Liver abscessation in pasture based beef bulls of New Zealand

Internationally, liver abscessation is reported as a major issue affecting the health and productivity of beef cattle under intensive total mixed ration production systems. With the increasing intensity of pastoral based production systems, there has been increasing anecdotal evidence of liver abscessation on bull beef farms across New Zealand, but to date, no research in the field. The research in this thesis was undertaken to establish the incidence of liver abscessation in pasture based beef bulls, as they are a typically at-risk livestock class; and establish if there were any differences in aetiology between lot fed and pasture based cattle. This was achieved by: identifying the species of bacteria present in abscesses in beef bulls from the South Island of New Zealand; establish the key rumen function features across the seasons in a production cycle with these bulls; quantify the changes in rumen papillae epithelium histology and ultrastructure across a production season; and measure the rumen population of Fusobacterium necrophorum in bulls managed under an industry typical intensive production system. Chapter 3 reported that the annual incidence of liver abscessation in a database of 137675 intensively managed bulls between 2000-2005 from the South Island of New Zealand was a mean of 9.5%. There were typically some variation across years due to environmental factors and a clear and significant seasonal influence was observed. Incidence of liver abscessation peaked across November- December (11.3 and 11% respectively) and then declined as the slaughter season progressed. The typical age of the bulls slaughtered appears to directly influence the seasonal incidence rate, with older bulls slaughtered during this the late spring and summer peak in incidence. Abscesses graded as severe (two or more abscesses with a diameter larger than 4 cm accounted for 67% of all abscessation recorded. Friesian and dairy type cross bulls had abscessation incidence approximately twofold greater than beef type bulls (10.3% and 4.71%, respectively). There was also a clear regional difference in liver abscessation incidence where it seems that those regions where animal performance is greatest also have the highest incidence of liver abscessation (Ashburton South district of 12.2%) Chapter 4 reports the use of abscesses of slaughtered bulls to identify the species of bacteria present across a production cycle, using a polymerase chain reaction assay of the V2V3 region of the bacterial 16S ribosomal RNA region. Fusobacterium necrophorum was identified in every abscess removed from affected livers across the production season of 2007-2008 and the partial sequence of the 16S ribosomal RNA gene was a 100% match to that of the F. necrophorum species identified in previously recorded international databases. In some of the abscesses removed, there was a mixed culture present of F. n. necrophorum and F. n. funduliforme and in 6% of samples other faculative anaerobic species of bacteria, namely Provetella and Porphyromonas species were also identified. Seasonal rumen function characteristics of four rumen fistulated bulls across a production cycle (chapter 5) revealed major changes in the concentration and production of short chained volatile fatty acids (SCVFA), ammonia and osmolarity of rumen fluid, particularly between the winter and spring grazing regimes. The total SCVFA concentration in summer was 62 and 55% greater than that recorded in winter and autumn but only 14% higher than recorded in spring with the SCVFA concentration between winter and spring increasing by 43% from a winter maintenance diet through to a near ad lib diet. Increases in the consumption of feed quantity and quality also revealed increasing proportions of propionic and butyric acid concentrations in the rumen with the proportion of both as a total of all SCVFA increasing by 28 and 89% for propionic and butyric acids respectively with the osmolarity of rumen fluid increasing by 23.3% between winter and spring seasons. Results of rumen pH assessment using indwelling rumen pH sensors revealed major diurnal fluctuations in rumen pH across each season with the highest mean rumen fluid pH occurring in spring (6.44 ± 0.01 pH units); in summer bulls recorded a mean pH of 6.29 ± 0.01 and spent 6.21% of each day under a pH of 5.8 units and 17.3% under a pH of 6.0 units. Estimated flow rates of rumen fluid were higher in spring and summer when compared to winter and autumn (22.7 ± 2.5, 21.3 ± 4.2 versus 10.8 ± 1.5 and 14.5 ± 1.0 %/h respectively) with the concentration of SCVFA in faecal fluid highest in spring (49.1 versus 37.2, 36.2 and 21.0 mMol/l for summer, autumn and winter respectively. Samples of papillae were removed from each of the four bulls post rumen evacuation and were subjected to scanning electron and light microscopy (chapter 6). The results from these revealed significantly thicker epithelial depths in winter compared to spring. Spring papillae widths were 25% thinner than winter, with cell counts decreasing by 34% between winter and spring (13.9 ± 1.11 and 10.4 ± 0.20 cells respectively). Light microscopy revealed extensive desquamation of the outermost epithelial tissue in spring. Thinner papillae were also observed in summer and autumn compared with winter, which may be a physiological adaptation of increased potential absorptive capacity to the greater concentration and production of rumen metabolites (principally SCVFA and ammonia). When compared to winter, papillae from each of the other season also had considerably greater populations of protozoa present on the surface. Chapter 7 reports the evaluation of the population of F. necrophorum in the rumen through the use of a quantitative polymerase chain reaction assay. Samples were collected from rumen contents of the four bulls across the seasons of the production cycle. The results from this assay revealed only very low concentrations of F. necrophorum in the rumen were present, with a viable population only detected in spring from one of the four bulls. In all other samples the assay was not able to detect of F. necrophorum in the rumen. Using a series of plasmid dilutions of F. necrophorum derived from the liver abscesses, the assay was able to confirm that the amplified rpoβ section of New Zealand F. necrophorum species is the same as that recorded internationally. The assay could detect the presence of F. necrophorum in the rumen fluid down to low concentrations confirming the assay was successful to detect F. necrophorum in the rumen and that the population of F. necrophorum in the rumen of the four bulls used in this study was below the level of detection of the PCR assay. To conclude, these results show evidence that liver abscessation in the New Zealand bull beef industry is present at 9.5% of all bulls slaughtered having abscessation. There is a clear seasonal trend of abscessation across the production season and F. necrophorum is the major microbial species present in liver abscesses in these pasture based bulls. The rumen function of beef bulls across a production season did not show evidence of sub-acute ruminal acidosis, nor is there any evidence of rumenitis upon rumen papillae removed from the ventral sac of the rumen. In three of the four seasons, the population of F. necrophorum in the rumen of the four bulls in this study was undetectably low. Further work is required to better understand the aetiology of liver abscessation in the context of intensive pasture based production systems as it appears that the traditional explanation of the sub-acute ruminal acidosis induced rumenitis- liver abscessation complex may not explain the relatively high incidence observed in these systems.