Wolbachia in filarial nematodes: evolutionary aspects and implications forthe pathogenesis and treatment of filarial diseases

The presence of intracellular bacteria in the body of various species of fi larial nematodes, including important parasites such as Brugia malayi, Diro filaria immitis, and Onchocerca volvulus, was observed as early as the mid- 1970s. These bacteria were shown to be transovarially transmitted (from the female worm to the offspring) and to be present in significant amounts in the body of the nematode. As highlighted by their discoverers, the potentia l importance of these bacteria is fairly obvious: (1) bacteria-derived mole cules should be considered as having an immunological and pathological role in filarial diseases; (2) the interaction between the bacteria and the fil arial host deserves investigation, in view of the possibility that the bact eria are needed by the host nematode and could thus represent a target for therapy. Other authors, independently from the discovery of these intracell ular bacteria, showed that the antibiotic tetracycline (which is well known for its efficacy on intracellular bacteria) had detrimental effects on two species of filarial nematodes (Brugia pahangi and Litomosoides sigmodontis ). It is therefore surprising that for more than 20 years, no further inves tigations focused on the bacteria of filarial nematodes, nor on the anti-fi larial properties of tetracycline. Recently, the bacteria of filarial nemat odes have been independently "rediscovered" by research groups from the sch ools of Hamburg, Liverpool and Milan. These bacteria are now classified as Wolbachia, and the basic aspects of their phylogenetic history and relation ship with the Wolbachia of arthropods have been reconstructed. In addition, their implications for the pathogenesis and treatment of filarial diseases have started to be uncovered. This paper, which is authored by representat ives of the three European schools who reopened this research area, reviews our present knowledge of these fascinating microorganisms, highlighting th e complexity of a symbiotic system which involves, in addition to the nemat ode and its bacterium, the vertebrate host. (C) 2001 Elsevier Science B.V. All rights reserved.