Understanding How BRD and AMR are Transmitted Among Feedlot Cattle

Background

Bovine Respiratory Disease (BRD) is caused by bacteria like Mannheimia haemolytica, Pasteurella multocida, Histophilus somni and Mycoplasma bovis. These bacteria exist naturally among other bacteria in the respiratory tract of cattle. In a healthy animal, the immune system and “good” bacteria keep the BRD-causing bacteria in check. However, when cattle get stressed, the “good” bacteria and the immune system get weakened. This allows BRD bacteria take over, move deeper into the lung, and make animals sick. We do the best with what we know and what we have, and in the case of BRD we use antibiotics to treat it. Unfortunately, antimicrobial resistant (AMR) bacteria can survive antibiotics treatment. They and also spreading contagiously from animal to animal.  

These researchers studied whether pathogenic and resistant Mannheimia and Pasteurella arise independently in each animal, whether they spread between animals, and the degree to which both factors may play a role. 

Objectives

• To better understand the degree to which BRD and AMR arise in individual calves, vs. the degree to which BRD and AMR spread contagiously from calf-to-calf.  

What they Did

This project used deep nasopharyngeal swabs collected from 400 calves purchased at a pre-sort auction sale and penned next to each other at the University of Saskatchewan’s Livestock and Forage Center of Excellence (4 pens x 100 calves). Calves in each pen came from between 30 to 70 different herds. Calves were processed following industry standards, vaccinated, and treated with tulathromycin (the antibiotic found in Draxxin, Increxxa, Lydaxx, Rexxolide, Tulamaxx, Tulaven, Tulinovet, and Tulissin). Nasopharyngeal swabs were collected on arrival, and again two and five weeks after arrival. Mannheimia and Pasteurella two bacteria that cause BRD, were isolated and saved from these swabs. 

They had the goal of finding out if BRD bacteria could spread contagiously from calf-to-calf within a pen, as well as between neighboring pens. The first two pens and the last two pens shared a water source. To see if the bacteria were spreading contagiously, they looked at the DNA sequences of the bacteria to see if they were related, and to also see if the bacteria had antibiotic resistance genes. They determined the whole genome sequences of every Mannheimia isolate collected from cattle from these four adjacent pens, and every Pasteurella isolate from one of the pens. 

What They Learned

Mannheimia: At arrival, 26 to 46% of calves were positive for Mannheimia. They found a lot of genetic diversity in the Mannheimia bacteria, but little antibiotic resistance. Two weeks later there was much less genetic diversity. Rather, one group of closely related Mannheimia dominated the isolates from each pen. The limited genetic diversity between the bacteria in each pen later in the feeding period suggest that the bacteria were spreading contagiously between animals. While most spread was within the pen, there was evidence that certain Mannheimia strains spread across pens as well. They also noted that antibiotic resistance genes were most common two-weeks post-arrival. This could be related to the antibiotic given on-arrival selecting for antibiotic resistance. 

Pasteurella: At arrival, about 44% of the calves were positive for Pasteurella, and there was a lot of genetic diversity in the bacteria. Two weeks later, only 13% of calves, were positive for Pasteurella, and there was slightly less genetic diversity as well. Calves that were positive for the bacteria at both times usually had the same exact strain both times.  

What it means

Mannheimia can spread contagiously. While metaphylactic antimicrobial treatment may help treat underlying BRD, there is potential that in some circumstances it is leaving behind bacteria that carry antibiotic resistance genes. Those strains can proliferate, spread and become the dominant strain of Mannheimia within some pens. The low genetic diversity observed in the Mannheimia isolated from individual animals and different pens suggests that contagious spread was likely the culprit for the proliferation of AMR in this group of pens rather than AMR developing independently in many different animals.  

Pasteurella appears to behave differently. Most of the calves that were positive at both time points did not switch the strains they carried, which means that they were not “catching” different strains from their penmates. The finding that there were fewer calves positive for Pasteurella at the later sampling may mean that the metaphylaxis drug given at arrival effectively killed the bacteria.  

These findings can allow producers to think differently about how to treat BRD and can potentially inform vets to customize disease prevention and treatment strategies for better health outcomes and to reduce the risk of AMR.