Plasmodium malariae blood-stage dynamics.

We examine the dynamics of parasitemia, fever, and gametocytemia reflected in the preintervention charts of 180 malaria-naive U.S. neurosyphilis patients infected with the USPHS strain of Plasmodium malariae, for malariatherapy, focusing on the 84 charts for which more than 35 days of patency preceded intervention and daily records encompassed 92% or more of the duration of each infection. Inoculum size did not influence any outcome variable. Fevers (days with temperatures > or =101 F) followed patterns that fit recognized brood structures more often than did our approximations of merogony cycles (via local peaks in parasitemia), but neither closely fit textbook quartan patterns. There were no discernable patterns in gametocytemia. Successful transmission to mosquitoes increased following subcurative drug treatment but did not depend on detectable gametocytemia.     

Gametocytemia and fever in human malaria infections

We examine the charts of 408 malaria-naive neurosyphilis patients given malaria therapy at the South Carolina USPHS facility, with daily records encompassing at least 93% of the duration of infection, and focus on the 152 patients infected with the St. Elizabeth strain of Plasmodium vivax, 82 with the McLendon strain of Plasmodium filciparum, 36 with the USPHS strain of Plasmodium malariae, and 15 with the Donaldson strain of Plasmodium ovale in whom gametocytes appeared before drug, or other, intervention. In P. vivax infections, fever and parasitemia were higher after gametocytes were first detected than before; in P. malariae infections, parasitemia was higher. In P. ovale infections, fever and parasitemia were similar before and after. In P. falciparum infections, fever, parasitemia, and fever frequency were lower after gametocytes were first detected than before. Parasitemia and temperature correlated in P. vivax infections, before and after gametocytes were first detected; parasitemia and temperature at first fever were not correlated in infections with any species. Gametocyte density correlated with parasitemia in P. malariae and sporozoite-induced P. falciparum and P. vivax infections. Fevers and detected gametocytemia coincided more often than expected by chance with P. vivax and P. ovale; fever temperature and gametocyte density were not correlated in infections with any species.     

A sub-microscopic gametocyte reservoir can sustain malaria transmission

Background

Novel diagnostic tools, including PCR and high field gradient magnetic fractionation (HFGMF), have improved detection of asexual Plasmodium falciparum parasites and especially infectious gametocytes in human blood. These techniques indicate a significant number of people carry gametocyte densities that fall below the conventional threshold of detection achieved by standard light microscopy (LM).

Methodology/Principal Findings

To determine how low-level gametocytemia may affect transmission in present large-scale efforts for P. falciparum control in endemic areas, we developed a refinement of the classical Ross-Macdonald model of malaria transmission by introducing multiple infective compartments to model the potential impact of highly prevalent, low gametocytaemic reservoirs in the population. Models were calibrated using field-based data and several numerical experiments were conducted to assess the effect of high and low gametocytemia on P. falciparum transmission and control. Special consideration was given to the impact of long-lasting insecticide-treated bed nets (LLIN), presently considered the most efficient way to prevent transmission, and particularly LLIN coverage similar to goals targeted by the Roll Back Malaria and Global Fund malaria control campaigns.Our analyses indicate that models which include only moderate-to-high gametocytemia (detectable by LM) predict finite eradication times after LLIN introduction. Models that include a low gametocytemia reservoir (requiring PCR or HFGMF detection) predict much more stable, persistent transmission. Our modeled outcomes result in significantly different estimates for the level and duration of control needed to achieve malaria elimination if submicroscopic gametocytes are included.

Conclusions/Significance

It will be very important to complement current methods of surveillance with enhanced diagnostic techniques to detect asexual parasites and gametocytes to more accurately plan, monitor and guide malaria control programs aimed at eliminating malaria.     

