Necrotic arachnidism in the northwest United States and its probable relationship to Tegenaria agrestis (Walckenaer) spiders.

Necrotic spider bites in the states of Washington, Oregon and Idaho, U.S.A., have become increasingly frequent during the past decade. The great majority of cases seen by physicians are tentatively diagnosed as 'brown recluse' spider bite on the basis of clinical manifestations. However, investigation of over 30 such cases from several northwest localities has revealed that Tegenaria agrestis, a spider introduced into the northwest U.S. from Europe, is almost invariably found at the locality where the bite probably occurred. Necrotic arachnidism was seen only in areas where populations of Tegenaria agrestis spiders were well established and did not occur where Tegenaria agrestis was absent. The geographical spread of Tegenaria agrestis spiders parallels the appearance of necrotic arachnidism in several areas. Evaluation of some individual cases strongly implicates Tegenaria agrestis as the responsible organism and Tegenaria agrestis is likely responsible for the majority of 'brown recluse' spider bites seen in the northwest U.S. Tegenaria agrestis should now be considered a species of clinical significance capable of producing marked dermal lesions and, not infrequently, severe toxicosis in humans. Case histories are presented and manifestations of poisoning are described.