CAPÍTULO II: ARQUITECTURA INTEGRADA DE UNA
2.2 Principales características
The overall aim of the studies conducted was to investigate whether D. nodosus persists on the sheep, on the pasture, or both. We used real-time PCR to investigate variations of bacterial load within and between sample types and developed a binomial mixed regression model for both studies to elucidate variables that drive the presence of D. nodosus, taking into account climatic changes.
The results of study 2 show that D. nodosus was carried from a flock were FR was present into a lesion-free flock when the study group was moved onto a separate pasture and that D. nodosus could be detected on healthy feet up to 5 weeks before onset of ID and SFR.
D. nodosus has been detected on healthy feet on a number of occasions (Moore et al., 2005; Calvo-Bado et al., 2011; Stäuble et al., 2014), but generally at lower loads than on diseased feet (Stäuble et al., 2014). Whereas this is true for both studies in this chapter, D.
nodosus was load was as high as 4.75 x 105 cells before onset of disease, indicating that
colonization took place and supporting previous evidence suggesting that D. nodosus plays an important role in the initiation of disease (Witcomb et al., 2014). This is further supported by the results of the two-binomial mixed effect foot models, that showed that presence of D. nodosus on feet was driven by D. nodosus load (and F. necrophorum presence) the previous week.
D. nodosus was detected in the gingival cavity in 25% of samples in study 1, whereas it was detected in only 1 sample in study 2. This is possibly attributable to the lower disease prevalence in study 2. Witcomb (2012) detected D. nodosus in 74% of all gingival cavity swabs, but disease prevalence in the study flock was high. The high likelihood of D. nodosus being present in the gingival cavity when a sheep had FR, shows that disease status of the sheep drives D. nodosus presence the gingival cavity rather than load on feet. These results suggest that D. nodosus transiently populates the gingival cavity when disease prevalence is high, but it is less likely that it persists.
For the first time, we show that faecal shedding of D. nodosus may be possible, as D. nodosus was detected in faecal samples directly collected from the sheep in study 2. Witcomb (2012) did not find D. nodosus in samples directly obtained from sheep, and did not link detection in faeces to disease status of the foot. In this study, D. nodosus was found in the faeces of the most diseased sheep in the flock and in the faeces of a non- diseased sheep in week 1. In this week D. nodosus was found in a larger proportion of
sheep, including healthy sheep, compared to any other week. Similar to D. nodosus presence in the gingival cavity, it is possible that D. nodosus presence in faeces is linked to the disease prevalence in the flock. The low quantity of D. nodosus detected suggests that it could be passaging through the digestive system, without colonizing. It may be beneficial to investigate faecal shedding a flock were disease prevalence is high.
For the first-time D. nodosus was detected in grass samples. This support existing evidence that D. nodosus is able to adhere to the environment at least for a short amount of time (Muzafar et al., 2015; Witcomb 2012). Hence grass is likely to act as a transitional medium for D. nodosus before it is passed to a new host.
D.nodosus has previously been detected in soil in both in vitro and farm-based studies (Witcomb 2012; Cederlof et al., 2013; Muzafar et al. 2015; Muzafar et al., 2016). Here we report for the first time that D. nodosus was detected in soil samples taken at an increased depth (4-5 cm) in addition to more superficial samples and that there were no statistical differences in detection frequency and D. nodosus load. It could be that the deeper layers of soil constitute a suitable environment for D. nodosus. There is however no evidence that suggests that the deeper layer of soil constitute a better environment for the survival of D. nodosus. Muzafar (2016) suggested that D. nodosus does not thrive in the outer layers of soil due to desiccation. Recent evidence indicates that D. nodosus may be able to survive at lower temperature than expected (Cederlof et al., 2013; Muzafar et
al., 2016) and Smith et al. (2014) showed that transmission of FR is possible at
temperatures below 10 °C. Deeper soil layers are colder than the soil surface, which absorbs more heat (Pepper and Gentry, 2015). Survival of D. nodosus in the soil would be more likely than survival on grass to due to the anaerobic nature of the bacterium (Rood et al., 2005) especially during wet weather when saturated soil is deprived of oxygen (Pepper and Gentry 2015). Whether deeper soil layers are exposed to the host, as a source of infection is unknown, but this may be possible during wet weather when pasture can be poached (heavily damaged by feet). Whether D. nodosus survives in the deeper layers of the soil and if so, for how long, is unknown.
