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Clinical Toxicology (2007) iFirst, 1–5
Copyright © Informa Healthcare USA, Inc.
ISSN: 1556-3650 print / 1556-9519 online
DOI: 10.1080/15563650701227729
LCLT
ARTICLE
Envenomation by the asp caterpillar (Megalopyge opercularis)
Envenomation by the Asp Caterpillar
DAVID M. EAGLEMAN
Baylor College of Medicine, Department of Neuroscience, Houston, Texas, USA
Background. The Asp Caterpillar, Megalopyge opercularis, is endemic to the southern United States and causes hundreds of human
envenomations annually. Envenomation from the spines of the caterpillar causes severe pain, burning, swelling, nausea, abdominal distress,
and headache. Despite the high prevalence of envenomations, little is known about the caterpillars, their geographical distribution, and the
symptoms they engender. Methods. We collected and analyzed 188 case reports of asp caterpillar envenomations over a three-year
period. Results. The geographical distribution of the caterpillars is confined to the southeast United States, largely in Texas, Louisiana,
and Florida. Symptoms of envenomation vary, but typically include burning pain, swelling, nausea, and itching. The peak months of
envenomation are July through November, with few cases reported at other times of year. Discussion. This study sheds light on the
caterpillars’ distribution and the range of symptoms caused in order to aid in diagnosing puss moth caterpillar envenomation.
Keywords Caterpillar; Megalopyge Opercularis; Asp caterpillar
Introduction
The best known venomous caterpillar in the American south-
west is the puss moth caterpillar, Megalopyge opercularis,
commonly called an asp, wooly asp, Italian asp, opossum
bug, wooly slug, and el perrito (1–5). It is considered one of
the most toxic caterpillars in North America (6,7). The cater-
pillar is abundant and may infest shade trees and shrubbery
around homes, schools, and in parks. They hold little impor-
tance as enemies of shade trees, but they present a real danger
when coming in contact with people. When a puss moth
caterpillar rubs against or is pressed against the skin, venomous
hairs break off and embed themselves, causing a severe burn-
ing sensation and rash. Within hours a clear pattern of hemor-
rhagic papules will arise, usually lasting for several days;
lymphadenopathy and swelling may develop (1–5,8–13).
Asp caterpillars resemble a teardrop-shaped tuft of cotton
or fur. Their hairs are long and silky. Their color varies from
white, gray, light tan, yellow, reddish-brown, or a mixture of
colors (Fig. 1A–1C). The mature caterpillar is 1 to 1.5 inches
long with seven pairs of prolegs (suction-cup-like claspers on
the rear of the body). The head and legs are not visible from
above. The hairs form a midline ridge along the back. Beneath
the hairs are concealed tubercular ridges: rows of sharp, short,
hollow spines (like porcupine quills) that penetrate skin and
discharge venom upon contact. Each hollow spine is con-
nected to a poison sac (1). In adulthood, the caterpillar
becomes a puss moth, which has blunt wings covered with
long, wavy hair and a wingspan of 1 to 1.5 inches. The wings
are orange at the base, fading to cream-colored at the tips.
Asp caterpillars pose a seasonal health hazard. Intense,
throbbing pain develops immediately or within five minutes
of contact with the caterpillar. Stings on the arm may also
result in pain in the armpit region. Erythematous (blood-
colored) spots typically appear at the site of the sting
(Fig. 1D–1F). Other symptoms can include headaches, nau-
sea, vomiting, intense abdominal distress, lymphadenopathy,
lymphadenitis, and sometimes shock or respiratory stress
(1,14,15). Pain usually subsides within an hour and spots dis-
appear in a day or so; however, with a larger dose of the
venom, it is not uncommon for the symptoms to last several
days or longer.
Often, those envenomated by the asp caterpillar find that
health care professionals have never seen nor heard of the
creature; the caregivers are left with little to go on, and the
victims are left with the distress of an unknown prognosis.
Existing literature often involves a single case study (15–17),
a self-reported envenomation (1), or a handful of cases (18).
The need for better information about these caterpillars is not
purely academic: at times public schools in Texas have been
temporarily shut down because of outbreaks of the caterpillar
(1), and more generally the problem of ignorance in the med-
ical community can lead to under-treatment or mistreatment.
Methods
To address these problems, this study was engineered to
determine the geographical distribution of these caterpillars,
Received 25 October 2006; accepted 18 January 2007.
