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 research article focuses on the application of Sartwell’s model of lognormal distribution of incubation periods to understand the age of onset and death resulting from Rhodococcus equi pneumonia in newborn horses. The findings indicate that these foals are likely infected with R. equi within the first few days of life, akin to point-source exposure.

Research Context

• The research applies Sartwell’s model, which suggests that incubation periods for infectious diseases that begin from single point exposures tend to follow a lognormal distribution. This pattern doesn’t usually apply to diseases linked to repeated or long-term exposures or diseases with strong environmental factors.
• The study focuses on Rhodococcus equi pneumonia, a serious bacterial infection primarily affecting foals. The model’s application aims to understand the pattern of disease onset and death in foals affected by this illness.

Research Methodology

• The researchers gathered data on the age at onset of clinical signs and age at death from 107 foals diagnosed with R. equi pneumonia. These foals were from breeding farms in Argentina and Japan.
• They applied Sartwell’s model to the collected data to examine whether the age at onset of disease and death in these foals follow a lognormal distribution, as predicted by the model in case of single point-source exposures.
• A comparison group made up of 115 foals with colic from the University of California was used to cross-compare the results. The onset of colic doesn’t fit into a lognormal distribution and hence served as a contrasting set.

Research Findings

• The study found the lognormal distribution pattern in the age of disease onset and death in foals diagnosed with R. equi pneumonia. Thus, it matched the distribution pattern of single point-source exposures according to Sartwell’s model.
• On the contrary, the comparison group with colic did not follow a lognormal distribution, reinforcing the validity of the findings.
• Consequently, the research corroborates the hypothesis that foals are likely infected with R. equi pneumonia during the few initial days of their life, reflecting a point-source exposure pattern.