The rise and fall of an epizootic of the diphyllobothriidean cestode Schistocephalus pungitii infecting the ninespine stickleback.

Epizootics of diphyllobothriidean cestodes appear to be simple, but deceptive similarity conceals the myriad ways in which these events are shaped by complex abiotic and biotic interactions. In Dog Bone Lake, Alaska, an epizootic of Schistocephalus pungitii infecting the ninespine stickleback (Pungitius pungitius) was short-lived. Its duration, with a peak that lasted only 1 yr, was shorter than for previously documented epizootics in Schistocephalus solidus. The ability of the ninespine stickleback to sustain infections, which appears to be related to species-specific characteristics of the host, may have played an important role in shaping the epizootic. Moreover, the epizootic of S. pungitii was not coincident with those observed for S. solidus in earlier studies within this region, supporting the hypothesis that processes involved in epizootics largely reflect local (lake-level) influences on population dynamics of the parasite. The outbreak occurred at a time when the host population was not relatively dense, which is inconsistent with epidemiological theory and may be a consequence of the parasite9s indirect life cycle. The variability of the unregulated and unstable epizootic events of diphyllobothriidean cestodes presents a challenge to understand the ecological and evolutionary factors influencing the prevalence of infections in host populations. Epizootics of diphyllobothriidean cestodes are deceptively simple. The number of infections temporarily rises markedly above anticipated levels, based on the recent history of infection, and then declines. Individual outbreaks, however, are shaped by complex biotic and abiotic interactions that are concealed by this deceptive similitude. For example, Heins et al. (2011) and Heins and Baker (2011) found that the extent of an epizootic of Schistocephalus solidus in Scout Lake, Alaska was limited by the deaths of juvenile host fish that did not survive to become adults, which are the major source of transmission of adult worms to the definitive host. The dynamics of an epizootic may, therefore, play out differently among age classes of intermediate host fish. Kennedy et al. (2001) found that epizootics of Ligula intestinalis coincide with rapid growth in the population of roach (Rutilus rutilus), its intermediate host fish. Epizootics are unregulated and, therefore, unstable and variable (Kennedy et al., 2001; Heins, Birden et al., 2010). Mega-epizootics result from a rare combination of circumstances, whereas less extreme and more gradual epizootics should be more common and play out in very many ways (Heins, Birden et al., 2010; Heins et al., 2011). Epizootic cycles appear to include mega-epizootics interspersed among typical ones. The decline of a mega-epizootic in Walby Lake, Alaska was followed by a less extreme event (Heins, Birden et al., 2010; Heins et al., 2011). In Slapton Ley (Devon, England), the second of 3 epizootics investigated by Kennedy et al. (2001) appears to have been a mega-epizootic. In the present study, we investigated the rise and fall of an epizootic in Schistocephalus pungitii infecting ninespine stickle- back (Pungitius pungitius) in Dog Bone Lake, Alaska. The life cycle of S. pungitii, like that of S. solidus (Smyth, 1962), involves transmission through predation of a free-living coracidium by zooplankton, a procercoid in a cyclopoid copepod, and a plerocercoid in a ninespine stickleback (P. pungitius), leading to an adult tapeworm in a piscivorous bird in which the parasite reaches sexual maturity and reproduces. Almost all of the parasite's growth occurs at the plerocercoid stage, the ninespine stickleback being the resource base for the growth of the parasite. As a result, S. pungitii imposes a substantial energy drain on the host fish because individual plerocercoids may attain relatively large sizes (D. Heins, pers. obs.), multiple infections can be common, and the total mass of all parasites in a single host fish can be large relative to the host mass (Heins et al., 2004). The epizootic event in S. pungitii is compared to those of S. solidus, including differences in the ability of the host species to sustain infections, which should explain the nature of the epizootic event. Other factors that might have influenced the epizootic are also considered.