Accelerated deactivation of Myxobolus cerebralis myxospores by susceptible and non-susceptible Tubifex tubifex

In the 1990s, the Tubifex tubifex aquatic oligochaete species complex was parsed into 6 separate lineages differing in susceptibility to Myxobolus cerebralis, the myxozoan parasite that can cause whirling disease (WD). Lineage III T. tubifex oligochaetes are highly susceptible to M. cerebralis infection. Lineage I, IV, V and VI oligochaetes are highly resistant or refractory to infection and may function as biological filters by deactivating M. cerebralis myxospores. We designed a 2-phased laboratory experiment using triactinomyxon (TAM) production as the response variable to test that hypothesis. A separate study conducted concurrently demonstrated that M. cerebralis myxospores held in sand and water at temperatures ≤15°C degrade rapidly, becoming almost completely non-viable after 180 d. Those results provided the baseline to assess deactivation of M. cerebralis myxospores by replicates of mixed lineage (I, III, V and VI) and refractory lineage (V and VI) oligochaetes. TAM production was zero among 7 of 8 Lineage V and Lineage VI T. tubifex oligochaete groups exposed to 12500 M. cerebralis myxospores for 15, 45, 90 and 135 d. Among 4 mixed lineage exposure groups, TAM production averaged 14641 compared with 2202495 among 12 groups of Lineage III oligochaetes. Among the 6 unexposed Lineage III experimental groups seeded into original Phase 1 substrates for the 45, 90 and 135 d treatments during the Phase 2 portion of the study, TAM production was reduced by 98.9, 99.9 and 99.9%, respectively, compared with the average for the 15 d exposure groups. These results are congruent with the hypothesis that Lineage V and Lineage VI T. tubifex oligochaetes can deactivate and destroy M. cerebralis myxospores.