MALARIA OF ORANG-UTAN (PONGO-PYGMAEUS) IN BORNEO

The life cycle and ecology of Plasmodium pitheci, a malaria parasite of the orang-utan were studied. The arboreal habitat of the orang-utan in the dipterocarp forests of eastern Sabah is described. Numerous species of anopheline mosquitoes are found in E Sabah. Anopheles balabacensis, which is attracted to orang-utans and to man, was caught at Sepilok. This species, the main vector of human malaria in N Borneo, may also transmit orang-utan malaria in this part of Sabah. Blood examinations on orang-utans since 1966 indicated a high prevalence of infection with P. pitheci. In 1966 10 of 19 animals had demonstrable parasitemia. Detailed case histories are presented to show the course of parasitemia in several orang-utans. Infections of P. pitheci ran a very chronic course. In 1972 a second, previously undescribed malaria parasite of the orang-utan was discovered, and was named P. silvaticum. The new parasite was successfully transmitted by blood inoculation and by sporozoite inoculation into splenectomized chimpanzees. Although both species of malaria parasite may cause transitory signs of illness, orang-utans generally appear to be little discomforted by the infection. Detailed descriptions and illustrations of P. pitheci and P. silvaticum are given. All stages of the life cycle of P. silvaticum are known (the tissue stages were described in the liver of a surrogate host, the chimpanzee). Only the blood and sporogonic stages of P. pitheci have been seen. This species was not infective to a chimpanzee. In the blood both parasites showed a tertian periodicity. From the appearance of the tissue schizonts on the 7th day, the complete pre-erythrocytic cycle of P. silvaticum in the chimpanzee lasts about 8 days. P. pitheci is readily distinguished from P. silvaticum, and most closely resembles P. hylobati and P. youngi of the gibbon. The sporogony of both orang-utan parasites in anopheline mosquitoes is described. Although P. pitheci produced a transient parasitemia in splenectomized gibbon following blood inoculation, P. silvaticum did not do so, and neither parasite was infective to Aotus or Macaca. P. pitheci was also noninfective to man. P. silvaticum clearly belongs to the P. vivax- P cynomolgi group and resembles P. eylesi of the gibbon. Several morphological and other features distinguish it from P. vivax with which a close comparison was made. A new plan of the evolution of the primate malarias in relation to their hosts is proposed. This agrees with other evidence linking the orang-utan with the gibbons, rather tahn with the other anthropoid apes, Pan, Gorilla and Homo. In considering the epizootiology of orang-utan malaria attention is drawn to the high prevalence of infection in relation to the relatively solitary habits and low population density of these apes. The hypotheses that P. silvaticum may produce zoonotic infections in man or, alternatively, that orange-utans may contract P. vivax malaria are considered, and rejected.