Assessments of the transmission of Onchocerca volvulus by Simulium sanctipauli in the Upper Denkyira District, Ghana, and the intermittent disappearance of the vector.

Following studies on the transmission of Onchocerca volvulus (Leuckart) by Simulium sanctipauli Vajime & Dunbar (Diptera, Simuliidae) in Upper Denkyira District in Ghana in 2001 and 2002 (Kutin et al., Med Vet Ent 18:167-173, 2004), further assessments were carried out in 2006 and 2013/2014 to determine whether transmission parameters had changed since community-directed ivermectin treatment (CDTI) began in 1999. There were no marked changes of the transmission intensities in 2006. Only slight, but non-significant, reductions were observed in infection rates of parous flies with larval stages (L1-L3) of O. volvulus from 44.1 % (of 1672 parous flies) in 2001/2002 to 42.1 % (506) in 2006 and from 6.5 to 5.9 % of flies carrying infective larvae in their heads. This suggested that there was an ongoing transmission in the area and the parasite reservoir in the human population was still high. Unexpectedly, further assessments conducted in October 2013 and March and October 2014 revealed that the vector S. sanctipauli had apparently disappeared and transmission had ceased, probably as a result of intensified gold mining activities along the rivers Ofin and Pra. The water of both rivers was extremely turbid, heavily loaded with suspended solids, probably preventing the development of blackfly larvae. Some breeding and biting of Simulium yahense Vajime & Dunbar was observed in a small tributary of the Pra, the Okumayemfuo, which is not affected by gold mining. However, the infection rate of flies was low, only 3.7 % of 163 parous flies were infected with first stage (L1) larvae of O. volvulus.

Similar blackfly attraction by onchocerciasis patients and individuals putatively immune to Onchocerca volvulus.

In areas endemic for onchocerciasis, a small number of individuals show no detectable infection with Onchocerca volvulus despite an apparently similar exposure to the transmitting blackflies. Such individuals have been termed putatively immune. Since several studies have indicated marked host differences in attractiveness for blood-seeking insects, putative immunity in O. volvulus infection may result, at least in part, from low vector attractiveness of the respective individuals. In an area hyperendemic for onchocerciasis (Guinea), where Simulium yahense is the predominant vector, we organized fly catches by putatively immune individuals and individuals with moderate-to-high worm counts. No differences were found between the 2 groups with respect to (i) the attraction of blackflies, (ii) the attraction of blackflies infected with O. volvulus, or (iii) the numbers of O. volvulus larvae carried by the attracted blackflies.

Delayed-type hypersensitivity reactions to Onchocerca volvulus antigens in exposed and non-exposed African individuals.

Although considered of critical importance, the mode of helper T-lymphocyte function in Onchocerca volvulus infection is still unclear including the role of the Th1/Th2 dichotomy. We studied the delayed-type hypersensitivity (DTH) reaction, which is the classical Th1 response, to O. volvulus antigens in Africans exposed and not exposed to the infection. DTH reactions were found in a small percentage of patients with generalized onchocerciasis, but in a high percentage of patients with localized onchocerciasis, in putatively immune subjects, and also in non-exposed individuals, which may be due to cross-reactivity with other nematodes. These findings support the notions of (i) prenatal influence of maternal O. volvulus infection preventing development of Th1 responses and/or (ii) suppression of Th1 responses by the infection itself.

Efficiency of Simulium sanctipauli as a vector of Onchocerca volvulus in the forest zone of Ghana.

The role of Simulium sanctipauli Vajime & Dunbar (Diptera: Simuliidae) as a vector of Onchocerca volvulus (Leuckart) (Spirurida: Onchocercidae) in the forest zone of central Ghana was studied in the Upper Denkyira district, where onchocerciasis is hyperendemic. Simulium sanctipauli was found to be a highly efficient vector, with a mean of 377 infective (L3) larvae in the heads of 1000 parous and 122 in the heads of 1000 biting flies. The overall infection rate of 44% of the parous flies with L1, L2 and L3 stages of O. volvulus (identity confirmed by polymerase chain reaction) demonstrates marked anthropophily. Female flies dispersed over a wide area and can transmit onchocerciasis up to at least 10 km away from their breeding sites. Annual community-directed treatments with ivermectin did not have a noticeable effect on the infection rates and parasitic loads of fly populations, which were as high 2 months after as 3 months before the distribution of ivermectin. This failure can be attributed to poor coverage, with treatment taken by only 24.4% of the population of the six study villages.

Cell-adhesion molecules expressed by activated eosinophils in Onchocerca volvulus infection.