Gametocytemia in Plasmodium vivax and Plasmodium falciparum infections

Two expert research microscopists, each blinded to the other's reports, diagnosed single-species malaria infections in 2,141 adults presenting at outpatient malaria clinics in Tak Province, Thailand, and Iquitos, Peru, in May-August 1998, May-July 1999, and May-June 2001. Plasmodium vivax patients with gametocytemia had higher fever and higher parasitemia than those without gametocytemia; temperature correlated with parasitemia in the patients with gametocytemia. Plasmodium falciparum patients with gametocytemia had lower fever than those without gametocytemia, but similar parasitemia; temperature correlated with parasitemia in the patients without gametocytemia. Hematologic data in Thailand in 2001 showed lower platelet counts in P. vivax patients with gametocytemia than in the P. vivax patients without gametocytemia, whereas P. falciparum patients with gametocytemia had similar platelet counts but lower red blood cell counts, hemoglobin levels, hematocrit levels, and higher lymphocyte counts than patients without gametocytemia.     

Intralymphocytic schizonts associated with an initial infection of Saurocytozoon tupinambi in Tupinambis teguixin

An initial natural infection of Saurocytozoon tupinambi in a juvenile Tupinambis teguixin from Venezuela was studied for 131 days following capture of the host. Intralymphocytic parasites appeared in this sequence: small uninucleate and binucleate stages (days 1–31 and again on day 41); schizonts with 3–102 nuclei (days 8–14 and 29–35); immature gametocytes (days 29–35) and apparently mature gametocytes of Saurocytozoon tupinambi from day 41. Maximum parasitemia of trophozoites and binucleate schizonts occurred on day 4 when 11% of lymphocytes were infected. Maximum parasitemia by larger schizonts occurred on day 8 at 0.13% of lymphocytes, while maximum gametocytemia was found on day 49 with 16.4% of lymphocytes parasitized. Two types of schizonts were observed: intralymphocytic and the same type free of host cells, and fragments of varying size which may have been torn from capillary endothelium.Due to presence of concurrent infection by a small Plasmodium species, identity of intralymphocytic asexual stages with S. tupinambi cannot be established. Presence of asexual and sexual stages in the same type of host cells (lymphocytes and close derivatives), sequential appearance of trophozoites, schizonts and gametocytes over a period of 40 days, and correlated fluctuations in lymphocyte density suggest they are conspecific, and that Saurocytozoon, which has a plasmodiid type of sporogony may prove to further differ from leucocytozoids by presence of an asexual cycle in circulating blood cells.     

Plasmodium cynomolgi: the comparative infectiousness of individual rhesus monkeys

Five rhesus monkeys were infected with the malaria parasite Plasmodium cynomolgi Mayer. Anopheles stephensi Liston mosquitoes were fed on each monkey over the period of prepremunitive gametocytemia. Individual monkeys did not differ significantly in either mean daily gametocyte count (median = 1300 gametocytes per mosquito blood meal volume per day) or mean daily oocyst production (median = 34 oocysts per mosquito per day). Significant differences among monkeys in daily oocyst/gametocyte conversion ratio were attributable to essentially random correlation effects. The observed range in duration of the period of prepremunitive gametocytemia was 14–43 days. Total oocyst production over this period, as calculated for a unit mosquito biting rate of one per day, ranged from 130 to 2800 oocysts. The overall efficiency of conversion of gametocytes to oocysts in A. stephensi was estimated at 0.02 oocysts per gametocyte.     

Plasmodium cynomolgi: serum-mediated blocking and enhancement of infectivity to mosquitoes during infections in the natural host, Macaca sinica

The infectivity of Plasmodium cynomolgi in its natural host, the toque monkey, Macaca sinica, to Anopheles tessellatus mosquitoes was studied in relation to the evolution of anti-sexual-stage immunity in the host during the course of a blood-induced infection. The effects of serum on the infectivity of gametocytes and the intrinsic infectivity of gametocytes to mosquitoes on each day were assessed in membrane feeding experiments. Mosquitoes were also directly fed on the animal on each day. Our results demonstrate that during the very early patent period, before the peak of gametocytemia, the infection serum enhanced the infectivity of gametocytes up to two to three times above their infectivity in normal monkey serum. Subsequently, serum drawn post-peak of parasitemia ceased to enhance, and began to suppress, infectivity. After 2-3 months, long after parasitemias ceased patency, the serum no longer suppressed and between 3 and 4 months the serum again tended to enhance gamete infectivity before losing any significant effect. Serum infectivity enhancing effects were consistent with low indirect immunofluorescence test antibody titers against blood stage parasites first during the very early days of a blood infection before reaching blocking levels, and again during convalescence when antibodies were declining. The serum infectivity blocking effects on gametocytes were seen at the peak of antibody titers from about Days 9 to 23 of an infection. From 78 to 95% of the total infectivity of the parasite to mosquitoes during an infection occurred when infectivity enhancing activity was present in the serum. Hence, the infectivity of the parasite to mosquitoes was largely dependent on infectivity enhancing antibodies in host serum.     

