RESPIRATORY DISEASES ARE A MAJOR welfare and economic issue in sheep-rearing countries. The economic losses are not only related to deaths, but also with condemnations, lower growths, downgrading of carcasses and treatment or prevention costs. These diseases can affect any age of animal, but they can present higher incidence in lambs aged over three weeks (Lacasta et al., 2008). The complexity of the disease lies in its multi-factorial nature. Respiratory diseases result from the interaction between the host, the agent (there are many agents capable of causing disease) and the environmental factors. For this reason, respiratory diseases are named as Ovine Respiratory Complex (ORC) in the same way as in other species such as bovine or porcine, as they cannot be understood as a singlefactor disease. According to the clinical presentation of the disease, we can differentiate three main lesional patterns at necropsy that could help us to guide where samples should be collected in order to get reliable results. Hyperacute forms are characterised by haemorrhages, mainly located in nasal cavities, epiglottis and retropharyngeal lymph nodes. In this case, samples should be taken from affected lymph nodes or from the brain if signs of infection can be seen on it. By contrast, the chronic forms are recognised by the presence of lung consolidation, which is normally accompanied by enlargement of the regional lymph nodes, while vascular phenomena are much less seen. In these situations, samples may be preferably collected from the consolidated area in the lung. Between these forms, acute presentations represent most of the cases and are characterised by a combination of both previous forms – lung consolidation, haemorrhages and fibrin deposition. Samples should then be collected from lungs, lymph nodes or the brain, according to the level of infection. The stressor factors are varied, but all of them are mainly associated with management, such as weaning, transport, presence of concomitant diseases, etc. All these processes have a repercussion over immune response, enabling the development of respiratory diseases.
Thermic stress
Climate and particularly thermic stress (due to cold or heat) is present in most of the small ruminant productive systems and it is probably the main
factor which may determine ORC development. This relevant role is particularly well-known in housing systems, where correct ventilation of the barn has demonstrated to be essential for the control of ORC. For instance, in feedlot lambs, the combination of immunosuppressive diseases, such as coccidiosis and viral infections, with improper building ventilation is seen as the most common cause of death in Mediterranean areas, related to respiratory diseases (González et al., 2016). Although several infectious agents have been associated with ORC, Mannheimia haemolytica (MH), Pasteurella multocida (PM), Bibersteinia trehalosi (BT) and Mycoplasma spp. are the most commonly isolated microorganisms in bacterial pneumonias and, generally, they are found mixed in the isolates with more than one bacteria species implicated (Gonzalez et al., 2016). Most of these bacteria
exist as commensal organisms of the nasopharynx, tonsil and lungs of healthy sheep (Glendinning et al., 2016). This is the reason why ORC should not be considered as an infectio-contagious disease, and in the same way the term “carrier” must be considered to describe the condition of these animals.
Treatment of respiratory diseases is one of the most frequent treatments
applied to sheep, as it was shown in a survey among sheep and goat farmers
and veterinarians in France (Gay et al., 2012). Even though the efficacy of antimicrobial treatments is good during an outbreak (Scott, 2011), their use in metaphylaxis is not a realistic way of controlling ORC. On the other hand,
the wide range of bacteria involved in the disease reduces the effectiveness of the antibiotic treatments. Moreover, antimicrobial resistance has been found in 72% of MH isolates and 50% of PM isolates of bovine respiratory diseases in the USA (Klima et al., 2014), and Mycoplasma bovis has proved to have acquired resistances for eight antimicrobial families over the last 30 years in France (Gautier- Bouchardon et al., 2014). For all these reasons, as veterinarians we must be careful in prescribing these treatments and voiding the prophylactic use of antibiotics in order to reduce the risk of developing
resistances. As prevention seems to be the key to our plan, vaccination and proper management and husbandry practices have to be present in successful ORC control. Vaccine choice has to be made carefully, taking into account
the bacterial species which are present in the farm and, in many cases, even its serotype (González et al., 2013), as some of them don’t offer crossprotection against each other. To be sure of its efficacy, it would also be advisable to know the type of vaccine we are working with, the antigens included and, if possible, the strain from which they were produced, as they frequently come from bovine isolates and could offer moderate results (Lacasta et al., 2015). In the same way, timing of vaccination has to be carefully determined according to the onset of the clinical signs on the farm. Finally, in order to perform the most suitable vaccination programme, we should remember that defenses against many of the bacteria involved in ORC are not well transmitted via colostrum; thus, if problems on the farm start early, affecting young lambs, it could be necessary to apply the first dose during the first days of life. Of course, all the boosters have to be given if we want to be sure of the success of the programme.
References
Gautier-Bouchardon, A. V., Ferré, S., Grand, D. l., Paoli, A., Gay, E. and Poumarat, F. (2014). Overall decrease in the susceptibility of Mycoplasma bovis to antimicrobials over the past 30 years in France. PLoS ONE 9, e87672.
Gay, E., Cazeau, G., Jarrige, N. and Calavas, D. (2012). Antibiotic use in domestic ruminants in France: results from surveys of practices among farmers and veterinarians. Bulletin épidémiologique, santé animale et alimentation 53: 8-11.
Glendinning, L., Wright, S., Pollock, J., Tennant, P., Collie, D. and McLachlan, G. (2016). Variability of the Sheep Lung Microbiota. Appl. Environ. Microbiol. 82: 3,225-3,238.
Gonzalez, J. M., Bello, J. M., Rodriguez, M., Navarro, T., Lacasta, D., Fernandez, A. and De las Heras, M. (2016). Lamb feedlot production in Spain: Most relevant health issues. Small Ruminant Research 142: 83-87.
Klima, C. L., Zaheer, R., Cook, S. R., Booker, C. W., Hendrick, S., Alexander, T. W. and McAllister, T. A. (2014). Pathogens of bovine respiratory disease in North American feedlots conferring multidrug resistance via integrative conjugative elements. Journal of Clinical Microbiology 52: 438-448.
Lacasta, D., Ferrer, L. M., Ramos, J. J., González, J. M. and De las Heras, M. (2008). Influence of climatic factors on the development of pneumonia in lambs. Small Ruminant Research 80: 28-32.
Lacasta, D., Ferrer, L. M., Ramos, J. J., Gonzalez, J. M., Ortin, A. and thenakis, G. C. (2015). Vaccination schedules in small ruminant farms. Veterinary Microbiology 181: 34-46.
Scott, P. R. (2011). Treatment and control of respiratory disease in sheep. Veterinary Clinics of North America. Food Animal Practice 27: 175-186.
