Density-dependent mechanisms in the regulation of intestinal helminth populations

Abstract

The regulation of helminth populations tends to occur primarily as a result of limitations imposed on the build up of parasite subpopulations within individual hosts (Anderson & May, 1979; May & Anderson, 1979). Considering the relevance of these factors to the success or otherwise of intestinal helminth control programmes, it is perhaps surprising that more information is not yet available concerning the particular mechanisms which may be responsible, and in particular, the population consequences of the immune responses which such parasites may precipitate. Density-dependence in a single rate parameter, if operative over the naturally observed numerical range, is sufficient to regulate parasite population flow throughout the life-cycle, whether direct or indirect (Anderson, 1976). For the genera given in Table 1, this could be provided by the observed pattern of parasite mortality and/or fecundity. It is of interest to note, however, that circumstantial evidence cited in the Table suggests that each of the 6 genera is also potentially able to induce host mortality under certain conditions. Whether this acts in a density-dependent manner in natural infections is almost entirely unknown. Rapid reproduction may be of great selective advantage to intestinal helminths, even if it is necessarily accompanied by pathogenicity (see Anderson, 1981). If the manner in which this pathogenicity acts in any way enhances the stability of the host-parasite interaction, then perhaps it may have contributed to the selection pressures which have led so many genera to continue to break the rules of the ‘well-adapted’ parasite (see, for example, Noble & Noble, 1971).

Experimental population dynamics of Rhabdias bufonis (Nematoda) in toads (Bufo bufo): density-dependence in the primary infection

“…Also Plasota (1969) showed that juvenile frogs became infected with R. bufonis shortly after metamorphosis and that worm numbers remain relatively constant up to, and throughout, adulthood. These data provide preliminary indications of temporal stability in worm numbers such as that suggested by Keymer (1982) which characterize many helminth-host interactions. Thus, in this system, it is possible that the observed density-dependent constraints which have been shown to occur under experimental conditions may also constrain, and potentially regulate, populations of R. bufonis in the field.…”

“…Density-dependent constraints on parasite survival, growth and fecundity are thought to be a result of either intraspecific competition for host resources or interactions with the host's immune response (Anderson & May, 1978;Keymer, 1982). The present data cannot distinguish between these two possibilities.…”

“…Experiments on host-parasite systems involving laboratory rodents and their helminth parasites have provided substantial evidence for density-dependent effects on parasite survival, growth and especially fecundity (Anderson, 1982;Keymer, 1982;Keymer & Slater, 1987). Such density-dependent effects play a central role in models of parasite population dynamics (Crofton, 1971;Anderson & May, 1978) and are proposed as key factors in generating stability in natural helminth parasite populations.…”

“…One of the central conclusions of these experimental systems and models is that helminth abundance is regulated through density-dependent feedback mechanisms which reduce the fitness of individual parasites within hosts as worm density increases. However, the relevance of theoretical studies and of experimental studies involving laboratory-reared hosts and parasites to density-dependent regulation of natural parasite populations is still a matter of controversy (Kennedy, 1975(Kennedy, , 1987Keymer, 1982). The controversy stems largely from a lack of supporting data from natural systems.…”

Aspectos quantitativos e morfométricos das infrapopulações de platynosomum illiciens (trematoda: dicrocoeliidae) em paroaria dominicana (passeriformes: emberizidae) do estado da Bahia, Brasil

“…Outros autores afirmaram que diferenças na capacidade reprodutiva entre os parasitos e a heterogeneidade existente entre hospedeiros podem gerar enganos em relação aos mecanismos dependentes da densidade (KEYMER;SLATER, 1987;POULIN, 1995;1996). No caso da reprodução, a densidadedependente pode ocorrer através dos efeitos diretos sobre a produção per capta de ovos ou, indiretamente, através da diminuição do crescimento, adiando a maturação e/ou reduzindo o perí-odo de vida do parasito (SHOSTAK; SCOTT, 1993).…”