Typically, when first introduced into a herd, CBPP is severe and mortality relatively high. A small proportion of cattle may die rapidly without showing any signs other than fever. It may be possible to link the onset of disease to previous contact with other cattle three to six weeks earlier, but this is not always the case as the incubation period can be as long as six months. Clinical signs may become apparent only several months after the contact. The disease can therefore become established in a herd before it is noticed, and tracing back to the origin can be difficult. This is particularly so where routine vaccination has been practised with long intervals between campaigns, and where antibiotics have been used to treat clinical cases. Both reduce the incidence of clinical disease, making its recognition more difficult.
After some time, the disease in the herd becomes chronic, clinical signs become less severe and the mortality rate falls. However, losses continue to occur. Not all the animals are affected in the same way and often the disease has a chronic course from its onset.
The hyperacute form, involving up to 10 percent of infected animals, may be observed at the onset of an outbreak. Death is sudden and is often without any other signs. The acute form is observed in approximately 20 percent of diseased animals. The course is usually from five to seven days. The earliest signs are a sudden onset of fever to 40°C or more and, in milking cows, a drop in milk yield. Sick cattle tend to isolate themselves from the herd and may stop eating.
A typical respiratory disease develops; breathing is laboured and obviously painful. Abdominal breathing with a respiratory rate of 50 to 55 breaths/minute may be seen and cattle may “grunt” when breathing out. Some animals develop a shallow, dry and painful cough, particularly noticeable on exercise. Application of pressure between the ribs is painful and resented by affected cattle, which sometimes react violently. On percussion, the ventral part of the chest sounds dull owing to the presence of fluid in the chest cavity. Acutely affected cattle stand with head and neck extended and forelegs spread apart (Plate 1), with dilated nostrils and mouth open panting for air. There may be nasal discharge, sometimes streaked with blood, and frothy saliva accumulates around the mouth. Some animals develop swellings of the throat and dewlap. Pregnant cows and heifers may abort, and diarrhoea has been recorded.
The subacute form occurs most frequently in about 40 to 50 percent of the animals affected. The symptoms resemble those of the acute form, but are less severe and fever is intermittent. This form usually becomes chronic.
The chronic form is a natural evolution of both acute and subacute forms but in some animals it may develop directly. The clinical signs regress but cattle can still have intermittent fever, together with loss of both appetite and weight.
Calves in the first six months of life more often show lameness from swollen, hot, painful limb joints.
The mortality rate is variable, rarely exceeding 50 percent, and depends on a range of factors, such as age, breed, nutrition, presence of other infections or infestations, and the type of management.
Many affected cattle appear to recover fully, yet the lesions in the lungs take a long time to heal completely. The causative agent can survive for as long as two years within the lesions. Up to 25 percent of affected cattle can become chronic carriers of infection. They are often referred to as “lungers” and are believed to play a role in initiating new outbreaks when they are introduced into susceptible herds.
In summary, look for one or more animals with:
Any chronic (persistent) mild cough in cattle otherwise appearing normal or losing weight should be a reason to suspect CBPP.
