Mycoplasmal pneumonia in calves

Vet Times The website for the veterinary profession https://www.vettimes.co.uk Mycoplasmal pneumonia in calves Author : Adam Martin Categories : Vets Date : November 2, 2009 Mycoplasma bovis is responsible for 25-35% of the economic losses associated with calf respiratory disease in Europe. The situation in the UK is similar to the majority of Europe. With conservative estimates attributing 25% of all calf respiratory disease costs to M bovis, the scale of these losses is enormous. In 1999, respiratory disease in UK cattle was estimated to cost the industry 54m. In addition, almost 150,000 calves die from pneumonia and related diseases annually; these calves have a potential market value of approximately 100m. Incredibly, despite its high prevalence and considerable economic impact, it is often overlooked as a cause of calf pneumonia. M bovis is a bacteria that belongs to the Mollicutes (a name derived from the Latin mollis soft and cutis skin) class, because they are enveloped by a complex plasma membrane, rather than the more typical cell wall observed in other classes of bacteria. When compared to other bacteria, they are tiny in size and are often found at mucous membranes in very close association with host cells. This is because of the limited biosynthetic capability that is allowed for by their correspondingly small genome. Despite this, Mycoplasma are capable of considerable immunomodulatory activity to help them evade their host immune response. Mycoplasmas can be primary or opportunistic pathogens, and are frequently commensal bacteria. M bovis was first recognised as a cattle pathogen almost 60 years ago, when it was isolated as a cause of mastitis in an American dairy cow. Since then, the pathogen has spread (via cattle movements) to many countries around the world, including most of Europe, arriving in Great Britain in 1975, Northern Ireland in 1993 and the Republic of Ireland in 1994. It is not the only Mycoplasma to have been isolated from cattle, either healthy or diseased. However, after M mycoides subspecies mycoides (the causal agent of contagious bovine pleuropneumonia), M bovis is the most important pathogenic mycoplasmal species in cattle. Some of the other Mycoplasma species isolated from cattle in the UK are listed below. Generally, these are commensal organisms that can opportunistically spread to the lungs when a calf experiences pneumonia. 1 / 7

M alkalescens M arginini M bovoculi M bovigenitalium M bovirhinis M californicum M canis M dispar For a long time, the role of M bovis as a pathogen in calf respiratory disease was disputed. This was probably because the bacteria exists as a commensal in many healthy calves upper respiratory tracts, combined with several pathogens often involved in disease outbreaks. In fact, in outbreaks of calf pneumonia caused by M bovis, the bacteria Mannheimia haemolytica, Pasteurella multocida and Histophilus somnus are often isolated before M bovis. Practical difficulties in culturing mycoplasmal species means that many laboratories will not perform mycoplasmal cultures unless specifically requested by the veterinary surgeon in the field. Clinical disease Clinical disease in young calves caused by M bovis infection typically presents as pneumonia, otitis media, arthritis or a combination of the above. The pathogen has rarely been associated with tendonitis, meningitis and abscessation. Mycoplasmal disease has been reported as occurring as early as four days after birth in calves, although more typically it is seen at one month of age (two to six weeks). Disease can be either acute or chronic, although the latter predominates as the diseases are often unresponsive to therapy. The clinical picture seen with M bovis-associated pneumonia is the same as one would expect with typical enzootic pneumonia. Elevated respiratory rate and efforts, pyrexia and inappetence are observed as the presence of a nasal discharge and coughing. Bronchial wheezing and crackling can be heard on auscultation of the lung fields. In chronic cases, areas of consolidation can be identified in the cranioventral lungs. Mortality rates of approximately 10% can be expected in M bovis-associated pneumonia outbreaks. Often, surviving calves will show poor weight gains after the acute phase has passed; many remain retarded until they are culled. Infrequently, the clinician will receive additional clues as to the aetiology of the disease outbreak, as individual calves will also develop either arthritis or otitis. Macroscopic and microscopic lesions found in the respiratory tract of calves experimentally infected with M bovis vary considerably. Generally, naturally infected calves exhibit an exudative bronchopneumonia, multifocal pyogranulomatous inflammation with centres of caseous necrosis and an extensive occurrence of foci of coagulative necrosis surrounded by inflammatory cells. 2 / 7

