**1. Introduction**

This chapter includes a thorough description of the main characteristics, life cycle, and distribution of chiggers. Moreover, a comprehensive review of the responsibility of chiggers as infectious disease vectors and agents of troublesome dermatitis is given. The following pages cover, for the first time in a unique chapter, the current knowledge of chigger mites.

#### **1.1. Taxonomy and distribution**

Trombiculid mites (Acari: Trombiculidae) are widespread ectoparasites of a wide range of vertebrates. More than 50 species have been recorded attacking humans, and about 20 of them are considered to be medically important because they cause dermatitis or due to their role as vectors of human pathogens. The most relevant species are *Eutrombicula alfreddugesi* in North and South America, *Neotrombicula autumnalis* in Europe, and *Leptotrombidium* spp. in Asia [1].

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Trombiculidae is one of the largest families in the Acari group, including more than 3,000 species [2]. Table 1 shows the taxonomic position of these mites.


**Table 1.** Taxonomic classification of the family Trombiculidae.

Trombiculids are distributed worldwide, but they show their greatest diversity in the sub‐ tropical, tropical and southern temperate zones [3]. Table 2 shows the main trombiculid species and their geographical distribution.



Trombiculidae is one of the largest families in the Acari group, including more than 3,000

Trombiculids are distributed worldwide, but they show their greatest diversity in the sub‐ tropical, tropical and southern temperate zones [3]. Table 2 shows the main trombiculid species

Russia, Sumatra, Malaysia, and Africa

(except for the southwest), South and Central America (including West

America, southwestern and southeastern United States

Tennessee West to Arkansas, Oklahoma and Kansas

Coast North to Massachusetts, Minnesota) and Ontario

Pacific Islands

Trombiculiasis

Trombiculiasis

Trombiculiasis

Trombiculiasis

Trombiculiasis

Trombiculiasis

**Species Distribution Disease**

*Eushoengastia koreaensis* Korea Scrub typhusa *Euschoengastia xerothermobia* Europe Trombiculiasis

Indies)

*Blankaartia acuscutellaris* Hungary, Spain, Moldova, Ukraine,

*Eutrombicula alfreddugesi* Canada, South of United States

*Eutrombicula batatas* Bolivia, Mexico, Central and South

*Eutrombicula lipovskyi* United States: from Alabama and

*Eutrombicula sarcina* Southeast Asia, Australia and the

*Eutrombicula splendens* Eastern United States (from the Gulf

species [2]. Table 1 shows the taxonomic position of these mites.

Phylum Arthropoda Subphylum Chelicerata Class Arachnida Subclass Acari

174 An Overview of Tropical Diseases

Superorder Acariformes Order Trombidiformes Suborder Prostigmata Superfamily Trombiculoidea Family Trombiculidae

and their geographical distribution.

**Table 1.** Taxonomic classification of the family Trombiculidae.


**Table 2.** Distribution and diseases transmitted by the main trombiculid mite species [1,4–15].

Members of this family are known by several names depending on their distribution (Table 3). They are often confused with other mites or insects and are mistakenly named as Mower's mites [common name of *Leptus autumnalis* (Acari: Erythraeidae)] [16,17] or jigger, chigoe, and niguas [common names of *Tunga penetrans* (Insecta: Siphonaptera)] [18,19].


**Table 3.** Common names given to trombiculid mites worldwide. [2,4,10,16,17,20–22]

### **1.2. Life cycle**

**Species Distribution Disease**

and Poland

Pacific Islands

Pacific Islands

Members of this family are known by several names depending on their distribution (Table 3). They are often confused with other mites or insects and are mistakenly named as Mower's mites [common name of *Leptus autumnalis* (Acari: Erythraeidae)] [16,17] or jigger, chigoe, and

*Neotrombicula japonica* Korea, Europe Scrub typhusa

*Neotrombicula nagayoi* Japan, China, and Russia Trombiculiasis *Neotrombicula zachvatkini* Europe Trombiculiasis

*Shoengastia hanmyaensis* Japan Scrub typhusa

*Trombicula toldti* Austria Trombiculiasis

Austria, Germany, Bulgaria, France, states of former Yugoslavia, Ukraine, Russia, Romania, Hungary, Slovakia, Trombiculiasis

Trombiculiasis

Trombiculiasis

Trombiculiasis

North-America, Asia

*Neotrombicula inopinata* Spain, Czech Republic, England,

*Odontacarus* spp. Southeast Asia, Australia, and the

*Schoengastia* spp. Southeast Asia, Australia, and the

**Table 2.** Distribution and diseases transmitted by the main trombiculid mite species [1,4–15].

niguas [common names of *Tunga penetrans* (Insecta: Siphonaptera)] [18,19].

**Common names Places**

Harvest mite Europe Aoutats, rouget, bête rouge France Orange tawny Ireland Augustelingen Germany Bicho colorado, coloradilla, ácaro rojo South America

Isango Peru Tlazahuate Mexico Coloradita, chivacoa Venezuela

**Table 3.** Common names given to trombiculid mites worldwide. [2,4,10,16,17,20–22]

Harvest bug, harvest mite, harvest lice, red bug, red mite, berry mite,

a

Not confirmed.

