Effects of the Pathogen Mycoplasma gallisepticum Transmission on Growth and Condition of Eastern Bluebirds Nestlings (Sialia sialis)

A critical aspect to understanding disease dynamics is to understand how transmission occurs between hosts. There are two basic modes of transmission: vertical and horizontal. Vertical transmission is the passage of a pathogen from mother to offspring before or during birth, whereas horizontal transmission occurs between an infected host to a susceptible host. Mycoplasma gallisepticum (MG) is a common bacterial pathogen of poultry that causes respiratory disease. Infection of MG often results in weight loss, slowed growth rate, and increased mortality. Both vertical and horizontal transmission of MG have been found to occur in poultry. In the mid-90s, MG underwent a dramatic host shift into songbird species. Little is known regarding the effects of transmission of MG between songbird parents and their offspring. In this study, we asked whether disease transmission was occurring between Eastern Bluebird adults with MG and their altricial nestlings. We found that approximately 9% of all nestlings sampled were positive for infection with MG. We further examined whether the presence of MG in the throat of 5 day old nestlings affected correlates of their future fitness, specifically body condition and growth rate. Our results demonstrate that parent-offspring transmission does occur, although we could not determine whether this was because of vertical or horizontal transmission. Surprisingly, we found that 5 d. o. nestlings testing positive for MG DNA were in significantly better body condition than those testing negative for MG DNA. A similar non-significant trend was found for both 11 d. o. and 14 d. o. nestlings. These findings may be due to increased feeding rates of infected nestlings, resulting in greater relative mass. However, parent-offspring transmission does not appear to be detrimental to nestlings quality, and remarkably, is even positively associated with body condition. It is likely, therefore, that parent-offspring transmission is not important for any negative effects of this disease on our Eastern bluebird population. Our data does not support a cost to nestlings as a result of the acquisition of an MG infection