食蟹猴疟原虫感染恒河猴的长期观察

本项研究证明,食蟹猴疟原虫感染恒河猴后出现的红细胞内期是一个长期的寄生过程,不论血传或子孢子感染,于原虫密度高峰后,均有较长期的低原虫密度阶段。恒河猴均可耐受食蟹猴疟原虫高密度的感染。     

Plasmodium gallinaceum: density dependent limits on infectivity to Aedes aegypti

In acute, blood-induced infections of chickens, the malarial parasite Plasmodium gallinaceum is most infective to the mosquito Aedes aegypti 1 day before gametocyte numbers peak. In an effort to account for this disynchrony , daily changes in parasite infectivity, parasitemia, hematocrit, and hemoglobin were measured during the course of infections. Three events were correlated with the loss of infectivity: (1) In the 24 hr between park infectivity and peak gametocytemia , schizont-induced hemolysis reduced the red blood cell volume 22%. (2) P. gallinaceum zygotes, fertilized in vitro and mixed with heavily infected red blood cells from which all viable, mature gametocytes had been removed, produced 67% fewer oocytes than when combined with uninfected red blood cells. (3) Zygotes fertilized in vitro on the day of peak parasitemia produced 47% fewer oocysts than zygotes prepared 24 hr earlier. It appears that high parasite density reduces infectiousness by destroying, through hemolysis and intraerythrocytic metabolism, a substance necessary to the sporogonic stages, and that there is also an intrinsic loss of infectivity, possibly due to decreased efficiency of fertilization.     

Enhanced gametocyte formation in young erythrocytes by Plasmodium falciparum in vitro.

Two gametocyte-forming clones, HB-3 and 3D7, were used. Concentrates of late stage parasites were mixed with bloods containing different proportions of young erythrocytes, and the parasitemia and proportion of gametocytes determined after 2, 3 or 4 days of culture. Significantly more gametocytes were formed in light cells than in heavy cells separated from the same normal blood samples. Up to seven times more gametocytes were formed in reticulocyte-rich bloods from patients with sickle cell anemia than in normal control blood.     

Plasmodium chabaudi: effect of antimalarial drugs on gametocytogenesis.

The proportion of asexual blood-stage malaria parasites that develop into transmission stages (gametocytes) can increase in response to stress. We investigated whether stress imposed by a variety of antimalarial drugs administered before or during infection increased gametocyte production (gametocytogenesis) in vivo in the rodent malaria parasite, Plasmodium chabaudi. All methods of drug treatment greatly reduced the numbers of asexual parasites produced during an infection but resulted in either no reduction in numbers of gametocytes or a smaller reduction than that experienced by asexuals. We used a simple model to estimate temporal variation in gametocyte production. Temporal patterns of gametocytogenesis did not greatly differ between untreated and prophylaxis infections, with rates of gametocytogenesis always increasing as the infection progressed. In contrast, administration of drugs 5 days after infection stimulated increased rates of gametocytogenesis early in the infection, resulting in earlier peak gametocyte densities relative to untreated infections. Given the correlation between gametocyte densities and infectivity to mosquito vectors, and the high frequency of subcurative drug therapy and prophylaxis in human populations, these data suggest that antimalarial drugs may frequently have only a small effect on reducing malaria transmission and may help to explain the rapid spread of drug-resistant geno-types.     