D. nodosus was detected in soil and grass samples from the pasture after it had been empty for 10 days. The pasture had previously been occupied by a group of ewes with some lameness (observations only, lesions were not assessed) and it is possible that D. nodosus survived on the pasture for 10 days as research suggests that it may survive on pasture from 7-10 days (Beveridge, 1941; Whittington, 1995). Bacterial loads on the pasture where low and given that D. nodosus loads on healthy feet where higher than
loads on pasture, it is more likely that the D. nodosus that was introduced from the original flock caused disease in this study.
The only time D. nodosus was detected frequently and at high loads in the environmet was during the first sampling visit of study 1, when the weather was wet and ground was muddy. After the first week of sampling, temperatures increased and the soil became very dry (authors observation), which was coupled with decreasing detection frequencies and decline of D. nodosus loads in soil, grass and faecal samples, which was not affected by the increase of disease prevalence in week 5. In week 5, D. nodosus was detected almost exclusively on feet that were diseased, but at higher loads. This could indicate that the wet weather in week 1 facilitated spread and subsequent colonization of D.
nodosus on some feet. This coincides with results from study 2 where D. nodosus could
be detected in high loads on the feet of a diseased sheep, but detection of D. nodosus in soil was reduced between week 9 and 16 following the dry period lasting from week 8- 14.
It is possible that D. nodosus does not thrive in drier environments. The optimum conditions for D. nodosus survival have been described as warm and damp (Beveridge, 1941, Whittington, 1995). Smith et al. (2014) found that the incidence rate of FR increased with increasing rainfall in the previous weeks, suggesting that moisture may play a role in preventing death of D. nodosus. It is however difficult to disentangle the contribution of different climatic factors to D. nodosus presence in the two studies based on the statistical model and associations between variables alone. There were only 4 sampling days in study 1 and the correlations between D. nodosus load and detection frequencies in all sample types are probably attributable to the high detection frequency of D. nodosus in week 1 and subsequent decline. In study 2, the effects of climate variables and soil moisture on D. nodosus presence and load on feet did not become clear due to the low number of sheep in the model. However, the lack of associations between soil moisture variables (and rainfall, as this was highly correlated) and D. nodosus presence or load on feet is maybe not surprising given that seemingly only FR lesions were affected by the dry period. However, the suggestion that D. nodosus loads are not affected by dry weather is tentative.
As ambient temperature and soil temperature increased, the number of D. nodosus positive soil samples decreased and decreased rainfall and soil moisture was associated with a decrease in D. nodosus positive soil samples. This is not surprising, as temperatures increased steadily throughout the trial, whereas detection in soil decreased
during the dry weather period. Although there is a high positive correlation between soil moisture and rainfall, it is more likely that the dry ground affected survival of D. nodosus in soil.
The results of model 1 in study 1 show that feet were less likely to carry D. nodosus when the foot was treated with antibiotic spray 2 weeks previously. This coincides with recent findings showing that D. nodosus load decreased significantly 1 week and 2 weeks after a foot had been treated with antibiotics (Willis, 2017, unpublished data).
The results also show that ewes were more likely to have D. nodosus on their feet than lambs. Muzafar et al. (2015) found that population loads of D. nodosus were significantly higher in on the feet of ewes than on the feet of their lambs and also reported a larger strain diversity in ewes. In Muzafar et al. (2015) lambs were only 5-13 hours, whereas in this study lambs were several months old and had been part the main flock for several weeks. It may be possible that age plays a part in the composition of the bacterial community on the feet of sheep and older animals that may have had FR previously may be more susceptible to D. nodosus colonization.
Whereas the dry climate in spring 2015 provided the opportunity to gain some insights into the peristence of D. nodosus, the combination of unusual climate and creation of a closed group of lesion-free sheep led to low disease prevalence and incidence and only 3 diseased sheep were included in the model (Study 2). To increase robustness of the data it would be beneficial to analyze additional samples from sheep, this was not within the scope of this projest due to practical restrictions.
Although FR disease patterns and disease prevalence varied between studies, the results suggest that D. nodosus is more likely to persist on the feet of sheep than other sheep sites (mouth and faeces) and in the environment, due to its transient presence at those sites and decreased detection in dry weather. It could also be possible that soil moisture is an important factor in D. nodosus survival, while rainfall may be associated with increased transmission.