Epidemiology and control of canine leishmaniosis: characterization of a previously undescribed endemic area in Catalonia and CaniLeish® vaccine field trial

Abstract

Leishmaniosis is an important vector-borne zoonosis caused by Leishmania infantum. The disease is widespread across several continents and endemic in the Mediterranean region. The domestic dog is the main vertebrate reservoir for the parasite and control of canine leishmaniosis (CanL) is deemed to be essential for the control of human cases of the disease. Due to the heterogeneous distribution of infection in endemic areas, epidemiological surveillance should be carried out focally, including both screening of canine populations and vector detection, the two determinant factors for parasite survival and expansion. CanL control measures are usually directed at the canine reservoir through the detection and treatment of infected individuals, as well as disease prevention through insecticide treatments and/or canine immunoprophylaxis. Vaccination against CanL is relatively recent and evidence of its impact in infection control at the community level is still insufficient. This is also the case for CaniLeish® vaccine, the first CanL vaccine to be licensed in Europe, in 2011. Pre-licensing studies were performed exclusively in homogeneous populations of beagle dogs, experimentally infected or introduced in endemic areas, and very little is known regarding this vaccine’s performance in native and heterogeneous dog populations from L. infantum endemic areas. The study presented in this thesis is divided into two parts. The first consists of a CanL epidemiological study in Girona province, a previously uncharacterized region of north-eastern Spain. The results obtained confirmed the endemicity of CanL in Girona province, characterized by a high prevalence of L. infantum infection in dogs (19.5%), together with the detection of a significant proportion of asymptomatic infected individuals (93.2%). The increase of dogs’ age and lower altitude of the kennel location were identified as risk factors. The two antigens tested to assess dog exposure to Phlebotomus perniciosus (SGH and rSP03B salivary antigens) proved to be suitable, with specific antibodies showing a marked decrease during the non-transmission season, which allowed detection of recent host exposure to vectors. In addition, detected levels of antibodies against both SGH and rSP03B were associated with seropositivity to L. infantum. The second part of this thesis describes a one year field trial of CaniLeish® vaccine, performed in a native heterogeneous canine population from Girona province. These dogs were kept in their natural housing conditions throughout the study and were naturally exposed to an L. infantum transmission season. Results showed that CaniLeish® vaccine induces the production of non-specific antibodies interfering with the serological diagnosis of L. infantum infection in dogs and that this interference could have a greater impact between one and four months post-vaccination. Vaccine trial results did not confirm CaniLeish® reported efficacy in preventing active L. infantum infection or clinical disease in dogs during the first year post-vaccination. These results were supported by an apparently short-lived vaccine-induced cellular mediated immunity, assessed in this study through the quantification of gamma-interferon (IFN-γ) produced by trial dogs at one and nine months post-vaccination. The results presented in this thesis support the need for maintaining and extending epidemiological surveillance in CanL endemic areas, in order to better characterize current CanL distribution and to anticipate possible L. infantum expansion trends. Additionally, further CaniLeish® evaluation studies are needed, together with active vaccine surveillance, to definitely assess the utility of this vaccine in CanL control at the community level in L. infantum endemic areas.

La leishmaniosis es una zoonosis de transmisión vectorial que en la región mediterránea está causada por Leishmania infantum y presenta al perro como principal reservorio. Su distribución heterogénea hace que la vigilancia epidemiológica deba realizarse de manera focalizada, abarcando tanto la detección de perros infectados como de los flebotomos vectores. Las medidas de control de la leishmaniosis canina (LCan) incluyen la vacunación de perros. En Europa, la primera vacuna para la LCan (CaniLeish®) se autorizó en 2011 y los estudios previos a la licencia se realizaron exclusivamente en poblaciones homogéneas de perros beagle, infectados experimentalmente o introducidos en áreas endémicas. La primera parte de la tesis incluye un estudio epidemiológico de la LCan en la provincia de Girona, previamente sin caracterizar. Los resultados obtenidos mostraron una alta prevalencia de infección por L. infantum (19,5%) y una proporción significativa de individuos infectados asintomáticos (93,2%). Se identificaron como factores de riesgo el aumento de la edad de los perros y la menor altitud de la ubicación de las perreras. El estudio de la exposición de los perros a los flebotomos a través del análisis de los antígenos salivales de Phlebotomus perniciosus (SGH y rSP03B) mostró ser útil. Los niveles de anticuerpos detectados mostraron una marcada disminución durante la temporada de no transmisión, lo que permitiría la detección de la exposición reciente a los vectores, y una asociación significativa con la seropositividad frente a L. infantum. La segunda parte describe un ensayo de campo de un año de CaniLeish®, realizado en una población canina heterogénea natural de Girona. Los perros se mantuvieron en condiciones habituales de alojamiento y estuvieron naturalmente expuestos a una temporada de transmisión. La vacuna indujo la producción de anticuerpos no específicos que interferirían en el diagnóstico serológico de la infección por L. infantum, con un impacto mayor entre uno y cuatro meses después de la vacunación. Los resultados no confirmaron la eficacia de CaniLeish® en la prevención de la infección activa por L. infantum o la enfermedad clínica en perros durante el primer año post-vacunación. Estos resultados fueron respaldados por una inmunidad mediada por células inducida por la vacuna aparentemente de corta duración, evaluada a través de la cuantificación del interferón gamma (IFN-γ).