2.3 The parasite
Ligula intestinalis (Linnaeus, 1758) is a cestode infecting a wide range of fish hosts particularly cyprinids (Kennedy, 1974). In infected fish like E. sardella it is found filling the body cavity. The parasite is trophically transmitted and has a complex life cycle involving two aquatic intermediate hosts, a zooplankton copepod and a fish (Dubinina, 1980). It reaches sexual maturity in the abdominal cavity of piscivorous birds that are the final hosts (i.e. the hosts where parasite reproduction takes place) (Dubinina, 1980).
The invasion of L. intestinalis in Lake Nyasa was first noted in the late 1990s during long-line research surveys where a milkish white worm was found in the body cavity of E. sardella (Mwambungu et al., 1996). The worm was identified to be the tapeworm Ligula intestinalis (L.). This parasite is believed to be introduced in Lake Nyasa by migrating infected fish-eating birds such as the White-breasted cormorant (Phalacrocorax carbo ), which is among the most abundant fish-eating birds in the Lake Nyasa basin (Linn and Campbell, 1992) and one of the final hosts of L. intestinalis (Loot et al., 2001; Rosen, 1920). Higher infection rates are observed in larger and olderE. sardella than in juvenile individuals (Msafiri et al., 2014; Rusuwa et al., 2014), which can be explained by diet shifts from phytoplankton to zooplankton as E. sardella reach maturity.