The efficacy of seven southern African ethnoveterinary medicinal plants in the mitigation of cutaneous myiasis

Abstract

Myiasis, the infestation of the skin of mammals by larvae of a variety of fly species, has been recognised as a major disease from ancient times. Despite being well known as a disease, it remains poorly controlled in the animal production industry with severe economic losses resulting thereof. The control of the myiasis-causing flies relies heavily on the use of pharmaceutical chemicals such as the organophosphates, pyrethroids or insect growth regulators. Unfortunately these chemicals are characterised by a high potential to cause human or animal toxicity in addition to being environmental contaminants. Another problem resulting from their continued use has been the development of resistance in the treated flies. Newer products need to be discovered. One source of these compounds could be the ethnoveterinary plants already in use by subsistence farmers where insecticides are either unavailable or unaffordable. This study focused on seven plant species used as such in South Africa and Zimbabwe: Aloe marlothii A. Berger, Aloe zebrina Baker, Calpurnia aurea (Aiton) Benth, Psydrax livida (Hiern) Bridson (Canthium huillense), Clausena anisata (Willd), Erythrina lysistemon Hutch, and Spirostachys africana Sond. In the first step of evaluation, the selected plant species were screened for their activity against selected wound contaminating bacteria, which are recognised as the inciting factor attracting myiasis-causing flies due to the characteristic odour they produce. Using the microdilution and bioautography methods, all plants had inhibitory activity with the acetone extracts being superior with MICs ranging from 0.04 to 0.6 mg/ml. Using the surface area of a general wound and the density of the extract, it was demonstrated that wounds treated with these extracts could reach these MIC levels. It was concluded that these plants could be beneficial in managing myiasis, in part, by reducing secondary infections and fly attraction. Two studies were undertaken to determine the in vitro effects of these plant species on the behaviour and development of blowfly larvae. For the first study, larvicidal activity of the seven plant species was determined on third instar larvae fed on a combination of meat (30 g) baited with the acetone leaf extracts (1 ml at 10 mg/ml). Aloe zebrina, C. anisata, E. lysistemon and S.africana, induced developmental anomalies in the larvae including paralysis, prolongation of the prepuparium stage, reduced pupation rates, pupae malformation and reduced adult emergence. The results suggested that the plant extracts interfered with the neuroendocrine control in the blowfly. For the second study the four active plant species from the first study were further evaluated at 10, 25, 50, 75, 100 and 150 mg/ml. Larval behaviour, larval development and emergence of adult flies were assessed after exposure to the baited meat. The increasing concentrations of the acetone plant extracts decreased ingestion of the meat by the larvae, pupae mass and adult emergence rates. For C. anisata and S. africana extracts the increase in the concentration was also associated with larvae circling on top of the testing cups, as far away as possible from the meat, possibly indicating repellency and the emerging adult flies being smaller. Clausena anisata was selected for field evaluation on populations of blowflies on two farms (one control and one test site) in Mpumalanga (South Africa) over 12 weeks when fly populations were highest. Larvae exposed to liver baits treated with C. anisata showed slow development, prolonged larval period, smaller body size, sluggish behaviour, delayed pupation and reduced eclosion rates in comparison to the controls. No significant differences were present between the numbers and sizes of flies on the treated and on the control farm. A difference in fly species was noted on the baits before and after treatment, which may be indicatory of a repellent effect. It is concluded that C. anisata could be beneficial in an overall control strategy through its ability to decrease blowfly populations in the long term by reducing the total number of life cycles completed in a year and perhaps as a surface repellent agent. From the beneficial in vitro and in field presence of efficacy, isolation of the active compound(s) from C. anisata was attempted. As a first step, due to the potential repellent effect on the farm and the in vitro result suggesting a similar effect, the plant was screened for the presence of pyrethrins which are known natural repellents. None of the pyrethrins were present although a terpene compound with an Rf value close to the pyrethrin II compounds was present. Using bio-guided fractionation and column chromatography, seselin was isolated and identified from the n-hexane fraction which was most active. This study therefore adds support for the use of the selected plants in myiasis and more importantly demonstrates that C. anisata may be valuable as a new agent in the control of farm fly populations.