RESUMO
Triatoma infestans nymphs have shown a good sensitivity for detecting trypanosoma cruzi in the bood stream of infected hosts when are used in the xenodiagnosis (XD). This method, with its natural limitations, using seven nymphs III of T. infestans, has been routinely utilized with a satisfactory yield. With the aim of an eventual improving of the yield of XD (with 7 nymphs), two series of 54 XD boxes each, containing a total of 378 nymphs III and 378 nymphs IV respectively, were applied one of each during three consecutive days to nine chronic chagasic patients. Each of the nymphs was weighted before an after the application of the boxes, and the intestinal content of them was examined 30, 60 and 90 days later. The main comparative results obtained with nymphs III and IV of T. infestans were: blood ingestion 40 versus 107 mg (2,7 higher), positivity of insects 35,8 percent versus 50,6 percent (15,8 percent higher), positivity of XD boxes (7 nymphs each) 46,3 percent versus 55,6 percent (9,3 percent higher), and mortality rates 28,6 percent versus 12,2 percent (16,4 percent lower). All these results demonstrate that nymphs IV of T. infestans, because their higher capacity of ingesting blood and higher tolerance to examination manipulations, are more suitable for been used in XD