Enzootic pneumonia caused by Mycoplasma hyopneumoniae is a difficult disease to control and has stretched the minds of farmers, veterinarians and academicians for several decades.

On its own the mycoplasmal infection can be quite mild causing occasional coughing, depression of growth rate, feed conversion efficiency and lung lesions can be from zero to moderate, affecting about 25% of the lung experimentally. Under today's intensive production systems, the primary infection is frequently invaded by secondary bacterial infections, especially Pasteurella multocida which can lead to bronchopneumonia, acute respiratory signs (difficulty breathing) and even death, with the lung lesions doubling in size upto 50% of the lung. In addition over recent years, with PRRS virus and PMWS (circovirus type 2), the poor pig's lung receives such an overwhelming infection leading to a complete swamping, with 70-80% of the lung with consolidated lesions.

Mycoplasmas are immunosuppressive, i.e. they confuse the immune defence system in the lung to stop it destroying them. It takes upto 4 weeks for the lesions to develop; they then consolidate for a further 4 weeks and then progressively heal from 10 weeks, as the pig's own immunity/resistance develops. Normally a respiratory infection (such as the common cold) is cleared within 14 days. This immuno-suppression allows the secondary bacteria and other primary bacteria such as Actinobacillus pleuropneumoniae, Haemophilus parasuis and Streptococcus suis to gain easier access to the lung and, accompanied by the virus damage also to the immune defences, can lead to the over whelming infections we see today - a real respiratory soup.

Control in the past, apart from improvements in husbandry and the environment, was primarily with the use of antimicrobial products, but they had their limitations. If you medicate early to prevent the infection, say in the grower stage, they could be challenged in the finishing house and break down with the infection there. Protection only really lasted during treatment and for a short period afterwards. The mycoplasma was not completely eliminated and the pigs had not necessarily developed much immunity. This lead to the development of pulse medication and strategic medication programs to prolong the medicinal protection or to target the stress periods when disease was likely to flare up. Many antibiotics only treated mycoplasma and not the bacteria so a variety of combinations have been used to broaden the control effect.

Unfortunately the development of antimicrobials took several decades and there are few good comparative studies to directly compare the efficacy of the products. In Table 1, a number of different trials are presented and the percentage reduction of lung lesions is used to make the comparison. In the prevention studies the medication is usually given around the time of infection and in the treatment trials it is usually 10-14 days after infection.

Table 1. Comparison of various antimicrobials for the prevention and treatment of enzootic pneumonia
(M. hyopneumoniae) using lung lesion reduction (%)

Generally there is a better effect with prevention than with treatment. Combination use with chlortetracycline and the pleuromutilins (tiamulin and valnemulin) does give an enhanced treatment effect.

Mycoplasmal vaccines have been taken up tremendously in the UK over the last decade. Initially the two shot approach was adopted (see Graph 1), with combination vaccines with H. parasuis also being widely used. As the vaccines developed the single shot vaccines emerged (see Graph 2) giving similar protection with obviously half the labour. The piglet's immune system is not fully mature below 4 weeks of age but is still sufficient in most cases to develop an immune response, even in the first week of life, and give protection throughout the growing and finishing period. High levels of maternal antibodies may interfere with vaccinal response however. The duration of protection is the major advantage of vaccines over antimicrobials.

Graph 1. Comparative protection of two-shot vaccines - reduction in their lung lesion scores in comparison with an unvaccinated control (Thacker and others, 1998)

 

Graph 2. Comparative protection of one-shot vaccines
reduction in lung lesion scores in comparison with an unvaccinated control (Groth et al, 2001)

Vaccines do have some drawbacks, i.e. the labour involved and protection is against only that organism (M. hyopneumoniae) and not against the wide range of disease-causing bacteria that we seem to encounter today. Whichever approach is used on the farm there are bound to be some advantages and disadvantages but I fear respiratory infections will still be with us for some time to come.

 

References:
Burrows, M.R., Morgan, J.H., Tasker, J.B. and Martin, M. (2002) In-feed Aivlosin for control of experimental enzootic pneumonia in pigs. Proceedings 17th International Pig Veterinary Society Congress, Ames, Iowa, USA, 2, p. 141.

Groth, D and Rapp-Gabrielson, V. (2001) Evaluation of M+PAC in one and two dose-regimens against competitor one-dose Mycoplasma hyopneumoniae bacterins. Proceedings of the Allan D. Leman Swine Conference, Recent Research Reports, Volume 28 Supplement, University of Minnesota, USA, p41

Hannan, P.C.T., Bhogal, B.S. and Fish, J.P. (1982) Tylosin tartrate and tiamutilin effects on experimental piglet pneumonia induced with pneumonic pig lung homogenate containing mycoplasmas, bacteria and viruses. Research in Veterinary Science, 33, 76-88.

Kubo, M., Yamamoto, K., Ibayashi, T. and DeKeyser, H. (1990) Study on the effect of feed medication with lincomycin against artificial infection of mycoplasmal pneumonia in swine. Proceedings 11th International Pig Veterinary Society Congress, Lausanne, Switzerland, p. 90.

Miller, D.J.S. and Stipkovits, L. (1991) Recent advances in the control of enzootic pneumonia in pigs. Proceedings of the World Veterinary Congress, Rio de Janeiro, Brazil.

Morgan, J.H., Aitken, I.A., Collins, P. and Burch, D.G.S. (1996) Prevention of experimentally-induced enzootic pneumonia in pigs using the novel compound SDZ PMD 296 (Econor - Sandoz Ltd). Proceedings 14th International Pig Veterinary Society Congress, Bologna, Italy, p. 433.

Schuller, von W., Laber, G. and Walzl, H. (1977) Chemotherapeutische Untersuchungen mit 81723 hfu (Tiamulin), einem neuen pleuromutilin-derivat, an der experimentellen Mykoplasma-Pneumonie des Schweines. Deutsche Tierartztliche Wochenschrift, 9, 345-349.

Simon, F., Semjen, G., Dobos-Kovacs, Laczay, P. and Cserep, T. (1990) Efficacy of enrofloxacin against enzootic pneumonia in swine. Proceedings 11th International Pig Veterinary Society Congress, Lausanne, Switzerland, p. 96.

Stipkovits, L., Miller, D.J.S., Glavits, R., Fodor, L. and Burch, D.G.S. (2001) Treatment of pigs experimentally infected with Mycoplasma hyopneumoniae, Pasteurella multocida and Actinobacillus pleuropneumoniae with various antibiotics. Canadian Journal of Veterinary Research, 65, 213-222.

Thacker, E.L., Thacker, B.J, Boettcher, T.B. and Jayappa, H. (1998) Comparison of antibody production, lymphocyte stimulation, and protection induced by four commercial Mycoplasma hyopneumoniae bacterins, Swine health and Production, 6, 3, 107-112

Thacker, E., Thacker, B. and Wolff, T. (2000) Efficacy of Aureomycin chlortetracycline against experimental Mycoplasma hyopneumoniae challenge. Proceedings 16th International Pig Veterinary Society Congress, Melbourne, Australia, p. 457.

 

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