Efficacy of Antimalarial Chemoprophylaxis for Travelers

Authors


Fabrice Simon, MD, Service de pathologie infectieuse et tropicale, Hôpital d’Instruction des Armées Laveran, BP 50, F-13998 Marseille Armées, France. E-mail: simon-f@wanadoo.fr

Abstract

Nine of 26 French tourists developed malaria after a 2-week stay under field conditions in a highly endemic place in Burkina Faso. A study of their preventive antimalarial measures identified a strong association of malaria attack with absence or inadequacy of chemoprophylaxis but not with mechanic measures.

The efficacy of protection against malaria for travelers to endemic areas is rarely evaluated for obvious ethical reasons. The particular situation of 26 French tourists traveling together in April 2004 directly from France to southwest Burkina Faso allowed the efficacy of chemoprophylaxis and physical protection against mosquitoes to be evaluated.

Methods

The observation of an unexpectedly high malaria attack rate in a group of travelers after their return to France led to a retrospective study of their living conditions, use of chemoprophylaxis, and practices of protection against mosquitoes. Data based on the medical records of malaria-infected patients and questionnaires sent to all members of the group in the first 2 months after the first reported cases of malaria attacks were obtained in June and July 2004 from 25 of the 26 participants.

Results

The group stayed in a rudimentary settlement in the gallery forest of Nasso, a tourist village located 20 km west of Bobo-Dioulasso. The average length of stay was 17 days (two departure dates at 1 wk interval). The median age of the travelers was 36 years (ranging from 11 to 57 y), with  a  male-to-female sex ratio of 0.67.

Self-declared individual protection against mosquitoes consisted of the occasional use of repellent by all the travelers but every evening only by 17 participants. The occasional use of covering clothing was made by 13 travelers; bednets were used by 19 travelers (of which 7 bednets were impregnated with insecticide) but only by 13 every single night. Nineteen individuals declared having taken chemoprophylaxis against malaria: chloroquine alone (1 individual), chloroquine–proguanil (13), atovaquone–proguanil (2), doxycycline (2), and mefloquine (1). Nine individuals followed adequate chemoprophylaxis, ie, use of an efficient product according to French guidelines at the time of the study, during the whole stay and at least 4 weeks after departure (1 wk for the atovaquone–proguanil combination).

Nine individuals suffered a malaria attack by Plasmodium falciparum, confirmed by blood slides in French hospitals (eight cases) or in Burkina Faso (one case). Its appearance varied between 16 and 40 days after arrival in the area of malaria transmission (average time: 24 d).

None of the nine individuals using adequate chemoprophylaxis at that time suffered a malaria attack, while 3 of 10 individuals using inadequate chemoprophylaxis and all 6 individuals without any chemoprophylaxis suffered malaria attacks. The use of protective clothing and impregnated or untreated bednets was not associated with effective protection against malaria in this group (Table 1).

Table 1.  Malaria attack rate based on the attitudes and prophylactic measures taken by the 25 travelers to rural Burkina Faso in April 2004
Behavior and prophylactic practicesMalaria attack rateRelative risk (95% CI)Fisher’s exact test (p value)
Cases/users, n/N (%)Cases/nonusers, n/N (%)
  1. CI = confidence interval.

Medical advice before traveling5/18 (27.8)4/7 (57.1)0.49 (0.18–1.38)0.205
Protective clothing use6/13 (46.2)3/12 (25.0)1.85 (0.59–5.79)0.411
Repellent use7/17 (41.2)2/8 (25.0)1.65 (0.44–6.21)0.661
Bednet use4/13 (30.8)5/12 (41.7)0.74 (0.26–2.12)0.668
Adequate chemoprophylaxis use0/9 (0)9/16 (56.2)0.008

Discussion

In the area where this group stayed, Anopheles can proliferate in March or April, taking advantage of the first rains of the dry season (“mango rains”).1 With a presumed malaria transmission intensity of 30 Anopheles bites per night and a sporozoite rate of 1% (s), the entomological inoculation rate per night would be 0.3. Then, according to the formula r= 1 − ema·s·t (where r= risk of infective bite, ma =Anopheles bites per night, s= sporozoite rate in Anopheles, and t= time of stay in days), the risk of receiving at least one infected bite in 17 days (t) is 0.994.2 The observed rate of 100% malaria attack for the six travelers without any chemoprophylaxis is in accordance with this calculated risk. For this small group, neither the medical advice before traveling, regular protective clothing, or the regular use of a bednet, whether impregnated or not, provided any protection against malaria. However, most of the bednets were not, as recommended, impregnated with insecticide, and infectious mosquito bites may have been acquired during the evening and morning periods outside the bednet.

So all individuals without any chemoprophylaxis suffered a malaria attack, whereas none of those with adequate chemoprophylaxis developed the disease. In an intermediate situation, 3 of the 10 travelers who did not use an efficient drug correctly experienced a malaria attack. The failure of chemoprophylaxis is easily explained by the use of an inefficient molecule (chloroquine in one case) and by the irregular use of chloroquine plus proguanil in the field and during the weeks following departure from the malaria-endemic area.

It is often difficult to compare directly the effects of various measures of antimalarial protection of nonimmune travelers because their malaria risk will differ depending on different living conditions and length of stay. Data on malaria attacks in a more homogenous group, military troops stationed in the tropics, demonstrated the importance of both the adequacy and the compliance of antimalarial chemoprophylaxis.3,4 Our observation of a group with very similar individual habits and travel schedules arrives at the same conclusion. Protection against malarial morbidity and mortality when traveling in an endemic area results from the cumulative protective effects of awareness, bite avoidance, chemoprophylaxis, and prompt diagnosis. As demonstrated in this study, compliance with appropriate  chemoprophylaxis  is  the  backbone  for  protection  in areas of intense malarial transmission.

Conclusions

Although conducted on a small group, this study underlines the fact that travelers can quickly become infected with malaria when living and sleeping under rough conditions in a highly endemic area. It also demonstrates the great benefits of good compliance with an efficient chemoprophylaxis regimen. Among measures recommended for reducing the risks of malaria transmission, the use of bednets, especially the ones impregnated with insecticide, can help reduce the Anopheles bite rate of individuals living in endemic areas but is not sufficient to avoid a malaria attack without adequate prophylaxis under such field conditions.

Declaration of interests

The authors state that they have no conflicts of interest.

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