Plasmodium ovale has developmental cycles in the human host and in the vector mosquito. Following introduction of sporozoites via the bite of infected mosquitoes, these forms rapidly invade the liver, where, within a single parenchymal cell, the parasite matures in approximately 9 days. Eventually, many hundreds of merozoites are produced. Upon release, these merozoites invade reticulocytes and initiate the erythrocytic cycle. The development of some of the parasites in the liver cells is delayed or suspended as hypnozoites, occasionally for many months. Following a developmental cycle in the erythrocyte that lasts, on average, 49 h, from 8 to 20 merozoites are released to reinvade other erythrocytes. As with other species of Plasmodium that infect humans, some of the merozoites that invade erythrocytes develop into two forms of gametocytes. The developmental time to maturity of gametocytes is the same as that of the asexual stage, approximately 49 h.
During feeding, mosquitoes take up both microgametocytes and macrogametocytes. Within the gut of the mosquito, exflagellation of the microgametocyte occurs, resulting in the formation of up to eight microgametes. Following fertilization of the macrogamete, a mobile ookinete is formed that penetrates the peritropic membrane surrounding the blood meal and travels to the outer wall of the midgut of the mosquito. There, under the basal membrane, the oocyst develops. After a period of several weeks, depending on the temperature, hundreds of sporozoites are produced within each oocyst. The oocyst ruptures, and sporozoites are released into the hemocoel of the mosquito. Circulation carries the sporozoites to the salivary glands, which the sporozoites invade and where they become concentrated in the acinal cells. During feeding, sporozoites are introduced into the salivary duct and are injected into the venules of the bitten human, initiating the cycle again.
Humans are the only natural hosts for P. ovale
. Much of what is known about this parasite was obtained during malaria therapy of naïve patients over 60 years ago. The prepatent period is the interval between sporozoite inoculation and the first detection of parasites in the peripheral blood. Sinton et al. (88
) reported a mean prepatent period of about 15 days, whereas James et al. (52
), working with six different strains of the parasite, reported a mean of 13.6 days. The Donaldson strain exhibited prepatent periods of 12 to 20 days, with a mean of 15.3 days; for the Liberian strain, prepatent periods of 13.5 to 15 days have been reported (37
). A retrospective examination of induced infections with P. ovale
was made by Collins and Jeffery (23
). These data were extracted from the records of patients that were given malaria therapy for the treatment of neurosyphilis between 1940 and 1963.
Prior to the introduction of penicillin for the treatment of syphilis, malaria was one of the most effective treatments for the disease (96). The range in prepatent periods following sporozoite injection was 14 to 20 days. A listing of prepatent periods (Table 1) for 30 patients infected via sporozoites with the Donaldson and Liberian strains indicated prepatent periods of 12 to 20 days, with a median of 14.5 days.
Fever. James et al. (52) reported that 15% of patients had 10 or more febrile paroxysms. With the Donaldson strain, only 10% of patients had over 10 paroxysms with peak temperatures exceeding 103°F (59). Mean maximum fever was 105.2°F. The median interval between peaks in the fever indicated that the periodicity (time for each developmental cycle) was approximately 49 h. A retrospective examination of records from induced infections (23) indicated that 47.1% of the fever episodes were 104°F. Patients reinfected with P. ovale rarely had fevers 104°F. An examination of fever episodes for 30 patients infected via sporozoites (Table 1) and 60 patients infected by the inoculation of parasitized erythrocytes (Table 2) indicated maximum fevers ranging from 102.0o to 107.0°F and 103.8o to 107.8°F, respectively. Mean maximum fevers were 103.3o and 105.4°F, respectively. For all patients, there were an average of 10.3 fever episodes of 101 and 4.5 fever episodes of 104°F.
Parasitemia. Maximum parasite counts are usually low compared to those of patients infected with P. falciparum or P. vivax (19), no doubt reflecting the restriction of P. ovale to development in younger erythrocytes. An examination of records from 90 patients (Tables 1 and 2) indicated maximum parasite levels ranging from 380 to 27,600/µl. The geometric mean maximum parasite level was 6,944/µl for sporozoite-induced infections and 7,310/µl for trophozoite-induced infections; median maximum parasite levels were 7,312 and 9,532/µl, respectively. Higher density parasitemia (1,000/µl) averaged 10.2 days and 12.4 days, respectively. The mean parasitemia curves for 30 sporozoite-induced and 60 trophozoite-induced infections (Fig. 1) indicated maximum parasite levels of 3,597/µl on day 9 for sporozoite-induced and 5,066/µl on day 10 for trophozoite-induced infections.
Previous infection with P. ovale did not prevent reinfection but resulted in reduced levels of parasitemia and fever. Previous infection with P. vivax (Table 3), P. falciparum, and P. malariae (Table 4) did not prevent infection; there was some reduction in the frequency and intensity of fever and parasite counts. Glynn and Bradley (42) reviewed archival records on 80 induced infections with P. ovale in nonimmune patients as regards inoculum size and severity of the resulting malaria. Patients with shorter prepatent periods had higher and more peaks of fever and longer-lasting infections.
The Duffy blood group does not appear to be a controlling factor for infections with P. ovale as it does with P. vivax. There appears to be no difference in susceptibility to infection between Caucasians and African-Americans (58, 59).
