Arthropods and parasites case challenge
A nursing assistant complains of increasingly severe itch for the last
month. It is particularly troublesome at night and affects her trunk and limbs. There has been no improvement with a topical steroid.
Which terms which describe the morphology of the eruption on her forearm?
#! Papule || A small palpable lesion. The usual definition is that papules are less than 0.5 cm diameter, although some authors allow up to 1.5 cm. They are raised above the skin surface, and may be solitary or multiple.
#! Excoriation || A scratch mark. It may be a linear scratch or a picked lesion (as in prurigo). Excoriations may occur in the absence of a primary skin lesion.
#! Erosion || A shallow moist lesion caused by loss of the surface of a skin lesion.
Explanation: The patient has an excoriated papular rash on her forearms and to a lesser extent elsewhere on trunk and limbs. Scalp is unaffected.
Rash on forearm
Careful inspection shows some discrete scaly lesions on the volar aspect of each wrist.
What sort of lesion is this?
* Scybala || The term “scybala” refers to packages of mite faeces sometimes identifiable using a light microscope to examine the contents of a burrow, which may also contain several eggs (3 are laid every day of the female's 30-60 day life-cycle).
What is the causative organism?
* Ankylostoma caninum
*! Sarcoptes scabiei
* Strongyloides stercoralis
* Cheyletiella yasguri
Explanation: The lesion is a typical scabetic burrow due to infestation with Sarcoptes scabiei var. hominis, and characterised by a wriggly scaly track about 1mm wide. In this case a female mite was extracted from the deeper end, which is mildly inflamed.
Explanation: Ankylostoma caninum is a cause of larva migrans, which results in wider (5mm) and longer (3cm) tracks under the skin. Strongyloides stercoralis causes larva currens, in which larvae may move 15cm/hour, usually perianally. Cheyletiella yasguri is the mite that causes animal mange; this almost never results in burrows when it affects humans.
The patient states that none of her family members have an itchy rash. Should they be treated with insecticide?
Explanation: All family members and close household contacts of a patient with scabies should be treated at least once with an effective insecticide, preferably on the same evening.
Which of the following topical insecticides may be used?
#! Gamma benzene hexachloride
Explanation: Although topical ivermectin is used to eradicate mites and helminths from domestic and farm animals, it is not licensed for use in humans. Oral ivermectin is effective but unlicenced for this indication. It not appropriate as first line treatment because of expense and potential toxicity. Permethrin, gamma benzene hexachloride (lindane) and malathion are available and marketed for treating scabies. However there is concern about increasing resistance to treatment with all these insecticides, particularly gamma benzene hexachloride and malathion. Gamma benzene hexachloride is neurotoxic and should not be used in young infants or pregnant females.
Lesion on wrist
The patient returns for a review a month later. She reports that although a week or so after treatment her itch was substantially improved, she still has a rash and moderate itching from time to time.
Which of the following are possible explanations?
#! Inadequate treatment
#! Dermatitis due to excessive washing
#! Dermatitis due to excoriation
Explanation: All the suggested responses are plausible and common explanations for continued itch after treatment for scabies. The patient should be very carefully examined, and if further burrows are detected or the clinical presentation suggests persistent infestation, she should be retreated with insecticide. Provide a careful explanation of how and where to apply the lotion, and the probable necessity for re-treatment of family members. The dermatitis should be treated with emollients and topical steroids, and she should be advised to use a non-soap cleanser.
She reports that she has heard of several staff members and patients at work being affected by a similar itchy rash. With her permission, it is advisable at this stage to contact the manager of the institution and advise that all staff and patients to be examined by a medical practitioner experienced in the diagnosis and management of scabies.
An elderly and bed-bound patient is identified at the rest home with a psoriasis-like crusted scaly eruption affecting her scalp, trunk and limbs. Scrapings are taken from under her fingernails and reveal innumerable mites.
Which of the following statements is true?
* The scalp never contains mites so does not need to be treated with insecticide
* She may have contracted her infestation from her pet dog, which lies on her bed
* She should be treated with oral prednisone and antihistamines
*! Crust and scale should be removed prior to repeated application of topical insecticides
Explanation: The description suggests crusted scabies (also known as Norwegian scabies), a severe infestation with human scabies that affects immune suppressed, mentally incompetent or elderly individuals. Crusted scabies is highly infectious because there may be thousands of live mites on the skin, probably because it is not particularly itchy or scratched and there is very little inflammatory response. Antihistamines and topical or oral steroids are not required. It is however important to treat the entire integument with topical insecticide and repeat every few days until all signs of rash have gone and scrapings are clear. Crusts and scales should be removed first including from the scalp and under the nails, using keratolytic agents such as salicylic acid and scrubbing. Alternatively, oral ivermectin 200mcg/kg can be used and repeated a week or so later with about 95% likelihood of cure. Secondary infection may require oral antibiotics.