Organochlorine pesticides are highly persistent in aquatic ecosystems. Amphibians, specifically anurans, play an intricate part in the aquatic food web, and have very permeable skin which makes them prone to bioaccumulation of persistent pollutants. In this study the bioaccumulation of various legacy organochlorine pesticides (OCPs)-including dichlorodiphenyltrichloroethane (DDT), currently used for malaria vector control (MVC)-was assessed along with a set of biomarker responses in Müller's clawed frog Xenopus muelleri collected from the lower Phongolo River floodplain in South Africa. Possible relationships between bioaccumulation and biomarkers (of exposure, oxidative stress biomarkers, and cellular energy allocation) alongside their temporal changes were investigated. The OCP concentrations showed a significant increase over time for the duration of the study. The increase correlated negatively with rainfall from the region. DDT levels were well below expected effects levels with p,p-DDE being the main contributing metabolite. The results of this study indicate OCPs actively accumulate at sub-lethal levels in aquatic frogs from the study area, while showing possible relations towards some of the biochemical stress responses measured. Most notable were negative relationships indicated between p,p-DDE and acetylcholinesterase, malondialdehyde, and carbohydrates and protein energy availability. Levels of DDT were not found to be significantly higher than other legacy pesticides in the frog tissue, although evidence of newly introduced DDT in the frog tissue was found. Further investigation about sub-lethal effects of these pesticides on anurans is required to gain better insight into their full impact on animal livelihood.

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Whilst frogs are generally considered as good indicators of environmental change, they are not widely utilised as indicators in ecological studies. With much of current research relying on model organisms as proxies for other animals or even humans, toxicology data on these model organisms is important for higher tier risk assessment studies and ecological modelling. The issue at hand is that for some topics, even when looking at model organisms, this empirical data does not seem to be as common as expected when doing literature research. Malaria vector control (MVC) is often a sensitive subject due to the paradox surrounding it (Bouwman et al. 2011). The lack of toxicological data mentioned feeds into the issues complicating debates on the chemical vector control interventions. Although being in use for decades, there are still many unknowns regarding the environmental impact of these pesticides. Informed decision making and policies require that this information first be available. The main research problem investigated in this review is this lack of toxicological and biomonitoring data available for one of the model organisms (Xenopus laevis) representative of aquatic ecosystems, and to a lesser extent, anurans. Species from the Xenopus genus are endemic to Africa and their distribution includes many areas where malaria vector control is applied. Reliable toxicology data on these frogs regarding malaria vector control pesticides (MVCPs) would strengthen the use of these animals as bioindicators.