Before discussing the relative merits or pitfalls of individual control techniques concerning myiasis it is important to understand firstly, what it is, and secondly, why control should be deemed necessary. A clear explanation of myiasis was given by Zumpt as “ the infestation of live human and vertebrate animals with dipterous larvae, which, at least for a certain period, feed on the host’s dead or living tissue, liquid body substances, or ingested food”.⁽¹⁾ There are three main dipteran families causing economically important myiasis in livestock and also occasional myiasis in humans. These families are the Oestridea, Calliphoridae (blowflies) and the Sarcophagidae (fleshflies). The justification for control is directly linked with the economic loss they inflict to livestock farmers. In 1989-1990, the UK alone had some half a million sheep infected with myiasis, mainly by the species Lucilia sericata.⁽²⁾ In humans, although myiasis cases are not common, caused mainly by Dermatobia hominis and Cordylobia anthropohaga, effective treatment is usually wanted by the patient mainly for the psychological reason of riding oneself of the maggot.⁽³⁾ The ensuing page will endeavour to give a succinct overview of available control methods.

Although there are a multiplicity of methods they can be broadly dichotomised into those for large scale control of livestock areas, and individual control in small focal regions. As is the case with any successful control campaign on finding a myiasis case the species has to be elucidated then the best, or most affordable control implemented.

The main control used against myiasis is the application of insecticides, using organophophrous and organocholrine compounds such as fenthion and dieldrin respectively. An example of their effect can be seen by studying the percentage of sheep infected by myiasis fifty years ago with that seen in 1990. The levels in the 1940’s were between 10-20%,⁽⁴⁾ this figure is now around 1.5%.⁽²⁾ This drop is due mainly to the advent of regular dipping regimens, and the topical application of insecticides at sites of infection. The success of a nation-wide insecticide based control regimen can clearly be seen in the UK eradication programme of Hypoderma bovis and H. lineatum. This began in 1967 with the use of topical applications of insecticides, such as Famphur, Crufomate, Phosmet and later with the drug ivermectin. Hypoderma has effectively been removed from the UK and control efforts are now concentrated on serological testing of imported cattle to prevent reintroduction.⁽⁵⁾ Control of Hypoderma was a direct result of its host specificity and availability of effective insecticides. The problem with species such as Calliphora vomitoria or Lucilia sericata is that carrion can also serve as breeding sites.⁽⁶⁾

The insecticide campaign in the UK for Hypoderma was implemented horizontally by farmers, backed by legislation. Although the results compare the following method, involving genetic control is implemented entirely differently. With a SIT ⁽⁷⁾ (sterile insect technique) campaign, explained below, the farmer has little direct input into the programme other than reporting any myiasis cases occurring in his livestock. The whole scheme is run by trained personnel in a hierarchical management style.

The screw-worm eradication programme in North and Central America has become a landmark project for entomologists in the effective use of genetic control over a large area. An effective campaign was initiated in 1959⁽⁸⁾ using males that had been sterilised by irradiating five day old pupae.⁽⁹⁾ Millions of sterile males where released in an area of high myiasis prevalence caused by Cochliomyia hominvorax. The males had to be able to compete with wild bred males for females, to copulate, and so cause females to lay batches of unfertilised eggs. The scheme was extremely successful and screw worm has been eradicated from large areas where previously it was a major problem. Certain factors made this approach attainable, the main one being that mass production of sterile males was possible, with production costs that could be justified by long term profits resulting from protected livestock. The production of huge numbers of sterile males is often the Achilles heel of SIT programs. Sterilised Hypoderma have been raised but the process is prohibitively expensive.⁽¹⁰⁾ These two control strategies have very different design structures and yet both have achieved excellent results.

The advent of ivermectin has also had a profound effect on myiasis in livestock as it is now possible to treat livestock for multiple parasites using single dose treatment. With the use of slow release boluses protection by ivermectin can be obtained throughout the year, this reduces the time needed to gather and treat herds when a topical insecticide regimen is being followed. The main problem with this drug is the long withdrawal period for lactating animals.

So far little has been discussed about human infection, this is because the control methods described thus far are not possible for treatment in humans. At present there are two options once a person is affected, either leave the larvae to develop and exit on its own accord, or remove it. Removal usually involves the occlusion of the spiracles of the larvae where they emerge from the skin. Oil and Vaseline have been used but very good results have been achieved using strips of bacon over the site, these strips entice the larvae out the cutaneous tissue far enough to be removed with small forceps. A method that can be used for humans but not livestock is education. Certain activities in endemic areas will increase the risk of infection, wearing damp clothes, or resting in sandy areas that are likely to have been soiled should be avoided. The problem with this advice is that many people travel to areas where myiasis can occur for their holidays. So telling people not to sit in the shade under a tree by the beach is not very practical advice. Repellents such as DEET can be used by tourists but this not practical for the indigenous people. It may be possible that livestock dipped with insecticide may increase human myiasis as a degree of repellencey may drive non-host specific dipteran species to seek alternative hosts, this would be a negative effect for local people, and a scenario not likely to occur with the SIT technique. Some work has been undertaken to create vaccines against myiasis as older animals often exhibit lower larval numbers when attacked. However to date a purified antigen for any of the problem diptera has not been produced.⁽¹¹⁾

The use of traps has been studied in detail and a commercially available lure, swormlure,⁽¹²⁾ is produced for use against Cochliomyia hominivorax. However population reductions did not compare favourable with either insecticide control or SIT. The production of traps and their maintenance was also higher than other conventional control methods. In the future traps may replace insecticide control if high attraction can be combined with a cheap durable design and a long lasting chemosterilant.⁽¹³⁾ For some species it is feasible to forecast the possibility of myiasis and so implement insecticidal control. The advantage of insecticides is the speed with which they can be implemented. At present the main stay of control is that of insecticides, due to the difficulties of employing a successful SIT campaign, and traps that are not yet effective enough. The main problem of insecticides is resistance which is occurring world-wide, this emphasises the need to improve other control measures.