The histology of "taches noires" of boutonneuse fever and demonstration of Rickettsia conorii in them by immunofluorescence

The recent increase in the incidence of boutonneuse fever in Italy provided the opportunity to study the pathology of six "taches noires," the lesions at the site of tick bite. The center of the lesion has either an ulcer or an area of necrosis of the epidermis and superficial dermis; in some cases the epidermis is intact. The alterations are mainly in the dermis and subcutaneous tissues where the small vessels show endothelial swelling and intramural and perivascular oedema and inflammation with macrophages, lymphocytes and smaller numbers of plasma cells, PMNs and eosinophils. In a few small arteries and fewer veins there are either nonocclusive mural or occlusive thrombi; there is no spatial or quantitative correlation between thrombosis and necrosis. We propose that cutaneous necrosis results from severe injury to many small vessels. Rickettsiae which had not been previously observed in "taches noires" were demonstrated in blood vessels by immunofluorescence, a finding that may be used as a means for early aetiological diagnosis of the disease. The "tache noire" is an excellent human model for localized rickettsial injury.