The Extraction of Sarcoptes scabiei from Mammalian Hosts

Abstract

Sarcoptes scabiei is an ectoparasitic mite that either burrows into the epidermis of mammalian skin or, in more populous infestations where there is strong host reaction, will live in the fissures and chambers of crusts formed from the host's exudates. Human infestations are usually of the former kind and mites are frequently discovered by scraping the skin (Muller et al., 1973, Arch. Dermatol. 107: 70), adhesive tape swab (Proenga, 1969, Rev. Paul. Med. 75: 303), or by histological section (Shelley and Wood, 1976, JAMA 236: 144-145). Material thus acquired has limited usefulness as the mites are often killed or damaged in the process. If live mites are required, it is necessary to dissect an inhabited papule with a fine needle and attempt to extract the mite intact. Humans rarely provide large numbers of mites for study. In a previous note (Sheahan and Hatch, 1975, J. Parasit. 61: 350) the difficulties of isolating large numbers of mites were outlined and a method (using scabetic crusts from pigs' ears) of extraction described. Crusts separated from underlying tissue were subjected to low heat and vibration for 6 to 24 hr. Although the method can be applied to animals other than pigs, there may be hosts and circumstances for which the modifications suggested below might be more appropriate. In a study that involved extraction of mites from red foxes and rabbits, it was found that in heavily furred mammals with fulminating infestations (e.g., red fox), cracks in the crusted, hyperkeratosed epidermis allowed the exudation of serum and the development of a dense, damp mat that was difficult to scrape. In contrast, very low populations of mites on rabbits that showed little host reaction presented difficulties of selecting sites for scraping and examination. The following technique gave good results in both cases. Infested or potentially infested skin was cut into strips small enough to cover about half the surface area of a petri dish. The dish was covered and left for about 12 hr at room temperature. If the room was warm and the skin previously chilled or damp with exudate, then the petri cover may have had to be cleared of condensation. After the 12 hr elapsed, the dish with its contents was placed on a black surface under a low magnification stereo microscope. A concentrated beam of light was aimed at an open area of the dish in such a way as to provide a bright spot 1 to 2 cm in diameter. A satisfactory spot of light can be provided by the illuminator of some types of stereo microscope. The combination of light and black surface created a warm spot in the petri dish which attracted the mites, and after about an hour or so most mites in the dish were concentrated in this small area. From there they were removed for study with the aid of a fine needle. As would be expected of a mammalian ectoparasite, the mites respond to warmth (i.e., they will move up a temperature gradient) and this taxis can be used to concentrate the mites in a dish. But it did not seem necessary to provide either heat or vibration (as did Sheahan and Hatch, 1975, loc. cit.) in order to encourage the mites to leave the skin in the first place. The procedure outlined has several apparent advantages. It is simple and may even be suitable in the field provided that electricity and a stereo microscope of a suitable kind are available. The limited time spent at higher temperatures prolongs the life of the mites and limits the risk of desiccation-also, as some host material reaches the laboratory in a state of partial autolysis, the treatment at room temperature means a lower rate of decay. The method appears satisfactory for smaller populations of mites. As Sheahan and Hatch (1975, loc. cit.) observed, most stages of the mite emerge and can be identified. Using the low power microscope, all stages (even eggs) are recognizable, except that male and female tritonymphs cannot be separated and adult males are hard to distinguish from nymphal stages. Where