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Also known as

Transit fever, shipping fever, pneumonic pasteurellosis, summer pneumonia


Pasteurellosis is most commonly seen in recently weaned beef calves, especially after transport. This disease is commonly associated with mixing and confining calves from multiple sources at cattle markets or new destinations—such as a feedlot--and is often seen within a couple of weeks after arrival.

The primary causative agents of pneumonic pasteurellosis are two bacteria: Mannheimia (Pasteurella) haemolytica (which some people feel is the most important one) and Pasteurella multocida.

The term pneumonic pasteurellosis refers to infection of the lungs with organisms of the genus Pasteurella. This term has been in use since the 1960s, referring to all pneumonias caused by Pasteurella haemolytica and Pasteurella multocia.

With the reclassification of P. haemolytica biotype A strains as Mannheimia haemolytica, pneumonic pasteurellosis is no longer an appropriate name for pneumonia caused by both agents but is still used by many people.

The terms pneumonic mannheimiosis and pneumonic pasteurellosis are more accurate--to differentiate the diseases caused by the two bacteria, M. haemolytica and P. multocida, respectively.

In some herds younger calves—still nursing their dams—develop pneumonia before weaning age, a condition called summer pneumonia. These are usually infections with multiple pathogens that may include bacteria as well as viruses. The bacteria may be Mycoplasma bovis and/or Pasteurella multocida.

These are both very common in the upper respiratory tract and nose of healthy calves, but if a calf has a viral infection or some environmental stress, those factors may open the way for these opportunistic bacteria to cause disease, resulting in pneumonia.

Calves with acute disease have a fever, rapid breathing and nasal discharge. Some deaths may occur and many of the sick animals remain dull and off feed for a while after recovery, resulting in significant economic losses in lower growth rates. Calves may also progress to chronic pneumonia, and these animals don’t make it to finish in a feedlot.

Early signs are often vague--with slight depression and lack of appetite--but can develop into more severe depression and refusal to eat, with fever, labored breathing, and rapid weight loss. A cough may or may not be present. Nasal discharge starts out as mostly mucous and develops into a purulent (pus) discharge.


  • Fever,
  • Dullness,
  • Rapid breathing,
  • Nasal discharge


Mannheimia haemolytica (formerly called Pasteurella haemolytica) is usually the primary pathogen causing shipping fever. There are two recognized serotypes (types A1 and A6) that cause disease.

These bacteria are found in the nasal passages of healthy carrier animals, with outbreaks of disease precipitated by stress and mixing of animals. Pasteurella multocida is also carried by healthy animals and is usually a secondary opportunistic pathogen, invading airways that have been damaged by viral or other infections.
Mannheimia haemolytica is present in the nose and sinuses of normal, healthy cattle and is readily available to become an invader. It can move down the windpipe and into the lungs if the cilia lining the windpipe are damaged by viruses. If these bacteria get down into the lungs, they cause pneumonia.

Although these organisms can cause disease in young calves, pneumonic mannheimiosis and pasteurellosis are a much greater problem in recently weaned beef calves shortly after arrival in a feedlot or after a long transport—as one component of shipping fever or bovine respiratory disease (BRD).

Economic losses to North American feedlots due to respiratory disease have been estimated as high as one billion dollars annually, and losses to the beef industry in the UK have been estimated at 70 million pounds annually.

Morbidity rates (number of animals in a group that get sick) of 15 to 45% and mortality rates of 1 to 5% are common in newly received feedlot calves. Studies have shown that about 75% of the morbidity and 50 to 60% of the mortality in feedlot cattle are due to respiratory disease.

M. haemolytica is the main bacterial pathogen involved in beef cattle respiratory disease. P. multocida is less frequently involved, but is often the major pathogen in young dairy calves that get pneumonia.

Mannheimia haemolytica and P. multocida are Gram-negative, non-motile, non-spore-forming bacteria. Pasteurella multocida has been classified into 5 serogroups (A, B, D, E and F). Most P. multocida bacteria isolated from bovine respiratory disease are the A serogroup.

The importance of M. haemolytica and P. multocida in pneumonia of feedlot cattle was suspected in early studies because there bacteria where frequently found in clinical cases and necropsy specimens. There were some doubts, however, regarding whether these agents were primary pathogens, due to difficulty in reproducing the disease experimentally, and because bacterins (vaccines using these bacteria) did not provide protection.

Identification of numerous pathogenic bovine respiratory viruses that cause lesions or impair the clearance of bacteria from the lower respiratory tract (by damaging the cilia lining the respiratory passages) led to the view that infection with viruses or other bacteria like Mycoplasma bovis was a necessary prelude to pneumonia involving M. haemolytica or P. multocida.

These bacteria are probably just one part of a complex group of bacteria and viruses which together cause shipping fever and fibrinous pneumonia—an aspect of pneumonia characterized by insoluble proteins in the lungs, formed from fibrinogen during clotting of blood. This forms a fibrous mesh that impedes blood flow. Caused by Mannheimia haemolytica, this is the most common lesion associated with BRD (bovine respiratory disease) in feedlot cattle.

Pasteurellosis occurs most commonly in weaned calves (usually 6 to 8 months of age) shortly after being weaned and placed in feedlots. Peak incidence of this disease is within 3 weeks of arrival at the feedlot. There is usually a history of stressful events associated with shipping fever, which include recently weaned calves, transportation, mixing groups of cattle from different sources, confinement, or inadequate shelter or ventilation in barns.


Prevention of pasteurellosis and shipping fever involves multiple strategies, which include stress reduction and careful preconditioning calves at weaning time. Calves moved directly from ranches to feedlots, regardless of their vaccination status, experience less stress and have lower morbidity and mortality--and better weight gains--than calves purchased from auction markets where they are mingled with calves from many sources before being shipped to the feedlot.  

Vaccination against Mannheimia haemolytica and Pasteurella multocida, along with other respiratory vaccines as part of a preconditioning program, can be of benefit when weaning calves.

The key to getting a good immune response to vaccination is to vaccinate before any stressful events occur, such as weaning and transport to feedlots. Low-stress weaning methods such as fence-line pasture weaning and two-stage weaning with nose flaps greatly reduce the incidence of disease in weaning-age calves.

For best vaccination results, veterinarians recommend two doses initially, 2 to 4 weeks apart.  Animals that are vaccinated young (less than 3 months of age) should be revaccinated at weaning or at 4 to 6 months of age.  

To prevent Pasteurella in young calves, choose a vaccine that protects against both types of Pasteurella and vaccinate them at about 1 to 2 months of age. This won’t protect them clear through weaning however; they need a booster just before weaning to provide protection through their first winter.  

If you only give one Pasteurella vaccination during calfhood it’s usually best to do it a couple weeks before weaning.  Most pneumonia vaccines are designed for weaning-age calves, to protect them during the stressful period of weaning. Discuss a vaccination program with your herd health veterinarian to design a program that best fits your situation.


Catching the disease in the early stages is key to successful treatment. Your veterinarian can advise you on the proper antibiotics and dosage. If the disease has progressed to more advanced stages, poor response to antibiotics can be expected, due to greater lung damage, resulting in loss of functional lung tissue. Some of that damage can be permanent even if the animal survives.

About the Author

EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.