1. Host factors, which refers to the characteristics of an animal that make it more prone to the disease, such as: age, immune status, prior exposure to the pathogens, genetics, etc. environmental factors such as transport, commingling, temperature fluctuations, crowding, ventilation, auction-sourced, etc.  
2. Infectious agents or pathogens that are necessary for causing 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). 
   • Bacteria, including Mannheimia haemolytica, Pasteurella multocida, Histophilus somni and Mycoplasma spp. 
   • Parasites, including lungworm.
3. The environment that the animal is in may increase the risk factors for disease. Animals in crowded or dusty pens, barns with poor ventilation, sourced from auction markets etc. are more likely to develop the disease.

Ultimately, a mixture of pathogens is usually responsible for BRD, but as previously stated a number of other factors influence the susceptibility to developing BRD. Any one risk factor alone may be insufficient to trigger cases of BRD, but together they form an additive effect that can predispose the animal to BRD.  

Environmental factors, particularly transport, have been associated with BRD for decades. Historically, 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”.  Today, we still rely heavily on the use of thermometer to identify animals with a ‘fever’, which is essentially a tell-tale sign of BRD. A study involving calves arriving at 21 US commercial feedlots from 1997 to 2009 concluded that distance traveled was correlated to the incidence of BRD. This, however, has not been substantiated in western Canada. 

Weather has always been implicated in the occurrence of BRD, presumably because the greatest incidence of BRD occurs during the fall. However, this finding 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 9 US commercial feedlots during September to November in 2005 to 2007, found that maximum wind speed, mean wind chill temperature, and temperature change were associated with an increased incidence of BRD. 

A number of 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 are pens composed of larger groups of calves. In addition, there is considerable anecdotal evidence that the quality of the calves purchased is highly associated with the incidence of BRD; with poorer quality calves having more BRD. 

A preponderance of studies found that lighter weight calves have a higher risk of developing BRD than do their heavier pen mates. 

A large-scale U.S. study involving 21 million animals found that from the years 1997-1999 females were at a greater risk of developing BRD than were males, but no difference in gender was found for the years 1994-1996. 

Most studies looking into the effects of dehorning and castration only examined performance parameters (ADG, feed:gain); however, it is inferred that these stressful events, and others like calving or the rapid  introduction to high grain rations, may precipitate cases of BRD. 

The risk factors associated with an outbreak in a particular feedlot may or may not exist in outbreaks involving similar types of animals in similar settings.  

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. 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. 

Nutrition may affect incidence of BRD. One review found that increased energy density (concentrates) will improve ADG without adversely affecting the incidence of BRD. Other studies found that the incidence of BRD tended to increase once concentrates exceeded 50% of the diet. 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, but none have significantly affected the incidence of BRD. 

Metaphylactic treatment strategies can help control BRD in high risk calves, but feeding oral antimicrobials, either by water or feed, may not be effective since stressed cattle may not be eating their feed. An analysis conducted in the early 1990s concluded that there was a lack of studies to justify the use of mass medicating with oral antimicrobials. Two subsequent studies found that inclusion of chlortetracycline and sulfamethazine in the ration reduced BRD treatments and morbidity. If treating on arrival, it is important to use antibiotics labeled for control of BRD. With the introduction of new regulations governing the use of antibiotics, it is important that producers consult with their veterinarian regarding the use of antibiotics for the control and treatment of BRD. 

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

The literature contains a large body of knowledge regarding the beneficial effects of antimicrobial therapy in the treatment of BRD. The question is not, whether to treat with an antimicrobial, but rather, “which antimicrobial works best?” There is no simple answer to this latter question. 

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 used on experimental models and there is a lack of data from well-designed, large-scale, clinical trials. That said, a more recent study found that meloxicam (NSAID) administered prior to castration significantly reduced the number of animals to develop BRD. 

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

Doing necropsies on sick animals early on in an outbreak can help to diagnose and treat the problem more quickly and accurately. If done early enough in the outbreak a necropsy can help your veterinarian define what the disease is and will give you enough time to develop protocols to deal with the disease appropriately.