Smallmouth yellowfish, Labeobarbus aeneus (Teleostei: Cyprinidae), as a potential new definitive host of the invasive parasite Atractolytocestus huronensis (Cestoda: Caryophyllidea) from common carp: example of recent spillover in South Africa?

Atractolytocestus huronensis Anthony, 1958 has been co-introduced with its cyprinid host Cyprinus carpio Linnaeus, 1758, common carp, to several continents. This cestode was only recently (2012) detected in South Africa and occur in two major river systems. In Africa, A. huronensis has only been reported from C. carpio . During routine parasitological surveys in the Vaal River system in central South Africa, unidentified cestodes were recorded from common carp at several localities. Using light and scanning electron microscopy, alongside genetic characterisation, they were identified as A. huronensis , greatly expanding the distribution of this parasite in the upper reaches of the Vaal River system and indicating rapid spread in the system. Thereafter, in November 2020, more caryophyllidean cestodes were detected infecting native smallmouth yellowfish, Labeobarbus aeneus (Burchell, 1822) just below the Vaal Dam wall. They were also morphologically and genetically identified as A. huronensis , indicating a possible new definitive host and spillover from carp. However, only juvenile worms (up to late stage 4) were detected in L. aeneus , suggesting a paradefinitive or accidental infection. Their pathological effect on the intestine of L. aeneus mimicked that described in acute infections in common carp, with damage limited to the intestinal epithelium and no prominent ulcerations. This apparent mild infection of an indigenous host needs to be monitored. The spillover to L. aeneus appears to be recent as no caryophyllidean cestodes were collected from this host species at the same locality and season the previous year, nor at any of the other well studied sites in the Vaal River system. Chronic infection of L. aeneus may still develop and indicate that the near threatened largemouth yellowfish, Labeobarbus kimberleyensis (Gilchrist & Thompson, 1913), may be at risk as well.