Changing bacterial populations evident in recent study

A recent article published in the journal Veterinary Microbiology sheds light on the antimicrobial resistance carried by bacteria that cause respiratory disease in western Canadian feedlot cattle.


The research was carried out at four commercial feedlots in southern Alberta by Dr. Edouard Timsit and colleagues at the University of Calgary’s Faculty of 
Veterinary Medicine and Agri-culture Canada research centre in Lethbridge.


The study set out to isolate respiratory bacteria from feedlot cattle using a technique known as a trans-tracheal aspiration. This technique allows researchers to culture bacteria retrieved from the lower airways rather than the upper respiratory tract. It makes it more likely that the bacteria isolated are responsible for causing bovine respiratory disease. 


Groups of high-risk calves arrived at the feedlot and received a standard processing protocol, which included respiratory vaccines, pour-on dewormer, and metaphylaxis with the long-acting antibiotic tulathromycin (Draxxin) to control respiratory disease.


In addition, the cattle also received some chlortetracycline in the feed as part of the feedlot’s protocol to control Histophilosis.


As pen riders checked the cattle after arrival, cattle with signs of respiratory disease were selected for the study and sampled via trans-tracheal aspiration. Healthy pen mates were selected for sampling in a similar fashion.


In total, 210 cattle with suspected respiratory disease and 107 cattle without respiratory disease were sampled in the four feedlots. Of the calves selected for treatment, 26 percent had a relapse within 60 days after first treatment and required retreatment. Four percent of the calves selected with respiratory disease eventually died.


The samples were cultured in the laboratory and provided interesting results. Bacteria were isolated from about 83 percent of the sick cattle. 


Surprisingly, the most common bacteria cultured was Pasteurella multocida, which was found in 55 percent of cattle, followed by Mannheimia hemolytica in 30.5 percent and Histophilus somni in 23 percent of the cattle.


In the healthy cattle, respiratory bacteria were isolated less frequently (in 48 percent of the time), but Pasteurella multocida was still the most commonly isolated bacteria. 


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Most previous studies suggested that Mannheimia hemolytica would be the most common bacteria isolated from cattle with respiratory disease. It is the bacteria traditionally associated with what many producers call shipping fever.


Pasteurella multocida is often considered a bacterium more commonly associated with pneumonia in young dairy calves or beef calves and is less commonly associated with weaned calf pneumonia. 


However, this study and others have shown this may no longer be the case in weaned calves. 


Another study author has suggested that the ratio of Mannheimia hemolytica to Pasteurella multocida has decreased from 3:1 to .8:1 between 1994 and 2000. This study shows that this trend may be continuing.


There are many factors that can change which bacteria are most commonly associated with respiratory disease in feedlot cattle.


Bacterial populations change and evolve. 


As well, there are changes in antimicrobial resistance that may alter the bacterial populations over time, and we have more efficacious vaccines for Mannheimia hemolytica than for Pasteurella multocida. These calves received those vaccines and that may change the bacterial populations as well.


Healthy cattle were also found to carry many of these pathogens, although at a lower frequency. It is common in studies looking at respiratory disease bacteria in cattle to see that healthy cattle are “colonized” by the same or similar bacteria but are not “infected,” which may be due to a lower dose of the bacteria or because of their own immune functions.


Researchers in the study were able to show high levels of resistance to several antibiotics. 


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The Mannheimia hemolytica isolates had 72 percent of isolates resistant to tulathromycin, 74 percent resistant to oxytetracycline and almost 80 percent resistant to tilmicosin. 


Resistant isolates were also found in healthy cattle but at a lower rate.


These results are not surprising given that all of the cattle sampled received both tulathromycin and chlortetracycline as part of the respiratory disease control program at arrival.


The authors suggest that resistance genes to these antibiotics are probably common in respiratory disease bacteria in our feedlot populations. The bigger question arises around where those resistance genes are acquired.


Do they originate within the cow-calf industry, the mixing that occurs in auction markets or in the feedlots once the cattle arrive? 


The authors also suggest that if these antibiotics are used as metaphylaxis, they should not be used as a first line treatment drug when treating cases of respiratory disease in feedlots. 


The authors noted that there was a high proportion of Histophilus somnus isolates resistant to tetracycline and this may bring into question current strategies used to control that disease in feedlots.


There was also some good news in that there were very low levels of resistance to florfenicol, and resistance to penicillin was rare. It should be especially noted that there was no resistance to ceftiofur or enrofloxacin, which are two antimicrobials deemed to be of very high importance to human medicine.


This study demonstrates how bacterial populations that cause respiratory disease continue to change in our cattle populations. We need to continue to learn from these studies so we can adapt our management practices to effectively control respiratory disease in cattle, as well as maintaining the usefulness of our antimicrobials for the treatment of sick animals.

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