution of Loa loa in the human host: population dynamics and epidemiological implications

Severe adverse events (SAEs) following ivermectin treatment may occur in people harbouring high Loa loa microfilarial (mf) densities. In the context of mass ivermectin distribution for onchocerciasis control in Africa, it is crucial to define precisely the geographical distribution of L. loa in relation to that of Onchocerca volvulus and predict the prevalence of heavy infections. To this end, we analysed the distribution of mf loads in 4183 individuals living in 36 villages of central Cameroon. Mf loads were assessed quantitatively by calibrated blood smears, collected prior to ivermectin distribution. We explored the pattern of L. loa mf aggregation by fitting the (zero-truncated) negative binomial distribution and estimating its overdispersion parameter k by maximum likelihood. The value of k varied around 0.3 independently of mf intensity, host age, village and endemicity level. Based on these results, we developed a semi-empirical model to predict the prevalence of heavy L. loa mf loads in a community given its overall mf prevalence. If validated at the continental scale and linked to predictive spatial models of loiasis distribution, this approach would be particularly useful for optimizing the identification of areas at risk of SAEs and providing estimates of populations at risk in localities where L. loa and O. volvulus are co-endemic.

Co-infection withOnchocerca volvulusandLoa loamicrofilariae in central Cameroon: are these two species interacting?

Ivermectin treatment may induce severe adverse reactions in some individuals heavily infected withLoa loa. This hampers the implementation of mass ivermectin treatment against onchocerciasis in areas whereOnchocerca volvulusandL. loaare co-endemic. In order to identify factors, including co-infections, which may explain the presence of highL. loamicrofilaraemia in some individuals, we analysed data collected in 19 villages of central Cameroon. Two standardized skin snips and 30 μl of blood were obtained from each of 3190 participants and the microfilarial (mf) loads of bothO. volvulusandL. loawere quantified. The data were analysed using multivariate hierarchical models. Individual-level variables were: age, sex, mf presence, and mf load; village-related variables included the endemicity levels for each infection. The two species show a certain degree of ecological separation in the study area. However, for a given individual host, the presence of microfilariae of one species was positively associated with the presence of microfilariae of the other (OR=1·79, 95% CI [1·43–2·24]). Among individuals harbouringLoamicrofilariae, there was a slight positive relationship between theL. loaandO. volvulusmf loads which corresponded to an 11% increase inL. loamf load per 100O. volvulusmicrofilariae. Co-infection withO. volvulusis not sufficient to explain the very highL. loamf loads harboured by some individuals.

Genetic polymorphism of the β-tubulin gene of Onchocerca volvulus in ivermectin naïve patients from Cameroon, and its relationship with fertility of the worms

Observations of low response of patients infected with Onchocerca volvulus to ivermectin suggest that the parasite may be under a selection process toward potential resistance. To limit the extension of this phenomenon, it is crucial to characterize the genes of O. volvulus that are involved. For this, O. volvulus adult worms collected before the introduction of ivermectin in an onchocerciasis endemic area of central Cameroon were genotyped for beta-tubulin. To derive a baseline to investigate the selective pressure of ivermectin, we analysed (1) the frequency distribution of the beta-tubulin alleles, and (2) the relationship between the different beta-tubulin related genotypes and the fertility status of the female worms. The frequency of allele b of the beta-tubulin gene was very low, as it was observed in West Africa. We observed a deficit of heterozygous female worms leading to Hardy Weinberg disequilibrium, which might be explained by a shorter life-span of these worms compared to the homozygous worms. Unexpectedly, our results also show that the heterozygous female worms were much less fertile than the homozygotes: more than two thirds of the homozygotes were fertile, whereas only 37% of the heterozygotes were fertile. These results will be further considered when analysing post-treatment data.

Impact of four years of large-scale ivermectin treatment with low therapeutic coverage on the transmission of Onchocerca volvulus in the Mbam valley focus, central Cameroon

The extent to which ivermectin treatments have an impact on onchocerciasis transmission is a matter of some concern. We investigated this issue in the Mbam valley, a hyperendemic focus located in a forest-savannah mosaic area of Cameroon. Parasitological examinations of 5-9-year-old children, who had never received any antifilarial drug, were conducted before the first distribution of ivermectin in 1991-1993 and again in 2002, after four annual rounds of mass treatments. After matching for gender, age and village of residence, the prevalence and intensity of microfilaridermia corresponded respectively, in 2002, to 66.2 and 42.0% of the initial values. The decrease was more marked among the youngest children who, compared with the older ones, were submitted to the reduced force-of-infection earlier in their life. The results of the present study suggest that the specific vectorial competence of Simulium squamosum cytotype B, the vector of Onchocerca volvulus in the Mbam valley, allows a significant decrease in onchocerciasis transmission after several years of treatment, despite low therapeutic coverage. Though these results are encouraging, efforts should be made to improve the therapeutic coverage in the area.

