Does BRD treatment strategy influence CPPS expression?
The objective of this study was to determine if two particular management factors (injectable antimicrobial metaphylaxis at arrival to the feedlot and/or treatment management strategy) have an effect on the incidence of chronic pneumonia and polyarthritis syndrome (CPPS).
Additional objectives for this study included: 1) to describe the ecology of mycoplasma within pens of feedlot calves; 2) to determine if specific strains of M. bovis are related to pathogenicity in the feedlot, and 3) to describe the antibiotic sensitivity of the isolated organisms.
The results of this study show that calves were less likely (0.77 times) to be treated for pneumonia if they were given oxytetracycline at induction. This was the anticipated outcome based on previous research of giving antimicrobials at the time of arrival to the feedlot (Schumann et al., 1990; Van Donkersgoed, 1992). Many feedlots in recent years have tend to use antimicrobials such as tulathromycin or tilmicosin for metaphylaxis, however, it is interesting to see that oxytetracycline was relatively effective in controlling subsequent pneumonia in pens of comingled auction market-derived calves. Contrary to the above findings, metaphylaxis appears to decrease the ADG during the first 100 DOF. This finding is opposite to what has been previously reported in the literature (VanDonkersgoed, 1992). We are unclear as to how to explain this result.
No difference could be found between the first relapse rates for calves treated and sent home immediately as compared to treated and held for 48 hours. However, we did find that calves held in a sick pen for 48 hours did have a lower ADG during the first 100 days in the feedlot as compared to calves that were returned to their home pen. Despite the relatively few animals that were housed in the sick pen at any one time, further disease transmission between sick calves could be quite plausible as the cause for this.
We would have preferred cattle of high risk of developing UBRD for this study, as treatment rates of less than 10% would suggest that these calves were of low to moderate risk. With treatment rates lower than anticipated in both years of the study, there were very few UBRD mortalities and cases of CPPS. No statistical differences could be found between either of the 4 treatment groups in these two outcome measures. Our results would suggest that both metaphylaxis and UBRD treatment strategy do not impact the occurrence of CPPS, but we must be cautious in this interpretation given the low power of this study. We had based our sample size on higher UBRD treatment rates from previous years in this feedlot. It is interesting to note that during the years of this study, cattle were brought into the feedlot over a longer period of time (September to February), which is quite different from the past (October to December).
BVDV persistently infected calves did not increase the treatment rate in their respective pens. This may seem contradictory to what has previously been reported in the literature (Booker et al., 2005). However, we were not able to test all calves adjacent to our study pens which may have influenced our findings. We had 6 calves die with signs of Mycoplasma pneumonia and arthritis on post-mortem examination, but none of them were persistently infected with BVD virus.
Only 22% of all the swabs cultured had Mycoplasma bovis isolated. The isolation rate was greatest from swabs collected at treatment and necropsy (45%) as compared to arrival (<1%). Given the retrospective selection of swabs to culture, it is quite plausible that more animals did have Mycoplasma bovis at arrival, but the freeze thaw associated with transport and storage decreased their viability. Other studies have also documented low isolation rates of M. bovis from nasal swabs collected from cattle at arrival to the feedlot. Our concern is the bias introduced into the cluster analysis given that most of the isolates were actually from animals with UBRD and only two strains from healthy animals at arrival. Serial sampling of cattle throughout the feeding period might reveal some further interesting findings.
Twenty clusters of M. bovis were identified in the feedlot over the two years from 54 strains. The overall similarity of the isolates was 61.7%. As might be expected, the similarity was greatest for isolates collected from the same individual calves (87.3%). This was followed by the similarity of isolates collected within a pen (78.2%). Just as notable, 11 separate clusters of M. bovis were identified in the swabs collected at necropsy over the two years. Based on the above findings, we believe that CPPS is not increasing in occurrence due to a particular disease causing strain. McAuliffe et al. (2004) describe two groups of isolates (A and B) with two subgroups within B (Ba and Bb). These two groupings were not as distinct in these groups of isolates. The variability in isolates as demonstrated by the large number of clusters is not surprising given how the cattle in this study were procured. Ribble et al. (1994) demonstrated that in each feedlot pen of 200 calves, approximately 60 different farms of origin are represented. This means that there is an incredible amount of mixing in a feedlot and auction market. When we examined the Canadian Cattle Identification Agency number for each calf, we found that on average 3 to 4 calves had a similar sequenced tag and entered the feedlot at the same time. This would confirm the same degree of mixing in these calves. Future research should look at the isolation of M. bovis from cows and calves on ranches to determine its prevalence and degree of similarity within and between farms.
The antimicrobial susceptibility of the M. bovis isolates was determined for 10 common antimicrobials (Tables 10 and 11). The interpretation of these minimum inhibitory concentrations (MIC) is difficult given that there are no cut-off values to determine resistance. However, when the MIC values are compared very little differences can be found between isolates, which suggests that antimicrobial resistance is also not the cause for the increased occurrence of Mycoplasma pneumonia and arthritis. The greatest proportions of these isolates were collected from cattle with UBRD, so it is quite plausible that they are quite similar, but still different than M. bovis from untreated cattle.
The prevalence of M. bovis isolated from nasal deep nasal swabs or bronchoalveolar lavages increases during the feeding period (Allen et al., 1991). Long-acting antimicrobials have significantly reduced the morbidity and mortality rates associated with shipping fever or Mannheimiosis. This means that more animals with acute pneumonia survive and perhaps the existing lung damage provides an adequate environment for Mycoplasma to become established. Given the lack of evidence for a particular disease-causing strain or antibiotic resistance, it would seem prudent to further evaluate the differences in virulence factors between isolates. Although AFLP has been found to discriminate well between strains of M. bovis, we can not rule out the possibility that there are virulence factor differences between these strains.
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