Fibrinosuppurative tracheitis has been reported. Chronic infections are often characterised by peribronchiolar lymphohistocytic cuffing, thickening of alveolar septa and atelectasis. Diagnosis The lack of specific pathognomonic M bovis pneumonia signs in live calves necessitates the use of laboratory diagnostics. Bronchoalveolar lavage offers a more reliable form of sampling than nasal swabbing for the majority of respiratory pathogens, and M bovis is no different. Prior contact with the laboratory that you intend to send the samples to (if you suspect mycoplasmal infection) is advisable, to check that the lab has the capability to look for the bacteria, and to discuss transport details. Isolation of M bovis may not be possible from a bronchoalveolar lavage sample, or even at postmortem examination of the lungs, for a variety of reasons. Commercial ELISA kits are available for the detection of M bovis by serological means. Alternatively, immunohistological techniques are available for use in the postmortem examination of lung tissue. Treatment Treating M bovis-associated pneumonia is notoriously difficult. Mycoplasmal species are not susceptible to beta-lactam antimicrobials, as they lack a cell wall, or sulphonamides. A number of pharmaceutical products in the UK have specific data-sheet claims to be effective against mycoplasmal pneumonia in cattle. Generally, these products are fluoroquinolones, although tylosin also has a claim. Additional antimicrobial products have a theoretical basis for action against Mycoplasma, but, to my knowledge, no specific licensed claims against M bovis or mycoplasmal pneumonia in cattle. The use of fluoroquinolones in veterinary medicine has always been a controversial issue, one that is now particularly topical after the comments of the chief medical officer, Sir Liam Donaldson, earlier this year. Worryingly, there is evidence that antimicrobial resistance to products traditionally used to treat Mycoplasma infections is increasing in Europe and North America. European isolates have shown an increasing resistance to tetracyclines, spectinomycin and tilmicosin. It is worth remembering that in-vitro test results do not necessarily reproduce themselves in vivo, and mycoplasmal pneumonia is regarded as being extremely difficult to treat effectively. Disease risk factors To instigate effective control measures, one must first understand a little about the transmission and risk factors for the disease. Generally, it is believed that M bovis is introduced to a farm by a purchased, clinically normal carrier animal. Calves have the potential to become infected in one 3 / 7

of several ways. The first opportunity for infection arises in the calving pen Mycoplasma are known to infect the reproductive tract, and exposure to vaginal or respiratory secretions shortly after birth is a possible route of infection for the newborn. Furthermore, in adult cattle, M bovis is known to colonise the udder (and was first isolated from a case of mastitis), so it is clear that colostrum also represents a theoretical exposure risk. At the moment, the exposure is indeed theoretical as, to my knowledge, it has never been isolated from colostrum despite researchers best efforts to find it. While infections at calving have been described, along with congenital infection, it is believed these routes play a minor role in the epidemiology of the disease. It is known that M bovis is a mastitis pathogen, and it is believed that the major means of transmitting the pathogen to young calves is through the feeding of infected milk. The fact that milk is frequently pooled from several cows before being fed to calves means the efficiency of pathogen spread is high, and that a single shedding cow can rapidly infect many calves. The likelihood of this occurring is even greater, as calves are often fed waste milk milk from cows that have high cell counts or have undergone antibiotic treatment. By employing this selection process for the feeding of calves, we are effectively selecting the cows most likely to be shedding pathogens to inoculate the calves. Calf-to-calf spread is clearly a potential route of infection. While relatively little is known about the duration and level of excretion in young calves, it is reasonable to believe that, after infection, the calves are likely to shed for a period of time. In the same way as contact with infected adults will increase the likelihood that a calf has Mycoplasma in its upper respiratory tract, it is reasonable to assume the same will happen with a calf. Therefore, spread within a group is a potentially important pathogen spread method. Spread between calves could also occur indirectly via fomites. It is known that Mycoplasma can be transmitted between udders by fomites. Consequently, care must be taken to ensure the same does not happen to calves through the use of poorly disinfected feeding buckets, teats etc. Perhaps more importantly, as with other causes of calf pneumonia, infected animals shed considerable amounts of pathogen into the air. Consequently, sub-optimally ventilated, overstocked and poorly designed buildings housing calves with M bovis typically have very high levels of airborne pathogens, which increases the likelihood of calf-to-calf spread. Despite the fact that Mycoplasma lack a cell wall, they are not fragile when free living in the environment. It has been demonstrated that M bovis can survive for two weeks in wood or water in calf pens, 20 days in straw, and more than two months in sponges and milk at 4 C. Increased temperatures reduce survival times, but as pneumonia is most commonly a winter disease, temperatures considerably below 10 C can be expected at the risk time of year. Herd size, in addition to a history of purchased cattle, are the only two commonly identified risk factors for mycoplasmal mastitis. Therefore, it is reasonable that these two risk factors will also be applicable to mycoplasmal pneumonia. In a national study undertaken in the USA, M bovis was 4 / 7

