Preliminary studies on the response of Plasmodium falciparum to chloroquine in the Philippines, with the in vitro technique.

Previous investigations had shown that resistance to standard regimens of chloroquine occurred in some cases of falciparum infection in the Philippines. More extensive investigations into this phenomenon were planned by the Malaria Eradication Service, by means of the in vitro technique developed by Rieckmann, in order to determine both the distribution of resistant strains throughout the country and their local incidence. Before these studies were undertaken, a series of observations were made on cases of falciparum malaria encountered in Manila and its environs, to assess the reaction of local strains of the parasite to the in vitro test. These cases were also treated with standard doses of chloroquine and some were followed up for 4 weeks to compare the predictions made as a result of the in vitro tests with the in vivo observations. Of the 34 in vitro tests carried out, 18 were followed up in vivo. In 8 cases, no recrudescence occurred, but in the other 10 recrudescences were detected during the 4-week observation period, thus indicating parasite resistance to the drug. In each of the 18 cases, the in vivo response followed the in vitro prediction.

Identification of malaria parasites by fluorescence microscopy and acridine orange staining.

The need for a technique that is more sensitive than the use of Romanowsky-stained thick blood films for detecting malaria parasites at low concentration in the blood is well recognized. One of the more promising methods appeared to be fluorochrome staining with acridine orange. However, reports on the efficacy of the technique were contradictory and it was not clear to what extent blood films taken under survey conditions would contain fluorescing artefacts that might confuse diagnosis. An investigation indicated that, provided reasonable care was taken, blood films made under survey conditions contained few confusing artefacts. However, it was found that, while acridine orange staining might have a slight advantage when large malaria parasites were present, it was inferior to routine Romanowsky staining for the detection of young trophozoites, the inferiority becoming more pronounced as the parasite concentration decreased.

A comparative study of thick and thin blood films in the diagnosis of scanty malaria parasitaemia.

In an attempt to explain the shortcomings of the routine thick-film examination in the diagnosis of scanty malaria parasitaemias, a direct comparison, in terms of positivity and parasite counts, was made between the results of routine thick-film study and long-term examination of thin films taken at the same time from the same individuals. Calculation of the average thickness of the thick and thin films prepared allowed these comparative results to be corrected according to the actual volume of blood examined. From these corrected figures it was observed that both parasite counts and positivity were significantly higher in the thin-film series, and it has been deduced that heavy losses in parasites, varying from 60% to 90%, occurred during the dehaemoglobinization and staining of thick films.The epidemiological implications of this finding in malaria practice are discussed. Emphasis is laid on the importance of further research in order to improve the sensitivity of the routine thick film in the diagnosis of the scanty parasitaemias met with in the later stages of malaria-eradication programmes.