Minimum inhibitory concentrations of doxycycline and oxytetracycline were determined against 55 Pasteurella multocida strains, 59 Actinobacillus pleuropneumoniae strains and 26 Mycoplasma hyopneumoniae strains isolated from the respiratory tract of pigs. An additional set of 76 P multocida strains isolated from pneumonic pigs was tested for their minimum inhibitory concentrations of doxycycline. The P multocida and A pleuropneumoniae strains were isolated in France and the minimum inhibitory concentrations were determined by an agar dilution method. The M hyopneumoniae strains were isolated in the United Kingdom and minimum inhibitory concentrations were determined by a serial broth dilution method. All the strains tested were susceptible to doxycycline whereas 15 per cent of the P multocida strains and 22 per cent of the A pleuropneumoniae strains were resistant to oxytetracycline. Doxycycline concentrations inhibiting 90 per cent of strains were 1 microgram/ml for P multocida and 2 micrograms/ml for A pleuropneumoniae. The ratio of the minimum inhibitory concentrations of doxycycline and oxytetracycline ranged between 1/1 and 1/4 for the oxytetracycline-susceptible strains and between 1/16 and 1/64 for the oxytetracycline-resistant strains. All the M hyopneumoniae strains were susceptible to doxycycline and oxytetracycline, the concentrations inhibiting 90 per cent of strains being 1 microgram/ml and 2 micrograms/ml, respectively. These data confirm that doxycycline has a higher in vitro activity against pig respiratory pathogens than oxytetracycline.

Actinobacillus seminis was isolated from the semen of five rams on four farms. Four of the rams had abnormal semen and three were also infertile. The isolates of A seminis showed similar phenotypic profiles and electrophoretic protein patterns to the type strain of A seminis but were distinct from Histophilus ovis previously isolated from rams with epididymitis in Scotland. The infection appeared to be subclinical but two of the five rams had palpable abnormalities of their testes. Three rams were treated with antibiotics but the infection persisted. No gross lesions were found in the genitalia of two of three rams examined post mortem but one had necrotic abscesses in the testes and epididymis. A seminis was isolated from the seminal vesicles and epididymis of one ram without gross lesions but not from the genitalia of the other two. On one farm the infection in a recently purchased ram led to the detection of another case as a result of the bacteriological screening of 11 stock rams not in contact with the initial case. These five subclinical cases, which included a supposedly healthy stock ram, suggest that A seminis infection may be widespread and should be considered in cases of infertility.

This study involved two experiments which examined the efficacy of 'in water' tilmicosin medication for the treatment of experimental Mycoplasma gallisepticum (MG) disease. The first experiment investigated tilmicosin concentrations of 50, 100, 200 and 400 mg/l administered for 5 days. In the second experiment, dose levels of 50, 75 and 100 mg/l tilmicosin administered for either 3 or 5 days were investigated. The first experiment suggested that tilmicosin was effective in reducing the incidence and severity of airsacculitis lesions caused by MG. In the second experiment tilmicosin treatment was found to significantly reduce the severity of airsacculitis caused by MG, although there was no effect on the morbidity. These experiments suggest that tilmicosin is effective in the treatment of MG infections when administered in drinking water. The minimum effective concentration of tilmicosin in drinking water appears to be at or less than 50 mg/1 when administered for either 3 or 5 days.

