[Focus On Public Health] Chigger Control

Larvae of mites in the family Trombiculidae, sometimes called "chiggers," "harvest mites," or "red bugs," are medically important pests around the world. More than 3,000 species of chigger mites occur in the world, but only about 20 species are significant pests.

Chiggers are mostly just a nuisance, causing intensely itchy bumps on the skin where they feed, but a few species transmit a disease called "scrub typhus" in parts of Asia. Larval chiggers crawl up on blades of grass or leaves and subsequently attach onto passing vertebrate hosts. On humans, they generally crawl to and attach where clothing fits snugly or where flesh is tender. Chiggers then pierce the skin with their mouthparts, inject saliva into the wound (which dissolves tissue) and then suck back up this semi-digested material. Feeding generally lasts a few days.

Most U.S. pest chiggers produce itching within three to six hours, followed by development of red, dome-shaped bumps. Itching increases in severity over the next 20 to 30 hours and may not subside for days. Not all medically important chiggers produce the familiar itch reactions; those serving as vectors of scrub typhus (central, eastern and southeastern Asia) are not associated with itching or skin reactions.

APPEARANCE/LOCATION. Most people have seen adult chiggers. They are oval shaped (about 1 mm long) with a bright red, velvety appearance. But it is the larval stage that attacks vertebrate hosts; adults do not bite. Chigger larvae are very tiny (0.2 mm long), round mites with numerous setae (see figure on next page). They are barely visible with the naked eye. They may be red, yellow or orange and have a single dorsal plate (scutum) bearing two sensillae and four to six setae.

Worldwide, chigger species are mostly tropical and subtropical, although they occur from Alaska to New Zealand and from sea level to more than 16,000 feet in altitude. Some particularly common pest chiggers are Eutrombicula alfreddugesi in the United States and parts of Central and South America, Neotrombicula autumnalis in Europe, and E. sarcina in Asia and Australia.

HOW THEY LIVE. Chigger mites are found in moist microenvironments within grassy, weedy or wooded areas, especially forest edges and wild blackberry patches (at least in the southern U.S.). Adult chiggers are predaceous. The female lays eggs singly on soil or litter; eggs hatch in about a week.

After hatching, the life cycle includes six stages: 1) the inactive prelarva (deuto-vum or maturing larva), 2) the parasitic larval stage, 3) quiescent protonymph/first nymphal stage (nymphochrysalis), 4) predaceous deutonymph/second nymphal stage, 5) quiescent tritonymph/third nymphal stage (imagochrysalis) and 6) the free-living adult stage. The entire life cycle may be completed in about 60 days.

In the northern U.S. the mites are active from about May through September, but in the South they may be active year round. The parasitic larvae normally feed on rodents, insectivores and ground-frequenting birds, but given the chance some species will avidly feed on people. Larval chigger mites secrete saliva while feeding that dissolves host cellular tissue. The mites then "slurp up" this mixture of lymph, dissolved body tissues and stray blood cells. They do not "suck blood" like mosquitoes or ticks.

HOW TO PREVENT AND TREAT. Customers should be advised that repellents containing DEET or permethrin work well against chiggers when sprayed around ankles and on socks. After exposure to infested outdoor areas, hot soapy baths or showers will help remove any chiggers. Once chiggers are attached, over-the-counter antiseptic, hydrocortisone and/or anesthetic solutions or ointments can be used to minimize itching and reduce chances of secondary infection.

Environmental sanitation around homes may help reduce chigger numbers. For instance, elimination of rubbish, rock or brick piles and other harborage will help reduce the wild rodent populations on which chiggers feed. Secondly, since the mites are fairly susceptible to desiccation, anything to allow a breeze and more sunlight into a yard will help. Things such as keeping the grass cut low, brush cut back, overhanging tree limbs removed and shrubs trimmed allow sun and wind to kill the mites. These efforts by themselves will not eliminate the mites, but they certainly will help. I personally have seen chigger infestations in well-kept yards, but not to the same extent as those in unkempt ones.

Pesticides may be used as well. Several products are available for spraying portions of yards, trails, picnic areas and other outdoor recreation areas. These often contain active ingredients such as cyfluthrin, cyhalothrin, deltamethrin and carbaryl. I have been involved with field studies of deltamethrin against chiggers and found it to be quite effective.

Images are courtesy of the author. The author is a medical entomologist for the Mississippi Department of Health, Jackson, Miss. He can be reached via e-mail at jgoddard@pctonline.com.