Enhanced Gametocyte Formation in Young Erythrocytes by Plasmodium falciparum In Vitro

ABSTRACT. Two gametocyte-forming clones, HB-3 and 3D7, were used. Concentrates of late stage parasites were mixed with bloods containing different proportions of young erythrocytes, and the parasitemia and proportion of gametocytes determined after 2, 3 or 4 days of culture. Significantly more gametocytes were formed in light cells than in heavy cells separated from the same normal blood samples. Up to seven times more gametocytes were formed in reticulocyte-rich bloods from patients with sickle cell anemia than in normal control blood.     

Gametocyte clearance in uncomplicated and severe Plasmodium falciparum malaria after artesunate-mefloquine treatment in Thailand

Artemisinin-based combination therapy (ACT) is currently promoted as a strategy for treating both uncomplicated and severe falciparum malaria, targeting asexual blood-stage Plasmodium falciparum parasites. However, the effect of ACT on sexual-stage parasites remains controversial. To determine the clearance of sexual-stage P. falciparum parasites from 342 uncomplicated, and 217 severe, adult malaria cases, we reviewed and followed peripheral blood sexual-stage parasites for 4 wk after starting ACT. All patients presented with both asexual and sexual stage parasites on admission, and were treated with artesunate-mefloquine as the standard regimen. The results showed that all patients were asymptomatic and negative for asexual forms before discharge from hospital. The percentages of uncomplicated malaria patients positive for gametocytes on days 3, 7, 14, 21, and 28 were 41.5, 13.1, 3.8, 2.0, and 2.0%, while the percentages of gametocyte positive severe malaria patients on days 3, 7, 14, 21, and 28 were 33.6, 8.2, 2.7, 0.9, and 0.9%, respectively. Although all patients were negative for asexual parasites by day 7 after completion of the artesunate-mefloquine course, gametocytemia persisted in some patients. Thus, a gametocytocidal drug, e.g., primaquine, may be useful in combination with an artesunate-mefloquine regimen to clear gametocytes, so blocking transmission more effectively than artesunate alone, in malaria transmission areas.     

Decreased hemoglobin concentrations,hyperparasitemia, and severe malaria are associated with increased Plasmodium falciparum gametocyte carriage

To determine factors influencing gametocyte carriage, a cross-sectional study was conducted among 512 patients admitted for Plasmodium falciparum malaria. After adjustments for potential confounders, hemoglobin concentrations were lower in gametocyte carriers 10.5 (+/-2.5) than in patients without gametocytes 12.5 (+/-2.3) (P < 0.0001). Hemoglobin concentrations were negatively correlated with peak gametocyte counts (Spearman's p = -0.37, P < 0.0001) and gametocyte carriage durations (Spearman's p = -(0.30, P < 0.0001). Adjustments for the duration of the malaria episode and other potential confounders did not alter the association (P < 0.0001). After adjustment for potential confounders, the median asexual parasitemia was higher in patients with gametocytes than in patients without gametocytes (P = 0.003). Severe malaria cases were more likely to have gametocytes (65%) than malaria with hyperparasitemia (38%) or mild malaria (31%) (P = 0.0001). These findings suggest that events surrounding anemia and tissue hypoxia stimulate Plasmodium falciparum gametocytogenesis.     

Long-term occurrence of Hemolivia cf. mauritanica (Apicomplexa: Adeleina: Haemogregarinidae) in captive Testudo marginata (Reptilia: Testudinidae): evidence for cyclic merogony?

Blood smears from wild-caught, long-term captive tortoises, Testudo marginata, revealed the presence of gametocytes of a Hemolivia mauritanica-like hemogregarine in the erythrocytes of 72% tortoises examined. Significant parasitemia was also found in animals living several years in captivity. Experimentally infected tortoises showed no evidence of a decrease in parasitemia level more than 15 mo after infection. Morphologically, stages found in tortoises' erythrocytes were indistinguishable from those referred to by previous workers as H. mauritanica from Testudo graeca. Moreover, successful experimental transmission to Hyalomma aegyptium confirms the conspecificity with H. mauritanica. The occurrence of H. mauritanica gametocytes in tortoise living up to 8 yr in captivity is suggested to result from continuous, long-lasting cyclic merogony in tortoises' parenchymatous organs, which is an unknown phenomenon in the life cycle of Hemolivia spp.     

