The following incident did not take place on the Link Trail, but rather a few miles away in Oneida. It is relevant to this journal because your child may encounter a cocoon from the same species of caterpillar on the trail.
Grace and I babysat Dylan and Abby for a few hours while their parents went out to celebrate their anniversary. While the kids played on the lawn I spent some time snapping pictures of spiders. In order to steady my hand, I grabbed a block of wood to prop up my arm. There happened to be a cocoon on this block of wood, and Dylan noticed it. The kids poked at it, which I though harmless enough. Little did I know what perils awaited our young charges. (CUE DRAMATIC MUSIC)
Well before his parents got back, Dylan was complaining about discomfort in his hand. I didn't think anything of it at the time, but he asked me to take a picture of his thumb soon after he touched the cocoon. I had forgotten all about this until weeks later, when I was going through my pictures from that day. There was a series of shots of the hairs embedded in his thumb - hairs I didn't even know were there when I took the shots!
Grace took a close look and noticed tiny hairs sticking out of both of the children's fingers and palms. I was shocked. The only time I'd heard of something like this was when, visiting Arizona in 2003, I was warned about the hairs of the teddy bear cholla. But I had no idea that hairs from a soft-looking cocoon could get stuck in the skin! I remembered that the best way to remove teddy bear cholla hairs is to apply Elmer's glue or duct tape and then peel it off, so I went and found some tape.
Thankfully the hairs from the cocoon don't burn like those from the teddy bear cholla; they seemed to cause the kids considerable itching, but not what you'd call pain. So they were patient and relatively fidget-free while we pressed the tape onto their little piggies again and again. It took a long time, but eventually we got most if not all of them out.
I thought that was the end of it, but no such luck. Two days later I was back at the office and I got a call from my sister. Dylan's hand had gotten worse! The discomfort had increased, and there were little red bumps on his hand. I called Madison County Cooperative Extension and posted a request for expertise on the BugGuide forum. Then I did a whole lot of furious Googling.
Among other things, I found this thread on Arachnoboards.com, in which the general consensus was that no one quite knows whether the hairs cause irritation via purely mechanical means, or whether they contain a poison like the urushiol in poison ivy. I think that was where I first saw the term "urticating hairs". It seems that tarantulas and some caterpillars have them, and that the cocoons of those caterpillars retain the hairs!
I called my sister back, conveying to her the general sense I got from skimming a number of sources: an injection may not do anything, so don't let the doctor stick the kid unless he can show you a good reason to do so. The best thing to do in these cases is to apply a topical steroid, which is exactly what the doctor ended up prescribing.
I spoke to my sister a few days later, and Dylan's hand was fine. The strange thing was that Dylan suffered much more than Abby, despite the fact that she got more hairs in her hand than he did. This might be because Dylan had been exposed to something similar before. If you read the Arachnoboards.com thread, you'll learn that the poison in urticating hairs effects people like the urushiol in poison ivy: most folks show no reaction the first time they're exposed, a moderate reaction the second time, and so on.
The irritation comes at least in part from the mechanical properties of the hairs, but it seems likely that the allergic reaction comes from a poison. This morning, nearly a month later, I read an article* about a moth with poisoned urticating hairs that I printed out from JSTOR**. It seemed to show conclusively that, although the initial skin reaction is due to the hairs' mechanical properties, there is a separate allergic reaction caused by poison.
OOH OOH!! I just went to Google the article so that I could see if it was available outside JSTOR, and look what I found: "Investigative Studies of Skin Irritations From Caterpillars"***. Note that you can download the entire .pdf file!
...pauses to read the article...
Interesting! And by "interesting", I mean the fraction that was not so densely technical that I had no clue what the authors were going on about. Here's a summary of the germane bits.
In clinical practice, skin exposure has been usually accidental in the woods or in the forest. In the home, caterpillars or cocoons or moths may get into sheets, pillows, etc. and irritate the skin even after prolonged periods of drying of the arthropod. In one area of Texas, according to Bishopp (8), schools had to close until the larva were under control. Katzenellenbogen (22), has described caterpillar dermatitis as an occupational disease in plantation workers. Ziprkowski, Hofshi and Tahori (23) report 600 cases of caterpillar dermatitis among 3000 soldiers encamped in a pine grove. Occasionally the irritant material may be even dustborne. The airborne factor is more important in the development of the irritation from the hairs of moths. "Yellow tail moth dermatitis" is well known among merchant marine personnel who use ports of Central and South America (24, 25, 26). Also well known among the troops in Korea is the papular dermatitis from the "Yellow Korean Moth".So, in summary: lots of live or caterpillars or their cocoons or even their remains can cause skin reactions of varying severity. The immediate reactions may be caused to a greater or lesser degree by the mechanical properties of the urticating hairs, but the more serious, and the more long-term, reactions are due to a toxin of unknown nature. So don't go nuzzling any caterpillars.
When setae were immersed in water, saline alcohol, etc., and dried out, although they were very brittle, the insertion of these into the skin did not produce any reactions. It was not possible on microscopic section to see a definite foreign body reaction around the inserted setae. Therefore,the seta itself produced no significant foreign body reactions. However, setae removed freshly from the caterpillar or left by the caterpillar were found to contain the irritant material. This is again additional proof for the basic idea that the setae, themselves, in spite of their barbed appearance serve merely as a tube to carry the irritant substance.
Although the caterpillar has been studied for some time, the exact nature of its irritant principle is still not known definitely. Its polypeptide nature is suspected; 5-hydroxytryptamine may be present. We, however, could not find this important material in our extracts. The poison glands of the caterpillar appear similar to the salivary glands of the other arthropods of dermatologic interest. The skin reactions vary according to the sensitivity of the individual. ...
*The Poison and Poison Apparatus of the White-Marked Tussock Moth Hemerocampa leucostigma Smith and Abbot
Paul M. Gilmer
The Journal of Parasitology, Vol. 10, No. 2 (Dec., 1923), pp. 80-86
**Helping put Grace through the School of Physical Therapy at Columbia has its perks, foremost among which is that I get access, through her, to pretty much any research article I want. JSTOR, an online library of many research periodicals, is the best of the sources.
***Investigative Studies of Skin Irritations From Caterpillars
Leon Goldman, M.D., Faye Sawyer, Ann Levine, John Goldman, Steven Goldman and Joan Spinianger, B.S.
Presented at the Twentieth Annual Meeting of The Society for Investigative Dermatology, Inc., Atlantic City, N. J., June 7, 1959.