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.