For more than four decades, cholera has recurred in Cameroon, affecting tens of thousands of people a year. Most recently, the West African country was one of four that had a death rate of more than five percent from the bacterial disease, exceeding the World Health Organization's target of less than one percent. Now, researchers have discovered one reason Cameroon has struggled to control the disease. Cholera follows different, distinct outbreak patterns in different climate subzones of the large country, the researchers reported in PLOS Neglected Tropical Diseases.
"The study highlights the complexity of cholera transmission, and its close link with environmental and climatic factors," said J. Glenn Morris, M.D., a professor in the College of Medicine and the director of the University of Florida's Emerging Pathogen Institute. "It underscores the fact that there is not a 'one size fits all' approach to cholera control; one has to be familiar with local environmental factors, and their impact on occurrence of cholera."
Cholera is a severe diarrheal disease caused by certain strains of the bacteria Vibrio cholera, which can be found free-living in the environment, often in bodies of water, or can be transmitted between people. Compared to many other countries where cholera is endemic, Cameroon has both a higher case and death rate. In part, this has been attributed to gaps in the knowledge of how, when, and where the disease spreads within the country.
In the new work, Moise C. Ngwa, of the University of Florida, USA, and colleagues used a unique health district level dataset to study cholera epidemiology throughout Cameroon. The data, collected between 2000 and 2012 by the Ministry of Public Health of Cameroon and the World Health Organization country office, include details on the more than 43,000 cases of cholera that occurred in Cameroon during the 13-year period. The researchers also obtained historic data on local rainfall and temperature, as well as information on highways, coastlines, lakes, and rivers.
"Cholera displays different epidemiological patterns by climate subzone," Ngwa said. "As such, a singleintervention strategy for controlling cholera within Cameroon does not appear to be feasible. Instead, there is a need to develop regionally or locally targeted prevention and mitigation strategies and plans."
During the 13-year span, on average, 7.9 percent of the cholera cases in Cameroon were fatal annually, and with an attack rate of 17.9 cases per 100,000 Cameroon inhabitants per year. But the seasonal patterns of illness varied greatly between different climate subzones of the country. In the northern Sudano-Sahalian subzone and the Tropic Humid subzone, the greatest number of cholera cases occurred during the peak rainy season, between July and September of each year; in the southern Equatorial Monsoon region, on the other hand, the lowest number of cases each year was during those peak rainfall months. Moreover, each region had distinct relationships between rainfall, temperature, and cholera cases -- in the Sudano-Sahalian and Guinea Equitorial subzones, increasing temperature and rainfall was found to be associated with higher rates of cholera transmission, whereas the opposite association was seen in the Tropical Humid and Equatorial Monsoon subzones.
"Environmental, natural, and structural determinants of cholera transmission differ in the various climate subzones, and these differences lead to different transmission incidence rate ratio," Ngwa said.
He suggests increased public health activity is necessary to reduce the transmission of the disease.
"Government officials should enable enhanced public health surveillance and rapid response to cholera outbreaks at the health district level in order to encourage a reduction in transmission," Ngwa said. "Improvements to water, sanitation and hygiene system deficiencies are needed in the long term, while the introduction of cholera vaccines are urgently needed in the interim. Improved personnel training and physical resources are also needed."
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Citation: Ngwa MC, Liang S, Kracalik IT, Morris L, Blackburn JK, Mbam LM, et al. (2016) Cholera in Cameroon, 2000-2012: Spatial and Temporal Analysis at the Operational (Health District) and Sub Climate Levels. PLoS Negl Trop Dis 10(11): e0005105. doi:10.1371/journal.pntd.0005105
Funding: The study was funded in part by a Supplement (awarded to MCN) to NIH grant R01AI097405 awarded to JGM, and JDS and YY were supported by NIH cooperative agreement NIH/NIGMS U54GM111274 awarded to Professor M. Elizabeth Halloran, without which this project would not have been realized. SL was supported in part by grants from NSF/EEID (EF-1015908) and NSF/WSC-Category 3 (1360330). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.