Malawi has been designated as a country whose population is at high risk of EC development [1, 2]. The reason for this elevated risk has not been determined, and data on risk factors in sub-Saharan Africa are limited. As EC treatments will not be available soon in low- and middle-income countries, the exploration of risk factors is needed to identify subpopulations at high risk of EC development, and to develop prevention strategies to reduce morbidity and mortality from this disease in the long term. In this study, we investigated associations of EC occurrence in Malawi with nutritional, lifestyle, and infectious factors.
We collected data on risk factors in 227 patients, of whom 157 presented with endoscopically proven EC. All biopsies that were taken showed histopathological characteristics of ESCC; no adenocarcinoma was found. We observed 100% accordance between the macroscopic appearance and histological diagnosis. These findings are consistent with those of another study conducted in Malawi, in which no esophageal adenocarcinoma was found in 82 EC biopsy samples taken between 2004 and 2008 .
In this study, the presence of EC was associated with smoking, age and the consumption of alcohol, hot food or tea, and smoked fish. In the logistic regression analysis, age and the consumption of hot food or tea remained significant.
The association of age with the presence of EC is not surprising and is consistent with other data from Africa. ESCC incidence rates increase with age, and approximately 80% of people with EC in eastern Africa are at least 50 years of age . On the other hand, 18.7% of patients in the tumor group in this study were aged ≤ 40 years. Thus, it is unlikely that the high occurrence rates of EC in Malawi can be explained primarily by age.
In Europe and the United States, alcohol and tobacco consumption are the predominant risk factors, with a strong synergistic effect on ESCC risk . Smoking and alcohol consumption were also identified as major risk factors for EC in a high-incidence area in South Africa . In this study, alcohol consumption and smoking were associated with EC, but 43.5% of patients with tumors consumed neither tobacco nor alcohol. Similarly, the consumption of alcohol and tobacco is less common in high-incidence areas in Asia than in Western countries, and seems to play a lesser role in the etiology of ESCC . Tran et al.  reported relative risks of 1.34 for cigarette exposure (ever-smoking) and 0.92 for alcohol consumption in a high-risk area in China. In a high-risk area in Iran, the history of alcohol consumption was negligible (1%) and the use of cigarettes was limited (27%) among patients with ESCC . Thus, other environmental factors also should be considered in high-risk areas.
Associations of ESCC with other risk factors have been discussed in the literature. Environmental factors such as the consumption of hot food and beverages and poor diet have been associated with elevated risks of EC and ESCC in high-risk areas . The association of EC occurrence with the consumption of hot tea and food in this study is similar to the increased risk of EC associated with this factor in high-risk areas in Iran , South America , and China . Hot beverages can harm the esophageal epithelium via thermal effects and chemical constituents . Local hyperthermia impairs the barrier function of the esophageal epithelium , and mild and moderate esophagitis has been associated with the consumption of boiling-hot beverages . In this study, we found no association of EC with the consumption of fish or vegetables. Studies conducted in high-risk areas in Iran and China revealed an inverse association of ESCC with the consumption of fresh vegetables [28–30] and fish . However, our results are consistent with those of a case–control study conducted in Zambia, which revealed no significant difference in the consumption of fruits, vegetables, and fish between EC cases and controls .
We investigated additional factors that are common in southern Malawi, such as the consumption of smoked fish and maize porridge; the presence of EC was associated with smoked fish consumption. Carcinogenic substances such as polycyclic aromatic hydrocarbons (PAHs) have been found in smoked fish, especially fish smoked heavily in traditional kilns . High exposure to PAHs in the general populations of high-risk areas in Iran [34, 35] and China  have been reported. PAH-DNA adducts have been detected in esophageal biopsy samples , and greater PAH exposure of the non-tumoral esophageal epithelium was detected in patients with ESCC than in controls in a high-risk area in Iran . Foods containing protease inhibitors, such as beans, which are frequently consumed in southern Africa, have been associated with EC . Furthermore, EC was associated with the consumption of purchased maize in South Africa , possibly due to nutritional deficiencies , fungal contamination , or increased intragastric production of prostaglandin E2, which leads to increased cell proliferation [42, 43]. However, we found no association between the consumption of maize porridge and EC in this study.
