# In vitro inhibitory effects of gentamicin and ceftiofur against Trypanosoma evansi: Promising antibiotic alternatives for equine trypanosomosis in Thailand

## TL;DR

This study shows that gentamicin and ceftiofur can inhibit the growth of Trypanosoma evansi in the lab, suggesting they may be useful as alternative or supporting treatments for equine trypanosomosis in Thailand.

## Contribution

The study provides the first in vitro evidence of gentamicin and ceftiofur's trypanocidal activity against the Thai strain of T. evansi.

## Key findings

- Gentamicin and ceftiofur significantly inhibited T. evansi growth in vitro.
- Ceftiofur had a lower EC50 value than gentamicin, indicating higher efficacy.
- Both antibiotics completely eliminated T. evansi at high concentrations after 48 hours.

## Abstract

Trypanosoma evansi infection (Surra) remains a major constraint to equine health and productivity in Thailand. The only available trypanocidal drug, diminazene aceturate (DA), has limited efficacy, poor blood–brain barrier penetration, and toxicity in horses. This study aimed to investigate the in vitro inhibitory effects of commonly used equine antibiotics, gentamicin (GMC), ceftiofur (CTF), and trimethoprim–sulfamethoxazole (TS), against T. evansi (Thai strain isolated from dairy cattle number 953; TEDC 953) to identify potential therapeutic alternatives or adjuncts for equine trypanosomosis.

An in vitro growth inhibition assay was conducted using the T. evansi TEDC 953 strain cultivated in Hirumi’s Modified Iscove’s medium 9 (HMI-9 medium) containing 20% horse serum under controlled conditions (37°C, 5% CO2, 75% humidity). Serial dilutions of DA, GMC, CTF, and TS were tested in duplicate across three independent experiments. Parasite viability was assessed after 48 h by microscopic examination, and the half-maximal effective concentration (EC50) was determined using nonlinear regression analysis in GraphPad Prism 5.

Among the three antibiotics, GMC and CTF significantly inhibited T. evansi growth in vitro, whereas TS showed no inhibitory effect. The EC50 values were 1.25 × 10-5 ± 3.90 × 10-6 mg/mL for DA, 0.22 ± 0.08 mg/mL for GMC, and 0.08 ± 0.05 mg/mL for CTF. Parasite viability assays confirmed that GMC (5 mg/mL) and CTF (0.2 mg/mL) completely eliminated T. evansi after 48 h of exposure. These findings provide the first in vitro evidence of the trypanocidal potential of GMC and CTF against the Thai strain of T. evansi.

GMC and CTF exhibited substantial inhibitory activity against T. evansi under in vitro conditions, supporting their potential use as repurposed or adjunct antibiotics for trypanocidal therapy in horses. This preliminary evidence underscores the need for in vivo validation, pharmacokinetic profiling, and mechanistic studies to explore synergistic effects with conventional trypanocides such as DA.