The review's objective was to evaluate the effectiveness of current treatments for scabies in order to inform practice and guide future research. The previous version of this review noted that clinicians faced considerable uncertainty when choosing the best treatment for scabies (Walker 2000). Ten years later the picture is a little clearer, but there are still considerable gaps in our knowledge.
All 22 included trials were designed to test the effectiveness of one or more treatments for scabies. Methodological quality varied between trials. Only two trials described both adequate randomization sequence generation and adequate allocation concealment, and the majority of the reports described neither adequately. The blinding was absent, or the degree of blinding was unclear in ten of the 22 identified trials, and losses to follow up were greater than 20% of the enrolled participants in four of the trials.
The results of this review suggest that, of the topical treatments for scabies, permethrin is most effective. Permethrin has been tested against topical crotamiton, topical lindane, and oral ivermectin in randomized controlled trials, and it appears to be superior to all three in terms of minimizing treatment failure in participants with a clinical diagnosis of scabies. In the one trial that tested permethrin against topical benzyl benzoate no difference in cure rate was detected, however this trial was small (53 participants) and the data used in the review related only to one week follow up.
In the subgroup of participants with microscopically confirmed scabies, permethrin was again superior to crotamiton, but there is uncertainty as to whether permethrin is superior to lindane. Permethrin also appears to be better at relieving itch than either crotamiton or lindane (itch was not reported as a separate outcome in the ivermectin versus permethrin trial). Unfortunately no trials comparing permethrin with either topical sulfur or topical malathion were identified; permethrin's relative effectiveness against these treatments therefore remains unknown.
In some countries natural pyrethrin-based topical treatments are available as an alternative to permethrin cream (Biele 2006). Pyrethrins are naturally occurring insecticidal compounds found in the Compositae family of plants (Wagner 2000), whereas permethrin is a synthetic pyrethroid analogue. Results from the two Italian trials included in this review suggest that pyrethrin is equivalent in effectiveness to both permethrin and benzyl benzoate.
Trials comparing crotamiton with lindane, lindane with sulfur, and sulfur with benzyl benzoate have all produced equivocal results, suggesting that there is no single most effective treatment out of these four topical options. In most countries the choice is in any case restricted, either due to lack of availability, or the lack of a licence for scabies.
Ivermectin is currently the only oral treatment for scabies that is in routine use. It appears to be more effective than both placebo and lindane, but less effective than permethrin. There was significant heterogeneity in the results of the five trials that compared ivermectin and benzyl benzoate, which may be explained by differences in drug regimen and length of follow up between the trials. After stratifying by drug regimen and length of follow up the relative effectiveness of ivermectin appeared to increase with increasing length of follow up. This may mean that ivermectin is slower in achieving cure than topical benzyl benzoate, however, this conclusion is rather speculative given these data.
An advantage of an oral antiscabietic treatment over a topical one is ease of use, particularly in hot humid climates, when engaging in mass treatment, or when treating children. However, ivermectin is not presently licensed for the treatment of scabies in most countries. Ivermectin's effectiveness, cost effectiveness, and safety in mass treatment in areas of high endemicity (preferably as a sustainable public health intervention) need to be further evaluated in larger trials of sufficient power.
Topical ivermectin has also been suggested to be effective after success in uncontrolled studies (Yeruham 1998; Victoria 2001). At present there is no commercially available topical ivermectin preparation available for the treatment of scabies, and randomized controlled trials are needed to evaluate this potential new treatment option.
There are still no published reports of randomized controlled trials that test the effectiveness of malathion against either placebo or another drug, despite over 30 years passing by since a non-controlled study first suggested that the drug was effective (Hanna 1978). The 2010 British National Formulary recommends malathion as the treatment of choice if permethrin is inappropriate (BNF 2010), despite the lack of evidence from randomized controlled trials. Such a trial comparing malathion with permethrin is needed to test their relative effectiveness.
We found trials of the herbal remedies toto soap (Alebiosu 2003) and lippia oil (Oladimeji 2000; Oladimeji 2005), but these trials did not meet the review's inclusion criteria. Both treatments look promising, but randomized controlled trials making direct comparisons with the existing best treatments are needed to assess their true relative effectiveness.
Treatment regimen was assessed in two trials. Maggi 1986 found that a one-hour application of lindane reduced itch compared with a much longer four-day application; the authors suggested that the itch may, at least in part, have been due to a dermatitis caused by the lindane treatment itself. Schenone 1986 compared two different regimens using decamethrin, a pyrethroid insecticide in the same class as permethrin. All participants were cured in both groups. Decamethrin is not commercially available for the treatment of scabies and we found no trials that tested its effectiveness against other treatments. Decamethrin (as deltamethrin) is usually used as an agricultural insecticide and its safety as an antiscabietic medication has not been established (WHO 1990).
