Respiratory diseases in young animals
Enzootic pneumonia in young calves is a multifactorial disease that occurs mainly in two different systems: in housed dairy calves reared for replacement or in housed calves reared for beef in a herd other than the herd of origin. Dairy calves are likely to suffer from the disease at any age, with it manifesting itself as a chronic, coughing pneumonia, or as a more acute, enzootic calf pneumonia. Older dairy calves are also vulnerable after housing in the autumn. Suckler calves are more likely to suffer from respiratory disease between two and five months of age, following weaning or transport from one herd to another. Outdoor reared beef suckler calves can also be severely affected by pneumonia.
In older calves, mainly in weaned suckler calves aged six months to two years, respiratory disease is likely to occur after transport or other environmental stress and is often called shipping or transit fever. This condition is discussed elsewhere in this compendium. Similarly, a respiratory disease caused by lung parasites, husk, occurs in older calves and is discussed elsewhere. A viral respiratory disease caused by the infectious bovine rhinotracheitis virus is also more significant in older animals and discussed under its own heading.
Respiratory diseases in young animals were ranked very low in importance in a survey of British organic beef and dairy farmers. Late weaning, a whole milk diet, the requirement for good housing standards and a closed herd policy reduce the risk factors for respiratory disease in calves in organic dairy herds. The Soil Association standards further prevent the sale of calves via the cattle markets when calves are sold for fattening. This should reduce the stress of moving from one farm to another and the risk of acquiring new infections.
Enzootic pneumonia in calves
Enzootic pneumonia in young calves may be a chronic disease with very few clinical signs apart from a dry cough and slightly increased respiratory rate. The acute form of the disease usually manifests itself in an outbreak involving several calves going down with the disease within a 48-hour period. Fever, dullness, inappetance and coughing, often combined with nasal discharge, are the most common symptoms.
There is very little data available on the prevalence of enzootic pneumonia in UK cattle herds, as recording of calf diseases is seldom carried out. Respiratory diseases are, however, considered the second most important cause of death and ill drift in calves. The condition is farm related, with some farms suffering serious losses due to calf pneumonia, while on others the disease is either very mild or non-existent. Sporadic outbreaks can, however, be experienced by farms that normally see very little respiratory disease in calves.
Causes of enzootic pneumonia
Enzootic pneumonia in calves is a multifactorial disease. Infectious agents, environment, management and the immune status of the calves are all-important factors in determining the outcome of an infection.
A multitude of infectious agents, including viruses, bacteria and Mycoplasma, are involved in different combinations on different farms. It is often suggested that the viral and mycoplasmal agents are the primary infections and the bacterial agents cause a secondary infection in an animal whose defences have been weakened by the first infection. The most common viral agents isolated from enzootic pneumonia cases are respiratory syncytial virus (RSV), parainfluenza III virus (PI3), infectious bovine rhinotracheitis virus (IBR) and bovine viral diarrhoea virus (BVD), some of which are discussed separately as herd problems elsewhere in the compendium.
Mycoplasmal agents are usually considered to be the most common agents causing the chronic form of enzootic pneumonia, even though Mycoplasma bovis has been identified as the causative agent in many acute outbreaks as well.
The most commonly isolated bacterial organisms are Pasterurella and Hemophilus subspecies.
The main environmental factor predisposing calves to respiratory disease is poor ventilation in calf housing. Cold, humid conditions, sudden changes in air temperature, stress due to different causes and change in the environment have also been associated with outbreaks of pneumonia in young calves. Inadequate intake of colostrum or poor quality colostrum will affect the calves’ defence against respiratory agents and make them more susceptible to infection. Weaning of calves before five weeks of age has been associated with increased respiratory disease. Rearing systems where calves of different origin are mixed together at a young age suffer from high levels of respiratory diseases. Large , shared air spaces, calves from different age groups and poor sanitation between calf batches often make these systems even more vulnerable. Calves that have suffered from diarrhoea are also more likely to suffer from respiratory disease. The stress associated with management procedures such as disbudding and castration may also be associated with a high respiratory disease incidence.