176 An Overview of Tropical Diseases

scrub-itch mite

Trombiculid mites undergo seven stages in their life cycle: egg, deutovum, larva, protonymph, deutonymph, tritonymph, and adult (Figure 1). This cycle is characterized by alternating active and inactive instars, being the larva, deutonymph, and adult the active ones. Active postlarval stages are soil dwellers that prey on various arthropods and their eggs. Deutonymphs look almost identical to adult mites. Both present eight legs, but deutonymphs are slightly smaller. Sexual dimorphism is not apparently evident [23]. Larvae parasitize all groups of vertebrates, except fishes, whereas the small mammals and birds are the main hosts [1,5,19,24]. There are just a few reports of chiggers feeding on invertebrates [3]. Humans are only accidental hosts. However, the question of the host specificity of trombiculids still arises. Most likely, trombi‐ culids are associated with specific habitats and attack and feed on the first available animal within their favorite habitat, although they can have preference for a particular host among the available ones [23,25].

**Figure 1.** Schematic description of the trombiculids' life cycle. (Adapted from Takahashi *et al*., 2003 with the permis‐ sion of the author.)

During their life cycle, eggs are laid in well-drained soil, and six-legged larvae emerge from them. The general term "chigger" refers to the parasitic larval stage, and this is the name commonly given to trombiculid mites due to the importance of this instar. Chiggers are usually reddish but can vary between yellow and orange [1,26]. These tiny larvae (about 200 μm) climb onto low vegetation, where they aggregate into clusters to wait for a suitable host. On the host, chiggers mainly move to areas where the skin is especially thin and feed on lymph and tissue fluids of the dermal layer (but not blood). Ears, head, armpits, abdomen, genitalia, and the area around the tail are preferred in animals [4,27]. In humans, bites occur mainly in body exposed areas and at sites where the clothing constricts [17,28]. Once engorged (development to subsequent stage cannot take place unless larvae have fed on the host), larvae fall to the ground and develop to the nymphal stages and subsequently to adults (900–1,200 μm).

Trombiculid mites live in moist soil covered with vegetation such as grassy and weedy areas. In general, optimal living conditions require a relative air humidity of 80% (what explains that chiggers are not typically found on vegetation higher than 30 cm off the ground) and neutral to slightly alkaline soil. The optimum activity of chiggers occurs at temperatures of 25–30°C [26,29].

Trombiculid mites often form localized "mite islands" (or "mite focus", "larvae focus") in suitable areas inhabited by potential hosts [30]. Therefore, chiggers have a patchy distribution on the vegetation. Mite islands are quite clearly defined, and larvae could not be detected in their immediate vicinity [26,31]. A possible explanation for this focalization may be that chiggers apparently do not move more than a few meters from where they hatched. Chiggers would temporarily disperse if a host approached. On the contrary, if physical contact were not managed or if the host were not close enough for them to drop on it, chiggers would invariably and promptly return to the cluster and would continue waiting [32].

The life cycle of trombiculid mites has been mainly studied in the laboratory. The most outstanding feature of the life cycle is the constant duration of quiescent periods and the variable duration of active stages. Trombiculid mites usually have one generation per year, but with overlapping generations, each well synchronized with the seasons because they can overwinter in most stages (egg, larva, deutonynph, and adult) and because the adult mites have a long life span [33]. In boreal species, an egg-to-egg cycle ranges from 150 to 400 days, but it is shorter in tropical species [23]. In nature, the life cycle is supposed to be completed in 2–12 months or longer, depending on the species and environmental conditions. In temperate areas, there may be 1 to 3 generations per year, whereas in tropical regions the life cycle is shorter and continuous throughout the year [1]. In Europe, the duration has been estimated in five to seven months under favorable conditions [26].

As mentioned above, trombiculid mites only act as parasites during their larval stage. Thus, the greatest attention has been paid to chiggers. In addition, adults and deutonymphs of the majority of trombiculid species have never been observed on the soil surface (in fact, their habitats are mostly unknown). Therefore, the taxonomy of trombiculid mites is based solely on their larvae [34]. It is estimated that only the postlarval stage of less than 10% of the total of Trombiculidae species are known [7]. This is the case of some tropical species in contrast to the difficulty of finding active postlarval instars in northern countries [23].

#### **1.3. Feeding process**

It is well known that when feeding on hosts, chiggers develop a characteristic feeding tube (stylostome) in the host's skin. The stylostome is mostly formed of the larval salivary secretions solidifying in the host's epidermis [7]. Larva cuts the stratum corneum with its rather short chelicerae and stylostome allows chigger to reach the underlying connective tissue layer from which it obtains nutrients. The host's tissues around the stylostome are destroyed and necrotized. Beneath the distal end of the stylostome, an interstitial food cavity containing lymphoid and epithelioid cellular liquid elements is formed [23]. The feeding period in animal host, both in nature and reared in the laboratory, usually lasts 3–6 days [26,35]. However, feeding on humans may typically vary from 3 - 8 h to 1–2 days for most non-infectious chiggers, but 2–10 days for scrub typhus vectors [1,29,36,37]. It is supposed that more than 6 h are required for the transmission of the bacterium [14]. During this period, the larva remains on the skin surface. For this reason, most trombiculid larvae can be classified as ectoparasites. Larvae of some genera, however, can partly or even entirely embed within the skin of different body cavities frequently forming various types of capsules during feeding on amphibians and mammals [23]. In such cases, the feeding time is prolonged up to several weeks or even months. In lizards, specific adaptive structures of skin, known as "mite pockets", may evolve to decrease the possible damage from mite feeding [38]. It is generally thought that the organi‐ zation and location of stylostome is species specific in trombiculid larvae, irrespectively of the host species and of the particular feeding site on the host, whereas the length of the stylostome is mostly a result of the width of the epidermal layer and the presence or absence of scabs at the attachment site [39].
