To obtain basic data for development of a new antimicrobial regimen against R. equi infection in foals, we performed two experiments using antimicrobial susceptibility tests that included disk diffusion tests and Etests. An initial examination of the susceptibility of 8 clinical R. equi isolates to 111 antimicrobial agents revealed that 18 agents in 6 groups performed effectively against all the isolates. We then examined the antimicrobial susceptibility to 6 representative agents of the 18 antimicrobials of 616 R. equi strains isolated from tracheobronchial aspirates of foals with respiratory disease in Japan. There were no resistant strains in these 616 R. equi isolates. However, antimicrobial-resistant mutants, which appeared in the inhibitory zone around antimicrobial disks, were also observed in ciprofloxacin, gentamicin, and rifampin. The minimum inhibitory concentrations of most of the ciprofloxacin-resistant mutants and all of the rifampin-resistant mutants exceeded the break points of both ciprofloxacin and rifampin. Our results suggest that in Japan, there are no strains resistant to the 6 antimicrobial agents, suggesting that the 18 agents in the 6 groups are potential therapeutic agents for new regimens against R. equi infection in foals. Our data also suggests a risk of the development of problematic antimicrobial resistance to ciprofloxacin and rifampin in R. equi.

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The anthelmintic efficacy of oral paste formulations of ivermectin against gastrointestinal parasites in horses was evaluated, using fifteen active racehorses in which nematodes had been detected in their gastrointestinal tracts by fecal examination. A single dose of ivermectin was given orally to twelve horses of the treatment groups at a dosage rate of 200 μg/kg in order to obtain and identify gastrointestinal parasites in their feces for 3 days after treatment. Small strongyles, strongyles, and Parascaris equorum were found passed in feces up to post-treatment day (PTD) 2, but they were no longer observed on PTD 3. Horse bot fly Gasterophilus spp. larvae were found on PTD 2 and 3. The percentages of small strongyles found on PTD 1 were 75-100% (88% on average), and the rest were found (12% on average) on PTD 2. The nematodes were 16 species from 7 genera, of which 11 species were identified as small strongyles. In the fecal examination conducted on PTD 14, all horses of the treatment groups showed negative for nematode eggs. No abnormalities were observed in general clinical signs of the treated horses for 14 days after treatment. Three non-treated horses of the control groups still showed positive for nematode eggs on PTD 14. These results confirm that when ivermectin was given orally to horses at a dosage rate of 200 μg/kg, small strongyles, strongyles, and P. equorum, were excreted in feces up to PTD 2, and Gasterophilus spp. larvae on and after PTD 2, demonstrating the potent anthelmintic efficacy of ivermectin against these parasites.

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To demonstrate the participation of the 95-kb virulence plasmid in the pathogenicity of S. Abortusequi in horses, the prevalence of the plasmid in S. Abortusequi field isolates was examined, and experimental inoculation of mice and horses was carried out using strains with and without the plasmid. Single, large plasmids were detected in 226 of the 242 field isolates (93.4%), and spvC was detected by PCR testing from all the isolates with the plasmid. Mouse virulence tests revealed that the LD₅₀ of 11 of 13 isolates harboring the virulence plasmid was less than 10⁴ CFU, whereas the LD₅₀ of 12 isolates not harboring the virulence plasmid was, without exception, over 10⁶ CFU. Experimentally, 6 horses inoculated with S. Abortusequi strains 17N1 or Sal-107 harboring the 95-kb virulence plasmid were infected, and showed similar clinical symptoms, even though Sal-107 is avirulent in IVCS mice. On the other hand, the 17N3 plasmid-cured strain did not infect horses. We conclude that the 95-kb virulence plasmid of S. Abortusequi plays a role as a pathogenic factor in mice, but is also important in horses.

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Tracheobronchial aspirates were obtained by bronchoscope from 29 two-year-old Thoroughbreds immediately after long-distance transportation for examination for cells and bacteria. A number of neutrophils were observed in the aspirates of 6 horses, and a number of different bacteria were isolated from most of the horses. Five of the 6 horses (83.3%) with numerous neutrophils showed a marked increase in rectal temperature (≥39.0°C) or peripheral leukocyte count (≥15,000 cells/μl) immediately after transportation, while only 2 of 23 horses (8.7%) with almost no change in neutrophil count showed a marked increase in peripheral leukocyte count (≥15,000 cells/μl). Results of this study indicate that measurement of rectum temperature and peripheral leukocyte counts after transportation can be used as a practical index for diagnosis of respiratory infectious disease associated with transportation.

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