Fatal spontaneous pneumocystosis in nude rats

Spontaneously generated fatal pneumocystosis was found in a congenitally athymic nude (rnu/rnu) rat colony. Severe pulmonary pneumocystosis was seen in rnu/rnu rats, and 10(7) to 10(8) cysts per lung were detected. No histologic changes were seen nor were Pneumocystis carinii organisms detected in heterozygous rnu/+ rats. Indirect immunofluorescence revealed strong reactivity in the sera from rnu/+ rats with rat-P. carinii cysts (originated from Wistar strain), compared with mouse- or human P. carinii cysts. The cysts isolated from rnu/rnu rats reacted strongly with antirat-P. carinii rat serum in comparison with antimouse-P. carinii rabbit serum and serum from a human with pneumocystosis. Immunoblotting studies using rat-P. carinii antigen showed that 116-kDa antigen reacted with the antirat-P. carinii serum and sera from rnu/+ rats but not with the antimouse-P. carinii serum and the serum from a human with pneumocystosis, whereas 52-kDa antigens of rat-P. carinii were recognized by all the anti-P. carinii sera tested. On the basis of our findings, the outbreak was caused by the P. carinii indigenous to the rat.