Mutations in Plasmodium falciparum dihydrofolate reductase and dihydropteroate synthase of isolates from the Amazon Region of Brazil

Since the late 1970s pyrimethamine-sulfadoxine (PS; FansidarTM Hoffman-LaRoche, Basel) has been used as first line therapy for uncomplicated malaria in the Amazon basin. Unfortunately, resistance has developed over the last ten years in many regions of the Amazon and PS is no longer recommended for use in Brazil. In vitro resistance to pyrimethamine and cycloguanil (the active metabolite of proguanil) is caused by specific point mutations in Plasmodium falciparum dihydrofolate reductase (DHFR), and in vitro resistance to sulfadoxine has been associated with mutations in dihydropteroate synthase (DHPS). In association with a proguanil-sulfamethoxazole clinical trial in Brazil, we performed a nested mutation-specific polymerase chain reaction to measure the prevalence of DHFR mutations at codons 50, 51, 59, 108 and 164 and DHPS mutations at codons 436, 437, 540, 581 and 613 at three sites in the Brazilian Amazon. Samples from two isolated towns showed a high degree of homogeneity, with the DHFR Arg-50/Ile-51/Asn-108 and DHPS Gly-437/Glu-540/Gly-581 mutant genotype accounting for all infections in Peixoto de Azevedo (n = 15) and 60 percent of infections in Apiacás (n = 10), State of Mato Grosso. The remaining infections in Apiacás differed from this predominant genotype only by the addition of the Bolivia repeat at codon 30 and the Leu-164 mutation in DHFR. By contrast, 17 samples from Porto Velho, capital city of the State of Rondônia, with much in- and out-migration, showed a wide variety of DHFR and DHPS genotypes.

The art of parasite survival

Parasites develop and survive in an environment which is often hostile to them. When facing aggressive conditions parasites are able to use various and complex strategies. Echinococcus granulosus, Toxocara canis, Pneumocystis carinii, Entamoeba or Toxoplasma gandii are able to seclude from the environment when stressed by surrounding (immunologic or non-immunologic) agressive factors. Specific antigens which exert a functional activity during a short period of time appear to be concealed from the immune attack at this crucial moment. This is the case for rhoptry or dense granule antigens of Plasmodium or Toxoplasma sporozoa involved in the formation of the parasitophorous vacuole which are released in a space perfectly isolated from the outside and therefore from antibodies. Some parasites like Schistosoma mansoni or Trypanosoma brucei reveal an amazing opportunistic behavior when they use cytokines of host origin induced by the infectious process for their own development. Leishmania, Toxoplasma and Trypanosoma cruzi are able to invade immunologically competent macrophages and to avoid the triggering of killing mechanisms of these cells. Parasites also take advantage of the genetic restriction of the immune response and it has been observed for Plasmodia that some high molecular weight antigens are unable to induce an immune response in particular strains of mice. Parasite receptors involved in the invasion of host cells by parasites can function in the presence of antibodies which can explain the failure of vaccination attempts targeting this type of molecules. Among the mechanisms developed by parasites to resist to drugs it appears that transmembrane transporters described in many protozoa or helminth parasites could play a role. Moreover, the description of parasite-specific enzymes able to protect them against the damaging effects of oxygen radicals suggests that parasites are potentially able to develop a resistance phenomenon against drugs acting via an oxidative burst