Trombiculidae

Trombiculidae (/trɒmbɪˈkjuːlɪdiː/), commonly referred to in North America as chiggers and in Britain as harvest mites, but also known as berry bugs, bush-mites, red bugs or scrub-itch mites, are a family of mites.[3] Chiggers are often confused with jiggers – a type of flea. Several species of Trombiculidae in their larva stage bite their animal or human host and by embedding their mouthparts into the skin cause "intense irritation"[4] or "a wheal, usually with severe itching and dermatitis".[5][6][7]

Not to be confused with Trombidiidae, similar in name and appearance, but a different family of mites.

Trombiculidae
Trombiculid mite larva
Trombiculid mite larva
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Class: Arachnida
Order: Trombidiformes
Suborder: Prostigmata
Infraorder: Anystina
Superfamily: Trombidioidea
Family: Trombiculidae
Ewing, 1929[1]
Type genus
Trombicula
Berlese, 1905
Genera[2]
The distribution of trombiculid species, which is nearly everywhere in the world.

Trombiculidae live in forests and grasslands and are also found in the vegetation of low, damp areas such as woodlands, berry bushes, orchards, along lakes and streams, and even in drier places where vegetation is low, such as lawns, golf courses, and parks.[8] They are most numerous in early summer when grass, weeds, and other vegetation are heaviest. In their larval stage, they attach to various animals, including humans, and feed on skin, often causing itching. These relatives of ticks are nearly microscopic, measuring 400 µm (1/60 of an inch) and have a chrome-orange hue.[9][10] There is a marked constriction in the front part of the body in the nymph and adult stages. The best known species of chigger in North America[11] is the hard-biting Trombicula alfreddugesi of the Southeastern United States, humid Midwest[12] and Mexico. In the UK, the most prevalent harvest mite is Neotrombicula autumnalis, which is distributed through Western Europe to Eastern Asia.[13]

Trombiculid mites go through a lifecycle of egg, larva, nymph, and adult.[14] The larval mites feed on the skin cells of animals. The six-legged parasitic larvae feed on a large variety of creatures, including humans, rabbits, toads, box turtles, quail, and even some insects. After crawling onto their hosts, they inject digestive enzymes into the skin that break down skin cells. They do not actually "bite", but instead form a hole in the skin called a stylostome and chew up tiny parts of the inner skin, thus causing severe irritation and swelling. The severe itching is accompanied by red, pimple-like bumps (papules) or hives and skin rash or lesions on a sun-exposed area. For humans, itching usually occurs after the larvae detach from the skin.[15]

After feeding on their hosts, the larvae drop to the ground and become nymphs, then mature into adults, which have eight legs and are harmless to humans. In the postlarval stage, they are not parasitic and feed on plant material. The females lay three to eight eggs in a clutch, usually on a leaf or under the roots of a plant, and die by autumn.[15]

History

Trombiculidae, from Greek τρομειν ("to tremble") and Latin culex, genitive culicis ("gnat" or "midge"), was first described as an independent family by Henry Ellsworth Ewing in 1944.[16] Then, when the family was first described, it included two subfamilies, Hemitrombiculinae and Trombiculinae. Womersley added another, Leeuwenhoekiinae, which at the time contained only Leeuwenhoekia. Later, he erected the family Leeuwenhoekiidae for the genus and subfamily, having six genera; they have a pair of submedian setae present on the dorsal plate.[17]

References to chiggers, however, go as far back as sixth-century China, and by 1733, the first recognition of trombiculid mites in North America was made (citation needed). In 1758, Carl Linnaeus described a single species, Acarus batatas (now Trombicula batatas). However, most information about chiggers came from problems that arose during and after World War II.[18]

Distribution

Trombiculid mites are found throughout the world. In Europe and North America, they tend to be more prevalent in the hot and humid regions. In northern Europe, including the British Isles where they are called harvest mites, the species Neotrombicula autumnalis are found during the summer and autumn (in French, harvest mites are called aoûtat because they are common in August[19]). In the United States, they are found mostly in the Southeast, the South, and the Midwest. They are not present, or barely found, in far northern areas, high mountains, and deserts.[20] In North America the species Trombicula alfreddugesi, and the species Trombicula (eutrombicula) hirsti, which are found in Australia and are commonly called the scrub-itch mite.[21]

