*2.1.1 Predisposing factors*

Bovine respiratory disease, also known as a shipping fever, is a complex multifactorial disease because its onset depends on the interaction between number of factors including host, environment, management and viral and bacterial infectious agents (**Figure 1**) [18]. The host factors predisposing cattle to BRD include age, body weight, immune status and genetics [18]. The age and body weight of the calves entering the feedlot are, in most cases, inversely correlated with disease

**19**

*2.1.2 Viral agents*

**Figure 1.**

bacterial pathogens [22].

*Bovine Respiratory Disease in Feedlot Cattle: Antimicrobial Resistance in Bovine Respiratory…*

susceptibility to BRD during the feeding phase [19, 20]. Shipping is the leading environmental risk factor for BRD due to the fact that almost all cattle placed in the feedlot are transported from elsewhere. Transportation distance has a negative impact on the animal resistance to the development of BRD owing to the stress and body weight loss that occur during the transportation [18]. In addition, commingling with other cattle in sale barns is an important management factor predisposing cattle to BRD. Because sale barn cattle have greater exposure to pathogens and stress as a result of mixing with cattle from multiple sources, feedlot cattle purchased from sale barn are often at greater risk for BRD compared to the ones purchased directly from the farm or ranch. Of note, the host, environment and management factors discussed above are necessary but not always sufficient to cause pneumonia, and thus, additional predisposing factors, such as viral infection,

The most common viral agents associated with BRD include bovine herpesvirus type 1 (BHV-1), parainfluenza-3 virus (PI3), bovine viral diarrhea virus (BVDV) and bovine respiratory syncytial virus (BRSV) [13]. These viral pathogens can induce primary infection with mild clinical signs of BRD and predispose cattle to bacterial infection [18, 21]. Viral infection can impair the mucosal barrier and respiratory pathogen clearance, damage the lung parenchyma and suppress immune responses in cattle. Combined, the effects of viral infection facilitate the proliferation of opportunistic bacterial pathogens in the upper respiratory tract and translocation of these pathogens into the lung and cause infections to the compromised lung [21]. A recent human study suggested that respiratory viruses can also affect the structure and composition of nasal microbiota, which may be another way through which virus weakens host resistance to

are often necessary to produce bacterial pneumonia [18].

*Schematic overview of the pathogenesis of bovine respiratory disease in cattle.*

*DOI: http://dx.doi.org/10.5772/intechopen.81718*

*Bovine Respiratory Disease in Feedlot Cattle: Antimicrobial Resistance in Bovine Respiratory… DOI: http://dx.doi.org/10.5772/intechopen.81718*

### **Figure 1.**

*Bacterial Cattle Diseases*

feedlots [9].

due to the growing red meat demand by an increasing world population, with an estimated increase from 7.6 billion people in 2017 to 8.6 billion people in 2030 [3]. Despite advances in veterinary medicine, animal husbandry and animal welfare, economic impacts of cattle disease on the beef cattle industry still remain significant, with BRD being the most significant health problem in modern feedlot industry in North America. Bovine respiratory disease is commonly associated with pneumonia in nursing beef calves and recently weaned feedlot cattle. Cattle are mostly affected by BRD within the first 45 days after feedlot placement [4, 5]. According to the Feedlot 2011 study conducted by the National Animal Health Monitoring System (NAHMS), 97% of feedlots across the USA reported having cattle with BRD, and 16.2% of the cattle in a feedlot were affected by BRD during the feeding period [6]. The annual financial loss attributed to BRD, including mortality, reduced feed efficiency and performance and treatment costs, has been estimated to be more than \$4 billion to the US beef industry [7]. This loss surpasses the economic losses incurred by all other cattle diseases combined [8], given that BRD accounts for 70–80% of all morbidity and 40–50% of all mortality in the US

Bovine respiratory disease is a complex disease with a multitude of stressors that predispose cattle to viral and bacterial infection. *Mannheimia haemolytica*, *Pasteurella multocida*, *Histophilus somni* and *Mycoplasma bovis* are the main bacterial pathogens involved in BRD and are, therefore, the main targets of antimicrobial treatments to control BRD [10]. In the North American feedlots, cattle considered at high risk for the development of clinical BRD signs are often given antimicrobial metaphylaxis upon feedlot arrival to prevent BRD [11]. However, recent studies have shown the emergence of BRD bacterial pathogens that are resistant to several classes of antibiotics used to both control and treat BRD [12–14]. The multidrug resistance in BRD pathogens towards particularly tilmicosin, tulathromycin and oxytetracycline has been increased in feedlot cattle in the last decade [15], and such increase may partially due to the increased use of these antibiotics as metaphylaxis. The continued rise in AMR in BRD bacterial pathogens necessitates the developing antimicrobial alternative approaches to mitigate bacterial pathogens associated with BRD in feedlot cattle. Recent research results suggest the potential use of probiotic and essential oil (EO) as antibiotic alternative approaches to mitigate bovine respiratory pathogens [16, 17]. The objective of this chapter is to provide a brief overview of pathogenesis of BRD, to review the current status of antimicrobial resistance in bacterial pathogens associated with BRD and to discuss the potential antimicrobial alternative strategies, including probiotic and EO approaches, to

Bovine respiratory disease, also known as a shipping fever, is a complex multifactorial disease because its onset depends on the interaction between number of factors including host, environment, management and viral and bacterial infectious agents (**Figure 1**) [18]. The host factors predisposing cattle to BRD include age, body weight, immune status and genetics [18]. The age and body weight of the calves entering the feedlot are, in most cases, inversely correlated with disease

mitigate bovine respiratory pathogens in feedlot cattle.

**2. Bovine respiratory disease (BRD)**

**2.1 Pathogenesis of BRD**

*2.1.1 Predisposing factors*

**18**

*Schematic overview of the pathogenesis of bovine respiratory disease in cattle.*

susceptibility to BRD during the feeding phase [19, 20]. Shipping is the leading environmental risk factor for BRD due to the fact that almost all cattle placed in the feedlot are transported from elsewhere. Transportation distance has a negative impact on the animal resistance to the development of BRD owing to the stress and body weight loss that occur during the transportation [18]. In addition, commingling with other cattle in sale barns is an important management factor predisposing cattle to BRD. Because sale barn cattle have greater exposure to pathogens and stress as a result of mixing with cattle from multiple sources, feedlot cattle purchased from sale barn are often at greater risk for BRD compared to the ones purchased directly from the farm or ranch. Of note, the host, environment and management factors discussed above are necessary but not always sufficient to cause pneumonia, and thus, additional predisposing factors, such as viral infection, are often necessary to produce bacterial pneumonia [18].
