Research Article: Pilot Field Trial of the EG95 Vaccine Against Ovine Cystic Echinococcosis in Rio Negro, Argentina: Second Study of Impact

Date Published: October 30, 2015

Publisher: Public Library of Science

Author(s): Edmundo Larrieu, Guillermo Mujica, Charles G. Gauci, Katherina Vizcaychipi, Marcos Seleiman, Eduardo Herrero, José Luis Labanchi, Daniel Araya, Luis Sepúlveda, Claudia Grizmado, Arnoldo Calabro, Gabriel Talmon, Thelma Verónica Poggio, Pablo Crowley, Graciela Cespedes, Graciela Santillán, Mariela García Cachau, Roberto Lamberti, Lilia Gino, Meritxell Donadeu, Marshall W. Lightowlers, Hector H Garcia.

Abstract: BackgroundCystic echinococcosis (CE) is an important zoonotic disease caused by the cestode parasite Echinococcus granulosus. It occurs in many parts of the world where pastoral activities predominate, including the Rio Negro province of Argentina. Although CE control activities have been undertaken in the western regions of Rio Negro for more than two decades, the disease continues to remain prevalent in both the human and livestock animal populations. Vaccination of animal intermediate hosts of CE with the EG95 vaccine may provide a new opportunity to improve the effectiveness of CE control measures, although data are lacking about field application of the vaccine.AimsEvaluate the impact of EG95 vaccination in sheep on the transmission of Echinococcus granulosus in a field environment.MethodologyTwo trial sites were established in western Rio Negro province within indigenous communities. Vaccination of lambs born into one trial site was introduced and continued for 6 years. Prior to initiation of the trial, and at the end of the trial, the prevalence of CE in sheep was determined by necropsy. Weaned lambs received two injections of EG95 vaccine, approximately one month apart, and a single booster injection one year later. Vaccination was not implemented at the second trial site. A total of 2725 animals were vaccinated in the first year. Animals from this cohort as well as age-matched sheep from the control area were evaluated by necropsy.Key resultsIntroduction of the vaccine led to a statistically significant in the number and size of hydatid cysts in comparison to the situation prior to the introduction of the vaccine, or compared to CE prevalence in the control area where the vaccine was not applied. The prevalence of infection in the vaccinated area was also significantly reduced by 62% compared to the re-intervention level, being lower than the prevalence seen in the control area, although the difference from the control area after the intervention was not significant possibly due to limitations in the numbers of animals available for necropsy.ConclusionsVaccination of sheep with the EG95 vaccine provides a valuable new tool which improves the effectiveness of CE control activities. Vaccination was effective even in a difficult, remote environment where only approximately half the lambs born into the communities were fully vaccinated.

Partial Text: Cystic echinococcosis (CE) is a zoonosis present worldwide produced by Echinococcus granulosus (EG). As the most common intermediate hosts (sheep and goats) that develop cysts in liver and lung [1] [2], human may be infected and develop cysts in the same organs. The dog is the definitive host.

An intervention study with a control group was defined. The regions chosen for the program were Anecon Grande, Rio Chico Abajo, Nahuel Pan, Manuel Choique, Blancura Centro and Lipetren. Each farm was defined as an Epidemiologic Unit (EU), each containing one house or houses for one extended family. The geographic region was the Rio Negro Province in Argentina comprising, in total, an area of 5820 Km2 [10] (Fig 1).

The numbers of animals involved in the vaccinations during the 6 years in which the program was continuing are shown in Table 1 (total 21447 sheep vaccinates). 9834 lambs received an initial dose of vaccine with a minimum and maximum coverage of 68.2% and 95.9% of the animals respectively, and 55.5 to 100% of the farms; 8082 received the second immunization approximately one month later with a minimum and maximum coverage of 43.3–87.8% of the animals respectively and 68.8–95.0% of the farms. 3531 received the third dose approximately one year later with a minimum and maximum coverage of 71.1 and 96.2% of the animals. To apply the third dose, the initial number of lambs was reduced by approximately 50% in each year (due to the animals being sold, consumed, predated, etc.).

The experience of the pilot vaccination program in Rio Negro with the EG95 vaccine described here found that vaccinated sheep had a significantly decreased prevalence of E. granulosus infection in adult animals, 21.1% in 2015 compared to 56.3% in 2009 (P = 0.03). A decrease in CE prevalence was also observed in the control area between 2009 and 2015 which was not statistically significant. Limitations in the number of animals available for necropsy may have contributed to there not being a statistically significant difference between the vaccination and control areas in 2015. There was an increased presence of veterinary staff involved in undertaking the trial activities in the control area as well as the vaccination areas over the duration of the study. This is considered to have been a likely to have led to greater compliance of the farmers in treating their dogs with praziquantel and this contributed to the reduction seen in the prevalence of CE there the non vaccination areas. In relation to the number and size of the hydatid cysts, 1.4 cysts per animal were found in the control area in 2009 (in the same communities as Larrieu et al., 2001) whereas 0.3 cysts were found per infected animal in the vaccinated area after 5 years of the program. The number of farmers with at least one E. granulosus infected animal was 94.7% at the start of the program and 23.5% in the evaluation described here. This suggests a substantial decrease of the infection risk to dogs due to a reduced availability of infected offal, which would be expected to translate to a lower infection in dogs and consequently incidence of human infection in the communities where the vaccine was used.



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