Does parasitism of Simulium damnosum by Mermithidae reduce the transmission of onchocerciasis?

Mermithid nematodes found in adult Anopheles from southeastern Senegal

Background

Over two dozen mermithid nematodes have been described parasitizing mosquitoes worldwide, however, only two species were found in Africa. Mermithid nematodes kill their mosquito host upon emergence, which suggests that they could be developed as biological control agents of mosquitoes. Both Romanomermis culicivorax and Romanomermis iyengari have been reared for mass release to control numerous Anopheles species vector populations, and in one instance this may have led to reduced malaria prevalence in a human population.

Methods

Anopheles mosquitoes were collected during a malaria study in southeastern Senegal. Two different adult blood fed mosquitoes had a single mermithid nematode emerge from their anus while they were being held post-capture. Primers from the 18 S rDNA were developed to sequence nematode DNA and screen mosquitoes for mermithid DNA. 18 S rDNA from the Senegalese mermithid and other mermithid entries in GenBank were used to create a Maximum Parsimony tree of the Mermithidae family.

Results

The mermithid was present in 1.8% (10/551) of the sampled adult Anopheles species in our study area. The mermithid was found in An. gambiae s.s., An. funestus, and An. rufipes from the villages of Ndebou, Boundoucondi, and Damboucoye. Maximum parsimony analysis confirmed that the nematode parasites found in Anopheles were indeed mermithid parasites, and of the mermithid sequences available in GenBank, they are most closely related to Strelkovimermis spiculatus.

Conclusions

To our knowledge, this is the first report of mermithids from adult Anopheles mosquitoes in Senegal. The mermithid appears to infect Anopheles mosquitoes that develop in diverse larval habitats. Although maximum parsimony analysis determined the mermithid was closely related to Strelkovimermis spiculatus, several characteristics of the mermithid were more similar to the Empidomermis genus. Future mermithid isolations will hopefully allow: formal taxonomic identification, laboratory colonization, determination of life history traits and species specificity, and characterize its usefulness as a biological control agent.

An 18S ribosomal DNA barcode for the study of Isomermis lairdi, a parasite of the blackfly Simulium damnosum s.l

The mermithid parasite, Isomermis lairdi Mondet, Poinar & Bernadou (Nematoda: Mermithidae), is known to have a major impact on populations of Simulium damnosum s.l. Theobald (Diptera: Simuliidae) and on their efficiency as vectors of Onchocerca volvulus (Leuckart) (Nematoda: Filarioidea). However, the value of I. lairdi and other mermithid parasites as potential means of integrated vector control has not been fully realized. This is partly because traditional taxonomic approaches have been insufficient for describing and analysing important aspects of their biology and host range. In total, rDNA barcode sequences have been obtained from over 70 I. lairdi mermithids found parasitizing S. damnosum s.l. larvae in three different rivers. No two sequences were found to vary by more than 0.5%, and cytospecies identification of mermithid hosts revealed that I. lairdi with identical rDNA barcodes can parasitize multiple cytoforms of the S. damnosum complex, including S. squamosum (Enderlein). Phylogenetic analysis using a partial sequence from the 18S ribosomal DNA barcode, grouped I. lairdi in a monophyletic group with Gastromermis viridis Welch (Nematoda: Mermithidae) and Isomermis wisconsinensis Welch (Nematoda: Mermithidae).

Onchocerciasis in Sierra Leone. I. Studies on the prevalence and transmission at Gbaiima village

A longitudinal survey--parasitological, clinical, immunological and entomological--of onchocerciasis is being conducted in Gbaiima village in Sierra Leone. The estimated Annual Transmission Potential (ATP) is 5863. More than 80% of the annual transmission occurs between October and December. Four species of the Simulium damnosum complex are known to breed in a nearby river. The relative role of these species as vectors has yet to be determined. The total population (598 persons) aged one year and over were examined. Based on microfilarial and nodular rates the prevalence of onchocerciasis was 68.6%. In persons above 15 years of age this prevalence was 88.9%. Microfilarial and nodular rates were related to age. Severe skin lesions occurred in 1.0% of persons. 24 adults (7.5%) were blind (but the cause of the blindness was not determined).

Mermithid parasitism of Simulium damnosum s.l. and the transmission of human onchocerciasis

Dissection data from a Simulium damnosum s.l. population in northern Ghana, heavily parasitized by the mermithid Isomermis lairdi, are presented. A simple calculation gives reason to suppose that this parasitism may significantly reduce the transmission of onchocerciasis at certain seasons.