HIV, CMV, HSV, and H. pylori infection are prevalent in Africa [12–15]. Our serological investigation revealed no association of EC presence with positive HIV serology, consistent with the finding of another study from Malawi , or with positive serology for CMV, EBV, HSV, or VZV. CMV is known to have onco-modulatory properties , and EBV has been detected, for example, in patients with nasopharyngeal carcinoma . Consistent with our findings, no association between CMV and EC was found in studies of tissue samples from China [47, 48]. Various prevalence rates of EBV in tissue samples have been reported; EBV DNA was found only in subsets of ESCC tissue samples [47, 49–51]. Similarly, various prevalence rates of HSV in EC tissue samples in high-risk areas of China have been reported [47, 48, 52]. The role of H. pylori in gastric malignancies has become evident , but its role in ESCC remains unclear . We found no association between antibodies against H. pylori and the presence of EC in this study, in contrast to the decreased risk of ESCC in individuals with H. pylori infection observed in other case–control studies [55, 56].
The lack of association between tumor development and the infectious agents investigated in this study may be attributable to high seroprevalence among control patients and low prevalence among patients with tumors, or to the small number of blood samples investigated. The latter can be considered a study limitation. Another technical limitation is that we investigated associations of infectious agents with EC only serologically, and not using tumor tissues. A further limitation of our study is that we used a self-generated questionnaire and not a validated questionnaire, as no such instrument was available for the Malawian setting in 2010, when the study began.
In our sample, many control patients reported the same nutritional habits as did patients with tumors. High PAH levels in urine samples from healthy subjects in high-risk areas in China, Iran, and Brazil have been reported . Thus, genetic susceptibility also needs to be considered in future studies. The risk of ESCC was elevated among patients with family histories of upper-gastrointestinal cancer and EC in a high-risk area of China . Furthermore, EC is more common in certain ethnic groups in high-risk areas, such as Kazakhs in China [57, 58], and the genetic background of migrants in China appears to influence the incidence of ESCC . Variants in several chromosome regions were associated with an increased risk of EC in China . Genetic diversity appears to be greater in African populations than in populations living on other continents . Data on the genetic susceptibility to EC in Africa are limited. In South Africa, certain variants of alcohol dehydrogenase were found to be associated with an increased risk of EC development . Bye et al.  suggested that the genetic contribution to ESCC differs in South African and Chinese populations due to differences in genetic architecture. A recently published transcriptomic analysis conducted with patients with ESCC in Malawi revealed genetic aberrations similar to those found in Asian and North American cohorts, such as TP53 mutations . Mutational signature analysis showed common signatures associated with aging and cytidine deaminase activity; a third signature was of unknown origin, and signatures of inhaled tobacco use, aflatoxin, and mismatch repair were notably absent. Thus additional factors that are characteristic in Malawi should be investigated and the genetic susceptibility to EC should be considered in further studies.
The results of this study should be viewed as only one component of a systematic understanding of risk factors contributing to the high incidence of EC in southern Malawi. Investigations of fungal co-infection and toxins, maize contamination, and the association with HPV are in progress or have been published . The performance of epidemiological studies in low-resource settings is always challenging. Thus, our results should be interpreted with caution, and more standardized investigations with larger patient samples are needed. However, our results suggest that more attention should be devoted to the development of primary prevention strategies for non-communicable diseases in sub-Saharan Africa. This effort may entail the addressing of other challenges, such as the shortage of healthcare workers; lack of infrastructure; insufficient access to early diagnosis, treatment, and palliative care; and poor public awareness of cancer .