Faunistique et biologie des Myxosporidies (Myxozoa : Myxosporea) parasites des poissons du lac de retenue de Maga (Extrême-Nord, Cameroun)

Abstract

Myxosporidia represent a great potential danger for fish host populations and fish farming because the diseases they cause are likely to lead to epizootics. In order to improve fish production, fish farmers need to control not only technical problems, but also pathological ones. It is therefore important to know which parasites are potentially harmful to the species being farmed, their pathological effects and their dynamism in the host fish. Thus, we undertook a faunistic study of the Myxosporidia parasites of the fish from the Maga Lake in Cameroon, as well as some biological aspects of the parasite species found in Labeo senegalensis which is widely consumed by the local populations. Between March 2015 and August 2019, six hundred and seventy-four (674) fishes belonging to four (4) orders, ten (10) families, seventeen (17) genera and twenty-one (21) species were captured and examined. The methodology followed was a set of classical methods concerning fishing, autopsy of potential host fish, search, mounting, light microscopic study of parasites and histological sections of heavily infested organs. The description of the species recorded was mainly based on the morphometric characteristics of the spores. A total of 36 species of Myxosporidia were recorded; these species belong to the genera Myxobolus Bütschli, 1882 (24 species), Myxidium Bütschli, 1882 (04 species), Henneguya Thélohan, 1892 (04 species) and Thelohallenus Kudo, 1933 (04 species). Among the parasitic species recorded, ten (10) were new to Science; these were: Myxidium tetraodoni sp. nov., Myxidium anisocapsularis sp. nov., Myxobolus dzeufieti sp. nov., Myxobolus magai sp. nov., Myxobolus kodjii sp. nov., Myxobolus hemibranchialis sp. nov., Myxobolus sp., Henneguya distichodii sp. nov., Henneguya magaense sp. nov. and Thelohanellus sp.. Twenty- six (26) species of Myxosporidia previously described in Cameroon or elsewhere were also found in the fishes examined. With 66.6% of the species collected (24 out of 36 species), the genus Myxobolus was the most widely represented. In the host examined, the Myxosporidia species of the genus Myxidium were essentially coelozoic while those belonging to the genera Myxobolus, Henneguya and Thelohanellus are all histozoic. Twenty-three (23) of the thirty-two (32) histozoic species (71.8%) develop on the gills, sometimes with heavy infesdtation, as in cas of Henneguya magai n. sp. which can develop up to 5 000 plasmodia on a single holobranch of Auchenoglanis occidentalis. In host fish, Myxobolus brachysporus and Myxobolus fomenai are muscles parasites; Myxobolus kainjiae develops in the ovaries, destroying mature oocytes and Myxobolus kodjii infestes the eyes. The study of the structure and population dynamics of fourteen (14) species of Myxosporidia was conducted from October 2018 to August 2019 in L. senegalensis. Among the parasitic species of this host fish, M. imami, M. nchoutnounensis, M. terengganuensis, T. bicornei and T. sanagaensis were appeared frequent (prevalence > 50%), M. bilongi, M. kodjii, M. hemibranchialis, M. kouoptamoensis, M. nanokiensis, M. njoyai, M. nokoueensis, T. assambai and Thelohanellus sp. were appeared intermediate (10% ≤ prevalence ≤ 50%) while Myxobolus nyongana and Myxidium nyongensis were appeared rare (prevalence < 10%). The average cystic load is high for M. imami (140.5 cysts), M. nanokiensis (124.1) and T. assambai (102.3), and very low for M. bilongi (6.3 cysts), M. njoyai (7.5), M. nyongana (4.9) and Thelohanellus sp. (5.7). Host sex had no effect on the parasitism in L. senegalensis. Of the parasitic species studied, Myxobolus kouoptamoensis and M. nanokiensis had a higher infestation rate in juvenile specimens (34.3% and 33.3% respectively), whereas infestation rates of M. imami, M. nokoueensis and M. nchoutnounensis were higher in larger specimens (80.0%, 82.2% and 20.0% respectively). Infestation rates and mean cystic load were higher during the rainy season with M. imami (respectively 60.9% and 144.2 cyts on average) and M. nanokiensis (respectively 26.4% and 196.7 cysts on average) while the dry season seems to be more favourable for the development of M. bilongi (32.4% vs 13.8%), M. hemibranchialis (19.0% vs 5.8%), M. kodjii (27.5% vs 9.2%), and T. bicornei (68.3% vs 48.3%). In L. senegalensis, a higher mean cystic load of M. imami (91.6 cysts), M. nchoutnounensis (16.0), T. assambai (90.9) and T. sanagaensis (7.8) was registred in the middle branchial sector. In the basalo-distal direction of the gills, mean cystic load of M. imami was higher in the distal 2/3 zone (87.0 cysts), while to T. assambai it was higher and statisticaly uniform in the basal and medial zones (55.8 cysts and 59.6 cyts respectively). Myxobolus kouoptamoensis and M. nanokiensis infested more the distal zone (24.3% and 17.0% respectively); while M. njoyai developed more in the basal zone with a prevalence of 18.4%. At the level of fins of L. senegalensis, M. nchoutnounensis was implanted more in the interlepidotrichal dermis of the basal zone with a rate varying between 3.6% on anal fin and 12.7% on caudal fin; T. sanagaensis infested more the basal and medial zones (respectively 5.1% and 7.6% on anal fin and respectively 38.6% and 33.0% on caudal fin), while T. bicornei developed more in the intrasegmental region of the medial zone with a prevalence varying varying between 6.1% on pectoral fin and 30.5% on caudal fin. This work has revealed an abundant and diverse fauna of Myxosporidia. The species that develop on the gills could disrupt, in the case of heavy infesdtation, respiratory functions in the host fish. Myxobolus brachysporus and Myxobolus fomenai could deteriorate flesh quality; Myxobolus kainjiae could induce a decrease in fertility and Myxobolus kodjii could affect eyesight in the host.