SASKATOON — The disease investigation unit at the University of Saskatchewan’s Western College of Veterinary Medicine has recently been involved in two separate incidents of adult cows dying unexpectedly.
One outbreak occurred in a dairy herd and another in a cow-calf operation. In both situations, the main cause of death in these adult cows was diagnosed as a severe pneumonia caused by the bacteria Mannheimia hemolytica.
Mannheimia hemolytica is a common cause of pneumonia in cattle. However, we more commonly find it associated with pneumonia in recently weaned beef calves or in pneumonia cases that occur in younger calves pre-weaning.
The classical presentation of Mannheimia hemolytica pneumonia would be described as occurring in feedlot calves shortly after arrival in the feedlot and was often traditionally called shipping fever.
The presentation of having adult cows die suddenly of a pneumonia caused by this pathogen is unusual. We certainly have seen a few cases over the years, but it seems like this may be occurring with greater frequency this year. I have spoken to several other veterinarians in the western provinces who have recently dealt with similar outbreaks.
It is important to understand that Mannheimia hemolytica can be found occasionally in the nasal passages of normal cattle. It often takes some other stressor, such as a viral infection, to create an environment that allows this bacterium to flourish and establish an infection in the lungs.
Bovine respiratory disease is often termed a “polymicrobial disease,” which means that it often develops because of a combination of infections with both viruses and bacteria. In addition, we know that many other stressors are important risk factors for the development of respiratory disease.
In fact, these risk factors, such as weaning stress in calves, mixing of cattle from various sources, transportation, exposure to dust, severe weather or poor immunity, are often necessary, in addition to the presence of viruses and bacteria, to cause the disease.
What has perplexed us in some of these latest outbreaks is that none of these other risk factors seem to be obviously present. We are still working on the investigations, but to date, we haven’t been able to identify any previous viral infections in these animals.
In addition, adult beef and dairy cows are usually not exposed to many of the stressors that a recently weaned beef calf entering the feedlot would be exposed to. There has not been a great deal of mixing of various sources of animals or transportation events that would be identified as an obvious stressor.
It should be noted that despite not being able to identify any viral infections, they may have occurred previously but because too much time had passed by the time these affected animals were identified, we can no longer find the virus present. In this particular case, both of these herds were well vaccinated for the typical viruses, such as infectious bovine rhinotracheitis, bovine respiratory syncytial virus and bovine viral diarrhea.
One potential explanation is that perhaps a new or different strain of Mannheimia hemolytica is causing these pneumonia cases in adult cows. Hopefully, someone will have the opportunity to do more research on this issue.
The good news is that in most of these cases that I have been aware of, the strain of the Mannheimia hemolytica bacteria identified was susceptible to most antibiotics and other cows that were identified with pneumonia in the early stages were successfully treated with antibiotic therapy.
What lessons have we learned? First, this is a relatively uncommon diagnosis in adult cows and not something most veterinarians would initially think of when we hear adult cows are dying in a herd. In fact, most producers are probably not watching for pneumonia cases in adult cows and as a result the first diagnosis often comes in the form of a dead or severely sick cow.
Having your veterinarian examine a sick cow or having them perform a post-mortem on a cow could help you diagnose the problem before it gets out of hand.
The lesions of Mannheimia hemolytica pneumonia are easy to identify in a postmortem because the pleura of the lungs are covered with yellow fibrin and the anterior lung tissue is often dark red or purple and firm to the touch.
In many outbreaks that I have seen, several cows had to die before a postmortem was performed. This can make the outbreak far worse and more animals may develop chronic pneumonia before a diagnosis is made. Tissues can be sent to the diagnostic lab to isolate the bacteria and to help to ascertain which antibiotics will be most effective.
Secondly, we don’t completely understand why these specific herds are dealing with an outbreak of bacterial respiratory disease in adult cows.
We will continue to investigate these outbreaks, but it would be prudent to minimize any risk factors that can contribute to respiratory disease, such as introducing cattle from other sources or confining cattle together in close quarters.
There is little doubt that these bacteria are passed from animal to animal and a form of bovine social distancing might be a useful means of controlling the spread.
As well, ensure that your cows have good nutrition and adequate trace mineral levels so that their immunity will be optimal.
Finally, if we continue to see more of these outbreaks, we might want to consider the use of a Mannheimia hemolytica vaccine in our adult cow herd. Some producers may already be doing this, but these vaccines are typically given to younger animals.
In a western Canadian study of vaccine use in cow-calf herds conducted as part of the Western Canadian cow-calf surveillance study, Dr. Cheryl Waldner and colleagues reported that only four percent of cow-calf producers were using MH vaccines in their adult cows, while 67 percent of producers used MH vaccines in un-weaned calves and 35 percent of producers used them in weaned calves.
You should already be vaccinating to protect against the common viral pathogens such as IBR, BVD and BRSV.
Speak to your veterinarian to determine if this is something you should consider adding to the vaccination protocol in your own herd.
John Campbell is a professor in the department of Large Animal Clinical Sciences at the University of Saskatchewan’s Western College of Veterinary Medicine.
