JOURNAL OF CLINICAL MICROBIOLOGY, Sept. 1998, p. 2789–2790 0095-1137/98/$04.0010 Copyright © 1998, American Society for Microbiology. All Rights Reserved.
Vol. 36, No. 9
Actinobacillus equuli Septicemia: an Unusual Zoonotic Infection CHRISTOPHER ASHHURST-SMITH,1 ROBERT NORTON,1* WENDY THOREAU,2 3 AND MARGARET M. PEEL Department of Clinical Microbiology, Townsville General Hospital,1 and Cardiology, Mater Hospital, Pimlico,2 Townsville, Queensland, and Microbiological Diagnostic Unit, Department of Microbiology and Immunology, University of Melbourne, Parkville, Victoria,3 Australia Received 1 June 1998/Accepted 11 June 1998
We describe the isolation of Actinobacillus equuli from the blood of a 53-year-old butcher with septicemia. This species of the genus Actinobacillus is primarily associated with animals and animal diseases, especially septicemia in foals. This is the first report of the isolation of A. equuli from a human with septicemia. Although the genus Actinobacillus includes species associated with human sources and diseases, such as Actinobacillus actinomycetemcomitans and A. ureae, most species are commensals or pathogens of animals, especially cattle, horses, and pigs (10, 13). As components of the oropharyngeal flora of these animals, A. lignieresii, A. equuli, and A. suis may cause bite wound infections in humans (4, 6, 9, 13). An aerogenic A. equuli-like bacterium has also been described from an infected horse bite wound (9). However, systemic human disease due to A. equuli has not been previously reported. We describe here the isolation of A. equuli as the causative agent of septicemia in a butcher who had cut his thumb. A 53-year-old man presented in an acute state of profound septic shock. Three days earlier, he had sustained a cut to his left thumb while at work as a butcher. Three years previously, he had received a mitral valve replacement for mitral regurgitation. He had remained well for the 2 days following the thumb injury but then developed fever, confusion, and nausea and collapsed. Upon admission, the patient was drowsy, confused, febrile, and hypotensive. His heart sounds were clear, with normal prosthetic sounds and a soft midsystolic murmur. The injury site did not show any obvious signs of infection. There were no clinical stigmata of infective endocarditis. An echocardiogram showed good function of his prosthetic valve, with no evidence of endocarditis. A computerized tomographic scan of the brain was normal. Treatment with intravenous flucloxacillin, gentamicin, and benzylpenicillin was commenced. Blood was collected and cultured (BacT Alert; Organon Teknika, Durham, N.C.). Within 48 h of admission and the commencement of antibiotic therapy, the patient improved, his fever resolved, and normal cerebral status returned. Treatment with benzylpenicillin was continued for a total of 4 weeks, after which the patient was discharged. After 24 h of incubation at 35°C, a gram-negative bacillus was detected in the blood culture system in two of the three blood culture vials. The motility test was negative. Overnight incubation of subcultures revealed small grey-white raised colonies on both the aerobic and anaerobic plates of Columbia agar (Becton Dickinson Microbiology Systems, Cockeysville, Md.) containing 5% horse blood. The colonies were sticky.
Slight alpha-hemolysis was observed. Growth on MacConkey agar (Becton Dickinson Microbiology Systems) was feeble. The oxidase reaction was positive, and the catalase test was weakly positive. Conventional biochemical reactions and other characteristics were determined as previously described (13), and acid production from carbohydrates was assessed with the API 20E identification system (bioMe´rieux, Marcy-l’Etoile, France) and Minitek carbohydrate-impregnated paper discs (Becton Dickinson Microbiology Systems). The growth characteristics and biochemical reactions (Table 1) suggested an identification of A. equuli (6, 13), which was confirmed by the Microbiological Diagnostic Unit at the University of Melbourne, Melbourne, Australia. Actinobacillus spp. are members of the family Pasteurellaceae. Close similarities exist among the genera within this family, especially between Actinobacillus and Pasteurella. Reexamination of some identified isolates has shown that misidentifications across the two genera have occurred (2, 8). Four biochemical tests are of particular value for their differentiation. These are b-galactosidase (as determined by hydrolysis of o-nitrophenyl-b-D-galactopyranoside), urease activity, and growth on MacConkey agar, which are usually positive for Actinobacillus spp., and indole production, which is always negative. All four tests should be used as an initial step in the identification of species belonging to these genera, as the test results for Pasteurella spp. tend to be more variable. Species differentiation within the actinobacilli may also present difficulties (1, 2). A. equuli can be differentiated from the closely related A. lignieresii by melibiose and trehalose fermentation (6, 9, 13). The former produces a positive result in both substrates, but the latter ferments neither. While A. suis also ferments melibiose and trehalose, it differs from both A. equuli and A. lignieresii in its hydrolysis of esculin and hemolysis of sheep blood (6, 9, 13). A. actinomycetemcomitans differs from other species in the genus in that it does not grow on MacConkey agar or in the absence of an atmosphere containing increased carbon dioxide, does not produce urease, and is usually oxidase negative. It has been suggested that this species should be removed from the genus Actinobacillus and placed in the genus Haemophilus (11), but this proposal has not been generally accepted (7). A. equuli is the causative agent of sleepy foal disease, an acute and often fatal septicemia of newborn foals (10). In adult horses, A. equuli has been associated with endocarditis, meningitis, metritis, and abortion. The species also causes disease in pigs (10). It has been isolated, along with A. lignieresii, from laboratory rodents (8). A. equuli is prevalent in horses in Australia, where it has been reported to cause equine neonatal
* Corresponding author. Mailing address: Department of Clinical Microbiology, Townsville General Hospital, Townsville, Queensland, 4810, Australia. Phone: 61 7 4781 9517. Fax: 61 7 4771 5002. E-mail:
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TABLE 1. Growth characteristics and biochemical reactions of case isolate of A. equuli Characteristic or reaction
Positive Aerobic and anaerobic growth Growth on MacConkey agar Catalase (weak) Oxidase Urease (Christensen’s) Nitrate reduction b-Galactosidase (ONPGa) Phosphatase Acid from slant and butt of TSIb Acid from glucose, lactose, maltose, mannose, melibiose, raffinose, sucrose, trehalose, and xylose Negative Motility Gas from glucose in MRS brothc Esculin hydrolysis Indole production Gas from nitrate Nitrate reduction Ornithine decarboxylase H2S from butt of TSI Acid from arabinose, cellobiose, inositol, salicin, and sorbitol a b c
ONPG, o-nitrophenyl-b-D-galactopyranoside. TSI, triple sugar iron agar. MRS broth, Oxoid CM359 (see reference 9).
