The Chesson strain is one of the classic parasites that has been studied for many years in humans and during the past several decades in monkeys and chimpanzees.1,4–15
During recent times, many isolates of this parasite have been stored frozen and subsequently passed through humans, monkeys, and chimpanzees. Whether it is now the same parasite as was studied in humans during the 1940s through 1960s is questionable. A molecular examination of preserved human material from that early era would indicate whether the present day parasites are similar to or the same as the one used in the early human studies. At the present time, we assume that we are working with Chesson stain of P. vivax
based only on its genealogy.
It was apparent from an examination of the data from feeding on infected Aotus monkeys that mosquito infection most often occurred before peaks in the asexual blood parasite count. Gametocytes were rarely observed on the blood films and could not be used as predictors of mosquito infection or the intensity of infection. In the monkeys, higher levels of infection occurred in the period preceding the peak in the asexual parasite count than occurred after the peak (). This finding contrasts with observations from archival data from human studies (), in which a higher percentage of mosquito infection occurred after the peak in the asexual blood parasite count.
In the human studies, the intensity of mosquito infection, (based on oocysts count) was at least 10-fold higher than that obtained by feeding on monkeys. In humans, gametocytes were often observed, although the number of gametocytes present was not always a predictor of the number of oocysts that would be found in the mosquitoes. This high level of infection had also been observed in previous studies with P. vivax
It is apparent that splenectomized Aotus
monkeys do not support the production of infective gametocytes at the same high density levels as do humans and splenectomized chimpanzees. In addition, it appears that in Aotus
monkeys, mosquito infection decreases once the peak in the asexual parasite count is reached, whereas in humans, mosquito infection appears to continue or increase beyond the peak in the asexual parasite count.
The Aotus monkeys, particularly splenectomized A. lemurinus griseimembra, mimic the human host for studies with the Chesson strain of P. vivax with regard to asexual parasitemia, susceptibility to infection by sporozoite inoculation, and recrudescences. However, these animals are apparently unable to produce infective gametocytes at levels approaching that of humans, which results in a much reduced level in the intensity of mosquito infection (oocysts per positive gut). In particular, microgametocytes are rarely seen during P. vivax infections in Aotus monkeys. Therefore, experimental feeding must be scheduled in relation to the anticipated peak in the asexual parasite count. The frequency of infection is reduced or aborted soon after the peak in the asexual count. This finding contrasts with archival data from human studies in which mosquito infection continues after the peak in the asexual parasite count.
This reduced intensity of mosquito infection and the limitation in periods of infectivity define the usefulness of the Aotus monkeys for studies with the Chesson strain where mosquito infection is a vital component. Currently, only A. nancymaae and A. vociferans are available in sufficient numbers for extensive studies. The higher median asexual parasite counts that were obtained in the A. vociferans and the higher level mosquito infection over that of A. nancymaae indicates that A. vociferans would be the preferred host for studies with this parasite. Of the laboratory mosquitoes tested for susceptibility to infection with Chesson strain, An. dirus was the most heavily infected. Thus, at the present time, studies with Chesson strain could best be conducted using A. vociferans monkeys and An. dirus as the vector mosquito.