Application of Sartwell's model (lognormal distribution of incubation periods) to age at onset and age at death of foals with Rhodococcus equi pneumonia as evidence of perinatal infection

The distributions of the incubation periods for infectious and neoplastic d iseases originating from point-source exposures, and for genetic diseases. follow a lognormal distribution (Sartwell's model). Conversely. incubation periods in propagated outbreaks and diseases with strong environmental comp onents do not follow a lognormal distribution. In this study Sartwell's mod el was applied to the age at onset and age at death of foals with Rhodococc us equi pneumonia. The age at onset of clinical signs and age at death were compiled for 107 foals that had been diagnosed with R equi pneumonia at br eeding farms in Argentina and japan. For each outcome (disease and death), these data followed a lognormal distribution. A group of 115 foals with col ic from the University of California were used as a comparison group. The a ge at onset of clinical signs for these foals did not follow a lognormal di stribution. These results were consistent with the hypothesis that foals ar e infected with R equi during the ist several days of life, similar to a po int-source exposure.