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By Robyn Elgie, DVM
Kirkton Veterinary Clinic
Over the past few years, there is a growing incidence of severe pneumonia outbreaks in dairy herds across Ontario and other parts of Canada. From a veterinarian’s perspective, the most troubling aspect of these outbreaks has been the speed and unpredictability with which it begins. During these sudden outbreaks it is possible to lose up to 5% of the herd and have 30% or more of the animals treated, all within the span of 1 week. Surprisingly, the implicating pathogen found in these herd outbreaks is not new. In fact, it can be found in almost all healthy cattle and is a well-known pathogen associated with pneumonia in feedlot cattle (1). The pathogen is known as Mannheimia haemolytica. Over the past three years, our practice has encountered 10 presumed and 7 confirmed cases of Mannheimia haemolytica herd outbreaks. This article is based on the collective clinical experiences of our veterinarians as well as discussions with veterinarians from other parts of Ontario and Canada.
What is Mannheimia haemolytica?

Mannheimia haemolytica is a bacterial pathogen that can be found in the nasal passages and larynx of clinically health cattle. Mannheimia, along with a few other bacterial pathogens, commonly live in the upper respiratory tract in low numbers. However, once a stressful event occurs it is able to rapidly reproduce into a large population and travel to the lower part of the respiratory tract (1). In the lungs, Mannheimia bacteria release a substance called leukotoxin which is designed to kill white blood cells (2). Under normal conditions white blood cells are able to consume and destroy bacterial cells like Mannheimia. When Mannheimia increases in number suddenly, the leukotoxin it releases can quickly eliminate the white blood cell supply, overwhelming the immune system (2). Once this occurs, Mannheimia bacteria can cause severe irreversible lung damage. Mannheimia haemolytica is one of the main bacterial pathogens in the Bovine Respiratory disease complex which is commonly associated with calves shortly after transport and arrival to a feedlot. Until recently, Mannheimia has not been associated as a common cause of pneumonia in dairy cattle.

There are 4 things that make Mannheimia 1960 “outbreaks” in particularly devastating in dairy herds:

  1. The apparent unpredictability of when an outbreak will occur.
  2. The high number of affected animals within the herd.
  3. The speed at which the animals become clinically sick and die.
  4. Affected animals do not always look like a cow with pneumonia.
Risk Factors

Our clinic gathered farm level treatment records and Dairy Comp herd level data from 3 herds that had recent outbreaks of Mannheimia haemolytica within a 21-day period of one another. While outbreaks do appear to be unpredictable and likely multi-factorial (3), there were some commonalities among these herds that may be considered risk factors.

1) Vaccine Protocol and Compliance

Classically, Mannheimia haemolytica is an opportunistic pathogen that will increase in number soon after a stressful event such as viral pneumonia (1). A well designed and executed modified-live vaccine (MLV) protocol can reduce the risk of viral pneumonia and abortion. In dairy herds this has been the main focus of vaccine programs. Until recently, Mannheimia haemolytica pneumonia was not identified as a major pathogen in Ontario dairy herds. As a result, most veterinarians recommended viral only vaccines. The theory was prevention of viral pneumonia would reduce the risk of a less likely bacterial pneumonia outbreak.

The effectiveness of a well-designed vaccine program is limited by compliance to the protocol. To achieve adequate protection using a 5-way Modified-Live vaccine, every animal needs to receive a proper primary series as a calf/heifer and a booster dose every 12 months after the primary series is completed. There are killed vaccines available that will provide immunity for the same viruses however they require more frequent booster doses.

Protocol compliance is dependent on accuracy. Every time an animal receives a dose there are 4 areas where error can occur. The correct cow, the correct product at the correct dose and the correct timing of administration are all variables where compliance can suffer. For example, a cow is bred and confirmed pregnant late in lactation so her calving interval will be greater than 12 months. This means the recommended time between her previous dose and the next dose of vaccine will exceed 12 months. Therefore, her previous dose may not be effective and theoretically she would need a primary series again to be 100% protected.

It is also important to note that vaccine handling, storage and proper administration route are also very important factors to achieve effective protection.

2) Ventilation

In lactating animals, we often think of ventilation primarily as cow cooling. However, ventilation is actually defined as bringing fresh air in and removing stale air at an adequate rate. Naturally ventilated barns are designed to achieve 4 air exchanges an hour during the winter when the side curtains are almost closed. This is achieved by the differences in air temperature between the outside and inside air.

Natural ventilation does not work in the same way when the outside temperature is more moderate. During these times we rely on open curtains and fans to move air. When there is extreme humidity with low natural air movement, less fresh air will come into the barn. Therefore, the fans can only push or move stale air.

During warmer weather, the curtains may be raised to reduce sunlight into the barn leaving less space for stale air to exit and for fresh air to enter the building. The type of fan can also make a difference. High volume, low speed fans create “dead spots” of air between fans. This type of fan moves air in a multi-directional manner, therefore it does not move stale air away from the cows.

3) Herd Health Status (Closed or Open)

The risk of bringing in a novel disease is significantly increased if animals are brought in from multiple other sources or, from somewhere where they would have been exposed to animals from other herds. Even if the vaccine protocol is well designed and the compliance is excellent, there is still opportunity for novel diseases to enter the herd. Even diseases that cause only mild sickness can be enough of a stressor to trigger Mannheimia haemolytica proliferation (3).

