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Revista brasileira de parasitologia veterinaria = Brazilian journal of veterinary parasitology : Orgao Oficial do Colegio Brasileiro de Parasitologia Veterinaria2024; 33(3); e003324; doi: 10.1590/S1984-29612024043

Reduction of egg reappearance period of cyathostomins in naturally infected horses after increasing doses of ivermectin in Brazil: a grim picture for sustainable parasite control.

Abstract: Cyathostomins are the largest group of parasites in horses that can be controlled by ivermectin (IVM). This study aimed to run a four-dose titration trial of IVM in 28 naturally infected Thoroughbred yearlings. The local Strongyle population had been recorded to be resistant to IVM (200 µg/kg). The parasite fecal egg count (FEC) was performed to investigate the egg reappearance period (ERP) of two and five weeks (w2pt and w5pt) after IVM treatment. FEC was > 1000 on day zero for all groups. Although 100% FEC reduction was reported at w2pt for all concentrations, the FEC at w5pt revealed < 83% efficacy. This study reports the reduction of ERP using the label dose as well as 300, and 400 µg/kg (double dose) of IVM. The protocol allowed IVM to significantly suppress FEC w2pt although not eliminating adult worms, failing to guarantee an extension of its protection period over 8 weeks. Moreover, the FEC at w5pt possibly means the infection was not cleared, and worms reestablished egg laying. We raised the possibility of withdrawing IVM of control programs when the drug has less than 80% FEC reduction at w5pt. Ciatostomíneos são o maior grupo de parasitos de equinos, controlados com ivermectina (IVM). Este estudo teve o objetivo de realizar um teste com quatro doses de IVM, em 28 potros Puro-Sangue Inglês. Os estrôngilos eram resistentes a IVM (200 µg/kg). A contagem de ovos por grama de fezes (OPG) foi realizada após tratamento (pt), para determinar o período de reaparecimento de ovos (PRO) após duas e cinco semanas (s2pt e s5pt). A OPG foi >1000 para todos os grupos. Observou-se 100% de redução na OPG na s2pt para todas as concentrações de IVM. A OPG na s5pt revelou < 83% de redução. Este estudo relata uma redução considerável do PRO utilizando a dose de bula, e as doses de 300 e 400 μg/kg (dobro da dose) de IVM contra ciatostomíneos em equinos. O protocolo permitiu observar a redução da postura de ovos na s2pt com IVM, falhando na garantia da extensão de proteção da droga. O reaparecimento da OPG significa que a infecção não foi eliminada. Foi levantada a possibilidade de restringir o uso de IVM, quando o medicamento atingir menos de 80% de redução da OPG na s5pt.
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Publication Date: 2024-08-12 PubMed ID: 39140496PubMed Central: PMC11340864DOI: 10.1590/S1984-29612024043Google Scholar: Lookup
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Summary

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Reduction of egg reappearance period of cyathostomins in naturally infected horses after increasing doses of ivermectin in Brazil reveals reduced efficacy and challenges for sustainable parasite control.

Background and Purpose of the Study

  • Cyathostomins are the most common intestinal parasites affecting horses.
  • Ivermectin (IVM) is widely used to control these parasites with a typical dose of 200 µg/kg.
  • Previous evidence had shown the local parasite population to be resistant to the standard IVM dose.
  • The study aimed to evaluate the effect of increasing doses of IVM (including double doses) on parasite control in naturally infected Thoroughbred yearlings in Brazil.
  • The focus was to measure the “egg reappearance period” (ERP), which is the time it takes for parasite eggs to reappear in feces after treatment, indicating a return of the parasite infection.

Study Design and Methods

  • 28 naturally infected Thoroughbred yearling horses were divided into groups receiving four different IVM dose levels: the label dose (200 µg/kg) and increased doses (300 and 400 µg/kg).
  • Fecal egg counts (FEC) were performed before treatment (day zero) and at two and five weeks post-treatment (w2pt and w5pt).
  • A high initial parasite load was confirmed with FEC > 1000 eggs per gram of feces in all groups at day zero.
  • ERP was assessed by recording the reduction in FEC over the post-treatment weeks and comparing them to pre-treatment counts.

