The effect of sulphadiazine, proguanil and 2:4-diamino-6:7-diisopro-Pylpteridine upon gametocyte production in Plasmodium gallinaceum (Brumpt, 1935)

Evolutionary Ecology of Avian Malaria: Past to Present

Avian malaria is the oldest experimental system for investigating the biology and transmission of Plasmodium parasites. Recent molecular protocols for detecting and characterizing avian malaria lineages in the field are providing an ever-growing picture of the prevalence, distribution, host range, and diversity hotspots of avian malaria across the world. The unparalleled genetic diversity uncovered rivals anything that has been found in other vertebrate malarias and seems to be matched by an equally rich phenotypic diversity, providing endless opportunities for exploring the selective pressures under which hosts and parasites evolve. We review the most important milestones in avian Plasmodium research and explain why this is a unique animal model to understand the ecology and evolution of malaria.

Higher gametocyte prevalence following failure of treatment of Plasmodium falciparum malaria with sulfadoxine-pyrimethamine and the combination of chloroquine plus sulfadoxine-pyrimethamine: implications for progression of anti-folate resistance

Chloroquine (CQ) treatment of CQ-resistant Plasmodium falciparum is associated with a significantly higher prevalence of post-treatment gametocytaemia which has been linked to the preferential transmission of CQ-resistant parasites. It is not known whether treatment failure (TF) with sulfadoxine-pyrimethamine (SP) is associated with the same higher prevalence of gametocytaemia as that seen with CQ TF. Using 1997 WHO in-vivo drug efficacy protocols for malaria, we therefore compared (in a study in 1999) the frequency of gametocytaemia in those with TF to the frequency seen in those with an adequate clinical and parasitological response (ACPR) following treatment with one of 3 regimens in Papua, Indonesia: SP monotherapy (n = 87; TF 20.7%), CQ monotherapy (n = 48; TF 83.3%), and the combination of CQ plus SP (n = 34; TF 38.2%). Following SP, day 7 prevalence was significantly higher in those with TF (67%) than with ACPR (38%, P = 0.03). Following combination treatment with CQ + SP, the day 14 gametocyte prevalence was significantly higher in those with TF (100% vs 38%, P = 0.016). The higher prevalence of SP TF-associated gametocytaemia may contribute to increased transmission of antifolate-resistant strains, and further cautions against the use of SP as monotherapy. Adding SP to CQ, after significant resistance has emerged to both drugs, may not prevent enhanced transmission of dual-resistant strains and progression of anti-folate resistance.

Formation of Plasmodium falciparum gametocytes in vivo and in vitro relates to transmission intensity

Plasmodium falciparum isolates were collected during a cross-sectional survey of fever cases in an endemic area of northern India, over a period of 6 months (July-December 1996). Smears of peripheral blood from 118 cases were found to contain Plasmodium falciparum. Isolates of the parasites were successfully cultured in vitro, under identical conditions, from blood samples from 58 of the smear-positives who had asexual parasitaemias of at least 0.05%. Gametocytes developed from 50 of the 58 isolates, with a variable range of gametocytaemia. An isolate was most likely to produce gametocytes and to produce high gametocytaemias (i.e. > or = 1.0%) in vitro if it had been collected when the intensity of transmission (as reflected in bloodsmear positivity) was relatively high. A young child (< 6 years) with fever was more likely to be smear-positive for the asexual and sexual stages of P. falciparum, but less likely to be seropositive for antibodies to these stages, than the older cases of fever.

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.

Antimalarial drugs and the mosquito transmission of Plasmodium

It is well-known that whenever possible, the treatment of patients with malaria should include measures to prevent them transmitting the infection to others. This is particularly important for P. falciparum, where the gametocytes can survive for a much longer period than the asexual stages. Not all antimalarials are gametocytocidal or sporontocidal and those that are may have particular disadvantages or lose their effectiveness because of resistance. Even drugs that have no obvious gametocytocidal or sporontocidal activity may have other effects. These include the possibility of increasing transmission, either by affecting the parasite within an individual host or by selection for parasite strains with increased potential for infecting the mosquito vector. This review summarises the available information on the properties of antimalarials in relation to mosquito transmission and highlights the need for more attention to be paid to this aspect of drug action.

