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The Impact of Mannheimia haemolytica on the High-Risk Herd

Mannheimia haemolytica is a bacterial contagion that is one of the main organisms responsible for the broader scope of bovine respiratory disease (BRD). M. haemolytica is frequently found in conjunction with Pasteurella multocida, with both bacteria typically presenting as secondary infections in compromised animals (most commonly following viral infections or significant stress events). It often leads to pneumonia, causing irreversible lung damage in the infected cow.

Prevalence of M. haemolytica in the Feedlot Setting

While M. haemolytica is fairly common to find in the living environments and upper respiratory tracts of most healthy cattle without causing issues, it does lead to secondary infections amongst compromised high-risk feeder cattle. In the case of M. haemolytica, it is responsible for pneumonia that can, at best damage the lungs, and at worst result in mortality of the infected individual.

In a study from the University of Georgia1, nasopharyngeal swabs (NPS) were taken during processing from a group of 169 high-risk feeder, sale barn bull and steer calves. They were also given the macrolide antimicrobial tulathromycin (2.5 mg/kg SQ) upon arrival at processing. Of those cattle, 27 (16%) swabbed positive for M. haemolytica (with 1 cow or 3.7% swabbing positive for a multi-drug resistant strain [MDR] of M. haemolytica). At 10 – 14 days a second NPS was taken from each of the 169 cattle with an alarming 123 (72.8%) showing positive cultures for M. haemolytica with 122 animals testing positive for a MDR strain of M. haemolytica. This offers a good idea of just how common it is to find M. haemolytica in high-risk stocker cattle and the waning or ineffective results seen with metaphylactic use on these groups.

How M. haemolytica Gains a Foothold in the Feedlot

Stocker cattle tend to be a mixed lot – some are healthy while others are in poor condition when they reach the sale barn. Knowing the background of any given sale barn calf is almost impossible.

  • Was it vaccinated properly?
  • Did the mother receive adequate nutrition during and after her pregnancy?
  • Was she vaccinated properly to ensure her and her calf’s immunity?
  • Did that calf have adequate weaning time?
  • How long has it been since its last meal? Is it already experiencing sub-acute ruminal acidosis (SARA)?

Unfortunately, these factors can have a huge effect on a calf’s ability to fight off disease and withstand the stresses of transitioning from its point of origin to the preconditioner’s lot.

M. haemolytica and Shipping Fever

“Shipping fever” is the common term for bovine respiratory disease (BRD), a complex of multiple diseases that can have detrimental impacts on stocker cattle. There are numerous viral diseases at play when looking at BRD – the most common include Infectious Bovine Rhinotracheitis (IBR), Bovine Viral Diarrhea (BVD – also a major contributor to scours), Parainfluenza Type-3 Virus (PI-3) and Bovine Respiratory Synctial Virus (BRSV).

Animals with weak or compromised immune systems are at greatest risk for developing shipping fever. With the stress of shipping – this includes handling, transport, lack of feed and water during transport, inhospitable weather and/or truck conditions, etc. – the calf’s immunity is put under significant stress. Because shipping fever is so prevalent, it’s common practice to administer a metaphylactic such as tulathromycin as they’re loaded for transport to help combat any secondary bacterial diseases that could contribute to “shipping fever”. Unfortunately, while this may help with bacterial infections that are already present, it doesn’t typically last long enough to address later-onset infections such as M. haemolytica.

When a cow is experiencing the ravages of shipping fever the immune system is under siege.  It is at this point that M. haemolytica can become a full-blown disease, invading the lungs, and causing pneumonia. As mentioned above, even with a metaphylaxis, the incubation time of M. haemolytica is such that it comes to fruition around the time the effects of the metaphylactic agent will be dwindling.

In some cases, cattle may contract M. haemolytica and recover without intervention; it’s often assumed that they “got over it” but the reality is that a good deal of lung damage may have occurred. This can become evident in that cow’s slower or reduced gain, stamina during handling, etc.

What Makes High-Risk Cattle Susceptible to M. haemolytica?

A high-risk cow is already in the “danger zone” for developing numerous problems. These cattle fall into several loose categories that can make them incredibly vulnerable to disease. It’s important to remember that the majority of a cow’s immune system lies within the rumen (about 80%). When viewed from that angle, it is no wonder that so many calves experience shipping fever and subsequent M. haemolytica secondary infections. Some of the factors that predispose a calf to issues include:

  • Unweaned calves. A calf that has not been properly weaned has not learned to eat on its own. It is also likely that its rumen is not fully developed, making it difficult for it to produce the necessary immunity to fight off disease. A calf that isn’t yet eating on its own can suffer from scours and acidosis.
  • Underweight calves. If a calf goes into the sale barn underweight it will surely arrive at the feedlot even more depleted. According to a review on transportation shrink by Ohio State University Extension, “Most of the shrinkage occurs during loading and in the first part (25 miles) of a trip. Cattle may lose half as much in 25 miles as they do in 200 miles. As the time increases, so does shrinkage, but at a slower rate than the first few miles.”2
  • Generally stressed calves. Calves that are being handled, moved to new environs, comingled with new cattle, etc., experience a high level of stress. Excessive stress releases cortisol which suppresses the immune system.
  • Undeveloped or under-developed rumens. Calves that have not been properly weaned and started on forage tend to have undeveloped rumens. This means that it is difficult to get them to start feeding and if and when they do, they don’t have the proper gut flora to adequately ruminate their feed.
  • Calves that refuse to eat or drink. In instances where a calf refuses to take in feed and water, they become quickly predisposed to being thrown into SARA.
  • Excessive antibiotic use prior to shipping. A metaphylactic program can be a vital part of keeping a herd healthy, however, the overuse or use of the wrong antibiotics can kill off beneficial gut flora, making it impossible for the rumen to function at peak levels.

