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Veterinary microbiology2013; 167(3-4); 623-631; doi: 10.1016/j.vetmic.2013.09.018

The effect of bacterial dose and foal age at challenge on Rhodococcus equi infection.

Abstract: While Rhodococcus equi remains the most common cause of subacute or chronic granulomatous bronchopneumonia in foals, development of a relevant model to study R. equi infection has proven difficult. The objective of this study was to identify a challenge dose of R. equi that resulted in slow progressive disease, spontaneous regression of lung lesions and age-dependent susceptibility. Foals less than one-week of age were challenged intratracheally using either 10(6), 10(5), 10(4), 10(3) or 10(2) cfu of R. equi. Two doses (10(3) cfu and 10(5) cfu) were used to challenge 2 and 3-week-old, and 3 and 6-week-old foals, respectively. Physical examination, thoracic ultrasound and blood work were performed. Foals were euthanized at the end of the study or when clinical signs of pneumonia developed. All foals were necropsied and their lung lesions scored. Foals challenged with low concentrations of R. equi developed slow progressive pneumonia and approximately 50% of the foals recovered spontaneously. Likewise, macroscopic (>1cm diameter) pyogranulomatous lesions were only observed when low doses of R. equi were used. Clinical pneumonia was not seen after low dose challenge in the 3-week-old foals or in the 6-week-old foals. This study demonstrates that the use of low doses of R. equi to challenge neonatal foals provides an improved model for studying this disease. Furthermore, susceptibility to R. equi infection was shown to diminish early in the foal's life, as has been reported in the field.
Copyright © 2013 Elsevier B.V. All rights reserved.
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Publication Date: 2013-09-24 PubMed ID: 24139178DOI: 10.1016/j.vetmic.2013.09.018Google Scholar: Lookup
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  • Non-U.S. Gov't

Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

This research aimed to develop a improved model for studying Rhodococcus equi infection in foals, a common cause of severe pneumonia. By varying the bacterial dose and the age of the foals, the study revealed that lower doses resulted in delayed development of disease and spontaneous recovery in approximately half of the cases, and that susceptibility to infection diminishes early in a foal’s life.

Research Objective

  • The primary goal of the study was to establish a challenge dose of R. equi that leads to slow disease progression, self-healing of lung lesions, and age-dependent vulnerability. The research also aimed to understand the effect of bacterial dose and the age of the foal at the time of infection on the nature and progression of the disease.

Methodology

  • Foals that were under one week of age were given doses of R. equi bacteria intratracheally at varying concentrations.
  • Options for the dosage were 10(6), 10(5), 10(4), 10(3) or 10(2) cfu of R. equi, with the middle to lower ranges also used to infect foals of 2-3 weeks and 3-6 weeks of age.
  • Physical examination, thoracic ultrasound, and blood work were conducted on the foals.
  • Once clinical signs of pneumonia developed, or at the end of the study, the foals were euthanized, necropsied, and lung lesions were scored to determine the severity of the disease.

Findings

  • Lower dosage levels of R. equi resulted in slower disease progression and in almost 50% of these cases, the foals recovered on their own.
  • Macroscopic pyogranulomatous lesions, which are a characteristic of R. equi infection, only occurred with the administration of lower doses.
  • In foals challenged with a low dose at 3 and 6 weeks of age, pneumonia symptoms were not detected.
  • The susceptibility to R. equi infection decreased as the foals got older, corroborating field observations.

Significance of the Study

  • The study offers an improved research model for studying R. equi infection in foals by introducing low doses to neonatal foals.
  • The findings validate field observations, contributing to a more extensive understanding of the disease and how to prevent or treat it effectively.

Cite This Article

APA
Sanz M, Loynachan A, Sun L, Oliveira A, Breheny P, Horohov DW. (2013). The effect of bacterial dose and foal age at challenge on Rhodococcus equi infection. Vet Microbiol, 167(3-4), 623-631. https://doi.org/10.1016/j.vetmic.2013.09.018

Publication

Veterinary microbiology
ISSN: 1873-2542
NlmUniqueID: 7705469
Country: Netherlands
Language: English
Volume: 167
Issue: 3-4
Pages: 623-631
PII: S0378-1135(13)00457-4

Researcher Affiliations

Sanz, M
  • Maxwell H. Gluck Equine Research Center, Department of Veterinary Science, University of Kentucky, Lexington, KY 40546-0099, United States.
Loynachan, A
    Sun, L
      Oliveira, A
        Breheny, P
          Horohov, D W

            MeSH Terms

            • Actinomycetales Infections / complications
            • Actinomycetales Infections / mortality
            • Actinomycetales Infections / veterinary
            • Age Factors
            • Animals
            • Animals, Newborn
            • Bacterial Load
            • Erythrocyte Count
            • Hemoglobins / analysis
            • Horse Diseases / diagnostic imaging
            • Horse Diseases / microbiology
            • Horse Diseases / mortality
            • Horse Diseases / pathology
            • Horses
            • Lymphocytes
            • Pneumonia / diagnostic imaging
            • Pneumonia / etiology
            • Pneumonia / mortality
            • Pneumonia / veterinary
            • Rhodococcus equi / physiology
            • Survival Analysis
            • Ultrasonography

            Citations

            This article has been cited 17 times.
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            16. da Silveira BP, Kahn SK, Legere RM, Bray JM, Cole-Pfeiffer HM, Golding MC, Cohen ND, Bordin AI. Enteral immunization with live bacteria reprograms innate immune cells and protects neonatal foals from pneumonia. Sci Rep 2025 May 25;15(1):18156.
              doi: 10.1038/s41598-025-02060-5pubmed: 40415003google scholar: lookup
            17. da Silveira BP, Cohen ND, Lawhon SD, Watson RO, Bordin AI. Protective immune response against Rhodococcus equi: An innate immunity-focused review. Equine Vet J 2025 May;57(3):563-586.
              doi: 10.1111/evj.14214pubmed: 39258739google scholar: lookup
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