Tarello, W · Comparative immunology, microbiology and infectious diseases · 2001 · DOI
This study describes four horses that had symptoms similar to chronic fatigue syndrome in humans—severe tiredness and other health problems that didn't improve with standard treatments. The veterinarian treated them with a low dose of an arsenic-based medication given intravenously, and all four horses showed complete improvement in their fatigue and symptoms. The horses also recovered from anemia and showed other positive blood changes after treatment.
This study is significant because it suggests that ME/CFS-like illness may occur across species and could have an infectious or parasitic etiology responsive to antimicrobial treatment. If similar bacterial findings and treatment responses exist in human ME/CFS patients, this could open new investigational pathways for understanding and treating the disease. The dramatic response to a single intervention in all four cases warrants further investigation into potential microbial involvement in ME/CFS.
This case report cannot establish that CFS in horses is identical to human ME/CFS or that the same bacterial infection causes human disease. The study does not prove causation between the observed bacteria and CFS symptoms, nor does it demonstrate that this treatment would be safe or effective in humans. Without controlled trials, placebo groups, or standardized diagnostic criteria, these results cannot be generalized beyond the four individual cases presented.
About the PEM badge: “PEM required” means post-exertional malaise was an explicit required diagnostic criterion for participant inclusion in this study — not that PEM was studied, observed, or discussed. Studies using criteria that do not require PEM (e.g. Fukuda, Oxford) are tagged “PEM not required”. How the atlas works →
The first block is for the primary paper and is the citation you should use in research work. The atlas-snapshot line only applies if you are specifically referring to this atlas’s reading of the paper on the date shown.
Primary citation
Tarello, W (2001). Chronic fatigue syndrome in horses: diagnosis and treatment of 4 cases.. Comparative immunology, microbiology and infectious diseases. https://doi.org/10.1016/s0147-9571(00)00013-8
BibTeX
@article{mecfsatlas-tarello-2001-chronic-fatigue,
author = {Tarello, W},
title = {Chronic fatigue syndrome in horses: diagnosis and treatment of 4 cases.},
journal = {Comparative immunology, microbiology and infectious diseases},
year = {2001},
doi = {10.1016/s0147-9571(00)00013-8},
note = {PubMed: 11131041},
url = {https://www.mecfsatlas.com/evidence/tarello-2001-chronic-fatigue},
}Atlas snapshot reference
ME/CFS Atlas. Generator v1 / Scanner v1.4 / policy v0.1. Accessed 2026-05-27. https://www.mecfsatlas.com/evidence/tarello-2001-chronic-fatigue
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