Fungal skin infection of the foot is a very common in podiatry practice and it is a topic which I have covered widely on this blog. Occasionally, you may come across patient’s who start out with fungal foot infection, but an eczematous eruption subsequently develops soon after at a remote site on the skin known as the ID reaction (“Idiosyncratic” reaction or “autoeczematisation”). This blog looks at this uncommon but relevant side effect of dermatophyte skin infection.
What is an ID reaction?
ID reactions are a form of acute, non-infectious, eczema or dermatitis which develops following an inflammatory or infectious event on the skin elsewhere. The ID reaction is normally remote to the initial event mostly occurring within two weeks from the appearance of the primary inflammation.
Typically, the reaction is localised, symmetrical but occasionally can be generalised. It is frequently itchy. The closer to the original site of exposure, the worse the ID reaction tends to be. It was first described by Whitfield in 1921 who described it as a form of auto-intoxication [1].
What triggers an ID reaction?
The majority of the ID reactions occur as a result of tinea pedis or stasis (varicose/gravitational) eczema but it has been reported to be caused a wide variety of skin disorders including other fungal skin infections such as tinea capitis [2, 3] and kerion [4], viral infections [5] such as molluscum [6], allergic contact dermatitis, infestations such as lice [7] and larva migrans [8], retained sutures and radiotherapy [9]. Re-exposure to the causative agent can result repeated ID reactions.
Why does it occur?
The exact reason is unclear, but evidence suggests that following exposure to a stimulus there is a rapid rise in primed T lymphocytes, as a result of keratinocytes producing pro-inflammatory cytokines (IL-25 and IL-33). The activated T cells migrate to the primary site but eventually may spread to remote parts of the skin to cause localised eczematous reaction [1].
Fungal foot infections and ID reactions
The most common form of ID reactions arise as a result of tinea pedis and fungal skin infections. These are labelled as “Dermatophytide” or “epidermatophytide” reactions. Typically, these appear remotely as acute vesicular eczema which an arise on the hands or feet of patients. Such reactions are most likely to be provoked by Trichophyton Mentagrophytes. It is important to remember that initiation of antifungal therapy has been known to trigger ID reactions as well, but treatment should be continued. Very occasionally, an ID reaction may present as an annular erythema or erythema nodosum [10].
Is it a genuine dermatophytide reaction?
To be labelled a true dermatophytide reaction, it must meet the following three criteria [11] :
1. A proven primary dermatophyte infection arising before the dermatophytide reaction.
2. The dermatophytide must be remote to the primary infection and free of fungal elements.
3. The dermatophytide rash will disappear upon treatment of the primary fungal infection.
Dermatophytide reactions to tinea pedis do arise but they are uncommon, and often not mentioned by the patient as the remote location means the two are related by the patient during history taking. Moreover, it is likely that such reactions are more prevalent when the primary fungus is a zoophilic species. As most tinea pedis in the western world is human-to-human (anthrophilic species) the reaction is not frequently observed.
Implications for practice
ID reactions are a recognised phenomenon in dermatology - the development of a remote area of acute eczema following a primary infection is the hallmark of the condition. For the practising podiatrist the most common presentation will be identical to pompholyx, with an eruption of small, itchy vesicles on the palmar and digital margins of the hands following inflammatory tinea on the foot. Treatment of the primary fungal infection should lead to rapid improvement of the pompholyx on the hands.
References
1. Bertoli, M.J., R.A. Schwartz, and C.K. Janniger, Autoeczematization: A Strange Id Reaction of the Skin. Cutis, 2021. 108(3): p. 163-166.
2. Jiang, Y., et al., Extensive tinea capitis and corporis in a child caused by Trichophyton verrucosum. Journal de Mycologie Médicale, 2019. 29(1): p. 62-66.
3. El Imene Ouni, N., et al., Tinea capitis by Microsporum canis in a 3-year-old girl with palmoplantar pustular dermatophytid reaction. Clin Exp Dermatol, 2022. 47(3): p. 596-597.
4. Ravichandran, K.R., P. Revankar, and S. Rahnama-Moghadam, Hand and Foot Dermatitis Secondary to Zucchini Exposure. Cureus, 2024. 16(5): p. e60359.
5. Haddock, E.S., et al., Extensive orf infection in a toddler with associated id reaction. Pediatr Dermatol, 2017. 34(6): p. e337-e340.
6. Jicha, K.I., E.L. Nieman, and D.S. Morrell, Granuloma Annulare-Like Id Reaction to Inflamed Molluscum Contagiosum: A Case Report. Clin Med Insights Pediatr, 2023. 17: p. 11795565231194819.
7. Nouh, A.H., Pediculid: Literature Review with Case Presentation. Skin Appendage Disord, 2022. 8(5): p. 419-423.
8. Del Giudice, P. and M. Reverte, Id reaction following the treatment of a cutaneous larva migrans. JAAD Case Rep, 2023. 38: p. 68-69.
9. Bosworth, A. and P.R. Hull, Disseminated Eczema Following Radiotherapy: A Case Report. J Cutan Med Surg, 2018. 22(3): p. 353-355.
10. Hay, R.J., Dermatophytosis (Ringworm) and Other Superficial Mycoses, in Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases, J.E. Bennett, Editor. 2020, Elsevier: London.
11. Hay, R.J., Fungal Infections, in Rooks Textbook of Dermatology, C. Griffiths, et al., Editors. 2024, Wiley-Blackwell: London.