BRD: Protecting Herd Health

Thursday, October 23, 2025

Respiratory Diseases Can Present Extra Challenges for Calves in Autumn

By Emily McKinlay

As calves are weaned in the fall months, many feedlots and backgrounders begin looking to fill their barns. The fall weather, along with a combination of other factors, can also set the stage for problems with respiratory disease in calves.

Bovine Respiratory Disease (BRD) can be caused by a combination of viral challenge, bacteria, and stress events.

Dr. Mac Littlejohn, a veterinarian at the Kirkton Veterinary Clinic, says that a viral challenge can predispose calves to a secondary bacterial infection, which then causes BRD symptoms.

“I can do a nasal pharyngeal swab on 100 calves, and I would find Mannheimia – the lead cause of shipping fever – on many of the animals that are healthy as well as those who are sick,” says Littlejohn.

If the bacteria are present even in healthy animals, what causes the sick calves to start presenting symptoms?

“What is the main factor that causes the bacteria to crawl into an animal’s lungs and cause disease? It’s stress,” says Littlejohn.

“The problem with BRD is that it is multifactorial. If our vaccines were bulletproof, we’d be golden – but they aren’t. It’s all stress-related.”

Littlejohn says when determining how to prevent respiratory disease, the first step is to assess risk.

“The most important thing to assess when buying in stockers or finishers is deciding their risk of BRD – or disease overall,” says Littlejohn.

“Assigning a risk level to the cattle coming in can help producers and their veterinarians decide which vaccines and prophylactic antibiotics to use on arrival.”

The main risk factors that Littlejohn recommends assessing are calf age, the number of source farms, calf sex, seasonality, previous nutrition and preconditioning, and the feedlot’s own history. Lighter calves, often being younger, have a higher risk of disease and are also more vulnerable to poor environmental conditions. Littlejohn notes that heifers are also at higher risk than steers for BRD.

Mixing calves from different sources also increases stress.

“How many sources did the calves come from? Are they all off one ranch, or has there been co-mingling?” asks Littlejohn.

“If you are not mixing and co-mingling any sources, those calves experience less stress versus calves from an auction. As soon as a pen of calves has more than six sources, the chance of having bacterial pneumonia issues is exponentially higher.”

Preconditioning of calves before sales is one strategy to mitigate the stress of entering a barn to be fed.

“All preconditioning helps lower stress. Have they seen a bunk before? Have they just been weaned off the cow?” Littlejohn says that previous nutrition also plays a factor in risk.

“If calves are in good shape nutritionally, you can almost tell based on the sheen of their coat. If they don’t have a shiny coat, don’t buy them. The immune system takes up a lot of energy to function properly. If you have poor nutrition, that’s less energy for the immune system.”

Fall weather can also play a role in BRD risk. The temperature fluctuations and dampness, particularly around November and December when many calves are selling, can predispose calves to pneumonias.

“You can almost draw a line on the number of BRD cases when winter sets in. Typically, after New Year’s, the number of BRD cases will drop because the weather becomes colder and more consistent,” says Littlejohn.

“In November and December, there is going to be worse weather that is cold, wet, and fluctuating.”

He cautions to watch for wet packs, particularly with smaller calves, as the energy required to keep warm will hinder the immune system.

Producers should also be mindful of stress when calves are being transported to the farm. Calves that are loaded calmly will experience less stress and are at lower risk.

When treating calves for pneumonia or BRD, Littlejohn says producers should discuss with their vets which class of antibiotic should be prioritized. During repeat treatments, a different class of antibiotic should be used.

BRD’s typical symptoms include dullness, lack of appetite, or droopy ears, but some bacteria can cause symptoms beyond those of pneumonia.

While Mannheimia haemolytica is often thought of as the main culprit of BRD, problems with Mycoplasma bovis and Histophilus somni have become more common.

Mycoplasma bovis

Like other types of pneumonia-causing bacteria, Mycoplasma bovis can be found even in healthy calves.

Dr. Jeff Caswell, a professor at the Ontario Veterinary College, says that unlike some other bacteria, Mycoplasma requires a living host to survive.

“Mycoplasma is highly dependent on being in a calf and can’t survive well in the environment,” says Caswell.

“It seems to spread quite readily through nose-to-nose contact from infected animals.”

He says that purchasing from multiple sources is a common way for feed-lots to become infected with Mycoplasma. Once a herd of cows or a barn of calves becomes infected, it is difficult to eradicate from the group.

“Mycoplasma is tricky to detect in live animals. It can be detected if animals are shedding a lot of bacteria in nasal secretions, but an individual animal sometimes may or may not shed the bacteria,” says Caswell, who notes that serology – blood samples – are a better indicator but are not useful on an individual animal basis.

