The effects of vector control on the antibody response to antigens of Onchocerca volvulus

Human Ascariasis: Diagnostics Update

Soil-transmitted helminths (STHs) infect over one billion people worldwide. Ascariasis may mimic a number of conditions, and individual clinical diagnosis often requires a thorough work-up. Kato-Katz thick smears are the standard detection method for Ascaris and, despite low sensitivity, are often used for mapping and monitoring and evaluation of national control programmes. Although increased sampling (number of stools) and diagnostic (number of examinations per stool) efforts can improve sensitivity, Kato-Katz is less sensitive than other microscopy methods such as FLOTAC®. Antibody-based diagnostics may be a sensitive diagnostic tool; however, their usefulness is limited to assessing transmission in areas aiming for elimination. Molecular diagnostics are highly sensitive and specific, but high costs limit their use to individual diagnosis, drug - efficacy studies and identification of Ascaris suum. Increased investments in research on Ascaris and other STHs are urgently required for the development of diagnostic assays to support efforts to reduce human suffering caused by these infections.

Assessment and monitoring of onchocerciasis in Latin America

Onchocerciasis has historically been one of the leading causes of infectious blindness worldwide. It is endemic to tropical regions both in Africa and Latin America and in the Yemen. In Latin America, it is found in 13 foci located in 6 different countries. The epidemiologically most important focus of onchocerciasis in the Americas is located in a region spanning the border between Guatemala and Mexico. However, the Amazonian focus straddling the border of Venezuela and Brazil is larger in overall area because the Yanomami populations are scattered over a very large geographical region. Onchocerciasis is caused by infection with the filarial parasite Onchocerca volvulus. The infection is spread through the bites of an insect vector, black flies of the genus Simulium. In Africa, the major vectors are members of the S. damnosum complex, while numerous species serve as vectors of the parasite in Latin America. Latin America has had a long history of attempts to control onchocerciasis, stretching back almost 100 years. The earliest programmes used a strategy of surgical removal of the adult parasites from affected individuals. However, because many of the adult parasites lodge in undetectable and inaccessible areas of the body, the overall effect of this strategy on the prevalence of infection was relatively minor. In 1988, a new drug, ivermectin, was introduced that effectively killed the larval stage (microfilaria) of the parasite in infected humans. As the microfilaria is both the stage that is transmitted by the vector fly and the cause of most of the pathologies associated with the infection, ivermectin opened up a new strategy for the control of onchocerciasis. Concurrent with the use of ivermectin for the treatment of onchocerciasis, a number of sensitive new diagnostic tools were developed (both serological and nucleic acid based) that provided the efficiency, sensitivity and specificity necessary to monitor the decline and eventual elimination of onchocerciasis as a result of successful control. As a result of these advances, a strategy for the elimination of onchocerciasis was developed, based upon mass distribution of ivermectin to afflicted communities for periods lasting long enough to ensure that the parasite population was placed on the road to local elimination. This strategy has been applied for the past decade to the foci in Latin America by a programme overseen by the Onchocerciasis Elimination Program for the Americas (OEPA). The efforts spearheaded by OEPA have been very successful, eliminating ocular disease caused by O. volvulus, and eliminating and interrupting transmission of the parasite in 8 of the 13 foci in the region. As onchocerciasis approaches elimination in Latin America, several questions still need to be addressed. These include defining an acceptable upper limit for transmission in areas in which transmission is thought to have been suppressed (e.g. what is the maximum value for the upper bound of the 95% confidence interval for transmission rates in areas where transmission is no longer detectable), how to develop strategies for conducting surveillance for recrudescence of infection in areas in which transmission is thought to be interrupted and how to address the problem in areas where the mass distribution of ivermectin seems to be unable to completely eliminate the infection.

