41-year-old man presents to a dermatology clinic with a three-day history of
pruritic lesions. Two lesions are located on the upper medial bicep of the
right arm (see FIGURE 1 for one of these lesions) and one is located on the
upper medial region of the right thigh. Each lesion consists of a central
vesicular area with a 2- to 3-cm red erythematous area. The patient does not
recall being bitten by any insects. He reports significant pruritus with
little pain; he has not experienced nausea, vomiting, diarrhea, or fever. The
patient has no significant past medical history.
Unlocking the Mystery
During the summer
of 2007, hundreds to thousands of Chicago-areaÜ residents went to emergency
rooms and physicians'
offices with itchy welts similar to those described in the case above.1,2
Residents of the western suburbs of Chicago, specifically, were dealing not
only with the mysterious bites, but also with the return of swarms of cicadas
after a 17-year hiatus. (Cicadas, sometimes confused with locusts, are large
insects with big, wide-set eyes; they produce one of the loudest insect sounds
known.3) According to an entomologist and mite expert from the
Illinois Natural History Survey, Ed Zaborski, there was a connection between
the bites and the return of the cicadas; it is his belief that the bites came
from a microscopic itch mite called
moat-eez") that is
known to cause dermatitis in humans.1 These mites appear to have
fed on cicada eggs.1 The Illinois Department of Public Health asked
for Zaborski's help in identifying the organism.1
Similar outbreaks had been
reported as far back as 1852, when an itch-mite outbreak associated with sacks
of wheat occurred in France.4 More recently, outbreaks were
reported in Kansas (2004) and Nebraska (2005) in which pin oak trees were the
source of the itch mite known as
In the Kansas and Nebraska outbreaks, the mites fed on midge (a small,
two-winged fly) larvae inside galls.6 Galls are irregular growths
that appear on plant branches and leaves and commonly occur on pin oak trees (
). Insects and mites manufacture growth-regulating chemicals that react with
plant hormones and produce the galls. These galls provide the mites with
nutrients and protection from pesticides and natural enemies.7
The two main Illinois counties
that were affected by the bites had very few oak trees, affording limited
information. Numerous cicada egg nests were found on the pin oak tree twigs
collected by Dr. Zaborski, however.1 Cicadas can potentially lay
hundreds of eggs per branch, and their nests of eggs provide needed protection
for the mites. Weather associated with spring cold fronts or human or animal
migration patterns may have carried the
Pyemotes to Illinois
from neighboring states, such as Kansas and Nebraska. The area of southeastern
Illinois, Indiana, and Tennessee is expected to experience the emergence of
the 17-year periodical cicadas in 2008; this may lead to another outbreak of
the P herfsi
Background on Pyemotes
20 different species of
Pyemotes. The oak
leaf itch mite (
P herfsi) is a
European mite species closely related to
commonly breed in stored in, dried beans and peas, wheat straw, hay, and other
dried grasses. Itch mites and straw mites have caused outbreaks in the past,
as previously mentioned. Species of
generally have similar life habits, as follows: The newly mated female injects
a neurotoxin-containing saliva into her host, paralyzing the host and enabling
the gravid female to feed on the host's hemolymph.9 Hundreds of
adult mites emerge from the gravid female. Studies have estimated the number
of mites dropping from large pin oaks in Nebraska to exceed 300,000 mites per
tree per day.8 The mite has a mouth like a soda straw; it jabs this
into whatever it's eating and sucks up cells through the straw. The chances of
finding a mite on a bite are rare;
P herfsi mites are
0.2 mm in length and barely visible to the naked eye, and the mite is gone
long before the initial reaction occurs.9
The life cycles of the
midge and the oak leaf itch mite are depicted in
Epidemiology of Pyemotes
characteristics associated with a 2004
Pyemotes outbreak in
Crawford County, Kansas, are given in
People living in urban areas were five times more likely to have been bitten,
with a higher likelihood occurring in individuals under the age of 34. The
prevalence of bites in this younger demographic is presumably due to greater
exposure from clothing choices and increased outdoor activity. Bites were
reported in humans and pets that rarely ventured outdoors, thus raising the
possibility that the mites, aided by the wind, entered homes through open
screen doors and windows.9 Residents with a pin oak tree in their
housing lot were at almost four times greater risk for bites than those
without one, and leaf galls on the pin oak trees were observed to be infested
with P herfsi
can be mistaken for mite bites. Some arthropods that commonly bite humans are
chiggers, also known as redbugs (
); mosquitoes; cat fleas; bedbugs; and scabies mites.10 Chigger
bites commonly occur beneath areas of restrictive clothing such as belts,
underwear, and socks. Morphologically, the chigger bite resembles the
Pyemotes bite except
that the chigger itself frequently appears as a tiny red dot in the center of
the urticarial lesion. Chigger bites are usually few in number, compared with
the large number of
Pyemotes bites on
many patients. Chiggers do not burrow; they feed at the base of a hair
follicle or in a pore. The bites commonly cause itching in three to six hours,
and dermatitis develops in 10 to 16 hours. In the South, chiggers are active
virtually year-round and commonly are encountered in woodlands and along
swamps. Chiggers feed on skin tissue over a period of several days.10,11
Mosquito bites are usually
associated with itchy wheals of short duration. They are generally found on
uncovered portions of the body and do not tend to be grouped. Considerable
variation in clinical appearance can be noted in the bites of the 160 species
of U.S. mosquitoes.10
) bites usually are located only on the lower extremities of the body--the
legs and ankles. The lesions are grouped, itchy, and raised, with a central
round minute spot differing in color from the surrounding tissue. Secondarily
infected lesions are commonly found. Generally the patient is aware of having
bites are usually found upon arising in the morning. They appear as grouped,
itchy papules around the waist or on other areas not covered by clothing.
