Trombiculiasis

Trombiculiasis is a parasitic skin disease in dogs and cats caused by the infestation of larvae of Neotrombicula autumnalis, a mite which, in the adult phase, lives in the external environment. The larvae can infest several animals, including humans.

MORPHOLOGY AND BIOLOGICAL CYCLE OF THE PARASITE

The hexapod larvae are ovoid, 250-500 µm long and characterized by a typical deep red-orange colour. The rostrum is well developed with two pincer-like palps, while dorsally there is a pentagonal-shaped scutum with five bristles. The body is covered by long, feathery bristles (Fig. 1). The mite manifests temporary parasitism since it only infests hosts during the larval phase of its biological cycle, while the adults and the nymphs live free in the external environment. The female lays its eggs in the soil, where they hatch after about 1 week. Once the larvae have emerged from the eggs and reached the host, they feed actively thanks to the formation of a special structure, the stylostome. The stylosome is composed of solidified salivary secretions of the parasite and, extending from the mouthpart of the mite to the skin of the host, allows the parasite to nourish itself with tissue fluids.

After feeding for 3-15 days, the larva abandons the host and completes its biological cycle in the environment. The biological cycle lasts around 50-70 days and is affected considerably by the season. In Europe the females usually lay their eggs in the spring and summer, so the larvae are especially abundant in late summer and in autumn; however, depending on climatic conditions, several complete cycles can take place during the course of a year and it is not unusual to find larvae in other seasons.

CLINICAL SIGNS

The larvae, present in the external environment, become localised mainly on body areas that are in contact with the soil, such as the interdigital spaces, the nail bed, the muzzle, the eyelids, the external ears (Henry’s pocket) and the abdomen, tending to gather in more or less numerous clusters (Figs. 2, 3 and 4). The infestation is often asymptomatic and the larvae are occasionally detected because of their characteristic red-orange colour, which is visible to the naked eye (Fig. 5). Other subjects experience itching of varying intensity, from moderate to very intense, most probably following the beginning of phenomena of individual hypersensitivity which can continue even after the larvae have abandoned the host. Some subjects exhibit papular-crusted lesions and, depending on the intensity of the itching, self-inflicted injuries (alopecia, abrasions, ulcers, scabs).

DIAGNOSIS

Trombiculiasis is diagnosed by observing the larvae under a microscope. In the presence of clusters of macroscopically recognisable larvae, the parasites can be collected using clear adhesive tape (Scotch tape test) (Figs. 6 and 7). Superficial skin scraping is another diagnostic technique that can be used for diagnosis when only a small number of parasites are present.

DIFFERENTIAL DIAGNOSIS

• Other ectoparasitoses
• Allergic diseases

TREATMENT

No compound is currently registered in Italy for the treatment of trombiculiasis in dogs and cats. Excellent results have been obtained from anti-mite preparations in both a topical form (fipronil spot-on in a single administration in dogs and cats, and permethrin spray, in combination with pyriproxyfen in a single administration in dogs) and a systemic form (selamectin spot-on in a single administration). These compounds appear to guarantee a good prophylaxis against environmental re-infestations, the true problem of this parasitosis.

Suggested readings

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