Plasmodium falciparum gametocyte formation in vitro: its stimulation by phorbol diesters and by 8-bromo cyclic adenosine monophosphate

Phorbol 12-myristate 13-acetate increased the number of gametocytes by 50 to 100% in well or petri dish cultures of the HB-3 clone of Plasmodium falciparum. Phorbol dibutyrate had a similar effect. The optimal concentration for each of these agents was 20 ng/ml or approximately 30 nM. No effect of forskolin was found, other than a general inhibition of growth at concentrations over 10 microM. An inhibitor of phosphodiesterase, 8-bromo cyclic adenosine monophosphate (at concentrations of 0.1 and 1.0 microM) also significantly increased the number of gametocytes formed by this clone.

Resistance to primaquine in Plasmodium gallinaceum, and the problem of resistance to quinoline compounds in malaria parasites

A tenfold enhancement of resistance to primaquine was obtained by maintaining a strain of Plasmodium gallinaceum in a state of acute infection by serial passage of infected blood through young chicks treated with gradually increasing doses of the drug.

No loss in resistance to primaquine was observed when the resistant strain was transmitted through mosquitoes, though there was some loss in resistance to the maximum tolerated dose, but not to lower doses, when the strain was maintained in a state of acute infection through untreated chicks for 41 weeks.

The primaquine-resistant strain was cross-resistant to lower effective doses of pamaquin, and slightly less sensitive to quinine than the parent strain but the loss in sensitivity to chloroquine was only marginal. Sensitivity to proguanil, dihydrotriazine and pyrimethamine was normal.

Attempts were made to produce a chloroquine-resistant strain of P. gallinaceum using different doses of the drug but no change in sensitivity was observed though the experiments were continued for more than a year. An attempt to produce a chloroquine-resistant strain by treatment of a proguanil-resistant strain with chloroquine also failed.

The problem of resistance to quinoline compounds in different species of Plasmodium is discussed.

Problems concerned with gametogenesis in Haemosporidiidea, with particular reference to the genus Plasmodium

Although many interesting observations have been made upon gametogenesis in Haemosporidiidea many fundamental problems await solution. Our knowledge of the nuclear changes occurring during the development of the gametes and the fertilization of the female gamete is still incomplete. Little is known of the factors which control gametocyte production, though there is evidence that in some species, i.e. Leucocytozoon simondi and Haemoproteus columbae in the duck, it is influenced by the physiological stage of the host. The type of host and the method of transmission have also been shown to affect the numbers of gametocytes formed in some species. No correlation has been found in Plasmodium, the genus in which this subject has been studied extensively, between acquired immunity and gametocyte formation. Although a broad relationship has been demonstrated in Plasmodium between the density of gametocytes in the blood and the oocyst rate and density assessed in susceptible mosquitoes, hosts heavily infected with apparently ripe gametocytes may prove non-infective to susceptible mosquitoes. In birds heavily infected with P. gallinaceum, serum taken at the peak of parasitaemia has been shown to affect adversely the infectivity of gametocytes to mosquitoes. Pamaquin and primaquine, in small doses, inhibit the development of gametocytes completely, whereas proguanil may not prevent the formation of oocysts though it inhibits their subsequent development. High densities of gametocytes have been observed in patients treated with proguanil or sulphamethazine. An actual in crease in the number of gametocytes produced, as compared with the untreated control strain, was observed in strains of P. gallinaceum in process of developing resistance to sulphadiazine, proguanil or 2:4-diamino-6:7-diisopropylpteridine as a result of treatment with those drugs.