Relapse. Plasmodium ovale is a relapsing infection in that secondary infections can be generated from latent parasites in the liver. These are often asymptomatic infections that are detected only by the continued examination of peripheral blood films. Relapses occurred as early as 17 days after treatment of the primary attack to as late as 255 days (16). Delayed primary attacks occur when the primary attack has been eliminated, usually with antimalarial drugs. Such infections have been reported after 4 years (94) and 1.3 years (17). A relapse of P. ovale after 45 months of incubation has been reported (65). However, Shute and Maryon (87) reported that of 200 cases of P. ovale experimentally induced by mosquito bite, only one patient had a detectable relapse of the infection.
Exoerythrocytic stages. The only demonstration of exoerythrocytic stages of P. ovale in the liver of a human was that of Garnham et al. (37, 38). A volunteer was fed upon by Anopheles maculipennis atroparvus mosquitoes infected with a Liberian strain of P. ovale. Infected mosquitoes were allowed to feed on the patient on three different days, 5, 6, and 9 days before a liver biopsy was performed. Exoerythrocytic bodies at different stages of development were demonstrated in liver tissue; parasites were observed in the blood of the volunteer 10 days after initial feeding. Only 17 schizonts were observed in the examination of over 4,000 serial sections. The size of the schizont was taken to indicate the age of the developing parasite. Eight schizonts, presumed to be 5-day forms, ranged in length from 28 µm to 60 µm. Nuclei were large, approximately 2 µm in diameter. Nine-day tissue stages measured from 70 to 80 µm by 50 µm. The nuclei of the exoerythrocytic stages had an uneven margin and the cytoplasm was granular. The cytoplasm was sometimes clumped around each nucleus so that it appeared to contain clefts. The merozoite of the schizont was large, spherical, and consisted of two portions, a larger portion of cytoplasm and a smaller portion being the nucleus.
Subsequently, exoerythrocytic stages were demonstrated in the liver tissue of chimpanzees following inoculation of sporozoites from Anopheles gambiae mosquitoes (11, 12). Seven-day exoerythrocytic stages in the liver measured an average of 36.6 by 30.3 µm. Three characteristics that have not been shown in the tissue stages of P. vivax or P. falciparum were a definite limiting membrane or periplast; peripheral nuclear bars tangential rather than radial; and a minor but distinct hypertrophy of the host cell nucleus. In a subsequent study, biopsy on the 19th day revealed bursting and mature schizonts suggesting the existence of a delayed generation (11). Exoerythrocytic bodies were also demonstrated in hepatic tissue of Saimiri monkeys (Fig. 2), 7 days following injection of sporozoites dissected from Anopheles dirus mosquitoes (75).
Sporozoites of P. ovale from Anopheles stephensi, Anopheles gambiae, and Anopheles dirus were introduced into primary cultures of human hepatocytes, rat hepatocytes, and cultures of a human hepatoma clone, Hep 5 A-1 (69). Maturation only occurred in primary cultured human hepatocytes. Parasites developed to 60 µm in length by day 8. The exoerythocytic stages of P. ovale were subsequently grown in primary cultures of hepatocytes from Saimiri sciureus boliviensis monkeys following introduction of sporozoites dissected from A. dirus mosquitoes (75). The morphology and size of the liver stages were similar to those previously described from humans and chimpanzees. By day 7, parasites contained over 100 nuclei; by day 9, parasites had a mean diameter of 68 µm and contained mature merozoites.
and A. funestus
are the likely natural vectors, based on enzyme-linked immunosorbent assay-based detection of infected mosquitoes (9
); Bray demonstrated their infection while working with chimpanzees in the Gambia (11
). Experimentally, A. atroparvus
was shown to be an effective mosquito host and capable of transmitting the infection to humans (38
). Other proven experimental hosts are A. albimanus
), A. quadrimaculatus
), A. freeborni
), A. maculatus
), and A. subpictus
); A. stephensi
and A. balabacensis balabacensis
(= A. dirus
) have also been shown to be experimentally infected (20
). In studies with the Donaldson strain of P. ovale
, A. quadrimaculatus
was the most susceptible to infection, followed by A. albimanus
from the Florida Keys and A. albimanus
from Panama (53
). In comparative studies with the West African strain, A. stephensi
was the most susceptible, followed by A. freeborni
, A. dirus
, A. quadrimaculatus
, A. maculatus
, and A. albimanus
). Anopheles farauti
has also been experimentally infected with P. ovale
The comparative rate of oocyst development of P. ovale in five species of anopheline mosquitoes (Anopheles balabacensis [= A. dirus], A. maculatus, A. freeborni, A. quadrimaculatus and A. stephensi) was determined (24). When held at 25°C, sporozoites were present in the salivary glands after 13 to 14 days. The mean diameter measurements of oocysts indicated that P. ovale was smaller than P. vivax and P. schwetzi (a parasite of chimpanzees and gorillas). A line of A. gambiae refractory for infection with P. cynomolgi was fed through a membrane on heparinized blood from a chimpanzee infected with P. ovale (21). There was 66% encapsulation of oocysts in the refractory line versus none in the susceptible line.
The development of monoclonal antibodies to detect mosquitoes infected with P. ovale has allowed a number of longitudinal entomological studies to determine the presence and biology of vectors of this parasite. Konate et al. (61) conducted a longitudinal survey in Senegal of Anopheles gambiae sensu lato and A. funestus in an area of Sudan-type savanna. Sporozoite typing indicated that 8.2% of the infected salivary glands were infected with P. ovale. This was calculated to represent eight infective bites per human the first year of observation and 25 infective bites the second year. In another report on the same study (95) it was estimated that the inoculation rate for P. ovale was 0.04 infective bites per person per night.