伯氏疟原虫Pb22截短蛋白免疫血清的传播阻断能力的研究

为探讨伯氏疟原虫(Plasmodium berghei)Pb22蛋白免疫血清的传播阻断能力,应用原核表达系统高效表达Pb22截短蛋白免疫小鼠后获得免疫血清。IFA实验证明Pb22截短蛋白免疫血清均可与疟原虫天然抗原结合。通过传播阻断实验,比较了Pb22截短蛋白和全长蛋白免疫血清的传播阻断效果,证明截短蛋白和全长蛋白对雄配子体出丝均有抑制作用,结果表明全长蛋白免疫小鼠的雄配子体出丝与对照组相比显著下降,截短蛋白免疫小鼠的雄性配子体出丝具有下降趋势但无统计学差异。全长蛋白和截短蛋白免疫小鼠的动合子形成数量较对照组均显著下降。以上结果表明抗Pb22截短蛋白免疫血清具有传播阻断能力,但阻断效果不如全长蛋白免疫血清。     

Schizogony and Gametogony of Leucocytozoon simondi and Associated Reactions in the Avian Host*

SYNOPSIS. Stages of development of Leucocytozoon simondi in White Pekin ducklings and their reactions to the parasite were studied on successive days after infecting them artificially with sporozoites from Simulium rugglesi. The minimum prepatent period was 5 days. The first asexual cycle occurred exclusively in the parenchymal cells of the liver. Progeny of these hepatic schizonts followed one of 3 courses: (a) invaded parenchymal liver cells to give rise to another hepatic cycle, (b) penetrated blood cells to form round gametocytes, and (c) were phagocytized by macrophages and grew into megaloschizonts thruout the body. The appearance of elongating gametocytes coincided with the period of maturation and release of merozoites from the megaloschizonts. Experimental evidence supports the hypothesis that the round gametocytes arise from the hepatic schizonts and the elongate forms from the megaloschizonts. Mature megaloschizonts released millions of merozoites, but a high 2nd peak in parasitemia did not develop because of retention of developing gametocytes in the deep circulation, particularly the liver and spleen, and a pronounced host reaction.     

Appropriate Time for Primaquine Treatment to Reduce Plasmodium falciparum Transmission in Hypoendemic Areas

Artemesinin-combination therapies (ACT) for falciparum malaria reduce gametocyte carriage, and therefore reduce transmission. Artemisinin derivatives will act against only young gametocytes whereas primaquine acts on mature gametocytes which are present usually in the circulation at the time when the patient presents for treatment. Both artemisinin derivatives and primaquine have short half-lives, less than 1 hr and 7 hr, respectively. Therefore, asexual parasites or young gametocytes remain after completed ACT. A single dose of primaquine (0.50-0.75 mg base/kg) at the end of ACT can kill only mature gametocytes but cannot kill young gametocytes (if present). Remaining asexual forms after completion of ACT course, e.g., artesunate-mefloquine for 3 days, may develop to mature gametocytes 7-15 days later. Thus, an additional dose of primaquine (0.50-0.75 mg base/kg) given 2 weeks after ACT completion may be beneficial for killing remaining mature gametocytes and contribute to more interruption of Plasmodium falciparum transmission than giving only 1 single dose of primaquine just after completing ACT.     

The effect of partial host immunity on the transmission of malaria parasites.

Experiments were carried out to determine the effect of partial host immunity against the rodent malaria parasite Plasmodium chabaudi on the transmission success of the parasite. There was a fourfold reduction in both the blood-stage, asexually replicating parasite density and the gametocyte (transmissable stage) density in immunized hosts. Some of the reduction in asexual parasite densities was due to strain-specific immunity, but there was no evidence that strain-specific immunity affected gametocyte densities. However, immunity did affect transmission in a strain-specific manner, with a fivefold reduction in gametocyte infectivity to mosquitoes in homologous challenges compared with heterologous challenges or non-immunized controls. This implies the existence of a mechanism of strain-specific infectivity-reducing immunity that does not affect the density of gametocytes circulating in peripheral blood. The proportion of asexual parasites that produced gametocytes increased during the course of infection in both non-immunized and in immunized hosts, but immunity increased gametocyte production early in the infection.