Key Findings

  • At two weeks post-treatment (w2pt), all groups showed a 100% reduction in FEC, indicating initial strong suppression of parasite egg shedding.
  • However, by five weeks post-treatment (w5pt), the efficacy dropped below 83% reduction in FEC across all dose groups.
  • This suggests that despite doubled IVM doses, adult worms were not fully eliminated and were able to resume egg laying earlier than expected.
  • The ERP was notably reduced even with higher doses, meaning the period of effective parasite control was shorter than desired (less than the typical 8+ weeks protection).
  • The reappearance of eggs at five weeks post-treatment indicates the infection was not cleared and the parasites quickly reestablished.

Implications and Recommendations

  • The findings highlight the problem of emerging ivermectin resistance in cyathostomin populations in Brazil.
  • Increasing the dose of ivermectin did not prolong the ERP or ensure a longer protective effect against these parasites.
  • The authors suggest reconsidering the use of ivermectin in control programs when the FEC reduction at five weeks falls below 80%, because this signals reduced drug efficacy and resistance development.
  • This could prompt the need for alternative parasite control strategies or drugs to maintain sustainable equine health management.
  • Continuous monitoring of ERP and FEC reduction can serve as crucial indicators for timely detection of ineffective treatments and resistance.

Overall Conclusion

  • The study provides evidence that cyathostomin populations in Brazil are becoming resistant to ivermectin, even at higher-than-label doses.
  • The shortened egg reappearance period and incomplete parasite clearance after treatment challenge the sustainability of current parasite control practices relying heavily on ivermectin.
  • Veterinarians and equine health managers should be cautious about ivermectin resistance, consider integrated parasite management approaches, and avoid over-reliance on this single drug.

Cite This Article

APA
Molento MB, Dall'Anese J, Pontarolo DV, Brandão YO, Yoshitani UY. (2024). Reduction of egg reappearance period of cyathostomins in naturally infected horses after increasing doses of ivermectin in Brazil: a grim picture for sustainable parasite control. Rev Bras Parasitol Vet, 33(3), e003324. https://doi.org/10.1590/S1984-29612024043

Publication

Revista brasileira de parasitologia veterinaria = Brazilian journal of veterinary parasitology : Orgao Oficial do Colegio Brasileiro de Parasitologia Veterinaria
ISSN: 1984-2961
NlmUniqueID: 9440482
Country: Brazil
Language: English
Volume: 33
Issue: 3
Pages: e003324
PII: e003324

Researcher Affiliations

Molento, Marcelo Beltrão
  • Laboratório de Parasitologia Clínica Veterinária, Departamento de Medicina Veterinária, Universidade Federal do Paraná - UFPR, Curitiba, PR, Brasil.
Dall'Anese, Julia
  • Laboratório de Parasitologia Clínica Veterinária, Departamento de Medicina Veterinária, Universidade Federal do Paraná - UFPR, Curitiba, PR, Brasil.
Pontarolo, Desiree Vera
  • Laboratório de Parasitologia Clínica Veterinária, Departamento de Medicina Veterinária, Universidade Federal do Paraná - UFPR, Curitiba, PR, Brasil.
Brandão, Yara de Oliveira
  • Laboratório de Parasitologia Clínica Veterinária, Departamento de Medicina Veterinária, Universidade Federal do Paraná - UFPR, Curitiba, PR, Brasil.
Yoshitani, Ursula Yaeko
  • Laboratório de Parasitologia Clínica Veterinária, Departamento de Medicina Veterinária, Universidade Federal do Paraná - UFPR, Curitiba, PR, Brasil.

MeSH Terms

  • Animals
  • Ivermectin / therapeutic use
  • Ivermectin / administration & dosage
  • Horses / parasitology
  • Brazil
  • Parasite Egg Count
  • Horse Diseases / parasitology
  • Horse Diseases / drug therapy
  • Horse Diseases / diagnosis
  • Female
  • Antiparasitic Agents / therapeutic use
  • Antiparasitic Agents / administration & dosage
  • Strongyle Infections, Equine / drug therapy
  • Strongyle Infections, Equine / parasitology
  • Strongyle Infections, Equine / diagnosis
  • Feces / parasitology

Conflict of Interest Statement

The authors have no conflicts of interest to declare.

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Citations

This article has been cited 1 times.
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