Antibody detection tests for Onchocerca volvulus: comparison of the sensitivity of a cocktail of recombinant antigens used in the indirect enzyme-linked immunosorbent assay with a rapid-format antibody card test

There is a need for a specific, sensitive, and practical diagnostic test to monitor onchocerciasis elimination campaigns. In April 2001, an enzyme-linked immunosorbent assay (ELISA) using 3 recombinant antigens and a rapid-format antibody card test (immunochromatographic test; ICT) using an individual antigen were compared in a Mexican population with onchocerciasis. The sensitivity of the ELISA and ICT was 97% and 86%, respectively.

Antibody responses in onchocerciasis as a function of age and infection intensity

Onchocerciasis is caused by the filarial nematode Onchocerca volvulus and is a major public health problem in West and Central Africa. With only partial and long-term treatment currently available, there is a need to develop a suitable vaccine. We analysed the antibody response to infective L3 larvae because this stage is thought to be associated with host protective immunity. In addition, we have related our findings to the age, gender and current infection intensity of our participants: variables that may significantly influence antibody production. Interestingly, whilst 90% of our study group were seropositive for adult specific immunoglobulin (Ig)E, only 23% produced L3 specific IgE. This is in contrast to IgG4 where seropositivity was comparable at 96% and 92%, respectively. Furthermore, IgG levels were significantly affected by age and the intensity of infection but unaffected by host gender. This finding is independent for the IgG subclass (IgG1, IgG2, IgG3 and IgG4) and its specificity (L3 versus adult antigen). In summary, we show that L3 larvae induce little specific IgE and the antibody response shows a different isotype balance than that against adult antigens. Both host and parasite variables can influence antibody production in this disease.

Onchocerciasis hyperendemic in the Unturán Mountains: the value of recombinant antigens in describing a new transmission area in southern Venezuela

A recently described hyperendemic onchocerciasis area, located in the Unturán Mountains (between the Siapa and Orinoco basins) of southern Venezuela was studied using a cocktail of 3 low molecular weight onchocercal recombinant antigens (OvMBP/10, OvMBP/11, and OvMBP/29). The resulting seroepidemiological data were compared with those from a hypoendemic community (Altamira) situated in the northern coastal mountain range. Parasitological (skin biopsy) and serological (enzyme-linked immunosorbent assay, ELISA) methods for the specific diagnosis of Onchocerca volvulus in these 2 very different endemic areas were, respectively, 88% and 96% sensitive in Unturán, and 57% and 91% sensitive in Altamira. The mean microfilarial load, the mean optical density (OD), and the seropositivity rates all increased significantly with age in both communities. The serological variables (mean OD and prevalence of anti-O. volvulus antibodies) were both significantly higher in Unturán than in Altamira for children and young adults (aged < 25 years), although above this age no differences between communities were detected. Seroprevalence had already reached 50% in the under 15 year-olds examined at Unturán but was just 5% at Altamira for the same age-class. The prevalence of specific antibodies (mainly a marker of exposure to risk of infection) exceeded 85% in the remaining age-categories at the hyperendemic area. This is in agreement with the high community microfilarial load recorded in Unturán (> 20 mf/mg) and the presence of sclerosing keratitis and hanging groin, suggesting that onchocerciasis is a public health problem in this community. The ELISA test used here, based on a cocktail of 3 low molecular weight onchocercal recombinant antigens, appears, therefore, to constitute a practical tool for the description of endemicity levels in remote areas, particularly given the fact that finger-prick blood samples are routinely taken from children in the Upper Orinoco region for surveys of malaria incidence. Such studies could aid in defining the true extent of the Amazon focus (still unknown) and providing priority indicators for the selection of communities where onchocerciasis control programmes should be implemented.