Because of their complex nutritional requirements, bedbugs--which emit an
offensive odor--come in contact with their human host only during feeding.
During feeding, injected saliva produces an allergic reaction. The reaction
causes delayed swelling, itching, and burning that may continue for seven days
Scabies mites (
burrow into the skin and produce a persistent, intensely itchy rash that often
takes the form of grayish lines. The adult female burrows into the outer layer
of the skin, where she feeds on tissue fluids and lays eggs that she cements
to the floor of the burrow. The rash and intense itching associated with
scabies usually occur several weeks to a month after the initial infestation.
The majority of scabies mites are found in lesions in the folds of skin
between the fingers, on the sides of the feet, on the wrists, on the genitals,
and in the bends of the knees and elbows. Diagnosis may require superficial
skin scrapings. Personal contact, particularly holding or shaking the hand of
an infested person, is a key method by which the mites are spread. Scabies
mites cannot survive off a human host for more than about 24 hours.11
Clinical Description of
The bite and
subsequent dermatitis are characterized as an itchy, raised, solid lesion
(papule) surmounted by a clear, pinhead-sized vesicle that later becomes
clouded by purulent material. Excoriated lesions may predominate due to the
extreme pruritus. A hyperpigmented area may persist for varying periods
following an early inflammatory process.10 There appears to be a
delayed reaction (10-28 hours) to the bites, with no initial prickling
sensation when bitten. The affected areas usually include the neck, arms,
shoulders, and upper torso.9,12 A majority of respondents to one
survey said that the rash resolved in one to seven days, with some reporting
resolution at 10 to 14 days.5,13 Few individuals (less than 20%)
with rash reported accompanying symptoms like fever, runny nose, and wheezing.
5 Ninety-five percent of respondents reported spending time outdoors 24
hours before the onset of the rash or lesion.5 Six percent of
respondents recalled an insect bite within 24 hours before the appearance of
the rash or lesion.5 Although no record exists of a human death
caused by the bites of these mites, secondary infections from excessive
scratching occurred. Many reports have documented the hospitalization of
recommendations for minimizing exposure to mites appear in
TABLE 2. For trees,
typical spray applications are not effective because the mites are protected
by the galls. Midges feeding on tree leaves provide a better target for
controlling mites. Two key preventive measures recommended by the World Health
Organization for humans are to avoid infested areas and to apply
diethyltoluamide (DEET).5 DEET has been shown to be the most
effective agent for repelling some mite species, although still with some
mixed results.5,8 Individuals also should wear clothing that covers
as much of their skin as possible when they are outside in infested areas.
treatment goal for mite bites comprises symptomatic relief and prevention of
secondary bacterial infections from excessive scratching (
symptoms vary depending on the type of insect bite. Typically, when bitten by
an insect, people experience localized pain, redness, swelling, or itching.
Occasionally, these symptoms are accompanied by burning, numbness, or
tingling. Mite bites generally go unfelt; they should be treated once the
signs (which include increased redness, swelling, and pain) appear. Start by
thoroughly washing the site with soap and water. This should be followed by
application of an ice pack and, possibly, the use of antihistamines or
hydrocortisone creams to relieve the itching. The rash typically resolves
after a few days through the use of topical steroids. Because a bite can
potentially progress to infection due to scratching caused by the extreme
itchiness, it is important to watch for signs.
bites that afflicted residents of some Chicago suburbs last summer were
probably from the
P herfsi mite. It is
difficult to positively confirm
Pyemotes because of
its microscopic size. The clinical suspicion of
Pyemotes should be
raised when a characteristic lesion (itchy, raised papule with a pinhead-sized
purulent vesicle) is identified. Utilization of preventive measures like DEET
and appropriate clothing are ideal for limiting exposure to self and others.
Secondary infections can occur from excoriation of the skin due to continual
scratching. Treatment should be with agents that relieve the itching
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