Prevalence of imported Plasmodium ovale curtisi and Plasmodium ovale wallikeri malaria in Parma (Italy)

Objectives. P. ovale curtisi (Poc) and P. ovale wallikeri (Pow) are two closely related but distinct species of human malaria parasites. Herein we report imported malaria cases caused by these two species, as detected by a newly designed Real-time PCR assay (Poc-Pow Real-time PCR), among the overall 259 cases diagnosed in our laboratory. Methods. From 2000 to October 2012, blood samples of 1,069 patients with the suspicion of malaria were subjected to microscopy and to different nested- and Real-time PCR assays targeting the parasite’s small subunit ribosomal RNA gene, alternatively used during the study period. Results. 259 cases of malaria were diagnosed: 206 P. falciparum, 12 P. vivax, 29 P. ovale, 3 P. malariae and 9 mixed infections, including 2 Poc and 1 Pow. In particular, by Poc-Pow Real-time PCR it was found that among the overall 32 P. ovale infections 20 were due to Poc and 12 to Pow. Conclusion. Our results confirm, as already reported by us in 2007, that P. ovale wallikeri is not confined to Southeast Asia, since the majority of the patients analyzed in this study had acquired malaria in African countries and that the two P. ovale species are sympatric in the countries where they occur. It is interesting that in this study 9 of the 32 patients presented to the hospital between 3 months and 2 years after their arrival to Italy. Thus, it is likely that the samples from these patients contained parasites from relapses, i.e. blood stage infections originating from the activation of a subset of the hypnozoites in the liver, indicating that relapses seem to occur in both species. The ability to detect and distinguish the 2 P. ovale species using the Poc-Pow Real-time PCR opens the way to epidemiological investigations of these parasites. As a matter of fact, P. ovale is one of the least studied of the Plasmodium species that infect humans. In the context of the goal of malaria control and eventual eradication, it becomes important to investigate P. ovale, given that its prevalence is likely to be substantially higher than previously thought and that it can maintain itself in the human host for long periods as a result of its capacity to produce hypnozoites. Any meaningful investigations of the true epidemiology and biology of the 2 P. ovale species, whose infections lead to only relatively scanty parasitaemias even in primary infections, will necessitate the application of sensitive and specific molecular methods of detection.