Address correspondence to David M. Eagleman, Baylor College
of Medicine, Department of Neuroscience, 1 Baylor Plaza Houston,
Texas 77030, USA. E-mail: eagleman@bcm.edu
Downloaded By: [Eagleman, David M.] At: 22:07 10 March 2008
2 D.M. Eagleman
their season, and the spectrum of experienced symptoms. To
this end, we constructed a laboratory website that contained
information about the asp caterpillar (http://eaglemanlab.net/
asp). On the website, we requested people who had been
envenomated to e-mail the author of the website to describe
the details of their location and symptoms. There were 188
cases reported (either by victims or their relatives) from
October 2003 to October 2006.
We are aware that this method of data collection leaves
open the possibility of response bias: those with the worst
symptoms may be more prone to look for information on the
Internet and e-mail the investigator of the study. Further,
there may be a socio-economic response bias: those living in
poor areas may not have access to the Internet. However,
with these caveats in mind, we can nonetheless learn a great
deal about the caterpillar’s geographical distribution, the
range of symptoms caused, and the seasons in which this
occurs.
Results
Geographical distribution
Figure 2 shows the geographical distribution within the
United States of cases reported through the author’s website
over a three-year period. Note the disproportionate number of
reports from Texas (n=124) as compared to the runner-ups,
Florida and Louisiana, with 10 reports each. Normalizing by
the land area of each state (Table 1), we find that Texas still
leads, but with only twice as many reports per square mile as
Louisiana, and two-and-a-half times that of Florida. Perhaps
because the author’s webpage has not yet been translated into
Spanish, no emails have been received from Mexico; thus,
this report sheds no light on statistics south of the Rio
Grande.
Note that collecting data over the Internet is a fast and
effective way to generate a snapshot of geographic distribu-
tion; in previous generations this was accomplished more
laboriously by travel and correspondence between experts.
These earlier methods left gaps in our picture of the distri-
bution; thus, the first reports of asp envenomation in Arkan-
sas only appeared 30 years ago (16), and in Florida 20 years
ago (18).
Fig. 1. (A-C) The Asp Caterpillar, Megalopyge opercularis, can vary in color from white to gray to brown, but is easily recognized by its
fuzzy appearance and teardrop shape. (D-F) Encounters with asp caterpillars are unmistakably evidenced by a grid-like pattern o
f
erythematous spots at the site of contact.
a
d f
cb
e
Fig. 2. Geographical distribution within the United States of cases
reported over a three-year period. Colors reflect numbers of cases
reported.
0
1–4
5–8
9–12
124
Asp Caterpillar Envenomations
Asp Caterpillar Envenomations
Asp Caterpillar Envenomations
Asp Caterpillar Envenomations
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Envenomation by the Asp Caterpillar 3
Season of the caterpillar
The caterpillars pass winter in cocoons on twigs. The moth
emerges from the cocoon in late spring and deposits her eggs
on shrubs and trees. Within days the eggs hatch, and whitish
larvae emerge. The caterpillars tend to darken as they mature.
The caterpillars feed on deciduous trees and shrubs (4).
To determine the time(s) of year when asp caterpillars pose
the greatest risk, we analyzed envenomations by the month in
which they occurred (Fig. 3). The double hump in the data is
consistent with a report by Goddard (6) that the asp caterpil-
lar has two generations a year, the first developing in spring/
early summer and the second in the fall. Also consistent is
Goddard’s observation that they seem to be especially
abundant from September to November (6).
Symptoms
Although the possible symptoms of asp caterpillars envenom-
ations have been previously reported, the statistics of those
symptoms (i.e., how common they are across individuals) has
remained unknown. Figure 4 shows the distribution of self-
described symptoms from 159 of 188 subjects whose reports
allowed such an analysis. Almost all victims described pain,
with many describing more specifically stinging or burning.
Typical reports included descriptions such as “it felt as
though my arm had been broken”; “it immediately felt like a
hammer hit me”; and “I have had kidney stones before, but I
believe the pain I am experiencing from the asp sting is
worse.” Twenty-nine percent described swelling of a limb,
similar to the 28% reported by Stipetec et al. (9). Twenty per-
cent or fewer felt nausea, headache/fever, and itching.
Approximately 9% report acute abdominal distress (as has
been reported before in Neustater et al. (17)), sometimes
lasting for days.
Other complaints not represented in the categories in Fig-
ure 4 include welts, hives, or sweating at the site of the sting,
and other scattered reports of symptoms such as muscle
spasm, faintness, tingling, and a red streak on the skin from
the sting site to the nearest lymph gland.
