Paederus dermatitis: an outbreak, increasing incidence or changing seasonal pattern?

Sir,

Insects of the genus Paederus are known to cause an irritant contact dermatitis. There are about 622 species of paederus or blister beetle distributed worldwide. The commonest species found in India is Paederus melampus.[1] The beetle lives in moist habitats and moves around in search of debris of food and other matter for its nutrition. Accidental contact with the skin of human being and consequent reflex brushing away or crushing of the insect on the skin causes release of Paederin from the haemolymph of the beetle resulting in the irritant contact dermatitis. The lesions are particularly seen on exposed parts of the body. Patients often observe the initial lesions in the morning on waking up or after bathing or changing clothes, especially if they sleep on the floor. Increase in incidence has been observed during the festival of Deepawali, possibly due to the fact that the insects get attracted by the bright incandescent light. The disease is characterized by sudden onset of erythematous, papular, and vesicular lesions [Figure 1] Linear lesions are caused by brushing away of the insect [Figure 2] and kissing lesions, especially on flexural areas occur due to apposition of normal skin to the lesion.[1] Though mucosal lesions are rare in India they have been reported more often from other countries. The irritant dermatitis may affect the genitalia or the eye (Nairobi eye)[2] by primary contact or by transfer of toxic chemicals from the cutaneous lesions.

Figure 1

Linear erythematous and pustular lesion on the neck resembling a whip lash

Figure 2

Oedematous papulovesicular lesions along a line

In our institution, which is an urban center located in the Eastern part of India the disease is seen throughout the year with an increase during certain parts of the year. In the spring and summer of 2011 our outpatient department (OPD) was flooded with cases of paederus dermatitis, the incidence being much higher than in previous years. In an earlier report from the same part of the country, the authors had opined that the incidence decreased in winter and spring.[3] Were we witnessing a change in seasonal pattern of incidence of paederus dermatitis? To determine this, we decided to conduct a retrospective statistical analysis of the number of cases seen during the years 2009 through 2011. The number of new cases seen during each of the 36 months, and the percentage of such cases in relation to the number of new OPD cases seen during the same period was determined. The results are tabulated in Table 1. The disease was diagnosed on the basis of clinical features. Identification of the insect species was not possible as none of the patients could capture and bring the insect to us, although all of them spoke of some initial irritation (probably after contact with the insect).

Table 1

Total number and percentage of paederus dermatitis cases (with respect to total number of outpatient department cases) seen during 2009-2011

In all the 3 years, there was an increase in the number of cases starting from March gradually rising to a peak in May, with a decreasing incidence from June onwards. A second smaller spike could be observed during November and December [Figure 3]. Hence, in contrast to the earlier study, we observed a rise in incidence during spring and summer (March-May) and again during autumn and early winter (November and December). Various investigators from different parts of India have reported different patterns of incidence: March to July in Orissa,[4] April in South India,[5] the post-monsoon season in Rajasthan,[6] and July in Punjab[7] indicating that the pattern may vary in different geographical and climatic zones of the country.

Figure 3

Pattern of incidence of Paederus dermatitis

The month of June is characterized by a decrease in the incidence as observed in 2009 and 2010. However, in the same month in 2011 the incidence remained at the same peak level as in May of that year. Additionally, it was observed that the total incidence in the months of March-June in 2011 (191) was much higher than in 2009 (47) and 2010 (107), with the month of June accounting for a major part of the difference. Outbreaks of paederus dermatitis have been reported earlier. However, these have been observed in closed communities such as army barracks,[8] schools,[9] etc., Our cases were seen in the OPD and the patients came from a widespread area, covering both urban and semi-urban localities. Therefore, it is debatable whether the rise in incidence could be designated as an outbreak or could be attributed to an increasing pattern of incidence.

Several authors have opined that proximity to paddy fields where the insect may be found in large numbers could be an important factor in outbreaks of the disease.[49] However, majority of our patients came from urban and semi-urban localities which were not located near paddy fields.

The insect paederus is known to be attracted by incandescent light.[1] Deepawali, the festival of lights is held during November and this could be one reason why the incidence was more during this month. However, what could be the reason for the rise in incidence during spring and summer and subsequent decrease during the rainy season? The insect is known to thrive in a moist habitat. The summer season is generally very dry and hot in our city, except for the occasional Nor’wester. It could be postulated that unable to bear the heat the insect moves indoors in search of a moist habitat which is not available in the parched soil outdoors during these months. The insect has also been seen to thrive in stagnant water.[4] The stagnant water in toilets and buildings under construction may also contribute to the tendency of the insects to move indoors. June signals the arrival of monsoon. It may be possible that the delay in arrival of the monsoon (June 16) in 2011 as compared to May 25 in 2009 and June 10 in 2010 could contribute in part to the high incidence seen in June 2011 as compared to the previous years.