“The really interesting thing is that in any group of animals, many of them could be infected with Mycoplasma in their nasal cavity or lungs, but the vast majority don’t show any evidence of disease. In other circumstances, animals show really severe chronic disease that they can’t seem to get rid of.”

From Caswell’s research, he believes that in the case of Mycoplasma, the difference between calves that show symptoms and ones that don’t is inflammation.

“Traditionally, people have thought that, similarly to shipping fever, maybe they are under stress and can’t mount a response, but there’s not a lot of good evidence for that. We’ve been finding that concurrent infections – multiple infections that occur at one time – that cause inflammation in the respiratory tract affect how calves respond to subsequent Mycoplasma infection,” says Caswell.

“If they have pneumonia from a more classic bacteria like Mannheimia, even if they are treated with antibiotics and fix that infection, the damaged tissue seems like a good place for mycoplasma to set up shop and cause disease.”

This also affects the timing and symptoms of calves that are infected with Mycoplasma.

“Often, shipping fever would appear in the first couple of weeks in the feed-lot, and it is usually quite acute – it shows up abruptly – whereas Mycoplasma often arises a bit later on,” explains Caswell.

“If calves are healthy when they are coming into the feedlot, Mycoplasma usually shows up the second month in the barn and tends to be a more smouldering condition.”

Calves can often appear to be poor-doers or may not respond well to treatment, often because of the lung damage and inflammation that has already occurred. Mycoplasma can also sometimes manifest as infection outside of the lungs, causing arthritis or middle ear infections.

“The lung infection is the source, but it then goes into the blood and infects the joint. Sometimes it can be cured with antibiotic therapy, but a much higher proportion of those calves would be euthanized because of the arthritis.”

Treatment of Mycoplasma can also be difficult. While antibiotic resistance is a concern in the industry, Caswell says that this is not the main reason for a lack of response to treatment.

“In post-mortems, areas of dead lung tissue are characteristic of Mycoplasma infection.

“Those seem to be areas that bacteria can hide in, so if they are treated with antibiotics, the bug may be resistant, but it might also be that the drug also can’t get into the dead lung tissue.”

Histophilus somni is another type of bacteria that has been challenging calf feeders.

Histophilus somni

Histophilus somni has become increasingly common in Ontario calves over the past five years, says Jacques van Zyl, general manager and head veterinarian at Metzger Veterinary Services.

“Histophilus is one of the most common bacteria causing pneumonia,” says van Zyl.

“It used to be more of a Western issue. In Ontario, it was not as common four or five years ago. Over the last three years, we have been seeing higher instances of Histophilus.”

He says that five years ago, Histophilus was the cause of only five to 10 per cent of pneumonia cases in calves that he treated. That number rose to nearly 50 per cent in 2024.

Histophilus is also commonly found in the airways of asymptomatic cattle, not causing disease until another stressor allows it to overgrow.

Van Zyl notes that Histophilus presents a few unique challenges. It begins to develop between 12 to 15 days after calves enter the feedlot, surpassing the highest worry period of the first week in the barn. Calves may also hide their symptoms until the disease progresses.

“Sometimes people call it a silent pneumonia because the cattle do a good job of hiding symptoms. When they do start showing symptoms, it’s often far down the disease process,” says van Zyl.

“The clinical signs are similar to other pneumonias. Calves are dull, off feed, panting, and might have a snotty nose. The bigger challenge is timing. By the time the clinical signs show up, it is hard to get ahead of.”

Similar to Mycoplasma, Histophilus doesn’t always show up as just a pneumonia.

“It can get into the bloodstream and cause other problems. It can get into the joint, which would make most producers think of Mycoplasma, but Histophilus can cause similar problems,” says van Zyl.

“It can also go to the heart and cause a heart infection – myocarditis – which can cause heart failure. In the brain, it can cause infectious thromboembolic meningoencephalitis (ITEME) or a brain infection. It’s not just a pneumonia.”

Van Zyl stresses that treatment time is very important with Histophilus.

“We did a big research project in the fall assessing antibiotic resistance, and none of our Histophilus samples came back resistant to florfenicol.

“Some people might say that they find Resflor is not working, but it could be too late in the disease process to get ahead, with too much fluid and damage in the lungs. Getting antibiotics into an animal as early as possible is huge,” says van Zyl.

Mycoplasma bovis and Histophilus somni create unique challenges for producers battling pneumonia in their calves. Doing the groundwork to minimize risk and set calves up for success can mitigate the chances of battling costly respiratory diseases. BF