Original ResearchHumoral Response in Naturally Exposed Horses After Leptospiral Vaccination
Introduction
Leptospirosis is a worldwide zoonotic bacterial disease, although its incidence seems to be higher in tropical versus temperate areas [1]. In horses, it is often associated with recurrent uveitis and decreased athletic performance [2], [3]. Regardless, reproductive disorders probably represent the most important losses due to equine leptospirosis, including abortions, stillbirth or neonatal mortality, and birth of weak foals [4]. Although serologic evidence of leptospiral infections is common in horses, acute clinical disease is infrequent, and many animals may have chronic or subclinical disease [5].
Horses are susceptible to a wide range of infective serovars of Leptospira. In this regard, members of serogroups Pomona, Grippotyphosa, Autumnalis, Sejroe, Canicola, Ballum, and mainly Australis and Icterohaemorrhagiae have been commonly reported worldwide [6], [7], [8]. Although strains of serogroup Australis, particularly serovar Bratislava, are regarded as being adapted to horses [4], the nature of the host-pathogen relationship in this species has not been fully elucidated [9].
Control strategies are rarely reported for equine leptospirosis, perhaps due to a paucity of commercially available vaccines for horses. That whole-cell leptospirosis vaccine has been associated with equine recurrent uveitis [10], [11], what probably reduces its use in adult horses under field conditions. Nevertheless, other studies did not confirm that association [12] and vaccination of horses has become increasingly adopted worldwide. In the United States, vaccines for horses have been approved in the last decade, although in Brazil, there is only one vaccine specifically approved for prevention of leptospirosis in horses. These vaccines are multivalent bacterins (2 mL), composed of various serovars of Leptospira. In general, the main limitations are that vaccine-induced immunity seems to be serovar-specific, and multiple doses may be required to confer protection [13]. Because vaccination was not recommended for horses for many years, very little is known about humoral response postvaccine, and there is a paucity of knowledge regarding the dynamics of the response in naturally infected horses under field conditions.
Section snippets
Study Design
This experiment was conducted in naturally exposed mares, at the Instituto Vital Brazil farm in Rio de Janeiro, between August and December 2014. Fifty-four randomly selected mares were used. All mares were clinically healthy and had no history of vaccination against leptospirosis, and no indications of uveitis (based on a detailed clinical ocular examination). There were 36 and 18 horses randomly allocated to the vaccinated (GI) and nonvaccinated (control; GII) groups, respectively. The
Results
Throughout the trial, none of the mares had any apparent side effects, and there were no indications of reproductive effects (abortions), systemic (jaundice, fever), nor ocular (uveitis) disorders.
Discussion
On D0, pathogenic Leptospira were circulating in the studied herd, as confirmed by both direct (PCR) and indirect (MAT) methods. As expected, at that moment, there was no difference between groups I and II. Furthermore, on D0, mares had IgG titers, but not IgM, consistent with natural exposure to leptospiral antigens. In agreement with another study conducted on the same herd 2 years previously, Australis and Icterohaemorrhagiae were the most frequent serogroups [19].
The most important outcome
Conclusions
To the best of our knowledge, such kind of trial depicting the humoral response postvaccine has never been conducted with naturally exposed horses before, what impairs the comparison with other reports. Nevertheless, development of expected and significant antibody curves, as well as absence of any clinically apparent side effects on vaccinated horses, provided the impetus for a wider evaluation of vaccination under field conditions.
Acknowledgments
The authors thank Dr Marco Alberto Medeiros (FIOCRUZ), staff of Instituto Vital Brazil farm, MSc Sabina Lemos (UFF), Junia Grapiglia (UFF) and Prof Felipe Brandão (UFF) for laboratory and technical support. G. Martins, A. Loureiro, and W. Lilenbaum are fellows of Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ). W. Lilenbaum is a CNPq fellow. None of the authors has any financial or personal relationships that could inappropriately influence or bias the content of the paper.
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Ethical/Animal care and welfare statement: All procedures were approved by Animal Ethics Committee of the Universidade Federal Fluminense, Brazil (protocol number 261/2014).
Conflict of interest: The authors declare no conflicts of interest.