cultured from bulk milk tank samples, from one in five herds with more than 500 milking cows. This dropped to one in 25 farms with cow numbers between one and 400, and to one in 50 farms with a herd of less than 100 cows. It is logical that a similar pattern would be found were similar research applied to mycoplasmal pneumonia. In addition to the specific risk factors for M bovis-associated pneumonia, the general risk factors for enzootic pneumonia also apply. These include ensuring that calves receive sufficient colostrum to confer adequate passive immunity, and that they are not subsequently placed under nutritional stress. Additionally, buildings should be constructed such that adequate ventilation can be achieved without the calves experiencing draughts and fluctuating temperatures. Calves should not change groups, or be kept in too large a group size, and should not share air space with older animals. Prevention The difficulties encountered in both the diagnosis and treatment of M bovis-associated pneumonia mean that prevention is certainly better than cure. Vaccinations are not available in the UK. Consequently, management procedures that minimise the risk of exposure to the pathogen, and prevent the onset of disease if the pathogen is present, should be employed. These begin by ensuring the environment in which the calves are housed is not a risk factor for disease (adequate air flow, stable temperature, correct humidity etc). The calves immune function should be maximised by ensuring adequate colostrum provision to all calves and, where possible, controlling other pathogens, such as bovine diarrhoea virus. Stressful experiences that may affect the calves should be minimised; particular attention should be paid to nutrition, transport, group mixing and overcrowding. If M bovis is not present on the farm, adequate biosecurity should be employed. In practice, this is likely to mean maintaining a closed herd. Once M bovis is on a farm, the aim must be to help reduce the spread of the pathogen. It is important that, as far as possible, calves are not fed M bovis. This means that colostrum should not be pooled before being fed to calves, and that either milk replacers or pasteurised whole milk is fed to unweaned calves. Buckets, pens and other equipment should be kept to individual animals/groups and adequately cleaned after use. Preventing exposure to sick calves will also help reduce disease spread. This involves creating a hospital into which sick animals are immediately placed (not in the same airspace as the healthy calves). Furthermore, when handling sick calves and helping calves to drink, disposable gloves should be worn. It is also logical to always handle the youngest calves first, progressing to the older calves, 5 / 7

thereby reducing the chance of spreading pathogens between the age groups. Conclusion M bovis-associated pneumonia is a common and under-diagnosed condition in the UK. Both diagnosis and treatment are problematic. In fact, one of the signs of M bovis-associated pneumonia is a poor response to antimicrobial therapy. As a vaccine is not available in the UK, sensible preventive measures are advised. First and foremost, these involve preventing the pathogen from arriving on farm by maintaining a closed herd. If this is not possible, or the pathogen is already on farm, the goal should be to minimise calf exposure to M bovis and maximise immune defences. This can be done by following a number of guidelines, the majority of which are generic for the control of calf enzootic pneumonia. References are available upon request to the editor. 6 / 7