Our study is the first one reporting on population wide sensitization to mopane worm in a rural community in Zimbabwe. While there were more adults (78%) than children (22%) in the study population, the proportion of males compared to females was similar in both age groups. There was a higher response rate for skin prick testing (93.15 %,) than in the feasibility study (39) (37%). We believe the response rate was increased by the continuous engagement with the community after completing the feasibility study as well as the use of the volunteer sampling method.
Sensitisation was predominantly to mopane worm (14.29%) and Tyrophagus putrescentiae (14.29%), closely followed by mopane leaves (13.42%) and both Alternaria alternata (6.49%) and Dermatophagoides pteronyssinus (6.49%). Sensitivity to Tyrophagus putrescentiae was almost exclusively in adults (17.22%) compared to children (3.92%). A high prevalence of Tyrophagus putrescentiae (72%) contrasted to mopane worm (50%) sensitisation was observed in the previously conducted feasibility study in Gwanda district (39). Tyrophagus putrescentiae sensitization has been reported in Europe and Asia, and its clinical relevance, particularly in elderly subjects, is increasingly recognised (49). Although co-sensitisation to Dermatophagoides pteronyssinus and/or Dermatophagoides farina is common among patients with sensitivity to Tyrophagus putrescentiae (50, 51), we are unaware of any studies addressing the molecular composition of mopane worms versus Tyrophagus putrescentiae. We speculate cross-reactive recognition of the two allergen sources as an explanation for the consistently high levels of T. putrescentiae reactivity in contrast with reactivity to D. pteronyssinus and D. farinae. Mite allergy due to D. pteronyssinus and D. farinae is common in Zimbabwe and has been associated with respiratory allergy (52). While these data informed the inclusion of these allergens in a screening panel in Gwanda district in order improve the detection rate of atopic individuals in the area, the dominance of Tyrophagus putrescentiae was unexpected and warrants further investigation.
Atopy and polysensitisation, which occur as a result of either cross reactivity or co-sensitisation (53), were more frequently observed in adults than children, particularly among women. Among the children, girls had higher sensitisation rates than boys. The proportion of self-reported allergy symptoms upon exposure to plants or furry animals and family history of allergy was generally higher in adults than in children. The cross sectional nature of the study design and the use of non-probability sampling, however, limits us from concluding that there is increasing sensitisation with increase in age. It is well documented, though, from longitudinal studies that the prevalence of allergic sensitization increases with increasing age from childhood to early adulthood and then starts to decrease thereafter (54). The high level of polysensitisation among adults, especially women, was a cause for concern because of the associated increase in risk and severity of allergic diseases (53). Allergy can develop at any point in life, including in adulthood, due to exposure to environmental, occupational and lifestyle factors (55). Furthermore, dependence on the abundant and diverse local flora and fauna for food and other purposes in African communities influences the observed allergy profile (56). It was not surprising that women, who are often responsible for several activities including mopane worm harvesting to ensure their families’ wellbeing, were the most sensitised subgroup in this study population.
Cluster analysis was used to identify the natural grouping of allergens amongst the participants. It was found that mopane worm, mopane leaves and Tyrophagus putrescentiae tended to cluster together in adults. While there was frequent co-sensitisation between mopane worms and mopane leaves, probably due to co-exposure, there were also some cases of monosensitivity to both extracts. The frequent co-sensitisation to mopane worm and Tyrophagus putrescentiae extracts could be due to the existence of cross reactive molecules or reactivity to pan-allergens such as tropomyosin (57). Such cross-reactivity is likely to involve allergens that are not represented in both D. pteronyssinus and D. farinae that were also tested in this population. Additionally, we also believe there could also be a case of co-exposure between the allergens. Tyrophagus putrescentiae, a type of storage mite, is a pest of many high fat or protein content foods (58) and could possibly contaminate mopane worm during storage (59, 60).
Mopane worm harvesting and post harvesting practices were similar among the 3 clusters and yet there was almost no mopane worm sensitisation (2%) in cluster 2 that consisted mostly of Tyrophagus putrescentiae and Alternaria alternata sensitised adults. It has been reported that pre-existing IgE reactivity profile does not seem to change substantially in adulthood and the limited neo-sensitisation to mopane worm that was observed in this cluster supports the phenomenon (61). Prevalence of mopane worm sensitisation, however, was high amongst adults in cluster 3 (96.88%) relative to cluster 2. The longer duration of exposure through harvesting and post-harvesting activities that was reported in cluster 3 could be the influencing variable. While cluster analysis was able to provide insight into underlying co-sensitisation patterns, there are some methodological limitations to consider. Cluster analysis is exploratory in nature and there are several different clustering algorithms to choose from. We selected Ward’s method of hierarchical clustering as it has been used in other studies assessing allergen clusters in different populations (62). Agglomerative hierarchical clustering includes all observations into a cluster and even though three clusters were optimally selected, there are likely smaller, but important, sub clusters within the main clusters (63). To establish whether a positive mopane worm skin prick test was due to true sensitization or cross-reactivity would require the use of component-resolved diagnosis (CRD), a more specific diagnosis which utilises pure allergenic molecules as opposed to a natural extract (64, 65).
There is substantial evidence from occupational studies that exposure to insect and insect-derived materials result in sensitization that subsequently leads to elicitation of respiratory allergy (33). While studies on edible insects have focused on sensitisation by oral exposure (32), it would appear that dermal or inhalation exposure are also important (66) as a result of processing activities that need to take place before the insects are actually consumed (67–69). The exposure scenario was useful in depicting the various opportunities of exposure to the mopane worms. The majority of participants (70.77%) reported that they eat mopane worm and 62.5% reported that they harvest mopane worm. An entomophagy survey in Zimbabwe concluded that consumption of mopane worm has remained undiminished over the years across the country (25). Interestingly, while it was found in this study that girls had higher sensitisation rates than boys, there were significantly more boys than girls who reported eating and harvesting mopane worm. Our exposure assessment, however, was limited since it was based on self-reports and prone to recall bias. As a result, there were a number of missing values particularly for variables that needed a fair amount of recall such as the frequency of harvesting per week and the lifetime duration of harvesting. A more robust longitudinal study is required to further explore these findings.
The results from the adjusted logistic regression models between mopane worm sensitisation and selected exposure variables were in line with our hypothesis that extensive contact with mopane worm through various activities could lead to sensitisation. Mopane worm exposure through harvesting and post harvesting practices increased the likelihood of mopane worm sensitisation regardless of age, gender, education, polysensitisation and family history of allergy. However, none of the reported adjusted odds ratios were statistically significant, suggesting insufficient evidence to reject the null hypothesis that there is no relationship between exposure and sensitisation to mopane worm. The use of non-probability sampling in the recruitment of participants could weaken the study, although this could not be avoided, it potentially limits the generalizability of the study findings. Notwithstanding this, we believe that our findings are a reasonable representation of sensitisation patterns in Gwanda district and other areas with similar environmental exposures. We also raise pertinent research questions with respect to cross-reactive recognition of other allergenic molecules including Tyrophagus putrescentiae.