Researchers and epidemiologists are observing a worrisome trend that shows Sarcoptes scabiei var. hominis, the human itch mite responsible for scabies, may be increasingly resistant to topical permethrin therapy. Findings from a real-world experience on the topic were published in the Journal of the European Academy of Dermatology and Venereology.
In a letter to the editor, a small team of clinicians described their experience with patients with scabies that were evaluated at an Italian clinic for sexually transmitted infections. According to the researchers, the clinic’s treatment protocol is more “aggressive” than that recommended by European guidelines.
These guidelines recommend patients with scabies, as well as coinhabitants, apply permethrin 5% cream from head to toe and wash off the cream after 8 to 12 hours. The regimen is repeated after 5 days to coincide with the mites’ life cycle which requires 4 to 7 days for the mites to reach adulthood. The Italian clinicians also prescribe benzyl benzoate 25% cream to patients who do not achieve a response to permethrin, while oral ivermectin is a third-line approach.
In their experience, the clinicians wrote that 34 out of 155 patients with scabies responded to permethrin, including 13 patients who responded to 1 cycle and 21 patients to 2 cycles. A total of 96 patients had no response after a median of 2 cycles, or 8 applications, and 25 patients were lost during the follow-up period.
A total of 75 patients at the clinic received benzyl benzoate 25% cream, with 47 achieving clearance and 8 who were lost to follow-up. Oral ivermectin was administered to 17 of 20 patients who were resistant to benzyl benzoate 25% cream, with 13 of these patients achieving cure and 4 lost to follow-up.
Although other researchers have posited lack of compliance to topical therapy as a reason for lack of efficacy, the clinicians of this experience state that nearly two-thirds of patients who did not receive benefit with permethrin responded to another topical agent, possibly indicating “a specific resistance to permethrin and excluding compliance issues related to the administration modality.”
Limitations of this study were the small sample size and single-center design. The researchers added that “no large cohort studies have been performed,” which further limit the understanding of the scabies’ potential insensitivity to first- and second-line treatment options.
Based on their findings, the clinicians wrote “we can answer that, considering present and past studies, scabies is clearly becoming less sensitive to permethrin therapy.”
Balestri R, Magnano M, Infusino SD, Rizzoli L, Girardelli CR, Rech G. Scabies is becoming less sensitive to permethrin therapy. J Eur Acad Dermatol Venereol. Published online July 26, 2021. doi:10.1111/jdv.17538