1. Host factors, which refers to the characteristics of an animal that make it more prone to the disease, such as: age, nutritional status, immune status, prior exposure to the pathogens, genetics (e.g., crossbreds generally have better health performance than straightbred cattle)  
2. Infectious agents or pathogens must be present to cause the disease. These can broadly be categorized as viruses, bacteria and parasites: 
   • Viruses, including bovine herpes virus (IBR), bovine parainfluenza virus (PI-3), bovine respiratory syncytial virus (BRSV), bovine viral diarrhea virus (BVD) and bovine coronavirus (BCV). These viruses usually cause the initial BRD infection and predispose the animal to subsequent bacterial BRD infections. Several of these viruses can also cause significant reproductive (BVD, IBR) or diarrhea (BVD, BCV) challenges in cow-calf operations.
   • Bacteria, including Mannheimia haemolytica, Pasteurella multocida, Histophilus somni and Mycoplasma bovis. 
   • Parasites, including lungworm.
3. The environment that the animal is in may increase the risk factors for disease. Animals in overcrowded conditions, in poor air quality (poor ventilation, dust, smoke), sourced from auction markets or stressed by transport, commingling, temperature fluctuations, etc. are more likely to develop the disease.

BRD is considered a polymicrobial disease, which means that it arises from infections with a combination of bacteria and viruses. In addition, several other factors influence the susceptibility of an animal to developing BRD. Any one risk factor alone may not trigger BRD, but several risk factors together form an additive effect that can predispose the animal to BRD. 

Management factors have been associated with BRD for decades. Regardless of herd size, outbreaks of BRD (20% of herds) were most common in herds that had purchased 10 or more bulls, used community pastures, bought cows or did not vaccinate newly purchased animals.³ Historically, feeder cattle were often shipped by train, particularly to large markets in the United States. It was recognized at the time that shipping was a risk factor for BRD, and hence pneumonia in cattle was termed “shipping fever.” A study involving calves arriving at 21 U.S. commercial feedlots from 1997 to 2009 concluded that distance traveled was correlated to the incidence of BRD.⁴ This, however, has not been substantiated in Canada.  

Weather has always been implicated in the occurrence of BRD, presumably because the greatest incidence of BRD occurs during the fall. However, this is confounded by the fact that it is also when the greatest number of calves is being assembled, mixed (commingled) and transported. A study involving 288,388 head of cattle, arriving at nine U.S. commercial feedlots during September to November in 2005 to 2007, found that wind chill and temperature change were associated with an increased incidence of BRD.⁵ 

Some studies have found a higher incidence of BRD in auction market versus ranch-derived calves. Furthermore, the incidence of BRD increases with the level of commingling; calves assembled from multiple lots are more likely to have BRD than pens composed of larger groups of calves from fewer sources. There is also considerable anecdotal evidence that the quality of the calves purchased is highly associated with the incidence of BRD, with poorer quality calves (e.g., lightweight, malnourished, uncastrated, unvaccinated) having more BRD. Studies also conclude that lighter weight calves have a higher risk of developing BRD than do their heavier pen mates.

Vaccines for respiratory diseases are routinely administered upon arrival at the feedlot, which may also be a difficult time for a stressed calf to mount an effective immune response. A recent review of the scientific literature found no clear benefit from vaccinating calves upon arrival at the feedlot.⁸ Another review relating to Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni vaccines concluded that there was a potential benefit for vaccinating feedlot cattle against M. haemolytica and P. multocida but not H. somni.⁹ 

Nutrition may affect incidence of BRD. One review found that increased energy density (concentrates) will improve ADG without adversely affecting the incidence of BRD.¹⁰ Other studies found that the incidence of BRD tended to increase once concentrates exceeded 50% of the diet.¹¹ Similarly, BRD morbidity increased once crude protein exceeded 14%. There is not enough evidence to conclude that injected doses of vitamins A, D and E will reduce BRD. A wide range of studies have also examined supplementing with potassium, thiamine, B-vitamins, copper, zinc, vitamin E, selenium and bypass protein. Although these studies haven't shown that supplementing with these nutrients significantly reduce the incidence of BRD, it's likely that significant deficiencies of these nutrients would almost certainly impair animal health.  

Metaphylactic treatment strategies using injectable antibiotics can help control BRD in high-risk calves, and likely are more effective than oral antibiotics since stressed cattle may not be eating or drinking regularly.¹² It is important that producers consult with their veterinarian regarding the use of antibiotics for the control and treatment of BRD, especially in terms of avoiding antibiotic resistance.

Good animal husbandry can also help reduce the risk of disease. Providing clean, dry bedding, protection from prevailing winds, avoiding overcrowding, using low-stress handling techniques, proper ventilation of barns and minimizing dust can help to reduce disease.  

There is a large body of scientific knowledge regarding the beneficial effects of antibiotics for treating BRD. The question is not whether to treat an animal diagnosed with BRD with an antibiotic, but rather, "which antimicrobial works best?” There is no simple answer to this latter question, but your veterinarian is a good place to start. 

Ancillary drugs, such as nonsteroidal anti-inflammatories (NSAIDS) and immunomodulators, have been used to treat BRD for decades. However, many of the studies have been small-scale experiments and there is a lack of data from well-designed, large-scale, clinical trials. However, a 2011 study found that meloxicam (NSAID) administered in water 24 hours before castration at the feedlot significantly reduced the number of animals that developed BRD.¹⁴ 

Even when working with the best recommendations, sometimes treatments fail. Common causes of treatment failure include: 

Doing necropsies on sick animals early in an outbreak can help to diagnose and treat the problem more quickly and accurately, potentially allowing time to develop protocols to deal with the disease more appropriately.