Deposition of pathogenic Mycoplasma gallisepticum onto bird feeders: host pathology is more important than temperature-driven increases in food intake

Abstract

Although ambient temperature has diverse effects on disease dynamics, few studies have examined how temperature alters pathogen transmission by changing host physiology or behaviour. Here, we test whether reducing ambient temperature alters host foraging, pathology and the potential for fomite transmission of the bacterial pathogen Mycoplasma gallisepticum (MG), which causes seasonal outbreaks of severe conjunctivitis in house finches (Haemorhous mexicanus). We housed finches at temperatures within or below the thermoneutral zone to manipulate food intake by altering energetic requirements of thermoregulation. We predicted that pathogen deposition on bird feeders would increase with temperature-driven increases in food intake and with conjunctival pathology. As expected, housing birds below the thermoneutral zone increased food consumption. Despite this difference, pathogen deposition on feeders did not vary across temperature treatments. However, pathogen deposition increased with conjunctival pathology, independently of temperature and pathogen load, suggesting that MG could enhance its transmission by increasing virulence. Our results suggest that in this system, host physiological responses are more important for transmission potential than temperaturedependent alterations in feeding. Understanding such behavioural and physiological contributions to disease transmission is critical to linking individual responses to climate with population-level disease dynamics.

Host Responses to Pathogen Priming in a Natural Songbird Host

“…3b). Because the severity of these clinical signs predicts the proportion of MG conjunctival load deposited onto bird feeders (Adelman et al 2013a), effects of prior exposure on disease severity may have independent effects on transmission potential in this system. Overall, these results suggest that bird feeders, in addition to their role as infectious fomites, may in some cases act as “immunizers” if they serve as environmental reservoirs of low doses of MG.…”

“…These concentrations of pathogen were selected because they represent novel doses for this system and fall within the range of levels of MG copies deposited onto feeder ports by an infectious bird (Adelman et al 2013a). Repeated “priming” exposures were given every other day and start dates were staggered such that the final priming exposure coincided for all individuals (Figure 1).…”

“…This isolate was selected because it has been used in the vast majority of past experimental studies to date (e.g., Dhondt et al 2008; Grodio et al 2012; Adelman et al 2013a), thus facilitating comparisons to past work. The day of use, stock inocula were thawed and immediately diluted using Frey's broth media with 15% swine serum (FMS) to approximate dosage.…”

“…However, MG is a cell-wall-less bacterium that appears to degrade quickly outside of its host, as it is only able to produce mild disease in naïve individuals exposed to contaminated feeders for up to 12 hours (Dhondt et al 2007a). In addition, rates of deposition of MG onto a single available feeder port from experimentally infected finches show that only a very small percentage (∼0.3%) of the conjunctival burden of infectious birds is deposited onto feeders during foraging (Adelman et al 2013a), resulting in mean deposition rates of 1.6 × 10 3 MG copies per day from hosts with the highest conjunctival pathogen loads (i.e., birds with a mean of 5.8 × 10 6 MG copies in the conjunctiva). Although it is extremely challenging to quantify MG exposure levels in natural populations, these lines of evidence suggest that free-living house finches are likely exposed to variable amounts of MG while feeding, with some individuals likely experiencing repeated exposure to relatively low doses of MG.…”

“…Although it is extremely challenging to quantify MG exposure levels in natural populations, these lines of evidence suggest that free-living house finches are likely exposed to variable amounts of MG while feeding, with some individuals likely experiencing repeated exposure to relatively low doses of MG. However, to date, experimental exposures of house finches to MG have used relatively high infectious doses (≥10 4 color changing units[=CCU]/ml) and/or single time-point exposures (Dhondt et al 2007b; Bouwman and Hawley 2010; Bonneaud et al 2011; Hawley et al 2011; Bonneaud et al 2012; Adelman et al 2013a), which are unlikely to be fully representative of the range of variation in MG exposure in the wild.…”