Body condition is negatively associated with infection with Syngamus trachea in the ring-necked pheasant (Phasianus colchicus).

The role that parasites play in regulating animal populations is debated, however recent research hints at their pervasiveness among free-living animal populations. Parasites exert both direct and indirect effects on host populations, and can act to regulate populations. The Ring-necked pheasant is an important game-bird species in the UK, and large numbers of birds are released annually. The impact of the ubiquitous tracheal nematode, Syngamus trachea on pheasant populations through effects on host condition was assessed on two pheasant estates in the south west of England. Pheasants infected with S. trachea demonstrated a significant reduction in host condition compared with uninfected controls, with as few as one pair of worms per bird. Although there was no difference in worm burden between sexes, analysis of regression slopes revealed there was a significant difference between sexes in the magnitude of the effect of increasing worm burden on host condition, with detectable effects observed in hosts with one and three pairs of worms for males and females respectively. The observed reductions in host condition in birds with even sub-clinical infections could be the cause of poor reproductive success and survival of pheasants post-release.

Density-dependent regulation of fecundity in Syngamus trachea infrapopulations in semi-naturally occurring ring-necked pheasants (Phasianus colchicus) and wild Carrion Crows (Corvus corone).

Previous work has highlighted increased opportunities for the transmission of Syngamus trachea within pheasant release pens, due in part to high levels of environmental contamination around communal areas. Despite this, the distribution of adult worms within their definitive hosts is not significantly different from predicted distributions under Taylor's power law. Therefore, density-dependent processes are probably acting to regulate S. trachea population dynamics. Patterns of nematode fecundity were investigated in a semi-naturally occurring population of ring-necked pheasants (Phasianus colchicus) and a wild population of carrion crows (Corvus carone). Worm length was a reliable indicator of nematode fecundity, and a negative association between mean worm length and mean worm burden was identified within both the species. The stunting of worms at greater parasite densities was present in both immunologically naïve and previously exposed pheasants, so is unlikely to be a function of age-dependent acquired immunity. Interestingly, the effect of parasite crowding in the crow population explained more of the variation in mean worm length, apparently driven by a greater mean worm burden when compared with pheasants. The findings of the present study suggest that fecundity is a function of parasite density, i.e. parasite-mediated competition and not host-mediated heterogeneities in immunocompetence.

Spatial distribution of infectious stages of the nematode Syngamus trachea within pheasant (Phasianus colchicus) release pens on estates in the South West of England: Potential density dependence?

The spatial distribution of the infectious stages of parasites with a direct life cycle is one of the most important factors influencing infectious disease dynamics, and acquisition rates will generally increase as the contact time between parasite and host increases. For animal species that are constrained by feeding opportunities, one might expect disease patterns to be highly skewed within confined systems. The aim of the present study was to identify to what extent, if any, eggs of avian parasites are aggregated within the release pen, and to evaluate what effect, if any, this aggregation had on the distribution of the adult stages within the host species. The abundance of Syngamus trachea eggs were highly aggregated within pens, with high levels of contamination driven by a combination of feeder placement, soil moisture and host-mediated heterogeneities in immuno-competence. The log mean and log variance of egg abundance was highly linear (R(2)=0.97-0.99), with an estimated slope (b) of between 1.79 and 1.97 for individual sites, and 2.11 when sites were combined, which indicated aggregation relative to an estimated Poisson slope of unity. Although the placement of feeders and environmental moisture could be contributing to parasite aggregation, density-dependent processes appear to be ensuring the population does not become too over or under-dispersed, in order to maintain the transmission-virulence equilibrium. To the best of our knowledge, this is the first paper to explicitly demonstrate the high spatial aggregation of eggs around feeding sites and the first to suggest possible density-dependent regulatory mechanisms stabilising disease dynamics between S. trachea and ring necked Pheasants (Phasianus colchicus).

Spatio-temporal factors influencing the occurrence of Syngamus trachea within release pens in the South West of England.

Syngamus trachea is a pathogenic tracheal nematode that causes syngamiasis in wild and game birds, especially when birds are managed at high densities. Despite its pathogenic nature, very little is known about its epidemiology and relationship with ambient temperature and humidity. The spatial and temporal modelling of disease was undertaken on two pheasant estates within the South West of England from April 2014 to August 2014. Significant differences between the mean numbers of eggs per gram of soil were identified between pens at both site 1 and site 2 but did not differ significantly between sites. Egg abundance was significantly associated with soil moisture content, with greater egg survival between years in pens with higher average volumetric soil moisture content. Previous years stocking density and pen age were also associated with greater egg survival between years with more eggs being recovered in pens with greater stocking densities, and pens that had been sited longer. The greatest model to explain the variation in the numbers of eggs per gram of soil per pen was a combination of soil moisture content, stocking density and pen age. Larval recovery differed significantly between sites. Larval abundance was significantly and positively associated with temperature and relative humidity at site 1. Similarly, temperature and humidity were also positively and significantly associated with larval abundance at site 2. Rainfall did not influence larval recovery at either site 1 or site 2. The model with the greatest ability to explain larval abundance at both sites, was a combination of temperature, humidity and rainfall. Infection status (positive faecal egg counts) was significantly and positively associated with larval abundance at both sites, but rainfall was only positively associated at site 1. Temperature and humidity were positively associated with infection status at site 2, but not at site 1. The present study highlights the influence of climatic variables on both egg survival and larval abundance, and could therefore be used to develop more targeted treatment strategies around periods of higher disease risk. The frequent use of release pens is a clear factor in the epidemiology of syngamiasis, and it is recommended that pens be rested and/or rotated in order to reduce infection pressure in subsequent flocks.