Life cycle
The life cycle of a harvest mite

The length of the mite's cycle depends on species and environment, but normally lasts two to 12 months. The number of cycles in a year depends on the region. For example, in a temperate region, only three per year may occur, but in tropical regions, the cycle might be continuous all year long.[14] Adult harvest mites winter in protected places such as slightly below the soil surface. Females become active in the spring, and once the ground temperature is regularly above 16 °C (60 °F), they lay eggs in vegetation, up to 15 eggs per day. The eggs are round and are dormant for about six days, after which the nonfeeding prelarvae emerge, with only three pairs of legs. After about six days, the prelarvae grow into their larval stage.[14]

Larva

The larvae, commonly called chiggers, are about 170–210 μm (0.007–0.008 in) in diameter, normally light red in color, and covered in hairs; they move quickly relative to size. The larvae congregate in groups on small clods of soil, in matted vegetation, and even on low bushes and plants, where they have more access to prospective hosts.

The larval stage is the only parasitic stage of the mite's lifecycle. They are parasites on many animals. About 30 of the many species in this family, in their larval stage, attach to various animals and feed on skin. This often causes intensely itchy, red bumps in humans.[14][22]

Chiggers attach to the host, pierce the skin, inject enzymes into the bite wound that digest cellular contents,[23] and then suck up the digested tissue through a tube formed by hardened skin cells called a stylostome.[24] They do not burrow into the skin or suck blood, as is commonly assumed. Itching from a chigger bite may not develop until 24–48 hours after the bite, so the victim may not associate the specific exposure with the bite itself.[15] The red welt/bump on the skin is not where a chigger laid eggs, as is sometimes believed.[25] The larvae remain attached to suitable hosts for three to five days before dropping off to begin their nymphal stage.[14] They tend to attach where clothing has restrictions, such as belt lines, or behind the knees when wearing jeans.

During the wet season, chiggers are usually found in tall grass and other vegetation.[26] During dry seasons, chiggers are mostly found underneath brush and shady areas.[25][27] Standing still or lying in tall grass gives them more time to climb onto a person.

Nymph

Once the larva has engorged itself on skin and has fallen off its host, it develops to its nymph stage. The nymph is sexually immature, but more closely resembles the adult.

This stage consists of three phases, the protonymph, deutonymph, and tritonymph. The protonymph and tritonymph morphologies are unusual in species of Trombiculidae. The protonymph phase combines larval and protonymph characteristics with deutonymph and tritonymph morphology.[28] The protonymph is an inactive transitional stage. The active deutonymph develops an additional pair of legs (for a total of eight). Lastly, it re-enters inactivity during its transitional tritonymph phase before growing to adulthood.[14]

Adult

As deutonymphs and adults, trombiculid mites are independent predators that feed on small arthropods and their eggs, and are also found to eat plant material.[14] They live in soil, and are often found when digging in yards or gardens or in compost bins.

Recently (2018), methods based on autofluorescence microscopy were developed to enable identification of trombiculid mites to the species level on the basis of morphological traits without any special preparation.[29]

Effect on humans

Trombiculosis
Main article: Trombiculosis

Trombiculosis, also called trombiculiasis and trombiculidiasis, is the term coined for the rash or infestation caused by trombiculid mite larvae.[30][31]

Treatment
Chigger-caused lesions on human skin showing characteristic welts
A Trombiculid mite larva attached to human skin

Because chigger wounds are a complex combination of enzymatic and the resulting mechanical damage, plus allergy and immune responses, plus possible secondary bacterial infection subject to local influences, no one remedy works equally well for most people.

