Development of a novel trap for the collection of black flies of the Simulium ochraceum complex

BACKGROUND

Human landing collections are currently the standard method for collecting onchocerciasis vectors in Africa and Latin America. As part of the efforts to develop a trap to replace human landing collections for the monitoring and surveillance of onchocerciasis transmission, comprehensive evaluations of several trap types were conducted to assess their ability to collect Simulium ochraceum sensu lato, one of the principal vectors of Onchocerca volvulus in Latin America.

METHODOLOGY/PRINCIPAL FINDINGS

Diverse trap designs with numerous modifications and bait variations were evaluated for their abilities to collect S. Ochraceum s.l. females. These traps targeted mostly host seeking flies. A novel trap dubbed the "Esperanza window trap" showed particular promise over other designs. When baited with CO2 and BG-lure (a synthetic blend of human odor components) a pair of Esperanza window traps collected numbers of S. Ochraceum s.l. females similar to those collected by a team of vector collectors.

CONCLUSIONS/SIGNIFICANCE

The Esperanza window trap, when baited with chemical lures and CO2 can be used to collect epidemiologically significant numbers of Simulium ochraceum s.l., potentially serving as a replacement for human landing collections for evaluation of the transmission of O. volvulus.

Epidemiological evaluation of onchocerciasis along Ogun River System, southwest Nigeria

BACKGROUND & OBJECTIVE

Epidemiological studies were carried out to assess the prevalence and community microfilarial load (CMFL) of onchocerciasis after repeated annual treatment with ivermectin along Ogun river System, southwest Nigeria.

METHOD

Skin snips were taken from consented participants in 11 selected communities along the River system. The microfilarial load of the community was estimated.

RESULTS

The prevalence and CMFL varied significantly in the communities (p <0.05). The prevalence of onchocerciasis ranged from 19.1 to 45.6%, while the CMFL ranged from 0.11 to 1.03 microfilariae per skin snip. The CMFL recorded was <5 microfilariae per skin snip, i.e. recognized by WHO as threshold value in certifying the communities to be free of onchocerciasis as public health problem, thus, signifying the possibility of onchocerciasis elimination in the study area.

CONCLUSION

Efforts should therefore be intensified to achieve improved ivermectin coverage and compliance in annual ivermectin treatment in order to completely eliminate onchocerciasis as a public health problem in the studied communities.

[Effectiveness of a polymerase chain reaction using seminested and multiplex strategy for the identification of the three main bacteria responsible for meningitis in Burkina Faso]

A prospective study (from August 2006 to April 2007) was carried out with 214 cerebrospinal fluid samples with suspicion of bacterial meningitis. The aim of the study was to assess the effectiveness of the simultaneous detection of Neisseria meningitidis, Streptococcus sp. and Haemophilus influenzae using seminested polymerase chain reaction strategy. Among the 214 samples tested by both PCR and culture, the overall confirmation rate was 64% for PCR and 40.1% for culture (P = 2 x 10⁻⁶). Taking culture method as the standard reference, the overall sensitivity of PCR was 98.8% and specificity, 59.4%. The sensitivity of PCR was 100, 97.3 and 100% respectively for N. meningitidis, Streptococcus sp. and H. influenzae with respective specificities of 70, 93.2 and 97.2%. In conclusion, the seminested PCR strategy is a sensitive method and it can be implemented in the reference public health laboratories for an exhaustive microbiological surveillance of bacterial meningitis.

Effectiveness of a trivalent serogroup A/C/W135 meningococcal polysaccharide vaccine in Burkina Faso, 2003

Following a large Neisseria meningitidis W135 (NmW135) epidemic in Burkina Faso (BF) during 2002, a newly licensed trivalent A/C/W135 meningococcal polysaccharide vaccine was introduced in 2003. We conducted a case-control study to assess the vaccine effectiveness (VE) against meningococcal disease. Thirty-two N. meningitidis A (NmA) and 3 NmW135 meningitis cases were enrolled and matched by age-neighborhood to 103 controls. After adjusting for confounding risk factors, VE against NmA or NmW135 was 83.6% (95% CI 31.8-97.0, p=0.01) for persons with verified vaccination. VE against probable/definite NmA alone was 94.0% (95% CI 58.7-99.0, p=0.0003). Low number of NmW135 cases did not allow estimation of VE against NmW135 alone. The vaccine was highly effective against the epidemic. Since 2003, the trivalent vaccine continues to be effectively used in Africa for the control of meningococcal disease epidemics.

