Dermatitis by Tropical Rat Mite, Ornithonyssus bacoti (Mesostigmata, Macronyssidae) in Italian city‐dwellers: a diagnostic challenge

Abstract

Editor Mesostigmata (Gamasida) is a suborder of mites within the order Parasitiformes.1 Some species can bite humans, typically causing pruritic, non-specific skin lesions. The mesostigmatic mites most commonly associated with cases of dermatitis are from the Macronyssidae and Dermanyssidae families.1 All are haematophagous non-burrowing mites of very similar size and shape, and can be seen with the naked eyed as they range from 0.5 to 1.5 mm in size.1 Adults are able to survive several weeks without a blood meal.1 The tropical rat mite, Ornithonyssus bacoti, Hirst (Macronyssidae) is one of these species. It can occur worldwide mostly as an ectoparasite of wild rats and mice,2, 3 but also pet and laboratory rodents may occasionally be the source of the mite.2, 4-6 As a nest parasite, it only visits its host to feed, mainly during the night; and then returns to its nearby hiding place during the day. In the absence of its natural host, human beings may become subject to mite infestation. Outbreaks of tropical rat mite (TRM) dermatitis have been reported in humans in urban areas in Europe, because of wild/pet rodent infestation of flats2-4 but also due to occupational exposure,5 including a single record from Italy.6 These scattered cases show that parasitization by the TRM is not obvious and skin lesions are usually misdiagnosed, unless the mite is discovered and correctly identified. To confirm this, we report an urban outbreak of TRM dermatitis diagnosed by identifying the parasite. In November 2013, veterinary entomologists from the Zooprofilactic Institute (IZSPB), Foggia were contacted to inspect a house for mites. Owner referred that her 2-year-old daughter suffered from multiple erythematous papules over a 9-month period. Cutaneous lesions were generalized but predominantly located in groups on the neck and body areas covered by clothes (trunk, upper extremities and abdomen) (Fig. 1). Her 71-year-old grandmother had also experienced the same symptoms after spending a single night in the child's home. Bites by arthropods/non-burrowing mites/wood-mite and Streptococcus spp. infection respectively, were diagnosed after two paediatric and two dermatological consultations. Symptomatic relief was also unsuccessful. Mites were detected in dust samples and in child's bedding, after careful investigation of the patient's living quarters by IZSPB staff. All of the parasites were identified as Ornithonyss bacoti based on morphological keys1 (Fig. 2). The source of the mites was a previously eliminated rats' nest in the living-room. Medical texts and parasitology manuals rarely mention mesostigmatic mites in association with human parasitization; detection and correct identification of these ectoparasites is also difficult. This makes it a challenge for physicians, including dermatologists, to recognize epizoonotic mite-associated dermatitis, including skin disorders caused by O. bacoti. This inevitably leads to failures in treatment. In addition to O. bacoti, mestostigmatic avian mites such as Ornithonyssus sylviarum and Ornithonyssus bursa (Macronyssidae), and more frequently Dermanyssus gallinae (Dermanyssidae) can be associated with urban dermatitis in Europe,2, 7, 8 including Italy.9 When unexplained pseudoscabious eruptions occur on patients in urban areas, investigation for mesostigmatic mites should always be undertaken. If small rodents are held as pets, animals and their bedding should be carefully examined by a veterinarian. Diagnosis and treatment of these infestations require accurate anamnesis, detection and identification of the parasite, removal of the source and disinfestation. As studies suggest that the rat mite may be a reservoir of various zoonotic pathogens and a vector of Bartonella henselae,10 its early diagnosis remains crucial. Dermatitis by mesostigmatic mites, including the TRM dermatitis, are without doubt more frequent than one might expect. Increased physicians awareness of the possible role of these ectoparasites in urban infestations of humans, and applying the One Health approach will be essential for diagnosis, treatment and prevention.