ABSTRACT
The biology of the ‘larial worms of the family Onchocercidae is highly specialized. They are tissue dwellers, their life cycle involves two hosts, one intermediate that serves as vector, usually a hematophagous species of Diptera, and the vertebrate de‘nitive host, but probably the most distinctive feature is that adult females lay mobile micro‘lariae (Figure 57.1). This characteristic is likely an adaptation to the constraint of adults to living within the host’s tissues with no direct contact with the exterior, and according to some authors it has probably evolved several times in evolution [3,5]. The adults of the genus Mansonella can be found embedded in different tissues of the de‘nitive host (see Anderson [7] and references therein). For example, adults of the human parasite M. perstans are found in the body cavities such as the peritoneal or pleural cavities, and less frequently in the pericardium; the adults of M. ozzardi commonly live in the subcutaneous tissues of the human host, although they have also been found in the peritoneal cavity; and those of M. streptocerca, another parasite of humans, dwell in the dermis and subcutaneous tissues. The female adult worms lay mobile embryos, known as micro-‘lariae, which migrate to more external tissues to facilitate their intake by the blood feeding vectors. Micro‘lariae of different species can also be found in different tissues. Thus, the micro‘lariae of M. ozzardi and M. perstans circulate in the periferal blood while those of M. streptocerca are found in the skin (see Anderson [7] and references therein). Once the vector ingests the micro‘lariae with the blood meal, they penetrate the vector’s midgut and migrate within 24 h to the thoracic muscles where they develop into infective L3 larvae between 6 and 7 days postinfection [8−11]. The L3 larvae migrate to the arthropod’s proboscis from where they enter a new de‘nitive vertebrate host when the vector takes a subsequent blood meal. The species that serve as vectors of Mansonella can vary within the genus and even within species. Thus, M. streptocerca and M. perstans are transmitted by members of the genus Culicoides (Diptera: Ceratopogonidae), M. streptocerca by Culicoides grahami, while M. perstans can be transmitted by both C. grahami
FIGURE 57.1 Micro‘laria of M. ozzardi stained with orcein. The head of the micro‘laria is on the lower right-hand corner.
