What is the relationship between asthma and worms?

Herrick (31) may have been the first person to recognise the association between asthma and helminthiasis, to question the significance of the relationship, and to attempt to investigate it. Nearly seventy years have passed since his paper appeared, but many of the problems that he raised still defy solution. Eosinophils have provided the longest historical link between atopy and worm infection. Twelve years after Erlich described the specific staining properties of these cells in 1879, Muller & Rieder (52) recorded an eosinophila in hookworm infection. This was confirmed two years later (82), then eosinophilia was found in rapid succession in a variety of infections with metazoan parasites including ascariasis, taeniasis (12), trichinosis (9), filariasis (30), and schistosomiasis (18). Meanwhile, eosinophils were being found in asthmatics. According to Herrick (31), Muller in 1889 observed many eosinophils in the sputum of patients with asthma. In the following year. Fink (20) described increased numbers of eosinophils in the blood of asthmatics and this was soon confirmed (21, 75). Despite these early observations, little attention was paid to the relationship between atopy and helminthiasis for many years. A new impetus was provided by the discovery of Johansson in 1967 that asthmatic individuals had raised levels in the serum of a new class of immunoglobulin, IgND, later renamed IgE (34). In the following year, elevations in the serum levels of this same immunoglobulin were also reported in children with helminth infections (35). These discoveries provided a further nexus between helminthiasis and atopy and stimulated a number of investigations over the next decade. Since asthma is a significant and common atopic disorder, these studies have been largely confined to an examination of the interaction between asthma and worm infections. Unfortunately, the data are sometimes conflicting, interpretations have differed and many of the issues have remained unresolved. This rex'iew attempts to analyse the information which has been reported from human studies. Limitations of space and uncertainties of significance have precluded any attempt to relate these findings to evidence obtained from experimental studies in animals. Three different strands of thought are discussed: 1) worms cause asthma, 2) worms ameliorate