Laboratory raised wild Norway rat males (Rattus norvegicus) were injected with leptospires of two serovars: icterohaemorrhagiae and grippotyphosa. The development of a carrier state was monitored serologically, culturally and histologically. Rats infected with icterohaemorrhagiae developed rapidly into a chronic carrier state and shed leptospires in the urine for the duration of the experiment (220 days). At the time of necropsy, histopathologic studies showed evidence of leptospiral infections in the lumen of proximal convoluted tubules of some kidneys.

Rats infected with grippotyphosa shed organisms for 40 days after inoculation; thereafter, they apparently cleared the infection. No organisms were detected histologically nor by culture at the end of the experiment (220 days). There appears to be a specific host-parasite relationship in the Norway rat towards becoming chronic carriers when infected with serotype icterohaemorrhagiae but not with grippotyphosa.