The current status of onchocerciasis in the forest/savanna transition zone of Côte d'Ivoire

Onchocerca volvulus exists in at least two strains in West Africa, while its black-fly vectors consist of sibling species, dwelling in the savanna and forest/transition zones. In transition and degraded forest zones both parasite strains and different sibling species of the vector can be sympatric. The strain of parasite in infected humans and in vector black-flies was determined in two bioclimes along the Bandama river of Côte d'Ivoire. The upper Bandama is located in the savanna bioclime while the Middle Bandama is located in a degraded forest zone. At both sites, savanna-dwelling sibling species of the Simulium damnosum sensu lato species complex predominated. The severe-strain of O. volvulus was the predominant strain at both sites. However, severe-strain parasites represented a significantly larger proportion of those found in the vector population than in the human population in the degraded forest of the Middle Bandama. These data suggest that in degraded forest areas recently invaded by savanna-dwelling species of S. damnosunz s.l. transmission of the severe-strain of the parasite might be more efficient than transmission of the mild-strain.

Detection of Onchocerca volvulus infection in low prevalence areas: a comparison of three diagnostic methods

The standard assay for onchocerciasis diagnosis is microscopical detection of microfilariae in skin snips. Skin snipping is painful, requires appropriate sterilization of equipment, and may fail to diagnose light infections. Two alternatives are a polymerase chain reaction (PCR) test which detects parasite DNA in pieces or scrapings of skin and a test based on allergic reactions to topical application of diethylcarbamazine (DEC). We compared these 2 diagnostics with standard skin snip microscopy in 313 individuals from 2 villages in Guinea, with low prevalence after over 10 years of control by the Onchocerciasis Control Programme. Lower and upper bounds on sensitivities and specificities of these 3 tests were estimated. In addition, these parameters were estimated using 5 different statistical models. Where prevalence was low, PCR and the DEC patch test appeared to be more sensitive than skin snipping which has low sensitivity. As the DEC test is non-invasive, simple and cheap, it may provide a good alternative to skin snipping alone for surveillance in low prevalence areas.

Eotaxin expression in Onchocerca volvulus-induced dermatitis after topical application of diethylcarbamazine

In persons with onchocerciasis, topical application of the anthelminthic diethylcarbamazine (DEC) induces clinical and histologic responses similar to acute papular onchodermatitis, including recruitment of eosinophils to the skin. To determine whether the eosinophil chemokine eotaxin is likely to be associated with eosinophil recruitment in onchodermatitis, DEC was applied to a 5-cm2 area on the skin of infected persons, and biopsies were taken from lesions 24 h later. Histologic analysis showed elevated dermal and epidermal eosinophils compared with tissue from an adjacent (untreated) site. Reverse transcription-polymerase chain reaction showed that eotaxin gene expression in DEC-treated skin was elevated 2- to 17-fold compared with control tissue. Eotaxin immunoreactivity was noted in mononuclear cells and eosinophils in the perivascular region of the dermis and in lymphatic and vascular endothelial cells. Together, these observations are consistent with a role for eotaxin in recruitment of eosinophils to the dermis in early stage onchocercal skin disease.

Detection of Onchocerca volvulus infection by O-150 polymerase chain reaction analysis of skin scratches

The standard assay for onchocerciasis diagnosis is microscopic detection of parasites in skin snips. Skin snipping is painful and may potentially transmit bloodborne infections. Thus, an alternative method for the diagnosis of onchocerciasis that does not require skin snipping is needed. A polymerase chain reaction (PCR)-based assay was shown to detect the presence of parasite DNA in superficial skin scrapings. Detection of parasite DNA in both skin snips and skin scratches was found to be more sensitive for detecting low-density infections than was microscopic examination of skin snips. The skin scratch PCR assay is minimally invasive and painless and does not present the risk of transmitting bloodborne infections. These properties make the skin scratch an attractive alternative to the skin snip for detecting O. volvulus infection.

Diagnostics in onchocerciasis: future challenges

The classical method of determining the prevalence and intensity of onchocercal infection is by the demonstration and counting of microfilariae in biopsies obtained by skin snipping. Although very specific, this technique is inadequate for detecting early, light or prepatent infections, and is also becoming increasingly unacceptable to the populations investigated. The prolonged clearing effect that Mectizan (ivermectin, MSD) treatment has on skin microfilariae also renders the skin-snip method of diagnosis less appropriate in areas with Mectizan treatment. Given all these factors, the greater challenge in the area of diagnostics for onchocerciasis is to develop a less invasive, adequately sensitive, and equally specific diagnostic test, either to replace or to be an adjunct to the present skin-snip method. This challenge is being addressed, with at least three new diagnostic tests for onchocerciasis under development: an immunological assay, based on a three-antigen cocktail; a PCR-based assay, which may also be used for 'pool screening' of blackflies; and the diethylcarbamazine (DEC) patch test. Of all these tests, the DEC patch test seems to fit best the criteria of an ideal test. The PCR assay would be better than the patch test if the cost of using it could be reduced substantially.

