Understanding Mycoplasma Bovis Pneumonia in Beef Cattle

Background

Mycoplasma bovis has emerged as a major cause of pneumonia and arthritis in Canadian beef cattle, but there has been a lack of progress in controlling these diseases because of the uncertainty of how they develop. M. bovis pneumonia is important because of its frequency (27-54% of mortalities), economic losses, and animal suffering from chronic pneumonia and arthritis that often responds poorly to therapy. Current vaccines are not effective, and control depends on metaphylactic/interventional mass medication with antibiotics. However, alternative control methods are needed because of increasing frequency of antimicrobial resistance and consumer concerns about antibiotic use. Nearly all beef calves are infected with M. bovis in the early feeding period, but only a minority develop pneumonia (Castillo 2012). Thus, understanding determinants of disease outcome (i.e., why some infected animals develop pneumonia while others remain healthy) is of key importance. The appearance of the lesions, patterns of M. bovis and Mannheimia haemolytica co-localization, and epidemiology all suggest that the inflammation and necrosis induced by prior M. haemolytica or viral infection creates a tissue environment favorable to development of M. bovis-induced tissue damage. Prior work suggested that the interaction between lung macrophages and M. bovis likely determines disease outcome, and this knowledge will be key to improved control strategies.

Objectives

• Determine how M. bovis causes damage to the lung, and what conditions are required for M. bovis to cause respiratory disease, in order to identify targets for disease prevention. 
• Determine why M. bovis infection leads to pneumonia in some calves but not others, by investigating how prior cell death or inflammation causes M. bovis-infected macrophages to promote harmful inflammation and tissue damage instead of tolerating and controlling the infection.

What They DID

Cells (macrophages) from the blood and the lung of calves were cultured in the lab and exposed to inflammatory stimuli (killed bacteria, or products of other cells stimulated with killed bacteria) and then infected with M. bovis. We measured the effect of the prior inflammatory stimulus on death of the macrophages, and on production of pro-inflammatory and lung-damaging substances by the macrophages. 

Experimental challenge studies were used to test the effect of inflammation and damage to lung tissue on the response to Mycoplasma bovis infection. Calves were infected with both Mannheimia haemolytica and Mycoplasma bovis bacteria to create inflammation and tissue death within the lung, but then treated with an antibiotic to kill the Mannheimia but not the Mycoplasma. Control calves received only Mycoplasma bovis and antibiotic. The severity and nature of the disease were monitored clinically and after euthanasia. 

An experimental study involving natural disease was done using 60 auction calves at high risk of BRD. On the morning after arrival to the feedlot, calves were aerosolized with either killed bacteria to induce inflammation in their respiratory tract or with saline as a control. Calves were monitored for 28 days for clinical signs, changes in the lung detected by ultrasound, need for antibiotic treatment, and death.

What They Learned

The lung lesions of M. bovis pneumonia develop characteristic areas of dead tissue that are thought to provide a refuge for the bacteria and are one reason that antibiotic treatment may be unsuccessful. This study found that these areas of dead tissue are formed by macrophages derived from the blood (not macrophages already residing in the lung, or other cells). This discovery will allow more precise investigations of the process using blood-derived macrophages cultured in the laboratory, in ways that cannot be done in live animals. Using this approach, they found that conditions representing lung inflammation or infection (killed bacteria, or the products of other cells stimulated by killed bacteria) make these macrophages respond differently when they encounter M. bovis. Specifically, the pre-stimulated cells responded by producing more harmful inflammatory mediators and lung-damaging substances, and the macrophages were more likely to die in a way resembling the dead tissue seen in the natural disease. 

Next, these concepts were applied to calf studies. Calves infected only with M. bovis developed mild signs of disease. In contrast, disease was more severe if there was an early episode of inflammation and tissue damage in the lung (induced by transient infection with M. haemolytica bacteria and M. bovis, with antibiotic treatment soon afterward to kill the M. haemolytica. Also, prior M. haemolytica infection increased the likelihood that M. bovis would spread to the joints to cause arthritis. 

Furthermore, we studied the same process in the context of natural BRD in high-risk calves. Thirty calves were aerosolized with killed bacteria to induce inflammation in their respiratory tract. These calves developed more frequent and severe mycoplasma pneumonia than high-risk calves that did not receive the aerosolized killed bacteria. This included greater fever and levels of blood inflammatory proteins, reduced weight gain, higher frequency of lung disease detected by ultrasound, more M. bovis in the upper respiratory tract, and higher frequency of mortality due to mycoplasma pneumonia. 

What It Means

Infection of healthy calves with Mycoplasma bovis does not cause severe disease by itself, but inflammation and tissue damage in the respiratory tract of apparently healthy calves creates a situation where a subsequent M. bovis infection is much more harmful. Respiratory tract inflammation on the day after arrival to the feedlot contributes to the development of BRD, and mortalities due to M. bovis occurring in the second month after arrival are influenced by factors initiated in the immediate post-arrival period. 

Furthermore, because it is impossible to precisely study some of these immune responses in detail in calves, identifying blood-derived macrophages as the main cell type forming the characteristic lesions of M. bovis pneumonia is important to further unravel how this disease develops, and to develop specific interventions. With this approach, we uncovered a sequence of molecular and cellular events by which early inflammation leads to worse disease. The findings of this study suggest the following approaches to preventing losses from M. bovis infection: 

• Reduce the prevalence of M. bovis infection. However, since this infection is currently common in Canadian beef cattle and spreads readily in groups of co-mingled calves, we expect it will be difficult to substantially reduce M. bovis infection prevalence in beef feedlots in the absence of an effective vaccine. 
• Control concurrent respiratory diseases such as viral respiratory disease and shipping fever pneumonia, which worsen the outcome of M. bovis infection. 
• Minimize inflammatory responses that may not cause disease on their own but predispose to worse outcomes of M. bovis infection. These subclinical inflammatory responses may be caused by viral infection, and perhaps by stress. Other work by this group also suggests that dust also stimulates respiratory tract inflammation in cattle. 
• Knowledge of how the early inflammatory response leads to “cellular suicide” of macrophages and the development of caseonecrotic lesions may lead to specific pharmaceutical methods to prevent this adverse outcome of M. bovis infection.