Malaria in Travelers to India: Is the Risk Decreasing?

By Brian G. Blackburn, MD, and Michele Barry, MD, FACP

Dr. Blackburn is a Clinical Assistant Professor in the Division of Infectious Diseases and Geographic Medicine at Stanford University School of Medicine.

Dr. Barry is the Senior Associate Dean of Global Health at Stanford University School of Medicine.

Dr. Blackburn and Dr. Barry report no financial relationships relative to this field of study.

Synopsis: Malaria incidence decreased substantially among recent travelers to India when compared to the early 1990s, with the risk of P. falciparum now reported as particularly low. The question is — just how low?

Source: Schmid S, Chiodini P, Legros F, et al. The risk of malaria in travelers to India. J Travel Med 2009;16:194-199.

Several European countries recently have revised their malaria prevention guidelines for travel to India in view of a perceived declining malaria risk. However, it is not clear if the data presented are robust, and further study should address this topic before guidelines are changed. In addition, even if the overall risk of malaria in travelers to India is decreasing, several states in India remain higher-risk areas and may require separate consideration.

Malaria has been a widely accepted risk for travelers to India. Current recommendations regarding malaria prevention for travelers vary, with U.S. authorities recommending prophylaxis for all travelers to most areas of the country, but with some European experts recently recommending only mosquito avoidance and standby emergency treatment, or seasonal chemoprophylaxis. Given the differing guidelines and changing incidence of malaria in India, the authors undertook a study to ascertain the incidence of malaria among travelers to India from developed countries.

Information on annual imported malaria cases were requested from the United Kingdom, United States, France, Germany, Australia, Italy, Singapore, Sweden, and the Netherlands for the years 1992-2005. The number of annual visits to India for the same years was obtained from the Indian Ministry of Tourism. From these data, malaria incidence rates were calculated for each year among travelers from these countries. In addition, data were obtained from the World Health Organization regarding malaria incidence among residents of India during this period.

Of the travelers to India during these years, 96% were classified as tourists. A decline was seen in the malaria incidence per 100,000 travelers, from 93 cases in 1992 to 19 cases in 2005. This paralleled the malaria incidence in the resident population of India, which saw a decrease from 260 to 180 cases per 100,000 per year during the same period. Notably, both groups (visitors and residents of India) had experienced temporary increases in malaria incidence in the mid and late 1990s, which resulted from several malaria epidemics during this time. Certain states had higher incidence rates throughout the study period among Indian residents (e.g., Orissa, Chhattisgarh, Jharkhand, West Bengal, Mizoram, Meghalaya, Tripura, Goa, Gujarat, Rajasthan, and Madhya Pradesh), from 200 to more than 1,000 cases per 100,000 persons per year.

The proportion of travel-related malaria that was due to P. falciparum was stated to be low, although precise data are not given. Among residents, the proportion of malaria cases due to P. falciparum was more than 50% in several states (Orissa, Chhattisgarh, Meghalaya, Tripura, Assam, Arunachal Pradesh, Mizoram, and Andhra Pradesh).

Commentary

The authors present interesting data that indicate a trend of decreasing malaria incidence among visitors to India during the past decade. This trend seems to mirror one seen in Indian residents, demonstrating that malaria incidence may be decreasing throughout the country, providing a reasonable biological basis for the observation. However, the data presented are somewhat preliminary in nature, and further study is needed before recommendations regarding malaria prophylaxis should be changed for visitors to India.

While the data in this study indicate that malaria incidence among travelers to India is decreasing, the rates are still higher than in many other Asian and Latin American countries for which malaria prophylaxis currently is recommended. Surveillance data covering American travelers returning with malaria in 2007 that was analyzed by country of acquisition indicate that malaria rates among travelers to India are above the median when compared to the 53 other countries in which American travelers acquired the disease that year.1 Most compellingly, certain states (e.g., Rajasthan, Gujarat, Karnataka, Goa, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Orissa, and the Northeastern states [especially Assam]) in India are associated with a higher risk of malaria, both among travelers and residents.2 Thus, even if chemoprophylaxis recommendations are changed to reflect the decreasing incidence of malaria overall in India, these, and possibly other, higher-risk states may require a more aggressive recommendation given this risk.

Another important consideration acknowledged by the authors is the issue of "VFR" (visiting friends and relatives) travelers. Such travelers repeatedly have been shown to be at higher malaria risk than casual tourists.3 As 96% of travelers were classified as tourists by the Indian Ministry of Tourism over the years of the study, it is likely that this separate category of travelers was not clearly identified. This is an important consideration, because even if the malaria risk was low enough among the general tourist population to recommend against universal chemoprophylaxis (which is debatable), this subgroup of travelers likely would be at high enough risk to justify chemoprophylaxis in many circumstances of travel to India. Teasing out this subgroup, therefore, is essential before comprehensive new recommendations can be devised.

In addition, the question of whether the data presented do indeed represent a declining malaria incidence in all travelers might be better defined with more robust data. One possible reason for the observations in the study is that more travelers are taking chemoprophylaxis, resulting in less malaria — an unlikely explanation, given the parallel decline in malaria among residents, but something not addressed by these data. In addition, the authors present several inconsistencies in the incidence data, leaving one to seek a more accurate and robust analysis before coming to broad conclusions. The authors also do not report any tests of statistical significance for their trends or for comparing the trends between travelers and residents of India. In addition, the results are highly dependent on denominator data regarding the number of travelers to India, obtained from the Indian Ministry of Tourism. It is not known how robust these data are, nor whether any change in data collection methodology occurred over the 14-year period of the study. If reporting bias is present, it could have increased the apparent number of travelers in the more recent years (as an example), which would make malaria incidence appear to decrease in those years artifactually. Finally, although one conclusion the authors come to is that most malaria in travelers to India is caused by P. vivax, the data regarding P. falciparum cases are not well presented; thus, more definitive data should be provided before this conclusion is reached. As the study notes, rates of P. falciparum can be high among residents of India, well above 50% of malaria cases in some areas.4 While tourists may not visit these areas frequently, more aggressive prophylaxis recommendations may be necessary when they are visited, even if other parts of the country have such recommendations relaxed in the future.

The authors recommend a flexible approach to malaria prevention in travelers to India, based in part on surveillance of the changing epidemiology in both the local population and travelers. This seems a reasonable approach, and while the data presented in this study are not robust enough to recommend a change in malaria prophylaxis recommendations for travelers to India at present, they do illustrate a compelling trend that likely signals a need for better surveillance and further study. Perhaps a broader revision of guidelines in the coming years will occur; it seems unlikely this would include a recommendation of withholding chemoprophylaxis for all travelers to India, but might base the chemoprophylaxis recommendation on region or season of travel to India.

References

  1. Mali S, Steele S, Slutsker L, Arguin P. Malaria Surveillance — United States, 2007. MMWR 2009;58(SS-2):1-16.
  2. Kumar A, Valecha N, Jain T, et al. Burden of Malaria in India: Retrospective and prospective view. Am J Trop Med Hyg 2007; 77(Suppl 6):69-78.
  3. Leder K, Tong S, Weld L. Illness in travelers visiting friends and relatives: A review of the GeoSentinel Surveillance Network. Clin Infect Dis 2006;43:1185-1193.
  4. Sharma SK, Tyagi PK, Padhan K, et al. Malarial morbidity in tribal communities living in the forest and plain ecotypes of Orissa, India. Ann Trop Med Parasitol 2004; 98:459-468.