Pilot field trial of the EG95 vaccine against ovine cystic echinococcosis in Rio Negro, Argentina: Early impact and preliminary data

Highlights

• A field trial of the EG95 vaccine against cystic echinococcosis was initiated in Rio Negro, Argentina.

• The prevalence of ovine echinococcosis is 61.3% in old animals at necropsy.

• Preliminary assessment identified a reduction in infection in the vaccinated animals.

Abstract

Cystic echinococcosis is endemic in the Rio Negro province of Argentina. After 30 years of control using praziquantel in dogs the transmission rate to humans and sheep has decreased significantly, however transmission persists. The objective of the study is to assess the impact of the inclusion of the EG95 vaccine for sheep in the control programme, including analysis of the vaccine's operative feasibility in field conditions. The vaccine was applied in an area comprising four communities of native people including 79 farms with 3146 lambs and 311 dogs in total. Seventy one farms were designated as control areas where no vaccinations were undertaken while vaccinations of lambs undertaken on 91 farms. Lambs received two vaccinations with the EG95 vaccine followed by a single booster injection when the animals were 1–1.5 years of age. Farm locations were defined using GPS coordinates for the houses. Evidence for Echinococcus granulosus transmission was monitored by coproantigen ELISA on samples of dog faeces, by E. granulosus-specific PCR using soil samples, and anti-E. granulosus antibody assessments in sera from 2 to 4 teeth lambs, purgation of dogs to detect E. granulosus worms and necropsy on adult sheep. Before the vaccine was introduced, 26.2% of sheep with 2–4 teeth were positive using ELISA/WB, the prevalence decreased to 7.8% at the third year following use of the vaccine. Necropsy of animals older than 6 years (not vaccinated) showed that 66.1% of animals were infected with E. granulosus. In dogs, 4% was found positive for E. granulosus using arecoline purgation and 24.7% of the farms were infected using coproELISA/WB. During the first year of vaccination 2721 lambs received the first vaccine dose and 2448 received a booster. In the second year 2138 lambs were initially vaccinated and 1745 received a booster, and 1308 animals received the third dose. During the third year 1110 lambs received the first dose from which 539 received a booster and 723 animals received the third dose. An analysis of advantages and limitations of the diagnostic techniques used and the ability of the geospatial analysis to detect risk area are included. Based in the immunodiagnostic techniques, the EG95 vaccine has been able to prevent the infection in animals up to 3 years old. Also, the difficulties in the field for the correct vaccine administration and the social features and habits that may impact on echinococcosis control are included in the analysis.

Introduction

Cystic echinococcosis (CE) is a parasitic zoonosis caused by infection with a taeniid cestode parasite, Echinococcus granulosus. The parasite requires two mammalian hosts to complete its life cycle. In South America the most important definitive host is the dog (which develops the adult tapeworm stage) and the most common intermediate host is the sheep (which develops the larval stage). Goats, pigs and cattle can be involved also as intermediate hosts (Eckert et al., 2001, Larrieu and Zanini, 2012). CE is one of the most prevalent zoonoses in Argentina, causing a significant burden for the health system due to the high costs of surgery and days spent in hospitals. It also causes losses in livestock due to the condemnation of infected viscera and decreases in production of wool, milk and meat (Budke et al., 2006).

Since the introduction of the cestocidal drug praziquantel, control programmes for CE have been based largely on dog treatment with this drug every 45–90 days in an attempt to eliminate the adult tapeworms. Hypothetically, if all dogs in a region were treated every 45 days the risk of transmission to humans and animals as intermediate hosts would decrease rapidly to reach a level where transmission was totally interrupted. In situations such as in the Rio Negro Province of Argentina, where sheep are responsible for acting as intermediate hosts, renewal of the sheep with animals born after interruption of transmission would cause the elimination of E. granulosus (Eckert et al., 2001).

However, where CE control has relied on frequent treatment of dogs with praziquantel, more than 10 years of intensive, compulsory intervention has been required before CE transmission reached a low level and almost 30 years has been required to achieve freedom from the disease (Craig and Larrieu, 2006). In other areas where CE control activities have been undertaken in the past century, the infrastructure required to treat the dogs 8–12 times per year for a long period of time (10 years or more) has not been sustainable, often because CE endemic areas are the poorest in each endemic country (Larrieu and Zanini, 2012).

A control programme for CE started in 1980 in the Rio Negro Province based on treatment of dogs with praziquantel every 90 days, using the existing primary health care infrastructure to deworm dogs. A group of health care assistants (non-professional staff) conducted home visits, while veterinarians from the health department lent support and managed the surveillance system. This network carried out four rounds of home visits annually. The health care assistants visited rural areas distributing praziquantel tablets to dog owners who were ultimately responsible for carrying out the deworming. Education activities were also undertaken in the community concerning the nature of the disease, lifecycle and the need to prevent dogs gaining access to sheep offal. Serological testing and untrasonography were undertaken on the human population in order to effect early diagnosis of CE infections (Larrieu et al., 2000, Larrieu et al., 2011, Larrieu and Zanini, 2012). The programme has been successful in decreasing the prevalence of E. granulosus in dogs and humans, although a proportion of infected sheep remain and this has been sufficient to lead to a continued incidence of CE in children (Larrieu et al., 2001, Larrieu et al., 2000).

Vaccination of potential intermediate hosts of E. granulosus with the EG95 recombinant vaccine (Heath et al., 2003, Lightowlers et al., 1996) could potentially be used to reduce the level of E. granulosus transmission and decrease the incidence of human infections (Bethony et al., 2011). Torgerson, 2003, Torgerson, 2006 and Torgerson and Heath (2003) have used mathematical models to predict the impact of various options for control of CE and considered that a programme involving vaccination of intermediate hosts together with 6-monthly treatment of dogs with praziquantel would decrease the time needed to achieve control of disease transmission.

The EG95 vaccine is a recombinant protein cloned from mRNA from the oncosphere life cycle stage of the parasite. The vaccine comprises the EG95 protein together with the adjuvant Quil A. Two immunizations in young animals protects against a challenge infection with E. granulosus by inducing specific antibodies against the oncosphere, eliminating it before it can establish and develop in the tissues of the intermediate host. EG95 is a defined protein, non-infectious, non-toxic and is produced using genetic engineering and expressed in Escherichia coli (Heath et al., 2003, Lightowlers et al., 1996, Lightowlers et al., 1999). However, other than some data about the use of the vaccine in China (Heath et al., 2003, Heath et al., 2006), there is little published information about the impact of the EG95 vaccine when used in field conditions nor about potential problems that could arise when the vaccine was applied on a large scale in livestock flocks (Larrieu and Zanini, 2012).

In order to assess the impact of the introduction of the EG95 vaccine in the control programme in Rio Negro, the authorities in charge of the programme decided to trial the introduction of the vaccine as an additional control tool. It is considered that optimal protection with the EG95 vaccine would be afforded by two immunizations in lambs followed by annual booster immunizations (Heath et al., 2003). However the resources available for hydatid control in Rio Negro were insufficient to support the requirements of this regime and it was decided to undertake a vaccination programme in which lambs received two vaccinations followed by a single booster immunization when the animals were 1–1.5 years of age. The objective of this work was as assessment of the baseline data in the regions selected for the control programme with vaccination and a preliminary assessment of the effects of EG95 vaccination on lambs born following the introduction of the vaccine. Secondary objectives include the assessment of the advantages and disadvantages of the diagnostic tests available for epidemiological surveillance of CE and also an analysis of the usefulness of the geospatial analysis in the CE control activities.