Versione Italiana

TROMBICULIASIS or Trombiculosis

(Also called: trombiculidiasis, harvest mites, scrub itch, chiggers)

Luciano Schiazza M.D.
c/o InMedica - Centro Medico Polispecialistico
Largo XII Ottobre 62
cell 335.655.97.70 - office 010 5701818

Trombiculiasis is an infestation with mites of the genus trombicula, which causes a rash similar to insect bites.

Trombicula autumnalis, commonly known as the “chigger”, is a parasite which lives in the woods. Chiggers are microscopic, round, reddish insects similar to sand grains.

They occasionally affect humans and animals during summer-autumn. In Italy it mainly affects people working in the woods: hunters, mushroom pickers and hikers.

Larva bites under close-fitting clothing and undergarments (ankles, waist and upper torso) causing welts and intense itching that can last for weeks.

Life cycle


The life cycle starts with eggs deposition in spring-summer. Eggs hatch in 10 days and the six-legged orange/yellow or light-red larva, 0,1-0,2 mm. long appears in summer-autumn (August/end of October). The larva is the parasitic stage that feeds on humans and animals.

Larvae feed on low vegetation, but need protein to develop. So they crawl on the soil surface until a suitable host is found. This is why they need to stick to men and animals.


Trombiculae do not feed on blood and only the larvae affect humans.

Trombicula do not burrow into the skin, as scabies acari. It sticks to the hairless areas of the body and  feeds for 2-10 days on cellular contents and tissue fluid of the host. After feeding they leave and go back on the ground. In 5-6 weeks they develop into eight legged nymphae, very similar to the adult stage of the parasite, which they will quickly reach.


Adults do not bite humans or animals, but they feed on plant fluids, insect eggs and larvae. Trombicula autumnalis prefers a warm, moist environment. As many as 1-5 generations may be produced per year depending on the temperature, moisture, and location.

During the winter the parasite is inactive.

Larvae can feed on a wide variety of animal species. In humans they attach in areas where the skin is tender and thin, as ankles, waistline, knees or armpits.

Ciclo vitale trobicula

Life cycle of Neotrombicula Autumnalis. 1-4. The eggs hatch into an ectoparasitic, six legged larval form and starts to feed on the skin of humans and animals (A). 5-6. After feeding the larva goes back on the ground and develops in proto-nimph (5), that develops in deutonymph(6) 7-8. After an inactivity pre-adult  stage as tritonymph (7) the adult sexual organs are developed (8). The beginning of copulation.

Larvae attach to the skin inserting their microscopic piercing mouth parts in the pores or hair follicles and all the parts where the skin is thin or wrinkled. The chiggers’ saliva contains digestive enzymes that break down the cells and their contents become a slurry that chigger ingests. Severe itching usually occurs in 1-3 hours. The itch is caused by the stylostome. In fact, the salivary secretions that break down cells also cause the surrounding tissue to harden. This creates a straw like tube called stylostome.

Irritation and inflammation in the surrounding tissue trigger itch.

The longer the chigger feeds, the longer the stylostome becomes and the deeper it penetrates into the skin.

After scratching the skin with chelicers (CH) a structure similar to a channel (ST) penetrating the skin from the mouth. Lymph takes nutrient from this channel. At the sides you can see an inflammatory infiltration (II). Z, legs; ST, stylostome.

Disegno trombicula

Trombicula  feeds for about  3-4 days: it is the only meal of its youth. Very seldom trombicula completes its meal on humans because it is often accidentally detached or scratched away. Once detached it cannot sting any more and death often comes. Itch gets stronger after 2-3 days.

Knowing that itch is caused by the allergic reaction to the stylostome, it’s easy to understand why the itch persists after the larva has detached from the skin. Stylostome is slowly eliminated by the body (7-10 or more days). Differently from ticks that patiently wait for their host, trombiculae constantly run.

They move towards any new object in their environment. You can make sure that there aren’t trombiculae in your lawn by inserting a piece of white or black card board in the ground: if any, they will run on the top edge. You can easily see them with a magnifying lens.