Vector-parasite transmission complexes for onchocerciasis in West Africa

BACKGROUND

In West Africa, there are two strains of the filarial parasite Onchocerca volvulus, which differ in their ability to induce ocular disease. Transmission studies have suggested that six sibling species of the parasite vector, the black fly Simulium damnosum sensu lato, allow development of the two strains of O volvulus with varying efficiency. We aimed to test the hypothesis of parasite-vector complexes, whereby the two parasite strains, known as forest and savanna, are preferentially transmitted by distinct groups of the species of S damnosum S l.

METHODS

During 1993 and 1994, wild black flies were collected from 11 river basins within the area covered by the Onchocerciasis Control Programme (OCP). The flies were dissected and filarial larvae, ovaries, and malpighian tubules removed. Genomic DNA was extracted from larvae, and PCR amplification was used to classify O volvulus parasites as forest or savanna strains. PCR-amplified DNA from ovaries and malpighian tubules was used to distinguish sibling species of S damnosum s l. S yahense and S squamosum were distinguished by body colour.

FINDINGS

214 of 105105 flies dissected were infected with filarial larvae; 84 of these were infected with mature O volvulus parasites. Of the 35 savanna-dwelling infected flies. 17 carried forest-strain parasites and 18 savanna-strain parasites. Of the 45 infected flies identified as the forest dwelling sibling species. 20 carried savanna-strain parasites and 25 forest-strain parasites. No significant differences were found in the numbers of mature larvae of each strain carried by the forest-dwelling species of fly or in the number of forest and savanna larvae in savanna-dwelling vector species.

INTERPRETATION

Vector-parasite transmission complexes do not currently play a part in the biology of O volvulus transmission in the area of the OCP in West Africa. This finding has important strategic implications for the future of efforts to control onchocerciasis in West Africa.

Onchocerca volvulus: comparison of field collection methods for the preservation of parasite and vector samples for PCR analysis

In recent years, methods for the identification of the filarial worm Onchocerca volvulus and its vector, blackflies of the Simulium damnosum complex (S. damnosum sensu lato (s.l.)), based on the amplification of parasite and vector DNA sequences with the polymerase chain reaction (PCR), have been developed. Routine application of these methods requires techniques for sample collection and preservation that are compatible with the limitations of field collection, yet preserve DNA in a form suitable for PCR. Two different methods for sample preservation were evaluated by the field collection teams and the DNA probe laboratory of the Onchocerciasis Control Programme in West Africa. The most successful involved the preservation of material from O. volvulus and its associated vectors in a dried state on microscope slides. Of over 1200 parasite samples preserved in this manner, more than 93% retained DNA yielding positive results in PCR analysis (1208/1291). Vector material (malpighian tubules and ovaries) preserved in the same manner on the same microscope slides also yielded DNA that was suitable for PCR.

Determining the prevalence of Onchocerca volvulus infection in vector populations by polymerase chain reaction screening of pools of black flies

An important variable in the epidemiology of arthropodborne diseases is the intensity of transmission, which is a function of host-vector contact and the prevalence of infection in the vector population. This latter value is often difficult to estimate. It is possible to envision the application of polymerase chain reaction (PCR) assays to this problem. To accomplish this, the assay must detect a single infected vector in a pool containing a large number of uninfected individuals. It must also be possible to calculate the prevalence of infection from the number of positive pools. A PCR assay for detecting Onchocerca volvulus in pools of vector black flies is described, and an algorithm is presented to calculate the prevalence of infection in the vector population, based upon the proportion of PCR-positive pools. This algorithm should be applicable to any disease for which a PCR assay is available.

Mitochondrial alleles of Simulium damnosum sensu lato infected with Onchocerca volvulus

Onchocerca volvulus infected Simulium damnosum s.l. were analysed by directed heteroduplex analysis. Of 73 infected flies, 68 produced heteroduplex products identical to those previously identified. All 6 major sibling species, except S. leonense, were present in this group. In the 5 remaining flies, 2 new heteroduplex patterns were noted. Molecular phylogenetic analysis of these samples suggested that they belonged to the S. squamosum/S. yahense subcomplex. The ability to reliably genotype adult flies will permit studies of the vectorial capacity of the sibling species of S. damnosum s.l. for the blinding and non-blinding strains of O. volvulus.