Oak processionary moth (OPM, Thaumetopoea processionea) larvae

The thermophilic oak processionary moth (OPM, Thaumetopoea processionea) is a native of southern Europe - however it is expanding its prevalence to encompass Central and Northern Europe

• the moth’s range has spread northwards and the species has now became established in the warmer regions of Northern France, the Netherlands, the UK, and Germany
• in the UK, the moth occurs in Surrey and in parts of west London
• the number of people affected will depend on the population density of OPM in a particular area and the density of the human population in that area
  • the types of interaction between OPM and the human population are also relevant
  • in circumstances where human activities result in direct contact with the insects - for example, where children are playing with the caterpillars – the release of large numbers of setae has been associated with a number of outbreaks
  • direct contact is not necessary for human health effects to occur and outbreaks of dermatitis have also been reported in the absence of direct contact with OPM caterpillars and their nests (2)
  • larvae of these caterpillars are equipped with fine, barbed hairs (setae, 0.2 mm), which can readily penetrate the human skin
  • one of the major issues with the hairs is that they readily detach and can then be blown via wind currents for considerable distances
  • the older larvae (fifth and sixth instars) are of particular concern as they can carry up to half a million of the urticating setae per caterpillar
  • spines can be active for up to 10 years, posing a long-term threat to human health

Open Image in Modal

Life cycle

• OPM tend to infest trees on the edges of woodland or those that are standing alone
• OPM eggs are laid in plaques on branches at the end of the summer (August/September) and hatch the following spring
• timing of the eggs hatching is coordinated with the budding of the oak trees; emergence can be delayed (embryonic diapause) if the budding of the oak tree is delayed
• caterpillar form passes through six larval stages (L1−L6) over the course of about three months
• during L3, which usually occurs in April−June, urticating hairs start to grow on the dorsal surface of two body segments and by L6 all body segments have setae. Setae density increases through the instar stages, with a significant increase in setae density from L5−L6
• at L4, the larvae establish a silk nest to which they return after feeding. These nests are usually built at the intersection of branches, on the protected side, and can reach up to 1m in size
• caterpillars are nocturnal, feeding on the leaves at the edges of the canopy at night and processioning back to their nest during the day
• larval development is usually finished by July/August, at which point each individual caterpillar spins a cocoon within the nest from which it emerges about 20 days later to reproduce

Health effects:

• systemic health effects caused by adult moths can result in a variety of medical conditions referred to as lepidopterism (Lepidoptera is the order of insects that includes butterflies and moths), while the skin conditions caused by the setae of the larvae are referred to as erucism or caterpillar dermatitis
• Dermal effects
  • after exposure the skin is always affected, particularly the neck, face, arms, and legs
  • several types of cutaneous reactions to processionary caterpillars have been described including urticaria, persistent itchy papules, localised pain, erythema, oedema, angioedema, cutaneous lesions, and bullous (blistering) dermatitis
  • papular urticaria and dermatitis are the most common manifestation of dermal symptoms. Maculopapular dermatitis is the typical delayed cutaneous reaction and occurs within about eight hours of exposure. Dermal symptoms generally resolve within three weeks (2)
  • been suggested that diffuse rash is associated with the toxic-irritative reaction while rashes that appear over a shorter period of time and persist for longer are caused by an IgE mediated mechanism
• Ocular effects
  • ocular effects following exposure to caterpillar setae are rare. Ocular irritation, conjunctivitis, keratitis, chorioretinitis, papillitis and ocular lesions have been reported
  • chronic ophthalmia nodosa (CON) has also been described and may occur months after exposure - characterised initially by conjunctivitis followed by pan-uveitis, with subsequent intraocular migration of the seta(e)
  • CON may be difficult to diagnose as it may be due to a single seta in the eye, which may not easily be seen. CON may require surgical intervention to remove the seta(e); treatment should be overseen by an ophthalmologist
• Respiratory effects
  • setae can become airborne and are small enough to penetrate the human respiratory system as far as the trachea and zones of the primary bronchi. Cough, shortness of breath (SOB) and asthma-like symptoms (wheeze) have all been described following exposure to OPM
    

Management

• symptomatic and supportive following recommended strategies of gently washing the urticating hairs off the skin or removing them using cellophane or
  adhesive tape
  • clothes should be removed immediately and washed at a minimum of 60°C
• ocular manifestations, eyes should be washed and an ophthalmologist contacted. Conjunctivitis should be treated locally and may include application of an antiseptic
  • patients with intracorneal setae should be informed about the risk of intraocular penetration and followed up for a minimum of 6 months (1)
• topical antihistamines and medium or high-strength corticosteroids can be applied to skin reactions (1)
• use of nebulised and/or systemic bronchodilators may be administered for asthmatic bronchospasms
  • in severe cases, parenteral application of corticosteroids may be necessary

Reference:

• Rahlenbeck S, Utikal J. The oak processionary moth: a new health hazard? British Journal of General Practice 2015; 65: 435–436.
• PHE (May 2015). Health effects of exposure to setae of oak processionary moth larvae - Systematic review