Sulphur as a treatment for scabies?

Often mistaken for eczema or impetigo, human scabies is a parasitic infestation caused by mites about 0.4 millimetres long, Sarcoptes scabiei variant hominis, which have the ability to penetrate the epidermis of their host to reproduce there. The average number of mites hosted by the host is usually less than 10, except in the case of "crusted scabies", where the host can harbour more than a million mites.

It is the adult female that plays a major role in the pathogenesis of scabies, while the adult male, nymphs, larvae, and eggs have a lesser effect in causing the disease. After fertilisation, the female settles just beneath the skin surface, beginning to burrow a tunnel in the horny layer at a rate of progression of about 1 to 2 millimetres per day, where she lays between 2 to 3 eggs per day. After 3 to 5 days, the eggs hatch and transform into adult mites within two weeks.

After 4 to 6 weeks, the infected patient develops an allergic reaction due to the presence of mite proteins and faeces, which leads to itching of varying intensity and with a nocturnal increase. Other clinical signs may also occur, but inconsistently, such as a skin rash (pruritic papules and nodules) or scabies burrows sometimes ending in a slight elevation, corresponding to the path taken by the mite. These lesions are generally located on the spaces between the fingers, the front of the wrist and elbow, the armpits, the lower part of the buttocks, the inner thighs or the male genital organs. The face is usually spared.

Contagious, scabies is transmitted in the vast majority of cases through prolonged and close skin contact, and this risk of transmission increases proportionally with the quantity of parasites present (the highest risk being associated with contact with individuals suffering from crusted or profuse scabies). On the other hand, indirect transmission through infested objects (clothing, bed linen, etc.) is a rarer occurrence in the case of classic scabies, but can be significant for individuals suffering from crusted or profuse scabies.

Given that there is an asymptomatic period of infestation, transmission can occur before the initially infested person develops symptoms. Present worldwide, scabies is more common in warm and tropical countries, as well as in areas with high population density. It is estimated that there are approximately 300 million cases of scabies worldwide each year.

Note : Outside of its human host, the mite responsible for scabies has a short lifespan, on the order of 1 to 2 days. Similarly, at a temperature below 20°C, the adult female loses her mobility and dies quickly (within 12 to 24 hours); above 55°C, the mite is killed in a matter of minutes.

The direct effects of scratching can lead to theinoculation of bacteria into the skin, which results in the development of impetigo. Impetigo can further complicate into a deeper skin infection, such as abscesses or a serious invasive disease, including sepsis. Moreover, in tropical regions, skin infection associated with scabies is a common risk factor for kidney disease and potentially rheumatic heart disease.

Similarly, individuals with a weakened immune system (immunocompromised), including those living with HIV, may develop a condition known as crusted scabies (Norwegian). This severe infection can harbour thousands, even millions of mites, and cause areas of dryness and scaliness (hyperkeratotic lesions) and widespread redness over most of the skin surface (erythroderma). Moreover, crusted scabies can spread very easily and thus lead to secondary infections.

A number of topical application products (acaricides) are available to treat scabies by destroying the mites and their eggs, such as the daily application of a sulphur cream/ointment with typical concentrations ranging from 5 to 10% for 3 to 5 consecutive days/nights, followed by a four-day break to allow the eggs to hatch before repeating the procedure for an additional 2 to 3 days for a high cure rate.

Numerous clinical studies have demonstrated the effectiveness of applying a sulphur ointment to eradicate scabies, often in a single dose. According to a study conducted on 420 patients with scabies, administering a single oral dose of ivermectin would be equivalent to a single application of a sulphur ointment after two weeks. A recent study also sought to investigate and compare the effectiveness of 5% permethrin cream and 10% sulphur ointment in the treatment of scabies on 218 patients. The researchers found that the mites show reduced sensitivity to 5% permethrin compared to the sulphur ointment.

Sulphur is the oldest known treatment for scabies, recognised throughout history. At the beginning of the 19th century, sulphur fumigation was used to treat those suffering from scabies. This method, conceived by Galès, involved placing patients in boxes and permeating them with sulphurous fumes. Several centuries ago, doctors also used it by painting the entire body of the infected patient with sulphur powder mixed with oil to treat scabies, as well as pediculosis and acne.

By what mechanisms?

Sulphur is believed to be toxic to the scabies mite (Sarcoptes scabiei). It is thought to work by inhibiting the growth of the parasite. According to studies, its parasiticidal activity may result from its conversion into pentathionic acid by skin cells (keratinocytes) and skin bacteria when applied to the skin. Furthermore, the keratolytic action of sulphur may aid in the removal of parasites from the stratum corneum.

However, the high content of sulphur can be associated with a high incidence of side effects, particularly if used over a prolonged period. It can cause intense itching and skin dryness, which in the most severe cases can even lead to ichthyosis (a severe form of skin dryness that involves excessive skin peeling).

Sulphur is safe for pregnant and breastfeeding women. However, to date, the safety of this treatment has not yet been tested on infants and is not recommended for children under six years of age.

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