Trombiculae are very fast thanks to the long legs. To reach the waist line, it takes around 15 minutes. In 15 minutes they cover a distance 5,000 times longer than the length of their body. Imagine a man able to climb a high mountain in 15 minutes on a empty stomach!


Once on the skin, the chigger takes one hour to find the right part of the body. If it finds an obstacle, such as a belt, it stops and starts to eat.

Trombiculae prefer women and children because they have a thinner skin and therefore a larger surface where they can feed themselves.
Chiggers are affected by temperature. They are most active in afternoon, and when the ground temperature is between 25 and 30 C° (66 – 86 F). Chiggers become completely inactive when substrate temperatures fall below 15/16 C° (60 F); temperature below 6 C° (42 F) will kill the chigger species that bite us.

If you can, plan your outdoor activities around your thermometer reading to keep chigger bites to a minimum. Chiggers actively avoid objects hotter than 37 C° (99 F). Rocks that have been baking in the sun are almost always free of chiggers, and make a safe place to sit when you are in a chigger-infested area.


Humans are infested while work or walk in the fields or in law vegetation areas.

The severity of the inflammation varies according to:

Larvae don’t have wings and attach the host’s body parts in contact with the ground. Most frequently infested animals are small rodents and dogs. However, many domestic animals may pass on chiggers, as well as birds and reptiles.

When the parasite attach the host, it moves towards areas where the skin is thinner, or where clothing fits closely: we will find it in the body areas where pressure keeps parassites in touch with the skin, such as waist line, arm pits, wrists, elbows, ankles, tighs. Clad fields workers often have lesions on hands, arms and neck, while children often have diffuse lesions all over the tegument (skin).

Dermatitis manifests with itchy papules. Initially it develops with a small red spot that after 24-48 hours develops in a papula, or papula-vesicule, sometimes haemhorragic. Itch is usually very intense. Lesions might persists for weeks. The intense irritation and subsequent scratching may result in secondary infection.

Also dogs may be infested with trombicula. On them you can observe a papulovesicular dermatitis, so itchy that the dog bites the infested areas of its body. Dermatitis is localized on the nose, the front legs, back of the knes, tighs, abdomen and ears. Secondary alopecia (hair loss), desquamation and lesions due to scratching.

Yu can observe chiggers on your dog with a magnifying lens. You will see little red-orange dots that rapidly move on the skin and objects in contact with the skin.


Symptomatic treatment gives a limited relief. Ich will disappear when the body eliminates stylostomes. Attention should be paid to superinfection. This is the only danger coming from trombiculosis.

There is no risk of transmission of typhus, borreliosis, tularemia, etc. It is not contagious and can be treated, in the case of intense itch, with symptomatic, such as antihistamine and topical steroid creams.

How to avoid it

The best defense against chiggers is to avoid them. Avoid wearing shorts, sleeveless shirts and sandals when going into chigger habitats. Tightly woven fabrics reduce the threat of chiggers penetrating through clothing.

Tuck pant legs inside boots, and button cuffs and collars tightly to keep chiggers on the outside of clothing. This increases the time that chiggers are exposed to any repellents you may have put there.

Remove clothing as soon as possible after exposure to chigger habitats and launder it before wearing it again. A warm shower with a vigorous skin massage, taken within an hour or two after exposure, greatly reduces the number of irritating bites. However, if itching has already started, it is probably too late for bathing to do much good. If you must enter chigger-infested areas, chemical repellents can be used with good results.

It should be applied to clothing from the feet up and must be repeated every two-three hours to maintain its effectiveness. Trombiculae hate sulphur.

It is effective when applied to the clothing. It might be applied on the legs and arms skin. It might be mixed to talcum powder: 50% sulphur and 50% talcum powder. But it would be better to contact own family doctor before apply it. Unfortunately sulphur has a very unpleasent smell and it becomes even more unpleasent when mixed with sweat.

It may also cause irritation to the skin. If you have never used it, test it on a small surface of the skin. To be absolutely avoided treatment with kerosene, ammonia, alcool, etc. Remember that chiggers can be removed with a light rubbing.