Konrád, P. Dermatology Clinic in Černošice
SUMMARY: The article describes thrombidiasis, a seasonal parasitic disease, typical in certain geographical areas in late August and September. The article identifies and classifies the causative agent of the disease, it describes the possible clinical courses of the parasitosis and it provides the most effective therapy.
KEY WORDS: thrombiculosis, Trombicula autumnalis, Tetranychus, permethrin, avenathramide.
Trombicula autumnalis is a species of mite from the Trombiculidae family (velvet mites). Its larvae live parasitically, they attack warm-blooded mammals and some ground-nesting birds. They live in humid climates and they are active from July to September. The mite larvae cause thrombiculosis.
The larvae are orange to brick-red with only six legs. In the nymph stage, they have eight legs. The larvae are 0.2mm. Adult mites are about 1mm, they have narrow bodies that are densely covered with velvet-resembling hair (Fig.1). Adults live in the soil and leaves, where females lay eggs in the autumn. A six-legged larva hatches from the egg, which is the mite’s only parasitic stage. After hatching, the larva climb up the grass blade or bush and wait for its prey, which are warm-blooded vertebrae like mice, voles, moles, dogs, cats, etc. but it also attacks humans.
Tetranychus – Incorrect definition
The laymen public oftentimes define the disease incorrectly as “Tetranychus“.
This confusion is caused by the similarity of the mites, Trombicula autumnalis and Tetranychus (Fig.2). However, Tetranychus are plant pests from the Tetranychidae family and from the Prostigmata order. They cause a silver hint on the leaves by creating air bubbles in the plant tissue (Fig.3). When the population develops more, they form fine or later denser cobwebs around the leaves or needles with small coloured spots. The cobwebs or fibres are often found where the leaf stem meets the twig or stem. If the course is invasive, the plant suddenly wilts and inexplicably dries out. Wooden plants may remain leaf-less or the \hanging leaves may have dried out. The mites live on the bottom of the leaf, where they can usually be seen under a magnifying glass.
Trombicula autumnalis mite larvae infestation, causes itchy acute inflammation of the skin with maculo-papules appearing in places, where clothes apply pressure on the skin – on the waist, above the ankles and in areas around the underwear.
Aetiology and Pathogenesis
Velvet mite larva is around 0.2mm and moves around the host’s body very quickly, making it difficult to see or catch. Therefore, it gets into specific places, where it can penetrate under the skin easily and discreetly. In humans, these areas especially include the groin area, genitals, inner thighs, abdomen, stomach, armpits, under the knees, or skin folds. The larvae like crawling under underwear. Here, they cling on to the skin and disrupt the surface of the skin with their sharp teeth (chelicerae). The instant the larvae attack the host is painless, thus the host pays no attention to it. The larvae then release highly lytic secretions from the salivary glands into the resulting wound. These secretions contain digestive enzymes. The larvae do not feed on blood but suck the resulting debris of the the host’s necrotic skin cells. After the sucking, which lasts 2–3 days, the larvae fall out and gradually develop through the nymph stage to the adult stage. The mite is harmless in the latter stages. Once the female larvae lay its eggs, it dies. The life cycle, including the velvet transformation lasts from the end of August to September, when the disease occurs. That’s why this disease is also called the August rash (ang. harvest mite). It tends to affect farmers and gardeners, usually living in the same areas (in the Czech Republic, this is mostly the Central Bohemia Region and Palava in Southern Moravia).
Bright red macules first appear after a harvest mite bite followed by puffiness and widespread popular manifestations with central haemorrhages (Fig. 4,5,6). A severe allergic reaction accompanied by severe itching may occur after several hours or days. The itching usually lasts one week and skin manifestations may last about 14 days if left untreated. Heat from a bed for example, increases the consequences of the bite, which may sometimes lead to excruciating pruritus! In the developed and untreated stage, the disease may spread all over the body. In exceptional cases, the allergic reaction is so severe that the patient needs to be hospitalized (Fig.7a,7b,7c).
It is determined by clinical findings and the course. It is characterized by when and where the finding is made.
It includes other parasitic diseases, like scabies, pediculosis, cimicosis and insect bites. The time and place of the diseases generally lead to differentiating the individual diagnosis. Typically, the indicators include cutting the grass and trimming bushes at the end of August and in September.
Although the skin manifestations may heal spontaneously, the accompanying pruritus really bothers the patients. Manifestations on the whole body are no exception. Therefore, there is a risk of mental alternations and the possible development of parasitophobia. The most effective one-time therapy is the antiparasitic, permetrín (Infectoscab ointment). The health insurance company does not cover this ointment to treat trombiculosis, so the patient has to pay for it out-of-pocket. The application is the same as in scabies, so for about 5 hours on the whole body. The course of the therapy is as follows. Permetrin is ideally applied at night. Before putting it on, the patient takes a shower using an anti-septic soap (for example, Cutosan® wash gel). This anti-septic soap prevents secondary impetiginisation, usually scratching foci. The patient uses the soap for hygiene until the manifestations are completely gone. Leave the anti-septic gel on for about 1 minute without the presence of water. In the morning, the patient washes the permetrín off using an anti-septic wash gel again and begins applying anti-pruritic externa. In severe cases, we choose corticoid substances. However, in minor cases, products with the anti-pruritic substances, avenanthramid combined with epithelization substances, dexpanthenol (for example, AD lotio Acut) suffice. This anti-pruritic externa is applied on the whole body twice a day, in the morning and in the evening until the symptoms of the disease are gone. For strong allergic reactions accompanied with pruritus, we generally prescribe antihistamines and in extreme cases, we administer intramuscular corticoids.
Thrombiculosis should be treated even though it may heal spontaneously. The goal is to prevent impetiginisation, alleviate subjective problems, mainly pruritus, shorten the duration of the manifestation and alleviate the allergic reaction, which may even lead to hospitalization in exceptional cases.
MUDr. et MUDr. Pavel Konrád
Mokropeská 2027, 252 28 Černošice
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