Occurrence of Leishmania infantum in Horses from Brazilian Amazon.

Overview

• This study investigated the presence of the parasite Leishmania infantum in horses in Manaus, a major city in the Brazilian Amazon, where human visceral leishmaniasis cases have not been reported despite being common elsewhere in Brazil.
• The researchers found a high occurrence of infection in horses, suggesting that these animals could serve as early indicators for the potential emergence of visceral leishmaniasis in the region.

Background

• Visceral leishmaniasis (VL) is a severe zoonotic disease caused by the protozoan parasite Leishmania infantum.
• The disease is widely distributed in Brazil and typically affects internal organs.
• Dogs are considered the primary reservoir host, responsible for maintaining the parasite life cycle and transmission to humans via sandfly vectors.
• Manaus is the largest urban center in the Amazon region with documented cutaneous leishmaniasis but no reported cases of visceral leishmaniasis in humans so far.

Purpose of the Study

• To assess whether horses in Manaus are infected with Leishmania infantum.
• Exploring the possibility that horses might play a role as sentinel animals for VL emergence.
• Filling a knowledge gap regarding the parasite’s presence in animal populations outside of dogs in the Amazon area.

Methods

• Sample collection: 178 horses were sampled from 16 different properties within Manaus.
• Diagnostic technique: Molecular diagnosis utilizing the L-like cathepsin gene specific to Leishmania infantum.
• The use of a molecular marker helps detect the parasite’s DNA accurately in host blood or tissue samples.

Key Findings

• Out of 178 horse samples, 82% tested positive for Leishmania infantum DNA.
• 15 out of 16 properties had at least one infected horse, indicating widespread occurrence across the city.
• Only one property had no positive horses, suggesting near-ubiquitous exposure or infection in the sampled regions.

Interpretation and Significance

• The detection of L. infantum in such a high percentage of horses suggests that these animals are actively exposed or infected in Manaus.
• The role of horses in VL epidemiology is still controversial—they are not established reservoirs but may be incidental hosts or amplifiers.
• Importantly, the infection in horses was found before any human VL cases appeared, indicating their potential as sentinel animals.
• Monitoring horses may allow early identification of Leishmania circulation in an area, enabling proactive public health interventions to prevent or contain potential human outbreaks.

Conclusions

• This study highlights the high prevalence of Leishmania infantum infection in horses in Manaus despite no reported human visceral leishmaniasis cases.
• Horses may serve as useful sentinel hosts, providing an early warning system for the emergence or establishment of VL in new areas.
• The findings call for enhanced surveillance strategies incorporating equine monitoring alongside traditional reservoir hosts like dogs.
• Further studies are needed to clarify the epidemiological role of horses in VL transmission cycles and their impact on human health risk.