This study compared the efficacy of continuous or pulsed-water medication with enrofloxacin, danofloxacin, and sarafloxacin in eight groups of 90 chicks each by using an infectious bronchitis virus-Escherichia coli model of colisepticemia. The model produced lesions of typical those occurring in birds with severe colisepticemia; for the infected, nonmedicated birds the mortality was 43.5% and the morbidity was 89%, 17.8% of birds had severe lesions, and the birds had a mean air sac lesion score of 2.58. This experiment showed that continuous dosing and pulsed dosing are clinically equivalent. However, for all fluoroquinolones studied, there was a trend for the continuously mediated birds to have lower mortality and less severe disease than birds receiving pulsed doses. Compared with infected, nonmedicated controls, only birds continuously medicated with enrofloxacin had a significantly lower morbidity (32%), and only birds medicated with enrofloxacin and danofloxacin (continuous and pulsed treatments) had significantly lower mortality (6.7 and 11.0% and 16.8 and 19.2% for continuous and pulsed treatments with enrofloxacin and danofloxacin, respectively). A significantly lower proportion of birds only in the groups medicated with enrofloxacin had severe lesions (for birds receiving continuous and pulsed treatments, 2.2 and 6.7%, respectively). Birds medicated with any of the three fluoroquinolones (continuous and pulsed treatments) except pulsed-water treatment with sarafloxacin had significantly reduced mean air sac lesion scores compared with the scores for nonmedicated birds (air sac lesion scores, 0.60 and 0.83, 1.38 and 1.63, and 1.80 and 2.05 for birds receiving continuous and pulsed treatments with enrofloxacin, danofloxacin, and sarafloxacin, respectively). The performance of the birds that survived the challenge or that recovered after receiving medication was not compromised compared to the performance of noninfected birds. Enrofloxacin was more efficacious than either danofloxacin or sarafloxacin for the treatment of colisepticemia in chickens by medication in drinking water. Similarly, danofloxacin appeared to be more effective than sarafloxacin in treating colisepticemia.

VALNEMULIN (Econor; Novartis) is a new semisynthetic pleuromutilin derivative related to tiamulin. In this study, minimum inhibitory concentrations (MICs) were determined for valnemulin and its effectiveness was compared to six other antimicrobial agents against four porcine pathogens. The antimicrobial agents used were drugs of choice for treatment of diseases caused by these pathogens.Ten strains of Mycoplasma hyopneumoniae isolated from porcine lungs between 1977 and 1990 were tested. In addition 10 strains ofSerpulina hyodysenteriae isolated from porcine faeces between 1989 and 1990, 15 strains ofActinobacilluspleuropneumoniae serotypes 1, 2, 3, 5, 6, 8 and 9, isolated from porcine lungs between 1980 and 1992 and 10 strains ofStreptococcus suis serotype 2 isolated from porcine disease (meningitis or septicaemia) between 1989 and 1990 were included in the study. All strains were isolated in the UK and were from diseased animals. Reference strains were used as controls and were induded in all tests. Tests were only considered valid if reference strain results were within internationally accepted ranges.The following antimicrobial agents were tested: valnemulin, tiamulin, oxytetracycline, lincomycin, tylosin, penicillin V and dimetridazole. Dilutions of these were prepared in the appropriate basal broth or solid media.The basal broth medium for M hyopneumoniae consisted of Friis' medium (Friis 1975), without antibiotics or PpLo broth base. The M hyopneumoniae strains were diluted to 104 colour change units per ml and these were inoculated to tubes containing the diluted antimicrobial agents. All tubes were incubated at 37°C and observed daily for seven days. The end-point value was taken as the colour change to match the control of pH 6-8, indicating growth. The MIC value was taken as the endpoint reading recorded after colour stabilisation for two days.For testing A pleuropneumoniae, S suis II and S hyodysenteriae, dilutions of the antimicrobial agents were added to the appropriate basal agar. Bacterial inocula were prepared in appropriate liquid media and were adjusted to an optical density equivalent to 1 x 108 colony forming units (cfu) per ml. The antibiotic-containing plates were inoculated in duplicate using a multipoint inoculator (Denley Instruments) which delivered a plate inoculum of approximately 1 VIl, giving an inoculum of 104 to 105 cfu per spot for all strains. Control M hyopneumoniae Pathogen S hyodysenteriae Antimicrobial agent MIC MICgo Range MICa, MIq, Range Valnemulin 0-0024 0-0049 0.0024-00098 01 4-0 0-025-4-0 Tiamulin 0-039 0.078 0039-0-16 0-3 1-0 0-025-1-0 Oxytetracycline 0-078 0.31 0-039-0-63 NT NT NT Lincomycin 0-31 0-63 0-31-0-63 50-0 100-0 1.0-100.0 Tylosin 0-16 0-31 0.16-0-63 200-0 >200-0 5-0->200-0 Dimetndazole NT NT NT 30.0 >40-0 1-0->40-0 NT Not tested (antimicrobial agent not applicable to the disease agent) Pathogen A pleuropneumonuiae S suis 11 Antimicrobial agent MIC5, MICgo Range MICso MIC9, Range Valnemulin liamulin Oxytetracycline Penicillin V 16-0 32-0 4-0-32-0 32-0 64-0 8...