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Acta veterinaria Scandinavica2025; 67(1); 8; doi: 10.1186/s13028-025-00792-y

Severe subcutaneous infection with Clostridium septicum in a herd of native Icelandic horses.

Abstract: Cellulitis due to infection with clostridia has not been documented in horses in Iceland. However, clostridia are well-known pathogens in Icelandic sheep, which have traditionally shared grazing land with horses. Clostridial infections of equine muscle or subcutis following injection with medicinal products have been described in other countries but have never been reported in Iceland. In this case report, we present the first documented outbreak of subcutaneous clostridial infection in horses in Iceland following subcutaneous injection. Methods: In November 2022, 16 out of 32 horses, that some days earlier had received a subcutaneous injection of Noromectin® 1% injectable solution, developed clinical signs indicating malignant oedema. The clinical signs included pyrexia, depression, swollen limbs, chest and neck, reluctance to move and dyspnoea, leading to the death or euthanasia of five horses. In addition, one horse was found dead with no previously noted clinical signs. Necropsy of one of the five horses revealed severe, acute cellulitis in the neck region, as well as lymphadenitis in regional lymph nodes. Abscesses, some with subsequent spontaneous drainage of seropurulent material, were observed at the presumed injection site on eight surviving horses approximately 2 weeks post-injection. Bacterial culture of samples from the necropsied horse and from abscesses from three surviving horses yielded the growth of C. septicum. Analysis of water samples from the pasture where the herd was kept also revealed the presence of C. septicum. Whole genome sequencing suggested that the isolates from the diseased horses contained the same C. septicum strain, whereas the strain isolated from the water samples differed from the disease-causing isolates. Conclusions: Clinical signs compatible with serious subcutaneous C. septicum infection were seen in over half of 32 horses injected with an ivermectin product, with the subsequent death of six of the horses. In the absence of other obvious sources, the outbreak suggests that C. septicum spores on the skin of these horses were introduced under the skin when they were injected. Such infections have not been reported in Iceland, although ivermectin products formulated for subcutaneous injection have been widely used for more than 30 years. The outbreak warrants further investigation into C. septicum in the environment of grazing horses in Iceland.
© 2025. The Author(s).
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Publication Date: 2025-02-06 PubMed ID: 39910631PubMed Central: PMC11800538DOI: 10.1186/s13028-025-00792-yGoogle Scholar: Lookup
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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.

The research article discusses the first documented case of a severe subcutaneous infection caused by Clostridium septicum in a herd of Icelandic horses following subcutaneous injections.

Background

  • The research focuses on clostridial infections, specifically from Clostridium septicum, in horses in Iceland. Although clostridial infections in Iceland’s sheep population have been well-documented, this is the first reported case involving horses.
  • The infection outbreak resulted from a subcutaneous injection administered to the horses. Previously, any similar cases in horses in other countries also traced back to similar injection routes.

Methods and Findings

  • The research team observed 16 out of 32 horses that had previously received a subcutaneous injection of Noromectin- a veterinary medicinal product. They showed symptoms of malignant oedema, including fever, depression, swelling on limbs and chest, reluctance to move, and breathing problems.
  • The severity of the infection led to the death or euthanasia of five horses. One horse was found dead without any previously noted symptoms.
  • On performing a necropsy on one of the horses, researchers found severe acute cellulitis in the neck region and lymphadenitis in the nearby lymph nodes. Abscesses were present at the injection site on eight surviving horses.
  • Researchers identified Clostridium septicum bacteria cultivation from samples taken both from the deceased horse and abscesses from surviving horses. The same bacterial strain was found in water samples from the horse’s grazing field.
  • Whole-genome sequencing established that the bacterial strain found in the infected horses was different from the strain found in the water samples.

Conclusion

  • The study concludes that serious Clostridium septicum infections resulted in the death of six out of 32 horses injected with an ivermectin product. This points to the possibility of bacterial spores present on the skin of the horses being introduced subcutaneously during injection. Similar incidents have never been reported in Iceland, despite widespread use of ivermectin products for over 30 years.
  • Given the severity and novelty of the outbreak, the research suggests a further investigation into the presence of Clostridium septicum in horse grazing environments in Iceland.

Cite This Article

APA
Oddsdóttir C, Sigurðardóttir ÓG, Friðriksdóttir V, Svansson V, Bragason BÞ, Björnsdóttir S. (2025). Severe subcutaneous infection with Clostridium septicum in a herd of native Icelandic horses. Acta Vet Scand, 67(1), 8. https://doi.org/10.1186/s13028-025-00792-y

Publication

Acta veterinaria Scandinavica
ISSN: 1751-0147
NlmUniqueID: 0370400
Country: England
Language: English
Volume: 67
Issue: 1
Pages: 8

Researcher Affiliations

Oddsdóttir, Charlotta
  • Division of Bacteriology and Pathology, Institute for Experimental Pathology at Keldur, Keldnavegi 3, 112, Reykjavík, Iceland. charlotta@hi.is.
Sigurðardóttir, Ólöf G
  • Division of Bacteriology and Pathology, Institute for Experimental Pathology at Keldur, Keldnavegi 3, 112, Reykjavík, Iceland.
Friðriksdóttir, Vala
  • Division of Bacteriology and Pathology, Institute for Experimental Pathology at Keldur, Keldnavegi 3, 112, Reykjavík, Iceland.
Svansson, Vilhjálmur
  • Division of Virology, Molecular Biology and Parasitology, Institute for Experimental Pathology at Keldur, Keldnavegi 3, 112, Reykjavík, Iceland.
Bragason, Birkir Þór
  • Division of Virology, Molecular Biology and Parasitology, Institute for Experimental Pathology at Keldur, Keldnavegi 3, 112, Reykjavík, Iceland.
Björnsdóttir, Sigríður
  • Icelandic Food and Veterinary Authority, Austurvegi 64, 800, Selfoss, Iceland.

MeSH Terms

  • Animals
  • Cellulitis / veterinary
  • Cellulitis / microbiology
  • Cellulitis / epidemiology
  • Clostridium Infections / veterinary
  • Clostridium Infections / microbiology
  • Clostridium Infections / epidemiology
  • Clostridium Infections / pathology
  • Clostridium septicum / isolation & purification
  • Disease Outbreaks / veterinary
  • Horse Diseases / microbiology
  • Horse Diseases / epidemiology
  • Horses
  • Iceland / epidemiology
  • Injections, Subcutaneous / veterinary
  • Injections, Subcutaneous / adverse effects

Conflict of Interest Statement

Declarations. Ethics approval and consent to participate: This study did not require official or institutional ethical approval. The animals were handled according to high ethical standards and national legislation. Consent for publication: Consent for publication was obtained from the owners of the horses in this report. Prior publication: Data have not been published previously. Competing interests: The authors declare that they have no competing interests. CO is editor-in-chief of Acta Veterinaria Scandinavica but has not in any way interacted with the review process or editorial decision making.

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Citations

This article has been cited 2 times.
  1. Ryu SH, Forbes E, Kim BS, Park KT. Injection site abscesses associated with commensal and environmental bacteria following intramuscular vaccination in horses. Vet Anim Sci 2025 Sep;29:100484.
    doi: 10.1016/j.vas.2025.100484pubmed: 40791848google scholar: lookup
  2. Huang X, Deng R, Huang H, Xie H, Chen A. Pseudomonas-Enterobacter Co-Infection Drives Cellulitis and Lymphangitis in Equines: A Case Report. Vet Sci 2025 Jun 11;12(6).
    doi: 10.3390/vetsci12060574pubmed: 40559811google scholar: lookup
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