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Background  In the absence of intermediate animal hosts, the process of embryogenesis leading to fecundity of adult female filarial worms is very critical for persistence of these obligate parasites in human communities. Embryogenesis in adult female filarial parasites involves fertilization of eggs or oocytes by sperms and their subsequent development into motile microfilariae inside the uterine cavity of worms. Development of assays for monitoring embryogenesis in adult female worms is a critical requirement in filariasis research – filarial worms are known to harbour endosymbionts such as  Wolbachia  which play a significant role in fecundity. Tetracycline or doxycycline treatment of the infected hosts effectively eliminates the endosymbionts resulting in inhibition of embryogenesis in female worms. Currently, inhibition of embryogenesis in adult filarial worms can be monitored only by microscopic examination of  in vitro  harvested intrauterine stages.    Methods  Adult female filarial worms of bovine filarial parasites,  Setaria digitata  were collected from the peritoneum of infected animals and intrauterine stages were harvested in culture medium and were analyzed for forward and side scatter by flowcytometry using a BD FACS Calibur. Different populations were gated, sorted and identified by phase microscopy. Binding of biotinylated lectins to intra uterine stages was monitored using FITC labeled Avidin and monitored by flow cytometry of gated populations. Similarly, binding of antibodies in human filarial sera to intrauterine stages was monitored using FITC labeled anti-human immunoglobulins.    Results  The forward and side scatter for intrauterine stages delineated 3 distinct populations labeled as R1, R2 and R3. The three populations were sorted and identified to be a) fully stretched microfilariae, b) early and c) late developmental stages of eggs respectively. Lectins such as Wheat Germ agglutinin or Concanavalin-A were found to bind strongly to egg stages and less prominently to intra-uterine microfilariae. Similarly the binding of antibodies in filarial sera to the three intra-uterine stages could also be precisely quantified.    Conclusion  The manuscript reports a novel flow cytometry based method to monitor progression of embryogenesis in adult filarial worms. Apart from relative quantification of different intra uterine developmental stages, the assay allows quantitative binding of lectins and antibodies to each of the intrauterine stages. It may now be possible to quantify levels of antibodies in infected and immune hosts to monitor anti-fecundity immunity in filariasis – the assay can thus be used as a powerful tool for drug development and in immunological studies in human and experimental filariasis.

A flow cytometry based method for studying embryogenesis and immune reactivity to embryogenic stages in filarial parasites