In this study, sub-clinical malaria was more prevalent in HIV-1 infected individuals than in HIV-1 seronegative populations when assayed by either smear or nested DNA PCR. Sub-clinical parasitaemia was less frequent in Maputo where malaria eradication efforts have been more intensive over the past several years in comparison with rural areas of the country. The significantly higher rate of sub-clinical malaria in asymptomatic individuals from Mocuba and the use of TS prophylaxis in HIV-1 infected persons with lower CD4 cell counts provided an opportunity to evaluate the impact of TS prophylaxis on the prevalence of sub-clinical malaria by both light microscopy and PCR. Although the practice of prescribing TS prophylaxis primarily to those with lower CD4 cell counts was a potential confounding factor, our data indicate that TS prophylaxis does not eliminate sub-clinical parasitaemia whether assessed by microscopy or PCR. Among those who were smear positive for malaria, more than twofold higher parasite density levels were observed in the HIV-1 infected population than among those who were HIV-1 seronegative, in contrast to what has been previously reported [
18]. Although both populations were recruited from people seeking care from HIV-1 testing and treatment sites in the same clinical facilities, caution in interpreting the differences in prevalence and parasitaemia levels between the HIV-1 infected and HIV-1 seronegative populations is warranted since other differences in the two populations might be unrecognized.
It is also possible that the observed differences in prevalence and levels of parasitaemia between the HIV-1 infected and uninfected populations might actually underestimate the effect of HIV-1 infection on malarial parasitaemia rates since HIV-1 infected individuals since those under care for HIV might be more likely to be in contact with the health care system and have more access to bed nets and other preventive measures. Furthermore, since syndromic treatment of fever with empiric anti-malarial therapy is not uncommon, persons with fever from HIV-1 or its complications could be more likely to receive empiric intermittent anti-malarial therapy for other disease states which might further lower the prevalence of sub-clinical malaria in this group. An additional limitation of the study was that HIV RNA levels are not routinely monitored in Mozambique and, thus, viral load data were not available.
TS prophylaxis in HIV patients reduces morbidity and mortality from malaria as well as from
Pneumocystis jirovecii[
19,
20]. In view of the low prevalence of malarial parasitaemia and of clinical malaria in Maputo, TS would seem to be less critical from the perspective of reducing morbidity and mortality from malaria than in places like Mocuba, where the risk of malaria remains higher. The potential impact of TS on other HIV-1 related opportunistic infections, such as
Pneumocystis jirovecii pneumonia would, nonetheless, continue to be important. It is well demonstrated that HIV infection predisposes to more frequent and more severe episodes of malaria [
3,
5,
9,
10]. Since this was a cross-sectional rather than a longitudinal study, time-dependent malaria-related morbidity and mortality rates were beyond the scope of this study. Nonetheless, in Mocuba where
P. falciparum elimination efforts have been less successful than in Maputo, it was found that the HIV cohort had 1.7 times more sub-clinical malaria than the HIV uninfected cohort as defined by PCR. Others have demonstrated that overt clinical bouts of malaria are associated with increased levels of HIV-1 RNA and of CD4 cell decline [
6,
21]. The implications of sub-clinical malarial parasitaemia for HIV-1 disease progression have not yet been delineated, but it would seem that such studies are warranted [
22,
23]. Finally, although the impact of sub-clinical malarial parasitaemia on the risk of clinical malaria in either the HIV-1 infected or the HIV-1 seronegative population or on the progression of HIV-1 in the co-infected population has not been fully delineated, the sensitivity of nested PCR of dried blood spots for detecting carriage of
P. falciparum might make this an attractive approach to monitor the success or failure of malaria control efforts in areas, such as Mocuba, as more concerted efforts to eliminate malaria are undertaken.