It is also reported in about a third of victims that the pain
radiates. For example, a sting on the hand or forearm can
radiate pain up the arm into the armpit and chest, leading
some to spuriously believe they are having a heart attack.
Similarly, if the sting site is on the leg, pain can radiate up the
leg to “coalesce” in the testicle or inguinal canal. In a previ-
ous study, radiating pain and systemic effects were present in
35.2% of M. opercularis envenomations (5). In a study of
phone calls to Central Texas Poison Control Center, 99% of
patients experienced local pain at the site of the sting and
27% experienced “intense radiating pain” (9).
Not represented in this analysis is the feeling of panic that
many sting victims reported to us. Because many people do
not know that caterpillars can be venomous, or sometimes do
not detect that they came into contact with a caterpillar, some
victims find themselves in intense pain without an explana-
tion. It is not unheard of that some victims are launched into
panic attacks by the unexpected onset of the pain.
Since patient histories were volunteered, and individual
follow-up was not always possible, there is a possible confla-
tion of descriptions like “pain” and “burning,” which in some
cases were used interchangeably, and in other cases were
described as different symptoms (e.g., “the burning was
accompanied by a terrible pain in my arm, as though my
entire arm had been charlie horsed”). Since not all patients
volunteered clear descriptions, the statistics in Figure 4 are
compiled from those reports from which the appropriate data
could be meaningfully extracted (n=159). As noted above,
the severity of the symptoms may reflect a reporting bias,
wherein only those with the worst symptoms volunteer their
stories; nonetheless, this report is useful for demonstrating
the range of symptoms.
Table 1. Number of asp caterpillar envenomations reports by state
(second column) and normalized by land area (fourth column)
State Reports Land area Normalized
AL 3 50767 0.12
AR 2 52078 0.08
AZ 1 113508 0.02
FL 10 54153 0.39
GA 4 58056 0.15
KY 1 39669 0.05
LA 10 44521 0.47
MD 1 9837 0.21
MS 2 47233 0.09
N
C 8 48843 0.35
SC 4 30203 0.28
TX 124 262017 1.00
VA 2 39704 0.11
172 of the 188 total case reports included clear geographical information
that could be used for this table.
Fig. 3. Asp caterpillar envenomations by month. n=163 of the 18
8
case reports which contained the month of envenomation in th
e
description.
0
5
10
15
20
25
30
J
a
un
a
r
y
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Envenomations (% total)
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4 D.M. Eagleman
Many victims report varying degrees of success with home
remedies. One successful strategy consists of applying tape to
the site and pulling it off to remove spines (“stripping”).
Other victims report applying ice packs, oral antihistamine,
baking soda, hydrocortisone cream, juice from the stems of
comfrey plants, or calamine lotion. Other papers in the litera-
ture have suggested relief through intravenous calcium glu-
conate (17) or systemic corticosteroids (19). As noted by
Stipetic et al. (9) and many of our case reports, most victims
have a difficult time finding any treatment that promptly
relieves pain.
As for what to do about local infestations of the Asp cater-
pillar, one patient reports success in killing the caterpillars with
the pesticide Cypermethrin (Viper Insecticide Concentrate). It
has also been suggested that Bacillus thuringiensis or plant oil
products can cut down on asp caterpillar infestations (7).
Discussion
This study reveals that the caterpillar M. opercularis covers a
larger range of the United States than previously reported,
spreading as far north as Virginia and Kentucky. This study
only takes into account the distribution of the asp caterpillar
in the United States; they are also known to be endemic to
Central and South America (6).
There has been some problem diagnosing Asp caterpillar
envenomation in the past because even health care providers
who know of the caterpillar are not aware of the potential
range of the symptoms. Symptoms such as nausea, severe
abdominal distress, and chest pain often confuse the clinical
picture in the emergency room, because clinicians are
unaware that these symptoms are commonly a part of the
constellation. More generally, many patients report that nei-
ther they nor their health care providers had any knowledge
at all of these toxic creatures. The high prevalence of caterpil-
lar envenomations thus calls for more study and understand-
ing of these arthropods. This study sheds light on their
distribution, seasons and range of symptoms in order to aid in
diagnosing puss moth caterpillar envenomation. Future stud-
ies will be required to expose the details of their venom with
the goal of providing an antidote.
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Envenomation by the Asp Caterpillar 5
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