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Submicroscopic double infection of PIasmodium falciparum in P. &ax malaria

T. Takaqi’*, S. Kane’, G. Masuda’ and M. SW& Department of Parasitology, Gunma Uni- versity School of Medicine? 2Department of f

Maebashi 371, Japan; In ectious Dtseases, Tokyo Metropolitan

Komagome Hospital, Bunkyo-Ku, Tokyo 113, Japan

Laboratory diagnosis of malaria is regularly made by microscopic examination of blood smears occa- sionally supplemented by serological tests. We had 2 patients diagnosed as having Plasmodium vivax malar- ia. To attempt in vitro culture of P. viva:, which has not yet been achieved, infected blood specimens taken from the patients were cultured according to the established method for P. falciparum (TRAGER & JENSEN, 1976).

The first patient, a 40 years old male from New Delhi. develooed 3 s&es of fever at around 40°C with daily in&vals while visiting Japan. By microsco- pical examination of blood smears, only P. vivax parasites were found; antibody titres were 1: 1024 for P. vivax and I:64 for P. falcibarum bv indirect immunofluorescence antibody test (IFAT). Thus, both clinical manifestations and laboratory lindings indicated vivax malaria in this case. The patient’s blood was withdrawn and subjected to cultivation. Parasites, seen in the culture in the following few days, disappeared soon, but on day 9 typical P. fakiparum ring forms were detected. These parasites multiplied continuously and, later, P. falciDarum gametocytes were seen. The antigen& of the oarasites was examined bv IFAT. usine standard wsitive sera; the results*also indicated that the parasites in culture were P. falciparum.

The second case was a 25 years old female visiting Tokyo from Madagascar. Before treatment, she twice had febrile episodes with a one-day interval. Her spleen and liver were palpable. Single infection with P. vivax was determined bv examination of thin blood smears. IFAT titre was l:IO24 for both P. vivax and P. falciparum. Blood from the patient was .again cultured. On day 1, malaria parasites were seen but they soon became undetectable. On day 12 of culture, typical ring forms of P. falciparum were seen, followed by P. falciparum gametocytes. Her 29-year-

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old husband developed a spiky fever simultaneously but immediate treatment prevented a second attack of fever. Both P. vivax and P. falciparum parasites were confirmed in his thin blood films. IFAT gave titres of 1: 1024 for both P. vivax and P. falciparum antigens. The husband’s blood was cultured, and parasites continued multiplication on subsequent days until reaching a plateau. By morphological features, rings and gametocytes growing in the culture were iden- tified as P. falciparum.

To date, double malaria infections have been determined by microscopical observations on blood smears. The present results indicated that submicros- copic double infections with P. falcipanrm have been regarded as single P. vivax infections. Clinical pictures of the 2 patients with submicroscopic double infections were comparable to those seen in vivax malaria, which is not in agreement with the accepted opinion. GARNHAM (1966) stated that, in vivax malaria, “The initiation of parasitaemia may be inhibited if P. falcibarum is also oresent. for the latter parasite takes the dominant role.” Recent report bv LOOAREESUWAN et al. (1987) support this view. MANSON-BAHR & APTED (1982) mention that P. vivax is usually superimposed upon P. falciparum, so that the patient’s illness appears typical of the latter infection. The present report shows that in some cases, or at some stage of double infections, P. vivax parasites may greatly predominate over P. falciparum in number and, in such cases, clinical manifestations are also dominated by those of vivax malaria. Our finding, together with the report by LOOAREESUWAN et al. (1987), strongly suggests that many more double infection cases exist than have been reported pre- viously. Especially in areas where drug resistance of P. falciparum exists., the possibility of submicroscopic double infection with resistant P. falciparum should be carefully considered if other malarias are diag- nosed. However. the isolates of P. falcibarum obtained during this study were all highly susceptible to chloroquine in vitro and in vivo.

References Gamham, I’. C. C. (1966). Malaria Parasites and other

Haemmporidia. Oxford: Blackwell Scientific Publica- tions, p. 130.

Looareesuwan, S., White, N. J., Chittamas, S., Bunnag, D. & Harinasuta, T. (1987). High rate of Plasmodium nivax relapse following treatment of falciparum malaria in Thailand. Lancer. ii. 1052-1055.

Manson-Bahr, P. E. ‘C. h Apted, F. I. C. (1982). Manson’s Tropical Diseases (18th edition). London: Bailliere Tin- dall, pp. 51-52.

Trager, W. & Jensen, J. B. (1976). Human malaria parasites in continuous culture. Science, 193, 673-675.

Received 1 December 1987; revised 5 January 1988; accepted for publication 5 January 1988 *Author for correspondence.