Infections of the airways of pigs are often caused by bacteria but these infections are often secondary to or complicated by viral infections. The most common primary infectious agents are Mycoplasma hyopneumoniae and Actinobacillus pleuropneumoniae. Infections of the respiratory tract occur most frequently in the fattening period but may also affect younger pigs. Secondary infections are com- mon, where Pasteurella multocida is a frequently identified micro-organism. Toxin producing P. multocida is part of the etiology of atrophic rhinitis.
Diagnostics of bacterial respiratory infections are best based on autopsies and/or serological testing of different age groups of animals. Culture of M. hyopneumoniae is difficult and time consuming. Isolation of A. pleuropneumoniae can only be done from acutely affected lung tissue. Since P. multocida is a common cause of secondary infections, concurrent with other diseases, serology is unsuitable
as a diagnostic tool. P. multocida should be cultured, preferably with a test for antimicrobial sensitivity. In order to diagnose atrophic rhinitis a toxin producing strain of P. multocida must be identified.
Choosing a suitable and effective antibiotic is not an easy task. Sampling for bacteriological culturing, complemented with a sensitivity test needs to be done whenever possible. Combating respiratory infections in pig herds demands constant follow ups and evaluations. If wide spread and severe outbreaks occur in a herd group treatment may be indicated; see Strategies for group therapy.
Infections with M. hyopneumoniae often progress with a weak cough in batches of fattening pigs. Other clinical symptoms are less common even if naive animals may suffer from acute pneumonia that may resemble an acute A. pleuropneu- moniae-infection. Infections caused by M. hyopneumoniae generally are chronic and should only be treated if animal welfare is at stake. The infection generally spreads slowly in a herd and often reaches its peak at the time of slaughter. Gene- ral condition is often unaffected and signs of disease only show up at slaughter.
Pigs that become infected with M. hyopneumoniae in the growing period may develop a dry cough. If general condition is affected, or the infection is widely spread, treatment should be contemplated. The drug of choice under Swedish conditions is tetracycline (TET) but M. hyopneumoniae is also sensitive to pleuromutilins, tiamulin for example. Tiamulin should, however, be reserved for treatment of Brachyspira hyodysenteriae-infections, but may be used strategically, for instance in eradication programs or sanitation of a herd. A third option might be enrofloxacin that often is effective against M. hyopneumoniae, but this anti- microbial ought not to be used without a prior bacteriological diagnosis and a test of sensitivity.
Clinical symptoms at infections caused by A. pleuropneumoniae vary from no symptoms where pathological changes – commonly pleuritis, are found at slaughter to severe, acute symptoms with high fever ( >40° C), anorexia and high mortality. A common find at slaughter are deposits of fibrin on heart and lungs. In mild outbreaks one finds only a few animals with cough, elevated body tempe- rature and depressed appetite. Recommended treatment of these pigs is individual injections of systemic BPP for 5–7 days. The Swedish Medical Products Agency recommends a higher daily dose than stated by the manufacturers at 25–3o mg kg and day (instead of 20 mg/kg SID3). Antibiotic treatment may be complemented with NSAID. In more severe cases, when the infection is widely spread in a herd, group therapy with TET may be indicated and in cases with high mortality and low effect of above mentioned antimicrobials enrofloxacin may be tried, but only if supported by results from bacteriological cultures and tests of sensitivity have been received from an accredited laboratory.
Since Pasteurella multocida is a common pathogen associated with secondary infections therapy should also be aimed at this pathogen. Under Swedish condi- tions P. multocida is often sensitive to BPP, which is the drug of choice.
Even though the above mentioned micro-organisms are the most frequently found in cases of infections of the respiratory tracts of pigs also others can be found. If the clinician experiences weak or no effect of therapy, samples for bac- terial culture and tests of sensitivity should always be obtained, in order to direct therapy properly and prudently.
In order to prevent respiratory tract infections in pigs a strict practice of rearing in batches in accordance with the all-in-all-out principle should be upheld.
Larger herds should divide the animals in age groups. Older “left over”-pigs, that have not been able to keep up with their ordinary group, shall never be allowed to move in with a new group of pigs. Environmental factors such as adequate temperature and quality of air must be optimal for both the newly weaned pig and the pig that is ready for slaughter, and since they have different demands
on their environment, this is another reason for strict sectioning according to age. Temperature in the barn for newly weaned pigs should not be below 22° C. When the pigs are half grown the temperature should be gradually lowered to 15° C. The aim should be to change the air, through proper ventilation, to keep a good quality of air. Thorough cleaning of barns, alleys and cubicles, proper disinfection and a long enough time for dry-up and leaving the stalls empty (at least 3–7 days depending on indoor temperature) between batches are necessary management practices to keep the pigs healthy and to achieve an economically sustainable level of income.