Experimental studies on the transmission of Onchocerca volvulus by its vector in the Sanaga valley (Cameroon): Simulium squamosum B. Intake of microfilariae and their migration to the haemocoel of the vector

As large-scale ivermectin distribution is becoming the mainstay of onchocerciasis control in Africa, the issue of its impact on local transmission is increasing in importance. The vector competence of Simulium squamosum B in the severe focus of the Sanaga valley, Cameroon, was therefore investigated, by feeding 1320 flies on 14 carriers of Onchocerca volvulus microfilariae (mff). The results enabled the relationships between skin microfilarial load, microfilarial intake by the flies, the proportion and mean number of ingested mff that succeed in reaching the fly's haemocoel, and the frequency distribution of the ingested mff to be described, as functions of time post-engorgement (p.e.) and parasite density (while taking account of possible measurement error in the predictor variable). The proportion of flies with haemocoelic mff and the mean number of mff/fly increased up to 3 h p.e. The proportion of flies with ingested mff was non-linearly related to mean intake, via the negative-binomial distribution, with the overdispersion parameter k best described as an increasing (power) function of the mean. Approximately one in every three ingested mff escaped imprisonment by the peritrophic matrix, irrespective of the skin microfilarial load or the intake of mff. The relationship between successful and input mff is nearly linear (indicating proportionality) in S. squamosum B. These results are compared with those from O. volvulus-S. damnosum s.l. combinations in other West African foci.

Neoplastic change in Onchocerca volvulus and its relation to ivermectin treatment

A pleomorphic neoplasm (PN) is described from sections of Onchocerca volvulus worms in nodules excised from Cameroonian patients. PN is confined to older, non-fecund, female worms, and those classed as moribund/dead. It is mainly composed of small, roundish, basophilic cells of diverse sizes, often forming a 'rosette' pattern around amorphous eosinophilic centres. The cells have a high nuclear/cytoplasmic ratio and up to 2-3 mitoses/high-power field; some become grossly enlarged, highly polymorphic and contain large, irregular blocks of chromatin. The eukaryotic PN cells first appear posteriorly in the pseudocoelom, probably from ovarian cells; they spread anteriorly, invading or compressing the uteri. Ivermectin treatment increased the prevalence PN from 3.7% of 1422 female worms in 637 patients before treatment to 17.5% of 1134 worms in 511 patients after 3 years treatment. Ivermectin at 400-800 microg/kg annually, or at 150 microg/kg or 400-800 microg/kg 3-monthly, over 3 years, did not increase the PN prevalence significantly, as compared with standard doses of 150 microg/kg annually. In other small series of African patients, PN prevalence increased in worms 2, 4, 6 and 10 months after ivermectin treatment; but there was no increase after treatment with amocarzine, albendazole or diethylcarbamazine and suramin. PN may partly account for the increased macrofilaricidal action of ivermectin on female O. volvulus in patients treated for 3 years at 3-monthly intervals.

Effects of a 3-day regimen of albendazole (800 mg daily) on Loa loa microfilaraemia

The encephalopathy that sometimes develops after ivermectin treatment in patients with high Loa microfilaraemias is probably related to a massive effect of the drug on the Loa microfilariae. A trial was therefore conducted to evaluate whether a course of albendazole would bring about a slower decrease in the Loa microfilaraemia, and thus could be used as a mass 'clearing' treatment, before the distribution of ivermectin in areas where onchocerciasis and loiasis are co-endemic. The Loa microfilarial loads were followed monthly for 9 months in two groups of subjects, one treated with albendazole (400 mg twice a day for 3 days), and the other with vitamin (B(1), B(6) and B(12)) tablets. There were no significant between-group differences in the microfilarial loads at any of the examination rounds. During the follow-up period, there was also no significant change in the overall loads among those treated with albendazole, although the counts in those with high initial microfilaraemias (>8000 microfilariae/ml) tended to decrease progressively during the first 3 months. Further trials should now be performed, to evaluate the effects on Loa loa of two courses of albendazole given 2-3 months apart.

Relationship between onchocerciasis and epilepsy: a matched case-control study in the Mbam Valley, Republic of Cameroon

Studies conducted during the past 10 years to investigate the possible relationship between onchocerciasis and epilepsy have led to contradictory results. In 1991-92 and 2001 we investigated 14 villages in central Cameroon to evaluate the relationship, at the community level, between the prevalence of epilepsy and the endemicity level of onchocerciasis. A case-control study compared the microfilarial loads of 72 epileptic and 72 non-epileptic individuals, matched according to sex, age, and village of residence. The prevalence of epilepsy and the community microfilarial load (CMFL) were closely related (P < 0.02), and the case-control study demonstrated that the microfilarial loads (microfilariae per snip) in the epileptic group (arithmetic mean = 288, median = 216) were significantly higher (P < 10(-4)) than in the control group (arithmetic mean = 141, median = 63). The results strongly support the existence of a link between onchocerciasis and epilepsy. The fact that such a relationship has not been found recently in some other West and Central African areas is probably due to the lowered endemicity of onchocerciasis following vector- and ivermectin-related control measures applied over the past 5-25 years. The socio-economic and demographic impact of onchocerciasis-related epilepsy should be evaluated, and taken into account as part of all onchocerciasis control programmes.

Excess mortality associated with blindness in the onchocerciasis focus of the Mbam Valley, Cameroon

The association between blindness, mortality and nutritional status was investigated in a retrospective cohort study in villages of central Cameroon where onchocerciasis is hyper-endemic. Overall, 101 blind subjects and 101 non-blind controls matched with the blind for age, sex and (generally) village of residence were followed for 10 years. Blindness gave rise to a significant increase in mortality (relative risk = 2.3; P = 0.012), the life expectancy of the blind adults being reduced by 4 years compared with that of their controls. For a given age, excess mortality was found to be associated with a late onset of blindness. The causes of death were similar for the blind and the controls but blind subjects had relatively low body mass indices, which may lead to relatively early fatal disease outcomes. These results are similar to those obtained in other parts of Africa and emphasise, once more, the demographic impact of blindness in developing countries.