28 Aug 2017
Adam Martin looks at various strategies to deter calf respiratory disease, including housing assessments and ventilation needs.
Adam Martin
Job Title
Image © CLM-bv / Pixabay
Calf respiratory disease (CRD) is a general term used to describe a number of different diseases, which essentially result in bronchopneumonia in animals typically aged between two and six months old. Its impacts are huge, both in terms of production (calf mortality, calf growth and long-term production) and welfare.
CRD is a complex, multifactorial set of diseases – the major risk factors and causal pathogens of which are relatively well understood by vets. Yet, despite this, it remains a common, and often ignored, disease on UK cattle farms.
Some studies have shown more than half of all treatments recorded as being given to calves on UK farms are treatments for CRD. For practical and farmer compliance reasons, CRD treatments often involve the use of “long-acting” antimicrobials, which some workers believe can facilitate the development of antimicrobial-resistant bacteria. Reducing the number of cases of CRD could, on certain farms, significantly reduce use of long-acting antimicrobials – and, therefore, the principles of responsible antimicrobials demand prophylactic measures are put in place to control CRD.
CRD is endemic on UK cattle farms; however, despite this, clear figures for incidence and prevalence are hard to find. Definitions between studies vary, as does farmer-defined CRD compared to veterinary-defined CRD.
Inter-herd variation in prevalence is large and in-farm incidence can range from one week to almost 1,005 a couple of weeks later.
It goes without saying that the point at which the “odd cough” becomes a clinical syndrome worthy of treatment also varies considerably between stockmen.
Although this question has been studied quantitatively, and various scoring systems have been produced to aid producers and vets intervention decisions, it is likely many treatments are unrecorded and, unless a conscious effort is made by a vet to actively follow up youngstock – particularly in the busy winter months – problems with relatively large, long-term economic problems can be missed.
CRD has a multifactorial aetiology involving complex interactions between the host, its environment and pathogens. A number of CRD pathogens are ubiquitous and only cause disease when the calf is under stress. Generally limiting the known risk factors will effectively control the CRD.
While this is very practical, as it allows treatment for the general diagnosis of CRD, it also limits the amount of time and energy put into establishing the cause in individual cases and outbreaks – particularly as a wide range of possibilities means establishing the definitive cause is, at best, difficult, and often virtually impossible under commercial conditions.
Viruses are regarded as the primary pathogens of CRD, despite the fact the secondary colonisers are often the most pathogenic. Viruses predispose the lung to secondary infection by, among other things, suppressing lung macrophage activity.
Viruses frequently implicated in the pathogenesis of CRD include:
The latter two viruses are generally only implicated in CRD when causing a problem in the adult herd.
A number of non-viral pathogens have been shown to act as the primary cause of CRD. Mycoplasma species and Ureaplasma species can be primary or secondary pathogens. Pasteurella multocida, Mannheimia haemolytica and Histophilus somni are commensal organisms of the nasopharynx, and the most important bacterial CRD pathogens. In the unstressed calf, respiratory defences (such as filtration, particle removal, secretary and cellular defences) prevent their spread to the lungs. However, stressors – including transportation – can compromise these defences, leaving the calf “open” to opportunistic infection.
Identification of the major pathogens involved in a CRD outbreak is important to correctly target therapy (improving success rates and reducing costs), and allow the development of an effective disease prevention programme. A number of diagnostic techniques can be used. Nasopharyngeal swabs are effective in identifying bacteria agents, although are ineffective in detecting viral causes of CRD (except infectious bovine rhinotracheitis). Identification of viral pneumonic pathogens can be improved if bronchoalveolar lavage is performed – particularly if the sample can reach the laboratory within two hours of collection.
Comparison of serological samples collected at the onset of the outbreak and two to four weeks later remain “the gold standard” to identify viral causes, along with Mycobacterium bovis and H somni.
However, paired serology is slow, can be difficult to interpret – as maternally derived antibodies may interfere with interpretation for four months – and is relatively expensive. Best diagnostic results are obtained by submitting an entire fresh carcase for postmortem examination at a veterinary laboratory.
The endemic nature of CRD pathogens means many calves will have already been exposed to them in the first two months of their life. This means it is essential farmers concentrate on making sure their calves receive adequate colostrum in the correct environment to allow for the successful transfer of passive immunity to the calf to occur.
