The impact of climatic risk factors on the prevalence, distribution and severity of acute and chronic trachoma

Methodological quality of the review: Medium confidence

Author: Ramesh A, Kovats S, Haslam D, Schmidt E, Gilbert C

Geographical coverage: Africa

Sector: Impact of climatic risk factors

Sub-sector: Acute and active trachoma

Equity focus: None specified

Review type: Effectiveness review

Quantitative synthesis method: Narrative synthesis

Qualitative synthesis method: Not applicable

Background:

Trachoma is the most common cause of infectious blindness worldwide. Hot, dry climates, dust and scarcity of water are thought to be associated with the distribution of trachoma but the evidence is unclear.

Objectives:

To evaluate the epidemiological evidence regarding the extent to which climatic factors explain the current prevalence, distribution and severity of acute and chronic trachoma.

Main findings:

Authors included eight observational studies in the review, all of which were undertaken in Africa. Three studies were conducted in West Africa: two in Mali and one in Burkina Faso. Five were conducted in East Africa: three in Ethiopia, one in Tanzania and one in South Sudan. Four studies were judged to be of moderate quality, and four were of lower quality.

Temperature was assessed in the four studies of moderate quality. In Mali, the prevalence of active trachoma was significantly lower in areas with higher annual coverage temperature and higher sunshine fraction. Monthly averages showed different results. In the study from Burkina Faso, authors noted the prevalence of active trachoma was lower in areas with higher minimum temperature. In Sudan, authors noted that findings did not predict the distribution of active trachoma.

In three studies included in the review, authors noted a lower prevalence of trachoma at higher altitude, but two did not take into account other climatic factors.

In relation to rainfall, authors noted that findings were mixed. Two studies – one conducted in Sudan and one in Mali – showed significant findings. However, two other studies had weaker evidence of an association between rainfall and trachoma. Authors also noted that there was consistent but weak evidence that the prevalence of trachoma was higher in savannah-type ecological zones. There were no studies on the effects of climate in low endemic areas, nor on the effect of dust on trachoma.

Authors concluded that current evidence on the potential role of climate on trachoma distribution is limited, despite a wealth of anecdotal evidence. They stated that temperature and rainfall appear to play a role in the transmission of acute trachoma, possibly mediated through reduced activity of flies at lower temperatures. Further research is needed on climate and other environmental and behavioural factors, particularly in arid and savannah areas.

Methodology:

Searches were undertaken separately for active and chronic trachoma (hereafter termed trachoma outcomes). The following electronic databases were searched: CAB Abstracts, EMBASE, Global Health, MEDLINE and Web of Science. Websites of international agencies were also searched: World Health Organisation (WHO), WHO Special Programme for Research and Training in Tropical Diseases (TDR), Inter-governmental Panel on Climate Change (IPCC), United Nations Children’s Fund (UNICEF), UN-Habitat, The Carter Center, the International Trachoma Initiative (ITI), Sightsavers, Helen Keller International, Fred Hollows Foundation and Christian Blind Mission.

Studies were only included if they quantified an association between a climate factor (temperature, rainfall, altitude etc) and a trachoma outcome. Papers were not excluded based on geographic location of study, age of participants or language of journal publication. Peer-reviewed journal articles and reports from leading international agencies (e.g WHO) published between 1 January 1950 and 1 April 2012 were included.

Screening of studies for inclusion, data extraction and quality appraisal of included studies were conducted by two reviewers independently. To quality assess each study, authors used a STROBE checklist for cross-sectional studies.

Authors used a narrative synthesis of cross-sectional studies, which seemed appropriate due to the heterogeneity of included studies.

Applicability/external validity:

Authors noted that all studies were undertaken in Africa, which limits the generalizability of the findings, and that it is possible that climate-trachoma associations are location specific.

Geographic focus:

Studies exploring the association between trachoma prevalence and climate type showed that the overall prevalence of trachoma appeared to be higher in semi-arid savannah areas. Authors noted this finding to be consistent with anecdotal evidence.

Review authors also noted that seasonality was not associated with the prevalence of trachoma in India. In addition, authors also stated that when using altitude as a proxy for temperature, there was some evidence that in highly endemic areas of East Africa, the prevalence of trachoma is lower at high altitudes, however two included papers found this to be non-statistically significant.

Summary of quality assessment:

Overall, there is medium confidence in the conclusions about the effects of this study.  Authors used appropriate methods to screen, extract data and quality assess included studies. Authors used a narrative synthesis, which seemed appropriate due to heterogeneity of included studies. However, authors did not conduct a thorough search of the literature to ensure all relevant studies were identified. In addition, authors did not report the summary of each criterion of each study included. Nevertheless, authors appropriately reported limitations of the review.