Taking animals off farm and then re-entering is also a risk but can be controlled by isolation, quarantine and booster vaccination of those animals.

It is very important to point out that although the above 3 factors increase the risk of a Mannheimia haemolytica outbreak, limiting or controlling all three does not guarantee immunity from a Mannheimia outbreak. We hypothesize that the unpredictable nature of the outbreaks we have encountered is likely due to a “perfect storm” of factors that coincide to cause an abnormal physiologic stress on the herd.

These coinciding factors can be difficult to prove in retrospect. In many cases, a proportion of these inciting factors may be invisible to our observation but may include sudden feed and weather changes, bedding type, season or other herd level health stressors. We have also seen cases where the herd was struggling with pink eye or mild viral exposure such as coronavirus which can cause dysentery or mild respiratory infection which is often evidenced by tearing around the eyes.

With regards to seasonality, our clinic has experienced a herd outbreak in every season on the calendar. There is a tendency to see more cases during the classic pneumonia season of the fall, however, there have been clusters of herd outbreaks in the winter and summer months as well.

At the cow level, our practice has not seen a stage of lactation or lactation number represented more than another overall. There does appear to be an increased risk in high production herds. We have seen it in both lactating and dry cows, with lactating cows being the most common. Other clinics in Ontario have experienced higher representation in fresh cows.

Rarely are heifers younger than those found in the close up group affected. In multiple cases, groups of differing pregnancy status or lactation were housed within nose to nose contact of the pen experiencing an outbreak and did not demonstrate any clinical signs consistent with Mannheimia haemolytica. This demonstrates that Mannheimia haemolytica is not contagious from animal to animal but requires a stressor within the animal, or a group of animals, to trigger the disease process.

Prevention and Control

In Ontario dairy herds, many calf vaccine protocols have incorporated intranasal vaccines which include Mannheimia haemolytica however, it has not been common to include it in the vaccine protocols for the lactating herd. Fortunately, there are a number of options that include Mannheimia haemolytica as part of a combination or on its own in both injectable and intranasal forms. Our current clinic principle outlines each animal should receive at least 2 doses annually of a vaccine with Mannheimia haemolytica as one of the components. The selection of vaccine and timing of administration is herd specific depending on management and facilities as well as known risk factors. It is best to consult with your herd veterinarian to tailor the protocol to your herd.

What to do in the Face of an Outbreak
1) Take Samples for Confirmation – Post Mortem animals that have died

In our experience, a farm will lose an animal unexpectedly proceeding the outbreak. This often occurs around 1 week before the majority of the cases appear. This does not mean that every animal that dies unexpectedly may be the beginning of an outbreak, but it is important to discuss with your herd veterinarian to determine whether a post mortem would be beneficial for any sudden deaths. Certainly, if morbidity and mortality have increased in adult animals in a short period of time this would be warranted.

2) Intranasal Vaccination

Due to the speed in which animals transition from clinically healthy to very sick, action needs to be taken immediately to reduce mortality and morbidity. This is almost always before results from the lab are available.

Administration of intranasal vaccines in the affected group and any other groups that may be potentially affected are the most effective and quickest way to boost immunity in the face of an outbreak. Injectable options are also available but have a slower immune response. They can be used as an alternative when restraint for intranasal administration is unsafe.

Herd level supportive measures can also include increasing the amount of chopped straw in the ration temporarily and bicarbonate freely available.

3) Treat Animals Immediately When Detected

Mannheimia haemolytica pneumonia often presents with very subtle clinical signs in the early stages. Producers who have experienced these outbreaks report a change in attitude is the first sign a cow will typically demonstrate. Clinically affected animals do not always have apparent increased expiratory effort or audible increase in lung sounds. Using tools like milk temperature, milk yield and rumination as well as walking the pens and observing the animals every 4-6 hours can help detect animals in the early stage of disease.

Mannheimia haemolytica progresses clinically very quickly. Antibiotics and anti-inflammatories can have successful cure rates but must be given early in the disease process to reduce morbidity and mortality. It is not uncommon to have up to a third of the affected group treated over the course of 1-2 weeks.

Depending on antibiotic selection and the number of animals treated within the lactating herd there should be clear communication with the client about the possibility of antibiotic residues and appropriate testing and withdrawals.

Take Home Messages

  1. Mannheimia haemolytica is a normal upper respiratory bacterial inhabitant of cattle that can cause sudden and severe pneumonia.
  2. An outbreak is often triggered by undetected stressful event(s) in a group of animals, up to one third of the herd can be affected.
  3. Herd level intranasal vaccination as well as early detection and treatment of affected individuals will significantly reduce morbidity and mortality during an outbreak.
  4. Reducing known risk factors as well as effective implementation of a regular vaccine protocol can considerably reduce the risk of an outbreak.


1. Radostits, O., Gay, C., Hinchcliff, K., Constable, P. (2007). Veterinary Medicine (10th Ed). Saunders Ltd.

2. Campbell, J. (2022). Merck Veterinary Manual. Merck & Co., Inc. Rahway, NJ, USA.

3. Caswell, J. and Gordon, J. (2020, November). Mannheimia in Dairy Cows, [Conference presentation] OABP Annual Fall Meeting Online Webinar.

This article was written for the Spring 2023 Dairy Eastern Dairy Grist. To read the whole Dairy Grist, click the button below.