Biology Reference
In-Depth Information
is present in several species of Ýlariae, including the main agents of human and
animal Ýlariasis. Examples of infected Ýlariae are
Wolbachia
Onchocerca volvulus
(the agent of river blind-
ness),
Wuchereria bancrofti
and
Brugia malayi
(agents of tropical elephantiasis), and
DiroÝlaria
immitis
, the
prevalence of infection appears to be 100%. Moreover, the infection appears stable along evolu-
tionary times: main branches of Ýlarial evolution are composed of species harboring
(the agent of canine and feline heartworm disease). In species harboring
Wolbachia
Wolbachia
(Casiraghi et al., 2001). In adult nematodes,
is present in the hypodermal cells of the
lateral chords; the cytoplasm of some of these cells is Ýlled with
Wolbachia
Wolbachia
and resembles insect
bacteriocytes. While in females
has not been
demonstrated in the male reproductive apparatus (Sacchi et al., 2002). The bacterium is vertically
transmitted through the cytoplasm of the egg, and there is no evidence of horizontal transmission
or paternal transmission. Indeed, the phylogeny of
Wolbachia
is also present in the ovaries,
Wolbachia
matches that of the host Ýlariae
(Casiraghi et al., 2001). It is also noteworthy that there are no indications for multiple infection of
a single host or for the presence of different ÑtypesÒ of
Wolbachia
in a given Ýlarial species. There
is thus overall evidence that the infection is stable and species-speciÝc.
Current information on the distribution and phylogeny of
Wolbachia
Wolbachia
in Ýlarial nematodes thus
suggests that
is needed by the host nematode (100% prevalence in infected species,
whole branches of Ýlaria evolution composed by infected species, consistency of host and symbiont
phylogenies). The results of experiments with tetracycline (and derivates) on animal hosts infected
by Ýlariae are in agreement with the idea that the
Wolbachia
Ýlaria association is obligatory.
Tetracyclines have indeed been shown to (1) inhibit the development from the infective larva (L3)
to the adult, (2) inhibit embryogenesis and microÝlaria (L1) production, (3) interfere with the long-
term survival of adult nematodes, and (4) interfere with L1ÏL3 development of the Ýlaria in
mosquito hosts (reviewed in Bandi et al., 2001b). It must be emphasized that some of the results
above have been obtained on more than one Ýlarial species (i.e., 1 and 2), while in other cases the
results are relevant to a single species (3 and 4). There is, however, overall agreement among the
results of treatment experiments using tetracycline and derivates, showing that these antibiotics, in
reducing the amount of
WolbachiaÏ
in nematodes from treated hosts, also have detrimental effects
on the Ýlarial nematode. Similar results have been obtained in
Wolbachia
in vitro
experiments, and other
antibiotics have been shown to cause attrition on Ýlariae harboring
.
It still must be demonstrated that there is a cause-and-effect relationship between the activity
of antibiotics on
Wolbachia
and the deleterious effects on the nematodes. It is, however, suggestive
that in a Ýlarial species that is free of
Wolbachia
), tetracycline
treatments do not interfere with L3-adult development and with microÝlaria production. The fact
that tetracycline and other antibiotics are detrimental to Ýlarial species harboring
Wolbachia
(
Achantocheilonema viteae
has
opened the way to new therapeutic strategies for the control of Ýlariases. For example, the
combination of a ÑtraditionalÒ antiÝlarial drug (ivermectin) associated with doxycycline (a
derivate of tetracycline) appears very promising for the therapy of human onchocerciasis (Taylor
et al., 2000; Hoerauf et al., 2001).
Wolbachia
E
S
I
F
N
A
VIDENCE
FOR
INGLE
NFECTION
IN
ILARIAL
EMATODE
NCESTOR
There is an important difference between arthropod and nematode
Wolbachia
. In arthropods,
Wolbachia
infects a wide taxonomic range of hosts, particularly insects, but also other classes
(Werren, 1997; Stouthamer et al., 1999; Jeyaprakash and Hoy, 2000). In nematodes,
has
so far been observed only in Ýlariae (which represent a single family, the Onchocercidae). Other
groups of nematodes may harbor
Wolbachia
in nematodes are in
progress in various laboratories (S. Bordenstein, personal communication). We must, however, keep
in mind that the
Wolbachia
, and screenings for
Wolbachia
we presently know in nematodes is restricted to the well-deÝned and
quite restricted taxonomic group of the Ýlariae. We may hypothesize that
Wolbachia
infection was
(or is?) a more widespread characteristic in nematodes, retained by Ýlarial nematodes. But we do
Wolbachia
