Haemobartonellosis in squirrel monkeys (Saimiri sciureus): Antagonism between Haemobartonella sp. and experimental Plasmodium falciparum malaria

A hemotropic parasite of the genus Haemobartonella (rickettsial parasite of the Family Anaplasmataceae) is responsible for latent asymptomatic infecti on in colony-born Saimiri monkeys, Indeed, many of these animals develop a patent Haemobartonella infection following splenectomy. Such patent parasit ism is characterized by an intense Haemobartonella parasitemia which peaks between days 12 and 14 after removal of the spleen and then decreases to be come undetectable between days 25 and 30. During the resolving phase of par asitemia, a moderate anemia associated with monocytosis and erythrophagocyt osis is observed. In certain Saimiri monkeys, Haemobartonella parasitemia r emains latent following removal of the spleen, This indicates that the sple en plays a role but is not necessary to maintain latent Haemobartonella par asitism. It also suggests the existence of heterogeneity in the host immune reactivity to the parasite. Latent or patent haemobartonellosis might rais e a problem when Saimiri monkeys are used as experimental hosts of Plasmodi um falciparum asexual blood stages, as already noticed with "rodent malaria ." Thus we investigated the relationship between Haemobartonella and P. fal ciparum in splenectomized monkeys. When animals harboring latent Haemobarto nella sp. were infected with P. falciparum, the former remained latent and exerted no influence on the course of the P. falciparum parasitemia. In con trast, when P. falciparum was initiated in animals which were in the proces s of developing patent haemobartonellosis, the course of the former was pro tracted and either the animal resisted longer, or it self-cleared the P. fa lciparum infection. Conversely, patent haemobartonellosis was delayed when splenectomy was performed at different times after initiation of P. falcipa rum infection in intact monkeys. Our results do not allow us to draw conclu sions as to the mechanism(s) of the antagonism between the two parasites, b ut they emphasize the need to monitor the presence of Haemobartonella when splenectomized Saimiri monkeys are used as experimentals hosts for P. falci parum parasitism. (C) 1999 Academic Press.