Drug-resistance transfer among rodent plasmodia

Current status of experimental models for the study of malaria

Infection by malaria parasites (Plasmodium spp.) remains one of the leading causes of morbidity and mortality, especially in tropical regions of the world. Despite the availability of malaria control tools such as integrated vector management and effective therapeutics, these measures have been continuously undermined by the emergence of vector resistance to insecticides or parasite resistance to frontline antimalarial drugs. Whilst the recent pilot implementation of the RTS,S malaria vaccine is indeed a remarkable feat, highly effective vaccines against malaria remain elusive. The barriers to effective vaccines result from the complexity of both the malaria parasite lifecycle and the parasite as an organism itself with consequent major gaps in our understanding of their biology. Historically and due to the practical and ethical difficulties of working with human malaria infections, research into malaria parasite biology has been extensively facilitated by animal models. Animals have been used to study disease pathogenesis, host immune responses and their (dys)regulation and further disease processes such as transmission. Moreover, animal models remain at the forefront of pre-clinical evaluations of antimalarial drugs (drug efficacy, mode of action, mode of resistance) and vaccines. In this review, we discuss commonly used animal models of malaria, the parasite species used and their advantages and limitations which hinder their extrapolation to actual human disease. We also place into this context the most recent developments such as organoid technologies and humanized mice.

Malaria parasite genetics: doing something useful

Genetics has informed almost every aspect of the study of malaria parasites, and remains a key component of much of the research that aims to reduce the burden of the disease they cause. We describe the history of genetic studies of malaria parasites and give an overview of the utility of the discipline to malariology.

Genetic studies on Plasmodium chabaudi: recombination between enzyme markers

Two lines of Plasmodium chabaudi differing in three characters have been crossed, using a technique previously described for P. yoelii. One line, termed 47AS, was characterized by an electrophoretic form of 6-hosphogluconate dehydrogenase, denoted 6PGD-2, a form of lactate dehydrogenase, denoted LDH-3, and was pyrimethanime-resistant. The second line, termed 10AJ, possessed enzyme forms of 6PGD-3 and LDH-2 and was pyrimethamine-sensitive. The cross was made by permitting mosquitoes to feed on a mixture of the two lines and infecting rodents with the resulting sporozoites. The products of the cross were cloned by dilution and examined for enzyme-type and drug-response. Results showed that recombination had occurred between each of the three characters. Clones characterized by 6PGD-2/LDH-2 and 6PGD-3/LDH-3 demonstrated recombination between the enzyme markers. The drug-resistance character segregated independently of either enzyme marker.

Genetic recombination in Plasmodium berghei

Crosses have been made between two lines of Plasmodium berghei yoelii differing in drug-sensitivity and enzyme-type. The two lines used were line A, which is pyrimethamine-resistant and contains an electrophoretic form of glucose phosphate isomerase termed GPI-1, and line C, which is pyrimethamine-sensitive and contains enzyme-form GPI-2. Equal numbers of blood forms of lines A and C were mixed and injected intravenously into a mouse. Mosquitoes were fed immediately on the mixture, and the resulting sporozoites used to infect further rodents. After treating these animals with pyrimethamine, drug-resistant parasites characterized by GPI-2 could be detected in the remaining infections. Controls showed that these parasites were recombinant forms, which had arisen by cross-fertilization of gametes in the mosquitoes, and not by mutation or ‘synpholia’. By cloning the products of crosses between lines A and C by dilution, parasite lines of two recombinant classes (resistant GPI-2 and. sensitive GPI-1) were isolated, together with lines of parental characteristics.

Altered dihydrofolate reductase associated with drug-resistance transfer between rodent plasmodia

Resistance to pyrimethamine in strains of Plasmodium vinckei and of Plasmodium berghei is attributed to changes in amounts and properties of the dihydrofolate reductases. The resistant strain of Plasmodium berghei was isolated from an experimentally induced mixed infection of drug-resistant Plasmodium vinckei and drug-sensitive Plasmodium berghei, through biological filtration in hamsters. The drug resistance in Plasmodium berghei appears to have been acquired through transfer of part of the gene or genes coding for dihydrofolate reductase from the resistant Plasmodium vinckei to the sensitive Plasmodium berghei.