The chiggers' digestive enzymes in the saliva cause "the intensely itchy welts".[32] The itching can be alleviated through use of over-the-counter topical corticosteroids and antihistamines. According to Mayo Clinic, the chiggers "fall off after a few days, leaving behind red, itchy welts", which normally heal on their own within one to two weeks.[33] Hot showers or baths also help reduce itching. In cases of severe dermatitis or secondary infection associated with chigger bites, a doctor should be consulted.[34]

According to an Ohio State University Extension Fact Sheet:[35]

... After returning from a chigger-infested area, launder the field clothes in soapy, hot water (125°F.) ... As soon as possible, take a good hot bath or shower and soap repeatedly. The chiggers may be dislodged, but you will still have the stylostomes, causing the severe itch. Scratching deep to remove stylostomes can cause secondary infections. For temporary relief of itching, apply ointments of benzocaine, hydrocortisone, calamine lotion, New Skin, After Bite, or others recommended by your pharmacist or medical doctor. ... (The sooner the treatment, the better the results.)

Home remedies to "suffocate" the mite, such as applying clear nail polish, rubbing alcohol, or bleach, may have little benefit since the mites do not burrow into the skin. However, since the mite may still be attached for up to 3 days, these treatments could possibly kill the mite, reducing further damage.

Chiggers as disease vectors
Further information: Scrub typhus

Although the harvest mite chigger usually does not carry diseases in North American temperate climates, Leptotrombidium deliense is considered a dangerous pest in East Asia and the South Pacific because it often carries Orientia tsutsugamushi, the tiny bacterium that causes scrub typhus, which is known alternatively as the Japanese river disease, scrub disease, or tsutsugamushi. The mites are infected by the Rickettsia passed down from parent to offspring before eggs are laid in a process called transovarial transmission. Symptoms of scrub typhus in humans include fever, headache, muscle pain, cough, and gastrointestinal symptoms.[36][37]

