Association of disease with isolation and virulence of Rhodococcus equi from farm soil and foals with pneumonia

Ronald J. Martens Department of Large Animal Medicine and Surgery, College of Veterinary Medicine, Texas A&M University, College Station, TX 77843.

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Shinji Takai Department of Animal Hygiene, School of Veterinary Medicine and Animal Sciences, Kitasato University, Towada, Japan.

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Noah D. Cohen Department of Large Animal Medicine and Surgery, College of Veterinary Medicine, Texas A&M University, College Station, TX 77843.

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M. Keith Chaffin Department of Large Animal Medicine and Surgery, College of Veterinary Medicine, Texas A&M University, College Station, TX 77843.

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Hui Liu Department of Large Animal Medicine and Surgery, College of Veterinary Medicine, Texas A&M University, College Station, TX 77843.

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Kimiko Sakurai Department of Animal Hygiene, School of Veterinary Medicine and Animal Sciences, Kitasato University, Towada, Japan.

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Hiromi Sugimoto Department of Animal Hygiene, School of Veterinary Medicine and Animal Sciences, Kitasato University, Towada, Japan.

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Sonia W. Lingsweiler Texas Veterinary Medical Diagnostic Laboratory, College Station, TX 77843.

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Abstract

Objective—To determine whether isolation and virulence of Rhodococcus equi from soil and infected foals are associated with clinical disease.

Design—Cross-sectional and case-control study.

Sample PopulationR equi isolates from 50 foals with pneumonia and soil samples from 33 farms with and 33 farms without a history of R equi infection (affected and control, respectively).

ProcedureR equi was selectively isolated from soil samples. Soil and clinical isolates were evaluated for virulence-associated protein antigen plasmids (VapAP) and resistance to the β-lactam antibiotics penicillin G and cephalothin. Microbiologic cultures and VapA-P assays were performed at 2 independent laboratories.

Results—VapA-P was detected in 49 of 50 (98%) clinical isolates; there was complete agreement between laboratories. Rhodococcus equi was isolated from soil on 28 of 33 (84.8%) affected farms and 24 of 33 (72.7%) control farms, but there was poor agreement between laboratories. Virulence-associated protein antigen plasmids were detected on 14 of 66 (21.2%) farms by either laboratory, but results agreed for only 1 of the 14 VapA-P-positive farms. We did not detect significant associations between disease status and isolation of R equi from soil, detection of VapA-P in soil isolates, or resistance of soil isolates to β-lactam antibiotics. No association between β-lactam antibiotic resistance and presence of VapA-P was detected.

Conclusions and Clinical Relevance—On the basis of soil microbiologic culture and VapA-P assay results, it is not possible to determine whether foals on a given farm are at increased risk of developing disease caused by R equi. (J Am Vet Med Assoc 2000;217:220–225)

Abstract

Objective—To determine whether isolation and virulence of Rhodococcus equi from soil and infected foals are associated with clinical disease.

Design—Cross-sectional and case-control study.

Sample PopulationR equi isolates from 50 foals with pneumonia and soil samples from 33 farms with and 33 farms without a history of R equi infection (affected and control, respectively).

ProcedureR equi was selectively isolated from soil samples. Soil and clinical isolates were evaluated for virulence-associated protein antigen plasmids (VapAP) and resistance to the β-lactam antibiotics penicillin G and cephalothin. Microbiologic cultures and VapA-P assays were performed at 2 independent laboratories.

Results—VapA-P was detected in 49 of 50 (98%) clinical isolates; there was complete agreement between laboratories. Rhodococcus equi was isolated from soil on 28 of 33 (84.8%) affected farms and 24 of 33 (72.7%) control farms, but there was poor agreement between laboratories. Virulence-associated protein antigen plasmids were detected on 14 of 66 (21.2%) farms by either laboratory, but results agreed for only 1 of the 14 VapA-P-positive farms. We did not detect significant associations between disease status and isolation of R equi from soil, detection of VapA-P in soil isolates, or resistance of soil isolates to β-lactam antibiotics. No association between β-lactam antibiotic resistance and presence of VapA-P was detected.

Conclusions and Clinical Relevance—On the basis of soil microbiologic culture and VapA-P assay results, it is not possible to determine whether foals on a given farm are at increased risk of developing disease caused by R equi. (J Am Vet Med Assoc 2000;217:220–225)

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