A newborn calf requires at least 100g of Igs to be absorbed from its first meal if the calf is to have the best chance of developing adequate passive immunity. The quality of dairy – particularly Holstein – colostrum is generally such that if a calf is to receive 100g of Igs, 3L to 4L must be consumed by the calf. In practice, this is an enormous volume for a newborn calf to manage to ingest by sucking alone.
Consequently, the majority of dairy calves left to suck do not ingest an adequate mass of Ig to successfully acquire immunity. As Ig is most efficiently absorbed from the intestines at the first meal, it is best practice to provide a one-off bolus of 3L to 4L of colostrum by tube before the calf begins to suck.
Beef suckler cows often have higher quality colostrum than dairy cows. This, combinedwith practical health and safety reasons,means unless a suckler calf is at increased risk of failed passive transfer of immunity – for example, experienced dystocia – interferenceis not advised.
Colostrum is not only important for passive transfer of immunity, but also an important nutrition source. Adequate nutrition is essential if CRD is to be prevented. Naturally enough, this includes micro minerals and macro minerals – particularly those associated with immune function, such as vitamin E and selenium.
However, the same is true for the diet’s major nutritional components, such as energy and protein, as without adequate supply of these, the immune system will also be compromised. Unfortunately, many young dairy calves are fed inadequate amounts of milk as traditional twice-daily feeding practices are still common – partly due to ignorance and partly due to the reality of time budgeting on dairy farms.
The calf’s environment is critical with respect to CRD – in particular, stocking density, group mixing and the age of animals in the same airspace. Ideally, the number of calves in a group should not exceed 12 and the number of calves in a given airspace should not exceed 30 animals. Increasing numbers beyond this will increase the infectious pressure on the group and the effect of any CRD outbreak that occurs. Calves in each group, and air space, should not vary considerably in age or origin.
Mixing calves of differing age groups increases stress for the younger animals as they are exposed to greater competition for food and other resources, which can lower their immune status. Furthermore, risk exists that older animals may be asymptomatic carriers of pathogens, which could infect younger calves in the group. Likewise, mixing of calves from different origins should also be restricted to a minimum due to the possible spread of pathogens from immune animals to naive ones.
The ventilation of calf housing is of critical importance. The air inside a barn can carry as many as 106 organisms/m3. Therefore, the air in calf housing should completely change a minimum of six times per hour and each calf (up to 90kg) should be afforded greater than or equal to 10m3 air, if CRD is to be avoided. The airflow through a barn can often be assessed by using one’s senses. The presence of cobwebs between lights or around the rafters indicates a poor airflow, as is the striping of rafters due to condensation forming on the roof and running on woodwork. Noticeable smells of ammonia also indicate it is unlikely air is circulating as it should, though bedding should, of course, also be checked.
The survival time of airborne CRD pathogens is shortest when relative humidity is 55% to 75%. Air humidity is likely to change with the weather and stocking conditions. It is important to note a proper environmental assessment can only be undertaken when the housing is stocked. Not only do animals directly influence the amount of moisture in the air, but they also heat the air, which causes the “stack effect” by warmer air rising. Consequently, an understocked or empty shed may well have a lower turnover of air than one correctly stocked.
When investigating calf housing, it is important to pay attention to the area the calf is in – that is, its level on the floor. Cold, damp and draughty environments are risk factors for CRD, as these can reduce the action of the innate immune defences in the upper respiratory tract. High concentrations of ammonia also have a similar effect. Therefore, the investigator should sit or kneel at calf level around the calf housing if he or she is to make an assessment of the environment the calf is in.
Once a picture of calf management from birth, pathogen identification, stocking practices and environment has been built up by the investigating clinician, immunoprophylactic products can be used as necessary. Many people have experienced that use of vaccines alone will not normally resolve a CRD problem. In fact, good protocols need to be in place to ensure calves receive the vaccine they have been prescribed the correct number of times, with an adequate supply chain to ensure product efficacy.
We often take these things for granted. However, clearly defined, repeatable, standard operating procedures tailor-made to each farm are essential if a vaccination programme is to be successful.
CRD is, and will continue to be, a major limitation on herd economic and welfare performance. Its avoidance requires a structured, proactive and dynamic approach to investigation and control. CRD’s complex multifactorial nature means staying on top of it is challenging, but together, vets, farmers and farm workers can keep it at bay.
