Megalopyge opercularis is tapered at the ends and can measure 2.5 to 3.5×1 cm at maturity. It is covered by silky, long-streaked, wavy hairs that may appear single colored or as a mix of colors—from white to gray to brown—forming a mid-dorsal crest.6 Beneath this furry coat, rows of short sharp spines are hidden. Upon contact with the human skin, these spines will break and discharge venom.1,6,8 Toxins contained within the hollow spines are thought to be produced by specialized basal cells, but there still is little knowledge about the dynamics and composition of the venom.1
Clinical Manifestations
The severity of the reaction depends on the caterpillar’s size and the extent of contact.1,4 Contact with M opercularis instantly presents with a throbbing or burning pain that may be followed by localized erythema and rash.1,6 A characteristic gridlike pattern of erythematous macules develops, reflecting each site of puncture from the insect’s spines (Figure 2).8,9 Skin lesions can progress from erythematous macules to hemorrhagic vesicles or pustules, usually self-resolving after a few days. The reaction also can present with radiating pain to regional lymph nodes and numbness of the affected area.1,6,8 Moreover, some patients may report urticaria and pruritus.9
Figure 2. Gridlike pattern of hemorrhagic papules and crusts on the palmar aspect of the right hand following Megalopyge opercularis envenomation.
Envenomation by a puss caterpillar also can present with systemic manifestations including fever, headache, nausea, vomiting, shocklike symptoms, and seizures.1,6,7 Anaphylactic reaction is rare but also can present.7 Uncommon cases have been reported with severe abdominal pain and muscle spasm mimicking acute appendicitis and latrodectism, respectively.7,9
Diagnosis
The diagnosis of M opercularis envenomation is made clinically based on the morphology of the skin lesions and a history of probable exposure. Coexistent leukocytosis is likely, but laboratory testing is not warranted, as it is both nonspecific and insensitive.9
Management/Treatment
The most commonly reported immediate approaches to treatment involve attempts to remove the spines from the skin with tape (stripping), application of ice packs over the affected area, oral antihistamines, topical and intralesional anesthetics, regional nerve block, and oral analgesics.6,9 There have been several cases detailing the successful use of parenteral calcium gluconate,5,7 and diazepam has been used to treat severe muscle spasms. Anaphylactic reactions should be managed in a controlled monitored setting with subcutaneous epinephrine.7 Despite their common use, some data suggest that ice packs and mid- to high-potency topical steroids are ineffective.9
Incidence
From 2001 to 2005, a mean average of 94,552 annual cases of animal bites and stings were reported to poison control centers in the United States, of which 2094 were linked to caterpillars in this 5-year period.10 There were 3484 M opercularis caterpillar stings reported to the Texas Poison Center Network from 2000 to 2016.5,6 Given their ability to sting throughout their life cycle, thousands of M opercularis caterpillar stings can occur each year.1,6 Existing literature on M opercularis caterpillar stings mainly involves case reports with affections of the skin and oral mucosa, self-reported envenomation, and case studies.5,6,8
Although multiple health concerns associated with caterpillar envenomation have been reported worldwide, the lack of official epidemiologic reports highly suggests that this problem remains underestimated. There also may be many unreported cases because certain reactions are mild or self-limited and can even go unnoticed.11 Nonetheless, there is an evident rise of cases reported in the United States. According to the 2018 annual report of the American Association of Poison Control Centers, there were 2815 case mentions from caterpillar envenomation.12
In 1921 and 1952, some public schools in Texas were temporarily closed due to outbreaks of puss caterpillar–associated dermatitis.8 Similar outbreaks also have been reported in South Carolina, Virginia, and Oklahoma.9 Emerging data suggest that plant oil products and the pesticide cypermethrin may be helpful in controlling local infestations of the puss caterpillar.8