1. General Considerations

   1. Trombiculidis are known as “chigger mites” or “harvest mites”.
   2. The six-legged larvae, which are characteristically dark orange in color, emerge from eggs that are laid by the free-living adults in moist and decaying vegetation and parasitize mammals.

1. 3. The parasitc larvae attach and feed on the skin of a variety of terrestrial vertebrates including humans.
   4. They tend to affect areas of the skin that are in contact with the ground, such as the ventrum, legs, paws, head (including the periocular and perioral regions), and base of the ears.

2. Cause

   1. Eutrombicula alfreddugesi (most common in the Southeast and South Central U.S.) and Neotrombicula autumnalis (most commonly found in Europe).
      1. Mites typically infest animals in the late summer and fall. Affected patients often have a history of environmental contact with woody areas and fields.
   2. Walchia americana is reported to affect squirrels and rodents in the Southwestern and Eastern U.S. and has been reported in the cat.

3. Clinical Signs

   1. Eutrombicula alfreddugesi and Neotrombicula autumnalis produce variable clinical signs but the infestation usually cause a pruritic papulocrustous eruption. However, in a retrospective study of 72 cats, pruritus was present in 43% of the cases and 57% of the cats were asymptomatic.
   2. Walchia americana also tends to affect the ventrum and ears but also the dorsum, medial legs and interdigital spaces. The paws may be swollen and the claws may be cracked. The host “reacts” by forming walled-off channels on the skin surface.
   3. Other reported lesions include nodular thickened skin with the surface appearing cracked and scaly. Papules and wheals are also reported.

4. Diagnosis

   1. Direct observation of the characteristic bright orange larvae.
   2. Skin scrapings can remove mites for microscopic examination. Mites are fast movers, so they should be placed in mineral oil immediately for direct microscopic evaluation.
   3. Biopsy may be considered in cases with unusual clinical signs.

5. Treatment

   1.  Various spot-on topical parasiticides applied once or twice should be effective. One report found one application of selemectin to be 100% effective in eight cats with naturally acquired Neotrombicula autumnalis infestation. Other options may include 0.25% fipronil spray or a spot-on formulation of moxidectin combined with imidacloprid (Advantage Multi®; Bayer Animal Health).
   2. Fluralaner (Bravecto® Topical Solution for Cats) was effective in treating two cats with trombiculosis. Most likely other isoxazolines will also be effective in treating this disease.
   3. Systemic glucocorticoids may be indicated short-term to alleviate pruritus (e.g. 2 to 3 days).
   4. Reinfestation is likely unless animals are prevented from accessing mite habitats. If this is not possible, it is advisable to continue treatment with a parasiticidal agent until the parasite season ends.

References

Leone F, Albanese F. Efficacy of selemectin spot-on formulation against Neotrombicula automnalis in eight cats.  Vet Dermatol 2004; 15 (Suppl. 1): 41-69.

Leone F, Di Bella A, Vercelli A et al.  Feline trombiculosis: a retrospective study in 72 cats.  Vet Dermatol 2013; 24:535-538.

Miller WH, Griffin CE, Campbell KL.  Small Animal Dermatology, 7th edn. St. Louis, MO: Elsevier, 2013; 297-298.

Nuttal TJ, French AT, Cheetham HC, et al. Treatment of Eornbicuh uutumnulis infestation in dogs and a 0.25 per cent fipronil pump spray. J Small Anim Pract 1998; 39: 237-239.

Ramiloa DR, Monteiro C, Carreira M, et al. First report of Neotrombicula inopinata infestation in domestic cats from Portugal. Vet Parasitol 2019; 267: 1-3.