[Mansonella ozzardi] [Mansonella perstans] [Mansonella streptocerca]
Three species of Mansonella, a vector-borne filarial nematode genus, are associated with human infections: M. ozzardi, and M. perstans (=Dipetalonema perstans), and M. streptocerca (=Dipetalonema streptocerca). These species vary in their geographic occurrence and localization within the host. Recently, a novel molecular clade of M. perstans, M. perstans “deux” has been reported from both Brazil and Gabon. On rare occasions, zoonotic species such as M. rodhaini and other unidentified Mansonella sp. have been detected in human infections.
Microfilariae of Mansonella perstans are unsheathed and measure 190—200 µm in stained blood smears and 180—225 µm in 2% formalin. The tail is blunt and nuclei extend to the tip of the tail. Microfilariae circulate in the blood.
Microfilariae of Mansonella ozzardi are unsheathed and measure 160—205 µm in stained blood smears and 200—255 µm in 2% formalin. The tail tapers to a point and the nuclei end well before the end of the tail, which is usually bent in a small hook-like shape. Microfilariae circulate in blood but may be rarely found in the skin, and should be thus distinguished from Wuchereria bancrofti and Onchocerca volvulus.
Microfilariae of Mansonella streptocerca are unsheathed and measure 180—240 µm by 3—5 µm. The tail is bent into a hook-like shape and the nuclei extend to the end of the tail. Microfilariae are found in skin and do not circulate in the blood, and should be distinguished from those of Onchocerca volvulus.
Rarely, Mansonella streptocerca may be found in tissue sections. Adults are very small (maximum diameter 80 µm for females, 50 µm for males) and have typical filarial features, including paired uterine tubes and a diminished intestine. The musculature is coelomyarian with a tall contractile portion. Microfilariae may be seen in various stages of development in utero.
Mansonella perstans and M. ozzardi are usually diagnosed by the finding of microfilariae circulating in blood whereas M. streptocerca is usually diagnosed by finding microfilariae in skin snips. The rare human cases of M. rodhaini were also diagnosed via skin snips. Concentration techniques (e.g. Knott’s concentration, polycarbonate membrane filtration) may increase sensitivity. Note that these filariae do not generally exhibit periodicity, though in some regions, M. perstans-infected patients may exhibit slightly elevated microfilaremia in morning hours.
Skin snips should be thick enough to include the outer part of the dermal papillae but not so thick as to produce bleeding (a ~2 mm wide section should be sufficient). Skin snips should be placed immediately in normal saline or distilled water, just enough to cover the specimen. Microfilariae tend to emerge more rapidly in saline, however in either medium the microfilariae typically emerge in 30—60 min and can be seen in wet mount preparations. For a definitive diagnosis, allow the wet mount to dry, fix in methanol, and stain with Giemsa or hematoxylin-and-eosin.
In the context of clinical diagnosis, the primary use of Mansonella molecular testing is to aid in differentiating microfilariae of M. streptocerca from Onchocerca volvulus in skin snips. In research settings, real-time PCR (RT-PCR) assays can be used to detect and quantify M. perstans and M. ozzardi microfilariae, and loop-mediated isothermal amplification assays (LAMP) have been developed for field use.
Serological assays developed for the detection of Mansonella spp.–specific antibodies or antigens are generally of limited utility due to cross-reactions with other filarial species and potentially other nematodes.
Standard precautions for the processing of blood and tissue samples apply.
Abrahim, C.M., Py‐Daniel, V., Luz, S.L., Fraiji, N.A. and Stefani, M.M., 2019. Detection of Mansonella ozzardi among blood donors from highly endemic interior cities of Amazonas state, northern Brazil. Transfusion, 59(3), pp.1044-1051.
Mathison, B.A., Couturier, M.R. and Pritt, B.S., 2019. Diagnostic Identification and Differentiation of Microfilariae. Journal of Clinical Microbiology, pp.JCM-00706.
Ta-Tang, T.H., Crainey, J.L., Post, R.J., Luz, S.L. and Rubio, J.M., 2018. Mansonellosis: current perspectives. Research and Reports in Tropical Medicine, 9, p.9-24.
Gobbi, F., Beltrame, A., Buonfrate, D., Staffolani, S., Degani, M., Gobbo, M., Angheben, A., Marocco, S. and Bisoffi, Z., 2017. Imported Infections with Mansonella perstans Nematodes, Italy. Emerging infectious diseases, 23(9), p.1539.
Lima, N.F., Veggiani Aybar, C.A., Dantur Juri, M.J. and Ferreira, M.U., 2016. Mansonella ozzardi: a neglected New World filarial nematode. Pathogens and Global Health, 110(3), pp.97-107.
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