Parasitism and evolution: opposing versus balancing strategies

Abstract

Parasitism refers to a particular symbiosis (deBary 1879) of organisms that live at the cost of their hosts. Virulent microbes (viruses and bacteria) proliferate unlimited causing toxic infections that end either by the host's death or its protective immunity. By this opposing survival strategy, via the population density, i.e. the contact rate, finally results a sequence of epidemic and endemic periods. The host population is kept at a reasonable level to resources, but host and pathogenic agent are wasted in the evolutionary arms race. Sparing such losses, eukaryotic proto-and metazoan parasites induce in natural hosts a delayed infection course by self-controlled propagation and avoid protective immunity. Hosts are 'immunized but not immune' and parasites propagate simultaneously with them. Metazoic and some protozoic parasitoses end with the parasite's natural death, pathogenic effects remain irrelevant. Nevertheless, the host's fitness becomes reduced by multiple re-infections and/or by increasing parasite loads. This balancing survival strategy controls host populations at lower costs. A network of parasitoses offers considerable selection advantages and explains the polyphyletic origin of eukaryotic parasites. Parasitism and other kinds of symbioses contribute to the stability of ecosystems and to stasis in long-term evolution but are invisible in the fossil record.

Prevalence, genetic identity and vertical transmission of Babesia microti in three naturally infected species of vole, Microtus spp. (Cricetidae)

“…In fact, in contrast to our expectations, the survival rate over three weeks after birth was lower among Babesia -negative pups. These findings support the ‘balancing strategy hypothesis’ [ 49 ]. The balancing strategy hypothesis proposes that long-term co-evolution of parasite-host interactions results in a ‘balanced’ system, with a low negative impact of parasites on the host population, low pathogenicity and mortality enabling simultaneous propagation of both parasite and host without epidemic periods that are characteristic in many viruses and bacteria systems which follow an ‘opposing strategy’.…”

“…The very low parasitaemia found in the pups with congenitally acquired B. microti infection (1–5 iRBC/200 fields of vision) in comparison to wild-caught voles (mean19.99 ± 16.91 iRBC/200 fields) supports this hypothesis, together with the known long-term survival of B. microti infection in rodent hosts under natural and experimental conditions [ 15 , 35 ]. Thus Babesia may be considered to be a master of a balancing strategy, together with Plasmodium falciparum , given as the example by Wenk & Renz [ 49 ].…”

High mortality associated with tapeworm parasitism in geladas (Theropithecus gelada) in the Simien Mountains National Park, Ethiopia

“…This dearth can largely be attributed to the scarcity of detailed, long‐term field data on primate life history and parasitism that would permit the detection of fitness costs (Kappeler & Watts, ). Additionally, the long evolutionary relationships between many parasites and hosts may result in selection against extreme fitness costs, both because parasites that rely on host survival and reproduction are under selection to temper their costs (Ewald, ) and because hosts are under selection to mitigate such costs (Boots, Best, Miller, & White, ; Roy & Kirchner, ; Wenk & Renz, ).…”