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Thrombicula favor feeding in areas where the skin is thin, hair sparse. Cats are bitten often on the tummy where the skin is particularly thin. Photo: brokinhrt2 - (CC BY 2.0)
Thrombicula favor feeding in areas where the skin is thin, hair sparse. Cats are bitten often on the tummy where the skin is particularly thin. Photo: brokinhrt2 - (CC BY 2.0)

Thrombiculid Mites
AKA Chiggers

From August to October we have to deal with
these microscopic blood suckers

Aug 23, 2019


By Dr. Michael Trapani

Not again! 

Here I am, again, going off on the topic of blood sucking insects. Is it me? Or are there are so many little blood suckers that I’ll never run out of material? You be the judge.

Thrombiculid Mites, AKA Chiggers.It’s bad enough that there are great big blood sucking bugs, like ticks and mosquitos, but from August to October we get to deal with a plague of microscopic blood suckers. I refer, as you undoubtedly guessed, to Thrombicula autumnalis, aka chiggers, or harvest mites.

To be fair, only the six-legged Thrombicula larvae are parasitic. These little buggers are about 0.2 mm (roughly 1/100th of an inch) in diameter. The mites are bright yellow-orange in color and pet owners (or sometimes unlucky humans) will sometimes observe them with the naked eye as they sit inside the incredibly itchy lesion where they feed. 

The mites are too small to bite through skin, but instead sink their tiny fangs into the host and inject enzymes that liquify the tissue. The mites feed on this fluid for a period of two to three days and will increase in size by as much as 400% before they are done. Once fed, the larval mites drop to the ground to complete their life cycle.

Adult mites feed on vegetation and do not parasitize other animals, so Thrombicula do not spread disease the way multiple host parasites, such as our local black legged ticks, carry and spread diseases like Lyme Disease. However, Thrombicula bites are extremely inflammatory and their bites itch like crazy!

The most common symptom is that your dog’s belly has lots of red spots. Photo:

Thrombicula favor feeding in areas where the skin is thin,  and hair is sparse. Humans are often bitten on the feet, ankles and lower legs. Cats are bitten on the ear margins or tummy where the skin is particularly thin. Dogs are more often bitten in the folds of the ear and other especially sensitive places. The bites themselves are usually bright red or crusted spots that are very often modified by self-trauma when the animal scratches the itchy, itchy feeding areas. Secondary bacterial infections are a common problem when Thrombicula bite, and large areas of moist bacterial dermatitis may develop when purulent fluid from the bites is pulled from hair shaft to hair shaft, rapidly spreading the infection.

Thrombiculid mites are most active on hot, dry late summer afternoons. They tend to localize in particular fields or gardens, but may not be present in spaces very close by. People and animals walking through infected areas can pick up scores of Thrombicula larvae in no time!

Prevention of Thrombicula bites is both easy and extremely difficult.  Modern systemic flea control products, such as fluralaner, suralaner and their relatives, are as effective against eight-legged insects (ticks and mites) as they are against six-legged insects (fleas, etc). Because these products enter the bloodstream, they reliably kill Thrombicula, but only after the mites have bitten and exposed themselves to the host’s body fluids. Older insecticidal products based on fipronil or pyrethrin are much, much less effective against mites, though they may sometimes help by repelling mites. Unfortunately, there are no products specifically tested or shown to be effective against Trombicula and we are unlikely to ever see such products developed. The small demand for anti-mite products makes it economically unfeasible that any manufacturer will ever make the investment required to develop and bring such a product to market. 

I have had some success using cat-safe flea sprays on cats and dogs. Pyrethrin-based spray products are gentle and can be sprayed on a cotton ball, then applied to the cat or dog’s sensitive ear and facial areas to help repel Thrombicula. Care just be taken to use only cat-safe products for this purpose, because some common insecticides can be quite toxic to cats. 

Of course, none of the things mentioned above are available for use by humans. Fortunately, we humans are tolerant of many aromatic oils, including menthol, peppermint, and wintergreen oils. In general, insects are repelled by pungent oils and these readily available products can be applied to clothing and directly to skin to encourage Thrombicula to feed elsewhere. Commercial products like DEET and picaridin have been suggested for use in repelling ticks, but I was unable to find any reference supporting their use against Thrombicula, though they may be helpful. A drug that repels ticks is expected to do the same with mites, since the groups are related. Once again, there are no guarantees. Remember, cats can be quite sensitive to essential oils and should not be assumed to tolerate human products.

When it comes to Thrombicula, we’re very much on our own. Use only safe, trusted products and then only with extreme caution.



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