The novel cuticular collagen Ovcol-1 of Onchocerca volvulus is preferentially recognized by immunoglobulin G3 from putatively immune individuals

The cDNA sequence encoding an Onchocerca volvulus collagen, Ovcol-1, has been isolated and the corresponding native antigen has been identified. The cDNA encodes an open reading frame of 96 amino acid residues containing an uninterrupted 66-residue Gly-X-Y repeat triple-helical (TH) domain (where X and Y may be any amino acids) flanked by a 26-residue amino non-TH domain and a 4-residue carboxyl non-TH domain. The size (9.7 kDa) and structure of the deduced molecule are unique among previously identified collagen chains. This novel collagen type has been designated "mini-chain collagen." Native Ovcol-1 is aqueous soluble and resolves by sodium dodecyl sulfate-polyacrylamide gel electrophoresis at 14.2 kDa under reducing conditions. Immunoelectron microscopy of adult female O. volvulus localized Ovcol-1 to the cuticles of both the adult worm and uterine microfilaria. A group of individuals from an area in Ecuador where O. volvulus is hyperendemic have been classified as putatively immune (PI) to O. volvulus infection. Analysis of the humoral immune responses to Ovcol-1 demonstrated that immunoglobulin G3 (IgG3) of PI individuals preferentially recognized this antigen in comparison to IgG3 of infected individuals.

Differential immune response to Onchocerca volvulus: IgG4 antibody responses differ in onchocerciasis patients from Guatemala and Ghana

Geographical differences exist in the clinical features of onchocerciasis in Central America and West Africa, which could be due in part from variations in the antigenic composition of the infecting organism. In an attempt to address this question, adult female worms of Onchocerca volvulus derived from nodules of patients from Guatemala and Ghana were compared in terms of polypeptide composition and the IgG4 antibody responses induced in patients. It was shown that a Tris-buffer soluble extract from the worms obtained in the two regions differ in polypeptide composition. Furthermore, the diagnostic polypeptides were found to be in the 30 kDa region but the recognition of these antigens was less intense and less frequently observed in the sera of microfilaria (mf) positive patients from Ghana than equivalent age and sex matched patients from Guatemala.

The effect of ivermectin treatment on the antibody response to antigens of Onchocerca volvulus

The effect of the microfilaricidal drug ivermectin on the antibody response to a detergent extract of adult Onchocerca volvulus (OvAg) and a number of specific recombinant peptides was examined. Three of the peptides were combined in a serodiagnostic 'cocktail' and the effect of ivermectin on the diagnostic performance of this assay was assessed. Immunoglobulin (Ig) G1 serum levels in response to OvAg significantly decreased following ivermectin treatment. The antibody response to only one recombinant peptide (OvMBP29) was significantly affected, with IgG levels decreasing following treatment. Levels of total IgE increased following treatment. No correlation was observed between initial antibody level (or change in antibody level) and any adverse reaction to treatment. The serodiagnostic 'cocktail' was 100% sensitive before and after the use of ivermectin. A serodiagnostic assay using specific recombinant peptides can be used to evaluate infection in the absence of dermal microfilariae in areas where ivermectin is used.

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Heterogeneity of IgG antibody responses to cloned Onchocerca volvulus antigens in microfiladermia positive individuals from Esmeraldas Province, Ecuador

The prevalence of IgG antibodies to three recombinant O. volvulus antigens, OvMBP/10, OvMBP/11 and OvMBP/29 was determined in a group of 94 microfilaria positive (mf+) individuals resident in the hyperendemic onchocercal area of Esmeraldas Province, Ecuador. Clone OvMBP/11 was the antigen most frequently recognized by patients sera followed by OvMBP/10 and OvMBP/29. When a cocktail of the three recombinant antigens was used the proportion of positive sera increased to 100%. Antibody responses to the fusion partner maltose binding protein (MBP) were low in comparison with those to the cloned antigens and no correlation of responses between individual antigens was observed. The relative level of antibody response to each of the clones in the cocktail varied between individuals. The distribution of IgG responses to OvMBP/11 was bimodal and those to OvMBP/29 and OvMBP/10 were positively and negatively skewed, respectively. When the three recombinant antigens were used in combination this variation was minimized and the pattern of responses showed a normal distribution as was also seen to crude O. volvulus antigen. The cocktail of recombinants thus offers excellent diagnostic sensitivity in combination with the parasite specificity demonstrated previously.