Treatment Efficacy and Malaria TRANSmission After Artemisinin Combination Therapy (TRANSACT)

2.1 MALARIA AND ITS TRANSMISSION TO MOSQUITOES Malaria is the most important parasitic disease in the world. Approximately one fourth of the world population is at risk of contracting the disease, and every year more than 2 million people, mainly young children in sub-Sahara Africa, die of malaria. Malaria is caused by single-cell (protozoan) parasites of the genus Plasmodium. Four species can cause human disease: P. falciparum, P. vivax, P. ovale and P. malariae. The parasites are transmitted between humans by the bite of an infected female mosquito (Anopheles). Inside the human body, the malaria parasites multiply rapidly in the liver cells. After approximately six days, parasites leave this organ and subsequently infect red blood cells (erythrocytes). A next wave of Plasmodium replication takes place in the erythrocytes, then the red blood cell bursts, followed by infection of new red blood cells by the parasites. Since this part of the malaria life cycle involves asexual replication, parasites in this phase are referred to as asexual parasites. A small fraction of these asexual parasites develop into sexual stage parasites (gametocytes). Asexual parasites are responsible for malaria morbidity and mortality, while gametocytes ensure transmission of the parasite from humans to mosquitoes. Malaria transmission takes place when mature gametocytes are ingested by mosquitoes that are taking a blood meal. Once ingested, male and female gametocytes merge to form a zygote that develops though an ookinete stage into an oocyst that can be detected on the mosquito midgut within one week after feeding. The oocyst will burst and sporozoites are released that migrate to the mosquito salivary glands. Once the salivary glands are infected with sporozoites, the mosquito is capable of infecting new human beings.

2.2 MALARIA TREATMENT WITH ARTEMISININ COMBINCATION THERAPY (ACT) Accurate diagnosis followed by prompt and efficacious treatment is the backbone of any malaria control programme. However, malaria treatment has been facing huge challenges in recent years. A number of affordable antimalarial drugs have been used to cure malaria since the 1940s: these include chloroquine (CQ), sulphadoxine-pyrimethamine (SP; Fansidar®), mefloquine, amodiaquine (AQ) and quinine. The emergence and spread of resistance to these commonly-used drugs has been largely responsible for the worsening of the malaria situation observed in the past few years.

Across the African continent, guidelines have recently been changed. The World Health Organization (WHO) recommends for falciparum malaria the use of combination therapies, preferably those containing artemisinin derivatives (ACTs - artemisinin-based combination therapies). Artemisinin derivatives, e.g. artesunate, artemether and dihydroartemisinin, being extremely potent antimalarial agents are the ideal partners in combinations with other antimalarials. ACTs have three demonstrable advantages over conventional therapy, i) they are efficacious in treating malaria patients, ii) substantially reduce post-treatment gametocyte carriaga and iii) "protect" the partner drug from selecting resistant parasites.

In Tanzania, both CQ and SP have lost clinical efficacy. CQ was replaced by SP in 2001 and in the year 2006, SP was officially replaced by Artemether-Lumefantrine (AL: Coartem®). The policy change to the artemisinin-based drug AL is in line with the WHO recommendations to shift to ACT as first line antimalarial treatment.

2.3 RESISTANT PARASITES, MALARIA TRANSMISSION AND ACT Parasite resistance against SP has a genetic background in mutations in the parasite dihydrofolate reductase (dhfr) and dihydropteroate synthetase (dhps) genes. Single nucleotide polymorphisms (SNPs) in these genes are associated with clinical treatment failure. There is now also accumulating evidence that these mutant parasite strains also have a transmission advantage compared to wildtype parasites. Gametocyte carriage is higher for parasite with mutations in the dhfr and dhps genes, even if parasites are successfully cleared due to a longer parasite clearance time. Importantly, these mutant parasites are also more infectious to mosquitoes. These are worrying findings that may explain the rapid spread of parasite resistance in the population. The findings also indicate that gametocytes may be used as an early warning system to indicate the development of parasite resistance: parasite strains that produce most gametocytes are likely to have a reduced susceptibility to the drug.

So far, ACT has proved to be an efficient tool to reduce the transmission of malaria to mosquitoes. Compared to monotherapy with SP, ACT reduces post-treatment gametocyte prevalence and density. This translates in a reduction in post-treatment malaria transmission. Compared to monotherapy, fewer individuals are infectious to mosquitoes after ACT treatment and the average number of infected mosquitoes and the oocyst burden in mosquitoes is reduced. Importantly, ACT does not completely prevent malaria transmission but may counteract the transmission of mutant parasite strains.

2.4 ACT RESISTANCE There is a genuine fear that resistance against ACTs may develop. Although there is no direct evidence of full-blown clinical treatment failure of artemisinin derivatives, there are some worrying findings suggesting a reduced susceptibility of parasite isolates for ACTs. An increased resistance of parasite isolates to different artemisinin derivatives was observed in vitro for P. falciparum field isolates from Cambodia, French Guiana, and Senegal. This resistance was associated with SNPs at codon S769N of the ATPase6 locus of P. falciparum. In addition, the lumefantrine component of AL may exert a selective pressure for parasites with a mutation in the parasite multi-drug resistance 1 gene (Pfmdr1). In general, there is fear that there may be a selection for the artemisinin partner drugs. Although ACTs are clearly giving hopeful results, it is not yet evident which combination of drugs provides the best results, especially in the light of possible artemisinin resistance18. Recently, the combination of dihydroartemisinin-piperaquine (DP) was found to be superior to AL in reducing the risk of recurrent parasitaemia and post-treatment gametocytaemia.

Based on the finding that SP resistant parasite strains exhibit a higher gametocyte production under drug pressure, we hypothesize that gametocytaemia after treatment can be used to screen for parasites that are most likely to have a reduced susceptibility to ACT.

1. JUSTIFICATION Studies on the development of resistance to ACT and the spread of ACT resistant parasite strains in the population are extremely relevant from a public health perspective. If resistance against artemether-lumefantrine or other ACTs develops, there will be no alternative drug available for first-line treatment. The identification of predictive markers for ACT resistance will be of great value for the protection of ACT. Studies on malaria transmission after ACT are of great importance in identifying those mutations that may eventually cause ACT resistance.

The current study determines the efficacy of two different ACTs with a specific focus on detecting markers for resistance or reduced susceptibility of parasites to ACTs and the transmission potential of mutant parasites.