**2.3 Current challenges associated with BRD prevention: antimicrobial resistance**

Despite advances in antimicrobials and vaccines, increased metaphylactic use and best management efforts to mitigate BRD, the prevalence of BRD in feedlot cattle continues to be increasing [9]. Although data are lacking, one contributor to increased BRD prevalence might be the development and spread of AMR in BRD pathogens. Recent studies have shown the emergence of BRD bacterial pathogens that are resistant to all classes of antibiotics used to treat BRD. For example, *P. multocida* strain isolated from a cow that died of BRD in Alberta has shown resistance up to five different antibiotics commonly used to control BRD (Alexander lab, unpublished data). *M. haemolytica* isolates isolated from Canadian and the US feedlots also exhibited resistance to more than three antibiotics [12, 13, 29]. Multidrugresistant *H. somni* isolates have also been detected in Albertan feedlots [13].

The prevalence of multidrug-resistant BRD bacterial pathogens is relatively high and increasing in both Canadian and the US feedlots over the years [30]. A recent study conducted in commercial feedlots in Alberta, Canada, revealed that there were significantly high levels of resistance (>70%) against tulathromycin and oxytetracycline in *M. haemolytica* and *P. multocida* isolates and high levels of resistance against oxytetracycline (67%) and penicillin (52%) in *H. somni* isolates isolated from the lower respiratory tract of feedlot cattle with (n = 210) and without (n = 107) BRD [13]. Likewise, Anholt et al. [31] observed that 100% of the *M. haemolytica* (n = 233), *P. multocida* (n = 117) and *M. bovis* (n = 226) and 67% of the *H. somni* (n = 75) isolates isolated from both living and dead BRD-affected cattle, originated from 60 different commercial feedlots in southern Alberta, exhibited resistance towards at least one antimicrobial class. Over 90% of all isolates (n = 745) displayed resistance to macrolide antimicrobials, which are the class of antibiotics

*Bacterial Cattle Diseases*

*2.1.3 Bacterial agents*

genetic differences among serotypes [25].

The main bacterial pathogens associated with BRD are *M. haemolytica, P. multocida, H. somni* and *M. bovis* [10]. *M. haemolytica* is the principal bacterial agent of BRD and has a considerable economic impact on the North American feedlot industry. It is a small, Gram-negative and facultative anaerobic bacterium that commonly exists as a part of nasopharyngeal and tonsillar crypt microbiota in healthy cattle and sheep [23]. To date, 12 different (1, 2, 5–9, 12–14 and 16–17) capsular serotypes have been identified within *M. haemolytica* [23]. Among these serotypes, serotype 1 (S1), serotype 2 (S2) and serotype 6 (S6) are most frequently isolated from feedlot cattle, with the S1 and S6 being the most prevalent in bovine infection [24, 25]. *M. haemolytica* residing in the upper respiratory tract of healthy cattle maintains a commensal relationship with the host due to the containment by the local microbiota and host immunity [23]. However, when the local microbiota and host immunity get disrupted by stress and viral infections, this opportunistic bacterium proliferates in the upper respiratory tract and then translocates into the lung where it induces acute infection characteristics to fibrinous pneumonia [23]. *M. haemolytica*-induced pathogenesis is accomplished through a combination of virulence factors including outer membrane proteins, leukotoxin (Lkt), lipopolysaccharide (LPS) and lipoproteins [23]. The outer membrane proteins, such as adhesion protein, facilitate attachment and colonization of *M. haemolytica* to the bovine respiratory cells. The Lkt, being the most important virulence factor, attracts neutrophils and macrophages to the site of infection when it is present in low concentration. High levels of Lkt, however, induce cell death of leukocytes and phagocytes, allowing *M. haemolytica* to evade the detection and destruction by the host immune system. The other virulent factors, including LPS and lipoproteins, are involved in hemorrhage, edema, hypoxemia and acute inflammation [10]. The virulence factors of *M. haemolytica* differ among different serotypes, and such difference has been reported to attribute to the

*P. multocida* and *H. somni* are also opportunistic BRD pathogens and are involved in the development of bronchopneumonia in cattle with clinical signs indistinguishable from pneumonia caused by *M. haemolytica*. The isolation rate of *P. multocida* and *H. somni* from clinically healthy cattle at feedlot entry is relatively high ranging from 15% up to 60% [10], suggesting they predominately exist as part of normal nasopharyngeal flora in healthy cattle. However, the isolation rate of these two pathogens is higher in the lower respiratory tract of feedlot cattle affected by BRD

compared to healthy cattle [13]. The main virulence factors identified in

variation and endotoxic activity of the LPS and biofilm formation [10].

**2.2 Current prevention and control strategies for BRD in feedlots**

*P. multocida* include a LPS, a cytotoxin, and iron acquisition proteins [10]. *H. somni* virulence factors include expression of immunoglobulin-binding proteins, survival in phagocytic cells, induction of apoptosis in endothelial cells, antigenic phase

Compared to the other three BRD bacterial pathogens*, M. bovis* is the least characterized BRD pathogen. This bacterium lacks a cell wall and is fastidious, requiring specialized media and techniques for its isolation and culture. *M. bovis* is often associated with chronic pneumonia, and its mechanism of actions remains

Prevention and control of BRD in large commercial feedlots in North America are aimed mainly at bacterial pathogens, through the use of antimicrobials and vaccination programs. Cattle considered at high risk for the development of BRD are often given metaphylactic antimicrobials upon feedlot arrival [26]. Metaphylaxis

**20**

poorly understood [10].

commonly used as metaphylaxis. Furthermore, Snyder et al. [14] also reported that a significant increase (from 3.7 to 99.2%) in the prevalence of *M. haemolytica* isolates resistant to tulathromycin in newly received feedlot cattle (n = 169) within 2 weeks after tulathromycin was given as metaphylaxis.

Three or more resistant genes have been detected from *M. haemolytica* and *P. multocida* [15, 29]. These resistant genes are most likely occurring from *de novo* mutation or being acquired from other bacteria. It has been reported that some resistant genes present in BRD bacterial pathogens are being encoded in selftransmissible conjugative elements [15]. Klima et al. [29] identified *M. haemolytica, P. multocida* and *H. somni* isolates from the US and Albertan feedlots that contain integrative conjugative elements (ICE) that conferred resistance up to seven different antimicrobial classes. These ICE can be transferred not only from one BRD pathogens to another BRD pathogen but also to other non-BRD-related bacteria (e.g. *E. coli*) via conjugation [29].

The resistant BRD-related pathogens can not only cause substantial profitability losses to the beef industry and animal welfare issues due to the higher disease relapse and mortality rate but also pose potential threat to the public health given the possibility of these transferable elements carrying resistant genes transfer into zoonotic pathogens. Therefore, counteracting measurements to reduce the development and spread of AMR in BRD pathogens are urgently needed.
