3.1. Pesticide Exposure and Poisoning
Pesticide poisoning can occur following acute or chronic (repeated) exposure. With the exceptions of pesticides such as warfarins, superwarfarins and coumarins that have delayed symptoms, an acute pesticide poisoning includes any health effect or illness that results from suspected or confirmed exposure to a pesticide occurring within 48 h [23
]. Chronic poisoning, on the other hand, results from low exposure levels occurring over a significantly longer time. Pesticide poisoning is a major contributor to the global burden of disease, especially in developing countries [24
]. Unfortunately, pesticide poisoning is often incorrectly regarded as a low public health priority area in many developing countries, where much emphasis is placed on infectious diseases.
Pesticide-related poisoning, with an estimated 168,000 deaths per annum, accounts for a significant proportion of 19.7% global suicides, while the proportion of pesticide-related suicides in Africa is 22.9% [25
]. Easy access to pesticides in Malawi increases the potential for pesticide self-poisoning, where pesticides were involved in 79% of reported cases [26
]. Furthermore, pesticides were reported to be among the leading causes of poisoning among children at the referral Queen Elizabeth Central Hospital in southern Malawi [27
]. Admittedly, these studies are very old, but studies from other countries [28
] indicate that pesticide-related illnesses still contribute significantly to poisoning cases.
Workers in developing countries are sometimes exposed to hazardous pesticides, including those that are severely restricted and banned in developed countries. For example, some pesticides such as ametryn, acetochlor, monosodium methylarsonate and profenofos are not approved in the European Union but are reported to be used on sugarcane farms in Malawi, where farmers were reported to suffer from skin irritation, headaches, coughing and running nose [31
]. Unfortunately, there appears to be no comprehensive and systematic monitoring of occupational exposure to pesticides in Malawi.
The public in Malawi could be exposed to hazardous levels of pesticides through food, water, and residential use of pesticides. The use of pesticides in food crops could result in significant exposure to pesticides through food [32
]. There have been reports of rather unusual risky practices among food-insecure communities of utilising pesticide-treated seeds for food consumption [33
]. Although the extent of this practice is not known, it is important to investigate factors that lead to this practice as well as potential mitigation measures. In addition to this risky behaviour, there are concerns of acute pesticide poisoning following exposure to a number of pesticides with a common mechanism of action, especially for foods that are consumed in large amounts [34
]. However, acute dietary pesticide poisoning is rare globally. On the other hand, the greatest concerns for pesticide exposure are associated with chronic exposure, especially since it has been shown that pesticide residues often exceed MRLs [36
]. Furthermore, use of pesticides in food crops for which they were not authorized is also common [38
]. Unfortunately, there appears to be no published studies conducted on pesticide residues in food in Malawi.
Pesticides can also contaminate surface and ground water through spray drift, surface runoff as well as drainage and percolation from treated areas. Cases of pesticide poisoning from drinking water are very rare [39
], although pesticides have often been detected in drinking water [40
]. Therefore, there are particular concerns for communities that utilize untreated drinking water from shallow wells located in agricultural areas. For example, atrazine and metolachlor were detected in surface water samples from Zomba and Bvumbwe areas in southern Malawi, especially with peak concentrations occurring in the first run-off event after pesticide application [42
]. Exposure from spray drift, run-off and percolation is expected to be significant in large commercial farms that utilize large amounts of pesticides using aeroplanes, booms and tractors.
Since forestry areas occupy over 30% of the land, the improvement of the productivity of trees, plantations, and forest areas can be a significant source of environmental pollution. Pesticides are mostly utilised for controlling pests and weeds in forest renewal areas, especially in tree nurseries, but are rarely used in fully-grown plantations [43
]. In the rare cases, Carbosulfan is used in the control of termites in miombo woodland forests to compliment “biopesticides” mainly made from of an extract of Zahna africana
, ash and animal dung [44
]. The use of the “biopesticides” is in line with the National Forest Policy [45
] which aims to conserve, establish, protect and manage trees and forests for the sustainable development of Malawi, with minimal environmental pollution.
In addition to exposure to pesticides from agricultural use, pesticide poisoning can also occur through residential use of pesticides. For example, positive associations have been shown between residential exposures to unspecified pesticides during pregnancy and childhood and childhood leukemia [46
]. Furthermore, household use of pyrethroid insecticides was shown to be a significant predictor of urinary pyrethroid metabolite levels in children [47
]. Therefore, measures are needed to manage residential exposure to pesticides. However, there appears to be no studies on residential exposure to pesticides in Malawi.
Humans and other species can be exposed to mixtures of pesticides from various sources. Exposure to mixtures of pesticides is recognized as an issue of great concern since even though the exposure levels for each individual pesticide may be below thresholds of concerns, the combined effect may be significant [48