Understanding How BRD and AMR are Transmitted Among Feedlot Cattle

Background

Bacteria that cause bovine respiratory disease (BRD) naturally exist among other bacteria in the respiratory tract of cattle. In a healthy animal, the immune system and “good” bacteria keep them in check. However, when cattle get stressed, the “good” bacteria and the immune system get weakened, and allows the BRD bacteria take over, move deeper into the lung, and results in animals getting 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, misuse of antibiotics can leave antimicrobial resistant (AMR) bacteria behind which may lead to resistant BRD bacteria spreading contagiously from animal to animal. These researchers want to clarify whether BRD and AMR arise independently in each animal, whether it’s due to spread between animals, or the degree to which they each 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). Per pen, calves represented anywhere between 30 to 70 different herds. Calves were processed and vaccinated like normal feedlot cattle, and also received a metaphylactic dose of 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 bacteria, one of the bacteria that causes BRD, were isolated and saved from these swabs.

This team 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. This study determined the whole genome sequences of every Mannheimia isolate collected from cattle from these four adjacent pens.

What They Learned

At arrival, the researchers found a lot of genetic diversity in the Mannheimia bacteria, with little antibiotic resistance. However, after two weeks, 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.

In addition, the researchers also noted a rise in antibiotic resistance genes in the bacteria that was most common in pens two-weeks post-arrival. The team speculates that this could be related to the metaphylactic antibiotic the cattle received on-arrival causing unintentional selection for strains of Mannheimia that have antibiotic resistance genes.

What it means

Mannheimia bacteria can spread contagiously, and 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 and allows those strains to proliferate and become the dominant strain of Mannheimia within some pens. The low genetic diversity observed in the isolated bacteria 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. This finding 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.