Mae Chaem District in Chiang Mai Province in northern Thailand is hilly and rural. More than 80% of the district's population are members of hill tribes. To study the seasonal prevalence of Plasmodium vivax malaria, we recorded the monthly rainfall in Mae Chaem District from July 2000 to June 2001. Ambient temperatures for these months are usually in the range of 15°C to 20°C. For the corresponding time period, the number of smear positive cases of P vivax diagnosed each month, by the medical technologist of Mae Chaem Hospital, was obtained from the hospital database. A diagnosis of P vivax was made in 105 cases. We studied the influence of rainfall on the prevalence of malarial infection by examining the relationship between the monthly rainfall and the corresponding number of P vivax cases each month, as shown in the Table.
Table.
Monthly Rainfall and Prevalence of Plasmodium vivax Patients
| Month | Prevalence of Plasmodium vivax* (n) | Monthly Rainfall (mm)†† |
|---|---|---|
| July 2000 | 20 | 150 |
| August 2000 | 17 | 130 |
| September 2000 | 15 | 110 |
| October 2000 | 5 | 50 |
| November 2000 | 6 | 60 |
| December 2000 | 5 | 50 |
| January 2001 | 3 | 60 |
| February 2001 | 2 | 40 |
| March 2001 | 1 | 40 |
| April 2001 | 1 | 30 |
| May 2001 | 13 | 60 |
| June 2001 | 17 | 110 |
According to the electronic database of Mae Chaem Hospital, Chiang Mai, Thailand
According to the data from the atmospheric monitoring station of Mae Chaem District, representative of the domiciles of the patient population
There was a positive correlation between rainfall and the prevalence of P vivax infection (Spearman's rho = +.94; P < .01). There was a similar observation in a nearby community in a previous study. Nacher and associates[1] reported a positive correlation between rainfall and the prevalence of P vivax infection in a big refugee camp on the Thai-Burmese border. They proposed that migration may be a contributing factor.[1] However, ours is a closed hilly community and all cases were in the native population. None of them had any history of travel prior to the illness.
Malaria is transmitted by the bite of the female Anopheles mosquito. It goes through 4 distinct stages: egg, larva, pupa, and adult. Most eggs hatch into larvae within 48 hours and mature to the point of being able to transmit the parasite within 10–14 days.[2,3] The rainfall can create collections of water (“breeding sites”) in which Anopheles eggs are deposited and larvae and pupae develop into adulthood.[4] Female Anopheles must survive long enough after they have become infected (through a blood meal on an infected human) to allow the parasites that they now harbor to complete their growth cycles (“extrinsic” cycle).[4] That cycle takes 9–21 days at 25°C or 77°F.[4] Hoshen and Morse[5] said that climate was a major driving force behind malaria transmission. Warmer ambient temperatures shorten the duration of the extrinsic cycle, thus increasing the chances of transmission.[4] The seasons also determine human behaviors that may increase contact with Anopheles mosquitoes between dusk and dawn, when the Anopheles are most active.[4] Hot weather may encourage people to sleep outdoors or discourage them from using bed nets.[4] Once an infected mosquito bites a human, the incubation period for the injected parasite until the onset of symptoms or clinical diagnosis of P vivax ranges from 1 week to several weeks.[2,3] Based on our observation, it can be proposed that the prevalence of vivax malarial infection may depend on rainfall, which varies among seasons.[4] Our finding on the season change of malarial prevalence, low prevalence in the period between February and April, is also concordant with the previous report on seasonal variation of the adult vector in the studied community.[6]
However, there are some limitations of this study to be mentioned. We could not imply for the incidence and the occurrence of new cases, because there may be some cases of chronic vivax infection. In addition, before concluding that the increased prevalence is a result of rainfall alone, other confounding factors should be considered, which also determine the transmission of malaria, including other seasonal factors (such as ambient temperature[5] and humidity).
Contributor Information
Viroj Wiwanitkit, Department of Laboratory Medicine, Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand. Email: wviroj@yahoo.com.
Akkaradej Suyaphun, Mae Jam Hospital, Mae Jam, Chiang Mai Province, Thailand.
References
- 1.Nacher M, Carrara VI, McGready R, et al. Seasonal fluctuations in the carriage of Plasmodium vivax gametocytes in Thailand. Ann Trop Med Parasitol. 2004;98:115–120. doi: 10.1179/000349804225003145. [DOI] [PubMed] [Google Scholar]
- 2.US Centers for Disease Control and Prevention (CDC) Malaria. Available at: http://www.cdc.gov/malaria/biology/mosquito Accessed March 8, 2005.
- 3.Campbell CC. Malaria: an emerging and re-emerging global plague. FEMS Immunol Med Microbiol. 1997;18:325–331. doi: 10.1111/j.1574-695X.1997.tb01063.x. [DOI] [PubMed] [Google Scholar]
- 4.US Centers for Disease Control and Prevention (CDC) Malaria. Available at: http://www.cdc.gov/malaria/distribution_epi/epidemiology.htm Accessed March 8, 2005. [Google Scholar]
- 5.Hoshen MB, Morse AP. A weather based model of malaria transmission. Malaria J. 2004;3:32. doi: 10.1186/1475-2875-3-32. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Wiwanitkit V, Suyaphan A. The survey of malarial mosquito of Mae Suk subdistrict, Mae Jam, Chiangmai Province: a short report. Lampang Hosp Bull. 2003;24:56–58. [Google Scholar]