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Onchocerca ochengi: morphological identification of the L3 in wild Simulium damnosum s.l., verified by DNA probes. 1998

G Wahl, and J M Schibel
Institut für Tropenmedizin, Eberhard Karls Universität, Tübingen, Germany.

In order to assess the prevalence of the cattle filaria Onchocerca ochengi in onchocerciasis vectors (Simulium damnosum s.l.) in North Cameroon, we searched for a means to morphologically identify its developing larvae, which closely resemble those of O. volvulus. To this end microfilariae of the 2 Onchocerca species were isolated from slaughter cattle in Ngaoundéré and injected into neonate Simulium species. Whereas the early developmental stages (sausage stage, L2 and pre-infective larva) were indistinguishable, the infective larvae (L3) of O. ochengi were longer (median: 740 microns), more slender (diameter = 19.3 microns = 2.6% of body length) and had a relatively shorter tail (4.9% of body length) than those of O. volvulus (680 microns, 20.5 microns, 3.0% and 5.8% respectively). The tail of O. ochengi L3 was thick and rounded, whereas it was slightly tapering in O. volvulus L3. O. ochengi L3 produced by feeding flies on infected cattle in a different area in North Cameroon (Sora Mboum) showed the same features as intrathoracically produced O. ochengi L3 from Ngaoundéré, but were even longer (785 microns). On the basis of the differences in length, relative diameter, length of the tail and shape of the tail, a simple key for the separation of O. volvulus and O. ochengi L3 was elaborated, and 248 L3 found in wild S. damnosum s.l. were separated into 'O. ochengi' (160 L3) and 'O. volvulus' (88 L3) following this key. Sequential dot blot hybridization of each of the 248 larvae with a DNA probe which reacts with O. ochengi and O. volvulus but not with other Onchocerca species (pOo5/1) and with an O. volvulus-specific DNA probe (pOv12) revealed that the morphological identification had been correct in 86-91% of the cases. Only a small proportion (6-9%) of the dot blots did not react with either probe. Since this proportion was equal in experiments using experimentally produced L3 and in experiments using wild L3, the nonhybridization was certainly due to a loss of L3 during washing of the filters and not due to the presence of other unknown L3 species resembling O. volvulus and O. ochengi. Our study shows that in Cameroon it is possible to identify O. volvulus and O. ochengi infective larvae during routine fly dissections by morphology alone.

UI MeSH Term Description Entries
D007303 Insect Vectors Insects that transmit infective organisms from one host to another or from an inanimate reservoir to an animate host. Insect Vector,Vector, Insect,Vectors, Insect
D007814 Larva Wormlike or grublike stage, following the egg in the life cycle of insects, worms, and other metamorphosing animals. Maggots,Tadpoles,Larvae,Maggot,Tadpole
D008018 Life Cycle Stages The continuous sequence of changes undergone by living organisms during the post-embryonic developmental process, such as metamorphosis in insects and amphibians. This includes the developmental stages of apicomplexans such as the malarial parasite, PLASMODIUM FALCIPARUM. Life Cycle,Life History Stages,Cycle, Life,Cycles, Life,History Stage, Life,History Stages, Life,Life Cycle Stage,Life Cycles,Life History Stage,Stage, Life Cycle,Stage, Life History,Stages, Life Cycle,Stages, Life History
D009693 Nucleic Acid Hybridization Widely used technique which exploits the ability of complementary sequences in single-stranded DNAs or RNAs to pair with each other to form a double helix. Hybridization can take place between two complimentary DNA sequences, between a single-stranded DNA and a complementary RNA, or between two RNA sequences. The technique is used to detect and isolate specific sequences, measure homology, or define other characteristics of one or both strands. (Kendrew, Encyclopedia of Molecular Biology, 1994, p503) Genomic Hybridization,Acid Hybridization, Nucleic,Acid Hybridizations, Nucleic,Genomic Hybridizations,Hybridization, Genomic,Hybridization, Nucleic Acid,Hybridizations, Genomic,Hybridizations, Nucleic Acid,Nucleic Acid Hybridizations
D009854 Onchocerca A genus of parasitic nematodes whose organisms live and breed in skin and subcutaneous tissues. Onchocercal microfilariae may also be found in the urine, blood, or sputum. Onchocercas
D009855 Onchocerciasis Infection with nematodes of the genus ONCHOCERCA. Characteristics include the presence of firm subcutaneous nodules filled with adult worms, PRURITUS, and ocular lesions. Onchocerciases
D002417 Cattle Domesticated bovine animals of the genus Bos, usually kept on a farm or ranch and used for the production of meat or dairy products or for heavy labor. Beef Cow,Bos grunniens,Bos indicus,Bos indicus Cattle,Bos taurus,Cow,Cow, Domestic,Dairy Cow,Holstein Cow,Indicine Cattle,Taurine Cattle,Taurus Cattle,Yak,Zebu,Beef Cows,Bos indicus Cattles,Cattle, Bos indicus,Cattle, Indicine,Cattle, Taurine,Cattle, Taurus,Cattles, Bos indicus,Cattles, Indicine,Cattles, Taurine,Cattles, Taurus,Cow, Beef,Cow, Dairy,Cow, Holstein,Cows,Dairy Cows,Domestic Cow,Domestic Cows,Indicine Cattles,Taurine Cattles,Taurus Cattles,Yaks,Zebus
D000818 Animals Unicellular or multicellular, heterotrophic organisms, that have sensation and the power of voluntary movement. Under the older five kingdom paradigm, Animalia was one of the kingdoms. Under the modern three domain model, Animalia represents one of the many groups in the domain EUKARYOTA. Animal,Metazoa,Animalia
D012680 Sensitivity and Specificity Binary classification measures to assess test results. Sensitivity or recall rate is the proportion of true positives. Specificity is the probability of correctly determining the absence of a condition. (From Last, Dictionary of Epidemiology, 2d ed) Specificity,Sensitivity,Specificity and Sensitivity
D012843 Simuliidae A family of insects in the order DIPTERA, which include black flies, buffalo gnats and simulies. Several species are intermediate hosts (vectors) for the parasitic disease ONCHOCERCIASIS. Black Flies,Simulium,Blackflies,Black Fly,Blackfly,Flies, Black,Fly, Black,Simuliums

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