The formulation of a topically applied product may influence its efficacy. For example, a 1% permethrin formulation marketed for the treatment of head lice appears to be less effective than the conventional antiscabietic 5% preparation, according to case reports (Cox 2000). None of the trials included in this review directly compared different strength formulations of the same treatment.
One trial compared different vehicles for the same drug (Avila-Romay 1991). Cold cream as a treatment vehicle for sulfur may be more effective than pork fat, with fewer adverse events. For resource-poor countries this could be a cheap and safe option, which in some circumstances might also be more culturally acceptable.
This review did not seek to assess the relative cost effectiveness of the various treatments for scabies; however, large cost differences are apparent. In the UK, costs are: permethrin £5.51 per 30 g of cream, benzyl benzoate £0.50 per 100 mL, crotamiton £2.99 per 100 mL, and malathion £2.96 per 100 mL (BNF 2010). When lindane was marketed in the UK it was a fifth the cost of permethrin per treatment (BNF 1997).
We did not specifically attempt to assess the effectiveness of treatments for crusted scabies, and none of the included trials selected participants with this diagnosis. Caution should therefore be exercised in generalizing the results of this review to the treatment of patients with atypical severe scabies infection. This is an important area where more research is needed.
Caution should also be exercised in generalizing these results, which were obtained from trials that recruited individual participants (mostly in the outpatient setting), to the management of outbreaks in institutions. Given the burden of disease caused by scabies within institutions, such as long-term healthcare facilities, the inclusion of such patients in randomized controlled trials of effectiveness would be beneficial.
Mass treatment of a community in order to eradicate scabies has been tested in two studies (Dunne 1991; Bockarie 2000), both of which used oral ivermectin. Unfortunately neither of these studies met the review's inclusion criteria (Bockarie 2000 was an uncontrolled trial, and Dunne 1991 recruited participants on the basis of a diagnosis of onchocerciasis). Further research is needed to test the effectiveness, safety, and practicality of this approach to the management of scabies, particularly in areas of high prevalence.
Serious adverse events leading to death or permanent disability were not reported in any of the included or excluded trials. This review did not seek to systematically review the literature on the safety of antiscabietic treatments, but a number of notable reports of serious adverse events that have been published elsewhere are discussed below.
Convulsions and aplastic anaemia have both been reported with the use of lindane (Rauch 1990; Elgart 1996); in some cases this being thought to be due to the application of the drug to non-intact skin. Lindane was withdrawn by the manufacturer from the UK market in 1996, but this was for commercial and not toxicological reasons. In 1995, the US Food and Drug Administration designated lindane as a second-line treatment due to its potential toxicity; only to be used in those who have failed to respond to, or who are intolerant of, other antiscabietic treatments (WHO 2003).
Ivermectin has been very widely used in the treatment of onchocerciasis (predominantly in adults) and even with repeated doses serious adverse effects have been rare (DeSole 1989; Pacque 1990). However, an increased risk of death among a group of elderly patients with scabies in a long-term care facility has been reported (Barkwell 1997). Whether this was due to ivermectin or to interactions with other scabicides, including lindane and permethrin, or other treatments such as psychoactive drugs was not clear and there was considerable discussion of the validity of this report (Bredal 1997; Coyne 1997; Diazgranados 1997; Reintjes 1997).
Rare adverse reactions have been reported with the use of both permethrin (dystonia, Coleman 2005) and natural pyrethrin (fatal asthma, Wagner 2000).
The relative purity of the active ingredients of certain topical treatments and their isomeric ratios may also affect drug toxicity. In particular, very little is known about the effects of exposure to different isomeric grades of permethrin. Clinical grade material is 25:75 cis isomer:trans isomer and agricultural grade is 40:60. The cis isomer has 10 times the acute toxicity and there could be dangers in people in resource-poor countries using agricultural-grade permethrin for treating human infestations (personal communication from Ian Burgess, Medical Entomology Centre, Cambridge). Similar problems have been reported with the inappropriate use of agricultural grade malathion for treating human infestations (Petros 1990).
A search of the WHO Adverse Drug Reaction Database in 1998 for a previous version of this review found reports of serious adverse drug reactions for convulsions (benzyl benzoate 4, crotamiton 1, lindane 38, malathion 2, permethrin 6) and death (benzyl benzoate 0, crotamiton 1, lindane 1, malathion 0, permethrin 5) (Walker 2000). A search for this update of the review of the UK Medicines and Healthcare Products Regulatory Agency database of suspected drug reactions found reports for convulsions (benzyl benzoate 1, crotamiton 0, lindane 3, malathion 0, permethrin 0, sulfur 0, ivermectin 1) and death (benzyl benzoate 0, crotamiton 0, lindane 1, malathion 0, permethrin 1 (intra uterine death), sulfur 0, ivermectin 3) (MHRA 2006). Extreme caution must be shown in interpreting these reports, as they are clearly influenced by the extent to which the products are used and by the quality of the reporting. Neither can a causal link be assumed for any of the reported events.