Because the majority of a cow’s immune system lies within the gut, any one of the points above, or a combination of, can spell disaster for a calf with a weak rumen. This usually indicates it will have an equally weak immune system.

Use of Metaphylactics and M. haemolytica

The use of metaphylactics in the stocker cattle industry has proven to be both a blessing and a curse. When used prudently and properly, metaphylactics can help prevent widespread disease amongst groups of incoming cattle. However, if used carelessly or in excess, the benefits of metaphylactic use quickly become null.

Metaphylactics are used throughout an entire group of cattle under the assumption that they may be sick or will become sick in the near future. They might be administered in the form of an injection or added to feed. This is a much less time-consuming process than pulling individual sick calves and treating them independently of the herd.

A healthy calf with a strong immune system should be able to fight off serious infection on its own. The help of a metaphylactic agent may nip in the bud many diseases that are lurking but haven’t taken hold. This is assuming that the calf actually receives a therapeutic dose of the chosen drug. In cases when the metaphylactic is added to the feed, the consumption and dosage can be quite variable.

Consider a calf that is not fully weaned or is stressed and not eating. It may consume small amounts of treated grain, but not enough to achieve therapeutic drug levels adequate enough to treat the problem. This is when antibiotic-resistant strains of bacteria develop and create a bigger issue than the one originally being treated.

Unfortunately, while metaphylactic use may be effective against many bacterial diseases, it is usually not particularly helpful in the case of M. haemolytica. This is because of the longer incubation time of M. haemolytica. If a calf is exposed to Mannheimia haemolytica at the sale barn or upon reaching its destination, it still has about two weeks before it might show any signs of illness. In the case of a calf given a metaphylactic at the time of sale or when arriving at the feedlot, that drug’s efficacy will be wearing off at around the same time that the incubation period for M. haemolytica will be ending. This creates the perfect timing for the bacteria to multiply and flourish in the body of the calf.

The other piece of this puzzle is the effect antibiotics can have on ruminal health. If a calf already has a weakened immune system with poor ruminal development, the likelihood that the metaphylactic agent used will destroy any remaining gut flora and thus throw the calf into scours and/or acidosis increases exponentially.

Why Immune System Development is Key in Preventing M. haemolytica

As previously mentioned, around 80% of a calf’s immune system lies in its rumen. Much of this immunity is received from its mother until it is naturally weaned. Because most calves must be prematurely weaned, this puts them at a disadvantage for developing a mature, fully functioning immune system.

The importance of a heifer’s health cannot be stressed enough when it comes to giving calves the best possible chance to start out healthy and realize maximum gains. A heifer that is depleted or experiencing SARA is not going to be able to put the necessary resources towards the development of her calf. Her immune system will be stressed, meaning the antibodies she is passing on, both in-utero and post-partum will not be nearly as potent as those that come from a healthy dam.

The milk quality will also be compromised – vital nutrients and other components that encourage proper development of a calf will be lacking. This makes it likely that the calf will require more milk to get its minimum requirements met, further depleting the dam.

Properly preparing a heifer for calving can save time and vital resources. A proper vaccination program for all cows is essential to help guarantee a calf’s latent immunity. This helps not only prevent scours in the post-natal calf, but also allows them to receive adequate antibodies throughout the duration of their nursing time.

Other Ways to Reduce the Prevalence of Shipping Fever and M. haemolytica

While there is no magic bullet for completely eliminating shipping fever or M. haemolytica, the risks can be greatly reduced.

The top way to keep the rumen functioning (and thus the immune system) is to get calves eating and drinking right away. They can be on transport trucks anywhere from a few hours to a few days without access to feed or water. During this time their ruminal health is incredibly compromised, predisposing them to acidosis and scours. As the acid concentration in the rumen increases, the appetite wanes, making it more and more difficult to get calves to consume feed and water. The best way to combat this is to bring the ruminal pH back into balance, thus stimulating the appetite.

It’s also important that a calf’s immunity start being built from day one. This includes ensuring that it is receiving adequate amounts of high-quality milk from the mother and not being exposed to stressors such as weather extremes, crowded living spaces, dirty environs or being subjected to improper or excessive handling. This naturally keeps the pH in check; when a calf is exposed to stress, administering buffering agents can stop the SARA process in its tracks.

Ensuring that the immunity continues to function benefits an animal in two ways – it keeps the calf on the right track for proper immune development and increases the efficacy of vaccinations. The chances of a healthy calf developing shipping fever and subsequent M. haemolytica go down markedly when a good care protocol is followed.

While it’s not possible to ensure that every calf is completely free of disease, it is possible to minimize the impact of shipping fever and M. haemolytica on the overall herd. The fewer animals that become sick, the greater the realized profits come harvest time. Prudence in antibiotic use and a keen eye on ruminal health can make the difference between a healthy, productive herd and one that sees more losses than gains.

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