Eutrombicula species

Trombiculid mites are free-living but their larval stages can infest a range of mammals, birds and people, causing sometimes severe skin lesions characterised by intense pruritus.  The clinical signs may persist after the mites have left because of the immune reaction to the feeding tube (stylostome) that they leave in the skin  Diagnosis is based on history and clinical signs and the identification of larval mites with plumose setae.

Phylum: Arthropoda
Subphylum: Chelicerata
Class: Arachnida
Subclass: Acari
Order: Prostigmata

The genera Eutrombicula (in North America) and Neotrombicula (Trombicula) (in Europe) are in the same Order as Cheyletiella and Demodex, which is different from that containing Sarcoptes, Notoedres, Psoroptes, Chorioptes and Otodectes, and from that containing Dermanyssus and Ornithonyssus.

The parasitic larvae of trombiculid mites (chiggers or harvest mites) are tiny – up to approximately 500 µm in length – and can be seen with the naked eye, in part because of their reddish colour. The larvae have six long legs and feather-like (plumose) bristles (setae) on the body and legs.

Eutrombicula sp. larva

Plumose setae

Trombiculid mites occur around the world.  In Canada, Eutrombicula and very occasionally other chigger mites infest dogs and cats and other mammals, birds, and less often people. Locally, the distribution of trombiculids can be very patchy, for example one area of a property is infested at a particular time, while others are not.

Trombiculid mites are free-living. The adults feed on decaying vegetable matter. The larval stages are accidental parasites, acquired when passing through areas where the mites are established. The entire adult-to-adult life cycle can be completed in two months, but the adult females can live longer and in North America have an annual cycle.

 

Infestations with trombiculid mites are acquired from the environment, particularly by walking through vegetation containing the larval mites. The parasite probably does not transmit directly from host to host.

 

Trombiculids are also called “harvest mites”, probably because clinical problems are seen more commonly in the fall. This may simply be because there may be higher levels of infestation in the environment at this time of year.

The reasons for the patchy spatial distribution of these mites are not understood, although it may be associated with local variations in microclimate.

Clinical signs associated with chiggers are usually seen in the late summer and fall when the larvae are most plentiful. While some affected hosts show no clinical signs, many will exhibit erythema and papulocrustous eruptions associated with the bites, together with often severe pruritus.  In severe infestations, the lesions can progress to exudation and ulceration.  Some infested animals are non-pruritic, and have nodules, pustules and crusts. There may also be scaling and alopecia. Lesions are most common on the feet, head, ears and ventral body surface, all areas that come into contact with vegetation and the ground. The larvae secrete a feeding tube (stylostome) which is pushed into the skin. It is the immune reaction to these tubes that stimulate most of the clinical signs seen in the host.  The infestation is usually self-limiting, so specific treatment may not be required, although measures to alleviate the pruritus would be beneficial.

History and clinical signs are useful. Identification of the six-legged larval mites with plumose setae (hairs with a feather-like structure - a main stem with multiple side branches) in a skin scraping or vacuuming, or recovered from the haircoat, is definitive. The larval mites can move very quickly and unless placed in a liquid medium may escape before examination is complete. Even fragments of mites with the characteristic plumose setae can be used to establish a diagnosis.

There are several products available in Canada for the treatment of trombiculid mites, although none are approved for this purpose and their use is extra-label. Remember when treating that these mites are essentially free living, and by the time that clinical signs are apparent the larvae causing them may no longer be present on the animal.

Because chiggers are essentially free-living, effective control is very difficult. Avoidance of areas known to be infested may be helpful, but detection of these usually results from a dog or cat acquiring the larval mites.

Larvae of trombiculids will infest people, with clinical effects similar to those in animals, but the parasites are acquired from the environment, not from other hosts.

Arther RG (2009) Mites and lice: biology and control. Veterinary Clinics of North America Small Animal Practice 39: 1159-1171.

Pilsworth RC et al. (2005) Skin Disease Refresher: Trombiculidiasis. Equine Veterinary Education 17: 9-10.