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.