septicemia (12), pleuropneumonia (3), and peritonitis (5). The species has not been reported in pigs in Australia. It is likely that our patient was occupationally exposed to the bacterium and that the cut on his thumb provided a portal of entry to his bloodstream. Correct identification of Actinobacillus species according to current schemes of classification depends on adequate biochemical characterization and awareness of the possible presence of these bacteria in sites such as bite wounds inflicted by horses, pigs, or sheep. Occupational or recreational activities may provide important clues for establishing an early diagno-
sis. This awareness, in conjunction with an appropriate range of biochemical tests, should lead to the correct identification of species of Actinobacillus, even in unusual zoonotic infections. REFERENCES 1. Bisgaard, M., K. Piechulla, Y.-T. Ying, W. Frederiksen, and W. Mannheim. 1984. Prevalence of organisms described as Actinobacillus suis or haemolytic Actinobacillus equuli in the oral cavity of horses. Comparative investigations of strains obtained and porcine strains of A. suis sensu stricto. Acta Pathol. Microbiol. Immunol. Scand. Sect. B 92:291–298. 2. Blackall, P. J., M. Bisgaard, and R. A. McKenzie. 1997. Characterisation of Australian isolates of Actinobacillus capsulatus, Actinobacillus equuli, Pasteurella caballi and Bisgaard Taxa 9 and 11. Aust. Vet. J. 75:52–55. 3. Collins, M. B., D. R. Hodgson, and D. R. Hutchins. 1994. Pleural effusion associated with acute and chronic pleuropneumonia and pleuritis secondary to thoracic wounds in horses: 43 cases (1982–1992). J. Am. Vet. Med. Assoc. 205:1753–1758. 4. Dibb, W. L., A. Digranes, and S. Tønjum. 1981. Actinobacillus lignieresii infection after a horse bite. Br. Med. J. 283:583–584. 5. Golland, L. C., D. R. Hodgson, J. L. Hodgson, M. A. Brownlow, D. R. Hutchins, R. J. Rawlinson, M. B. Collins, S. A. McClintock, and A. L. Raisis. 1994. Peritonitis associated with Actinobacillus equuli in horses: 15 cases (1982–1992). J. Am. Vet. Med. Assoc. 205:340–343. 6. Holmes, B., M. J. Pickett, and D. G. Hollis. 1996. Unusual gram-negative bacteria, including Capnocytophaga, Eikenella, Pasteurella, and Streptobacillus. p. 499–503. In R. R. Murray, E. J. Baron, M. A. Pfaller, F. C. Tenover, and R. H. Yolken (ed.), Manual of clinical microbiology, 6th ed. ASM Press, Washington, D.C. 7. International Committee on Systematic Bacteriology Subcommittee on Pasteurellaceae and Related Organisms. 1987. Minutes of the meetings, 6 and 10 September 1986, Manchester, England. Int. J. Syst. Bacteriol. 37:474. 8. Lentsch, R. H., and J. E. Wagner. 1980. Isolation of Actinobacillus lignieresii and Actinobacillus equuli from laboratory rodents. J. Clin. Microbiol. 12:351– 354. 9. Peel, M. M., K. A. Hornidge, M. Luppino, A. M. Stacpoole, and R. E. Weaver. 1991. Actinobacillus spp. and related bacteria in infected wounds of humans bitten by horses and sheep. J. Clin. Microbiol. 29:2535–2538. 10. Phillips, J. E. 1984. Genus III. Actinobacillus Brumpt 1910, 849AL, p. 570– 575. In N. R. Kreig and J. G. Holt (ed.), Bergey’s manual of systematic bacteriology, vol. 1. Williams & Wilkins, Baltimore, Md. 11. Potts, T. V., J. J. Zambon, and R. J. Genco. 1985. Reassignment of Actinobacillus actinomycetemcomitans to the genus Haemophilus as Haemophilus actinomycetemcomitans comb. nov. Int. J. Syst. Bacteriol. 35:337–341. 12. Raisis, A. L., J. L. Hodgson, and D. R. Hodgson. 1996. Equine neonatal septicemia: 24 cases. Aust. Vet. J. 73:137–140. 13. Weyant, R. S., C. W. Moss, R. E. Weaver, D. G. Hollis, J. J. Jordan, E. C. Cook, and M. I. Daneshvar. 1996. Identification of unusual pathogenic gram-negative aerobic and facultatively anaerobic bacteria, 2nd ed. Williams & Wilkins, Baltimore, Md.
