DISCUSSION

Myiasis is defined as a disease caused by the infestation of body tissues by larvae of several fly species of veterinary and medical interest. The disease occurs predominantly in rural areas and is associated with poor hygienic practices. Myiasis producers can be divided into three classes; obligate parasites requiring living tissue for larval development, facultative parasites whose larvae usually develop on carrion but may invade wounds, and accidental myiasis producers whose eggs or larvae are accidentally ingested and not killed in the intestine. The fly species that infest healthy tissues are called biontophage and are responsible for furunculoid myiasis, the cutaneous form of this condition. Among these species are Dermatobia hominis (the most frequent), Cochliomyia hominivorax, and Oestrus ovis. The larvae that infest necrotic tissues (cavitary myiasis) include those species of the genera Sarcophaga, Lucilia, Calliphora, and Musca.

Dermatobia hominis is widely distributed in both South and Central America. The adult fly usually lays its eggs, enclosed in a form of cement, on the surface of foliage or thorax of certain insects, such as day flying mosquitoes and flies. These insects may transfer the eggs of Dermatobia hominis to the skin of humans or other new animal hosts via phoresis (“hitchhiking”). The larva hatches from the egg when it senses warmth and is capable of penetrating the human skin within a further 5–10 minutes. There have been several approaches to control of myiasis—for example, Dermatobia hominis has been controlled with insecticides including DDT and Toxaphene in Brazil.¹ Based on the knowledge that female flies mate only once, a control programme using male flies, sterilised by radiation, has successfully eradicated Dermatobia hominis in Curaçao.¹

In the furunculoid form of the disease, the larvae penetrate the skin leading to the formation of painful, inflammatory nodules that can form fistulae to internal organs or to the surface of the skin. The lesions are usually single and are most common on the head. Ophthalmomyiasis may ensue when eggs are deposited in moist eyelid margins and larvae migrate beneath the conjunctiva from where removal is difficult; in severe cases, loss of the eye can occur. Furunculoid myiasis usually has a good prognosis, although tetanus and secondary infection may occur as complications. Frank suppuration is less commonly seen as a possible result of the bacteriostatic activity of the larval gut.

Examination of the central pore for a typical serous exudate and the characteristic spiracles of the larva provides the diagnosis. The treatment consists of the removal, often surgical, of the whole parasitic larvae and thorough cleansing of the affected area. Furunculoid myiasis is usually treated by the occlusion of the fistula opening with vaseline ointment, nail polish or, as a traditional remedy, pig fat,² making larval respiration impossible. Following the administration of local anaethesia, gentle compression of the sides of the nodule and the use of surgical tweezers usually results in successful removal of the larvae. It is important to remove the entire larva otherwise the cavity will not heal and can become chronically infected. Afterwards, the cavity should be cleaned and topical/oral antibiotics administered to prevent secondary infection.

Although myiasis is infrequent, many reports can be found in the medical literature.

Often it affects children and occurs in regions other than the genitalia.^3–7 The published cases of genital myiasis are very few in number and these reports refer to patients of various ages, the youngest being 7 months old.^8–16

We found only two case reports of scrotal myiasis following a literature search. The first case occurred in a 48 year old man attending a medical facility in the Unites States after spending a week in Costa Rica.¹⁷ This patient’s scrotum contained two orifices with one larva in each. The second case was a 21 year old man who attended a urology clinic in Turkey with a mass in his scrotal tissue of 3 weeks’ duration.¹⁸ Neither the clinical examination nor the ultrasonography elucidated the cause of the problem which was only determined after the surgical removal of the peripheral testicular mass. When the nodular lesion was opened, it was possible to observe the motile larva inside. In both cases, the larvae were those of the fly Dermatobia hominis. We also found a citation from Norway where the authors mention, in a letter, acute balanoposthitis caused by infestation with Cordylobia anthropophaga.¹⁹

Genital myiasis should be considered as a differential diagnosis for nodular inflammatory lesions on the genitalia, particularly when patients have returned from tropical countries where myiasis occurs more frequently. Careful history taking and examination of the genitalia may allow correct diagnosis and successful treatment without the need for a tissue biopsy and/or excision of the lesion.

CONTRIBUTORS

The paper was written by MP and DL; MP, RV, GR were involved in the treatment and follow up of the case; NB provided the laboratory expertise; DL took the photograph in figure 1.