What – if anything – is a strain?

Abstract

Two worldwide variants of the Plasmodium vivax circumsporozoite protein (CSP), VK210 and VK247, induce distinct antibody responses and can co-infect individual humans and Anopheles. The prevalence of VK210 and VK247 can change on a seasonal or a year-to-year basis. Different sympatric species of Anopheles might be more susceptible to each variant. VK210 could be associated with higher parasitaemia and chloroquine resistance. This is a relatively obscure line of research, but it could provide crucial insights into a question that has long beset parasitologists: what is a strain? Or, as Shute asked, albeit rhetorically in 1958, ‘is not the word strain in regard to malaria used much too loosely?’The latest reports from Mexico 1xPlasmodium vivax: ookinete destruction and oocyst development arrest are responsible for Anopheles albimanus resistance to circumsporozoite phenotype VK247 parasites. Gonzalez-Ceron, L. et al. Exp. Parasitol. 2001; 98: 152–161Crossref | PubMed | Scopus (20)See all References1 and Colombia 2xVariants of the Plasmodium vivax circumsporozoite protein (VK210 and VK247) in Columbian isolates. Gonzalez, J.M. et al. Mem. Inst. Oswaldo Cruz. 2001; 96: 709–712Crossref | PubMedSee all References2 present their results from experiments on local VK210 and VK247 parasites in Anopheles albimanus, which contrast in an intriguing manner. In the first instance, A. albimanus is susceptible to VK210 and near refractory to VK247 parasites (opposite to Anopheles pseudopunctipennis). VK247 survivorship can be tracked to the ookinete and early oocyst stages. In the second instance, VK210 and VK247 develop successfully in A. albimanus derived from Guatemala, but only VK247 parasites survive in A. albimanus from the Pacific Coast of Colombia (also applies to a recent report from Brazil). The VK247 produces more sporozoites than VK210. The Mexican group proposes that factors innate to the vector population mediate the local prevalence of VK210 and VK247. The Colombian group suggests that frequency-dependence in transmission-blocking antibodies or relative immunogenicity mediates prevalence. Both groups reveal a remarkable, perhaps exemplary, parasite–host–vector system.The VK210 and VK247 dichotomy includes phenotypes (entities distinguished by their apparent properties or interactions with their environment, rather than their genetic constitutions per se) at least for antibody response, which presumably comprise a collection of genotypes. These current papers suggest that there could be geographic elements to variation in the suite of other supposed traits, but probably not congruent with the recently proposed split of P. vivax into Old and New World subspecies (based on sequence data in addition to relative infectivity to A. albimanus). Thus, P. vivax now seems to be the surprise frontrunner for addressing the recurring question: is a strain a heuristic concept without a tangible biological basis, or a discrete biological entity that can be distinguished by a modern technique? Are VK210 and VK247 strains?