References
  1. "Trombiculidae Ewing, 1929 (Family)". SysTax - database query. Universität Ulm. Retrieved 2009-03-06.
  2. Shatrov, Andrey B.; Kudryashova, Naina I. (2008). "Taxonomic ranking of major trombiculid subtaxa with remarks on the evolution of host-parasite relationships (Acariformes: Parasitengona: Trombiculidae)". Annales Zoologici. 58 (2): 279–287. doi:10.3161/000345408X326591. S2CID 83569187.
  3. G. A. Smith, V. Sharma, J. F. Knapp & B. J. Shields (1998). "The summer penile syndrome: seasonal acute hypersensitivity reaction caused by chigger bites on the penis". Pediatric Emergency Care. 14 (2): 116–118. doi:10.1097/00006565-199804000-00007. PMID 9583392. S2CID 37926004.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. Merriam-Webster's Online Dictionary, chiggers cause intense irritation
  5. chigger: Medical dictionary: "produces a wheal, usually with severe itching and dermatitis"
  6. Chigger: Archived 2012-09-26 at the Wayback Machine American Heritage Dictionary
  7. chigger: Dictionary.com, http://dictionary.reference.com/browse/chigger
  8. Ballantine, Todd (1991). Tideland treasure: the naturalist's guide to the beaches and salt marshes of Hilton Head Island and the southeastern coast. Columbia, South Carolina: University of South Carolina Press. pp. 1–4. ISBN 978-0-87249-795-5.
  9. Mandell, Gerald L.; Bennett JE; Dolin R (2005). "294". In 6th (ed.). Principles and Practice of Infectious Diseases. Philadelphia: Elsevier Churchill Livingstone. ISBN 978-0-443-08686-1.
  10. Goldman, Lee; Dennis Arthur Ausiello (2007). Cecil Medicine (23, illustrated, revised ed.). Elsevier Health Sciences. p. 1032.
  11. Eutrombicula (Trombicula) alfreddugesi is the most familiar [chigger] in North America."
  12. "Eutrombicula alfreddugesi." "...from the Atlantic coast to the Midwest and southward..." Encyclopædia Britannica Online, 2011. Web. 10 Aug. 2011.
  13. N. autumnalis "has not been found in the Nearctic region...." http://www.vetstream.com/lapis/Content/Bug/bug00357
  14. Durden, Lance A. (2002). Medical and veterinary entomology (3rd ed.). Academic Press. p. 458. ISBN 978-0-12-510451-7.
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  16. Scarborough, John (1998). Medical and Biological Terminologies. Oklahoma: University of Oklahoma Press. p. 122. ISBN 978-0-8061-3029-3.
  17. H. E. Ewing (1946). "Notes on trombiculid mites with descriptions of Walchiinae n. subf., Speotrombicula n. g., and Eutrombicula defecta n. sp" (PDF). Journal of Parasitology. 32 (5): 435–440. doi:10.2307/3272913. JSTOR 3272913. PMID 20277195.
  18. Bowman, Dwight D.; Hendrix, Charles M.; Lindsay, David S.; Barr, Stephen C. (2002). Feline clinical parasitology. Wiley-Blackwell. pp. 385–386. ISBN 978-0-8138-0333-3.
  19. "Aoûtat : Definition" (in French). Vulgaris - medical. Retrieved 2009-05-19.
  20. Vater, G. (2006). "The geographical distribution of the harvest mite Neotrombicula autumnalis (Acari: Trombiculidae)" (in German). CABI: 1–2. {{cite journal}}: Cite journal requires |journal= (help)
  21. Hirst, A. (1929). "On the "scrub itch mite" of North Queensland (Trombicula hirsti Sambon) — a possible carrier of tropical pseudotyphus" (PDF). Transactions of the Royal Society of Tropical Medicine and Hygiene. 22 (5): 451–452. doi:10.1016/S0035-9203(29)90067-5.
  22. "ACES Publications : CHIGGERS : ANR-1109". Retrieved 2008-06-24.
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  24. "Chiggers". Retrieved 2008-06-24.
  25. About.com: Chiggers Pediatric Dermatology Basics
  26. "ArmaXX Pest Control". Retrieved 2008-06-24.
  27. University of Florida: IFAS Extension
  28. Takahashi, M; Misumi, H; Urakami, H; Misumi, M; Matsumoto, I (2003). "Life cycle of Leptotrombidium pallidum (Acari: Trombiculidae), one of the vector mites of scrub typhus in Japan (Author abstract)". Ohara Sogo Byoin Nenpo. 45: 19–30. ISSN 0285-3671.
  29. Kumlert, Rawadee; Chaisiri, Kittipong; Anantatat, Tippawan; Stekolnikov, Alexandr A.; Morand, Serge; Prasartvit, Anchana; Makepeace, Benjamin L.; Sungvornyothin, Sungsit; Paris, Daniel H. (2018). "Autofluorescence microscopy for paired-matched morphological and molecular identification of individual chigger mites (Acari: Trombiculidae), the vectors of scrub typhus". PLOS ONE. 13 (3): e0193163. Bibcode:2018PLoSO..1393163K. doi:10.1371/journal.pone.0193163. ISSN 1932-6203. PMC 5832206. PMID 29494599. open access
  30. Baumann T (March 2001). "New treatment for harvest mite infestation". Archives of Internal Medicine. 161 (5): 769. doi:10.1001/archinte.161.5.769. PMID 11231715.
  31. An "infestation" with Trombicula mites (chiggers). http://medical-dictionary.thefreedictionary.com/trombiculiasis
  32. Missouri Department of Conservation
  33. Mayo Clinic, Healthy Lifestyle, Slide show: Poison ivy and other summer skin irritants - Chiggers "fall off after a few days, leaving behind red, itchy welts."
  34. 2006, Field Guide to Venomous and Medically Important Invertebrates Affecting Military Operations: Identification, Biology, Symptoms, Treatment
  35. William F. Lyon, Ohio State University Extension Fact Sheet, Entomology, Chiggers], HYG-2100-98 - Ohioline, --gives other "Control Measures" for chiggers, including: "Keep moving since the worst chigger infestations occur when sitting or laying down in a sunny spot at midday with temperatures above 60°F."
  36. Service, Mike (2008). Medical Entomology for Students (4, illustrated, revised ed.). Published by Cambridge University Press, 2008. pp. 250–252 of 289 pages. ISBN 978-0-521-70928-6.
  37. "CDC - Scrub Typhus Reemergence in the Maldives". Retrieved 2008-06-24.
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