Cell-adhesion receptors mediate the interaction between host effector cells and cellular or multicellular targets, including opsonized parasites. Our recent finding of a deposition of plasma proteins, including fibronectin, on the surface of infective larvae of the helminthic parasite Onchocerca volvulus led us to investigate the possible expression of cell-adhesion molecules (CAM), particularly fibronectin receptors, on eosinophilic granulocytes from persons infected with O. volvulus. Immunofluorescence analyses showed that freshly isolated eosinophils strongly expressed the beta 2-integrin CR3 and exhibited to a lower degree CR4 and the integrin-associated carbohydrate determinant Le(x), as well as antigen p24 (CD9). Eosinophils exposed to the eosinophil-active cytokines recombinant human interleukin 3 (rhIL-3) and granulocyte/macrophage colony-stimulating factor (rhGM-CSF) in addition to CR3, CR4, and CD9 expressed the beta 1-integrins VLA-4 and to a lesser extent VL-5 (both fibronectin receptors) as well as the intercellular adhesion molecule ICAM-1. Low-level expression of these adhesins was also observed on eosinophils cultured in the presence of these interleukins on confluent fibroblasts. The presence of VLA-4 as well as VLA-5 on activated eosinophils was confirmed by demonstration of the formation of immune rosettes using antibody-coated microspheres and by their attachment to fibronectin-coated surfaces. These results indicate the possibility of an involvement of previously unidentified antibody- and complement-independent mechanisms in cellular interactions with the parasite O. volvulus.

Mastomys natalensis or Mastomys coucha. Correct species designation in animal experiments.

Haemoglobin electrophoresis and chromosome analysis indicate that the widely used multimammate rat (chamois coloured) as experimental animal belongs to the species Mastomys coucha (Smith, 1836) and not, as commonly assumed, to Mastomys natalensis (Smith, 1834).

Characterization of human immune responses to the cytosolic superoxide dismutase and glutathione S-transferase from Onchocerca volvulus.

In onchocerciasis patients and in O. volvulus-exposed individuals without signs of onchocericiasis, T- and B-cell responses to two recombinantly expressed O. volvulus enzymes were analysed and compared to responses to total protein extract of adult parasites. The cytosolic enzymes Cu/Zn superoxide dismutase 1 (OvSOD1) and glutathione S-transferase 2 (OvGST2) represent 2 detoxifying molecules which may play an important role in parasite defense against host-induced oxidative stress. The T-cell response to the two recombinant proteins was analysed by investigating the cytokine responses of peripheral blood mononuclear cells. Induction of IL-5 at the mRNA level and IL-5 and IL-10 at the protein level was demonstrated in patients with the generalized form of onchocerciasis and endemic normals without clinical manifestations. IFN-gamma was not found to be induced by either antigen. This pattern of lymphokine expression is indicative of a Th2-type response. Compared to patients with the generalized form, a higher level of cytokine induction was observed in the group of endemic normals. Low but significant IgG levels were observed against OvSOD1 in patients with onchocerciasis; higher antibody levels were found against OvGST2 in patients and endemic normals. The highest IgG levels were detected against the crude O. volvulus extract. These results indicate that the two recombinant O. volvulus proteins induce moderate T and B cell responses.

Strong IgG isotypic antibody response in sowdah type onchocerciasis.

Persons exposed to Onchocerca volvulus express differences in manifestations of onchocerciasis ranging from hyporeactive (generalized) to hyperreactive (sowdah) forms; absence of disease is seen in endemic normal persons. Analysis of the IgG isotypic antibody response to O. volvulus extracts and nonparasite ubiquitous antigens in 92 West Africans revealed highest anti-O. volvulus IgG1 and IgG2 responses in sowdah, high levels in the generalized form, and low antibody levels in endemic normal persons. Nonexposed persons had no antibodies. A significant IgG3 antibody response was detected only in sowdah, while high IgG4 levels occurred in both polar groups but were absent in both control groups. Isotypic responses to antigens unrelated to O. volvulus were similar in all groups but showed higher IgG1 and IgG2 levels in sowdah. Sowdah patients had high levels of Ro/SS-A antibodies, circulating immune complexes, and eosinophil cationic protein. These results document a strong B cell response in African sowdah and indicate variations in immune responsiveness.

Epidemiological studies of onchocerciasis in southern Benin.

We studied the prevalence of human onchocerciasis in four geographically different regions of the southern part of Benin in West Africa. In a total of thirteen villages 1596 individuals were examined for clinical and parasitological signs of onchocerciasis. Prevalence of microfilariae of Onchocerca volvulus in skin snips was 29% in region I (lower Oueme river), 64% in region II (Mono river), 56% in region III (upper Oueme river) and 70% in region IV (Okpara river). Based on endemicity criteria of the WHO regions II and IV were found to be hyperendemic, region III mesoendemic and region I hypoendemic for onchocerciasis. The community microfilarial load ranged from 4 mf/mg skin in the hypoendemic region to 10.5 mf/mg skin in the hyperendemic regions. The prevalence of nodules was 21% in region I, 30% in region II, 17% in region III and 41% in region IV. The overall prevalence of chronic onchocercal dermatitis was 12%. Of 689 individuals infected with O. volvulus 388 were treated with a single dose of ivermectin.