Taenia solium oncosphere antigens induce immunity in pigs against experimental cysticercosis
Introduction
Taenia solium is a common parasite in pig raising areas of developing countries (Aluja, 1982, Garcia et al., 1993). The life cycle of T. solium includes the pig as the normal intermediate host, harboring the larval vesicles or cysticerci, and man as the definitive host, harboring the adult tapeworm (Nash and Neva, 1984). Humans can also serve as the intermediate host and develop the cystic form when they ingest T. solium eggs in food or water contaminated with feces (Schnaas, 1972). Human cysticercosis is an important contributor to neurologic pathology in endemic areas. (Garcia et al., 1993, Garcia et al., 1999, Medina et al., 1990). In Latin America, an estimated 75 million persons live in areas where cysticercosis is endemic, and ∼400,000 have symptomatic disease. Seizures are the most common manifestation of symptomatic human cysticercosis and 1 in 200 have seizures associated with neurocysticercosis (Bern et al., 1999). In addition, cysticercosis is an important porcine disease frequently present in over 30–60% of free ranging pigs in Peru and other Latin American countries (Bern et al., 1999). Porcine cysticercosis is also responsible for widespread economic losses for pig farmers (CWGP, 1993, Murrell, 1991).
In the usual transmission cycle, man acquires intestinal tapeworm infections by ingesting T. solium cysticerci in undercooked pork. In the intestine, the protoscolex evaginates and attaches to the intestinal wall by means of suckers. The tapeworm grows by forming segments termed proglottids from the caudal end of the scolex. The terminal proglottids contain approximately 50,000 eggs each and are shed intermittently in the feces. The intermediate host, normally the pig, is infected by ingesting eggs or proglottids from human feces. The eggs hatch and are activated by the action of gastric and intestinal fluid. The hatched larvae, also called the oncospheres, penetrate the intestinal mucosa and migrate via the blood stream throughout the body of the intermediate host and preferentially develop in muscle and brain. Over a period of 3 weeks to 2 months, the oncospheres enlarge and mature into cysticerci. The life cycle is completed when humans consume undercooked pork containing the cysts (White et al., 1992).
In the life cycle of T. solium, the pig is an obligatory intermediate host. Intervention with an effective porcine vaccine should interfere with the transmission of T. solium and may provide an useful adjunct for the control of cysticercosis (Gonzales et al., 1994).
The number of eggs needed for challenge is a major limiting step in the evaluation of T. solium vaccine candidates. Relatively large numbers of eggs (15,000–50,000) are needed to challenge each pig orally. Mature eggs and proglottids are difficult to obtain in large numbers, firstly because the prevalence of human tapeworm is low and difficult to detect. Secondly, even when detected, treatment often results in breakage of the tapeworm and the recovery of immature proglottids. We have developed a porcine model to evaluate vaccine efficacy. This model is based on infection of pigs by the intramuscular injection of oncospheres and is called the intramuscular oncosphere infection assay (IMOA; Verastegui et al., 2000). After a period of 3 months the oncosphere develop into viable cysts. The advantages of this model are that multiple sites can be infected in the same pig and the number of oncospheres needed to obtain infection, generally below 2000, is smaller than that used for oral infection (Verastegui et al., 2000). However, even this model usually requires the use of over 15,000 oncospheres and over 20,000 mature eggs per pig. In this study, we examined whether vaccination with either oncosphere or metacestode antigens would produce immunity for cysticercosis. We also observed what specific antigens were associated with immunity for T. solium oncosphere challenge.
Section snippets
Experiment 1—protection against reinfection in naturally infected pigs
Naturally infected pigs with cysticercosis were treated with oxfendazole (OFZ), a medicine known to completely clear cysts from pig muscle (Gonzales et al., 1997a) and were then challenged with viable oncospheres. This study was performed to determine if infected pigs that were cured could develop immunity to reinfection. Three tongue test positive pigs, generally heavily infected, aged 6–8 months old, were treated with two doses of 30 mg/kg of OFZ separated by a week. Given at this dose, OFZ is
Experiment 1
None of the T. solium infected pigs treated with OFZ on challenge developed cysts at any injection site. The three control pigs that were injected with 3×1500 viable oncospheres, in three different sites all developed cysts. The majority of cysts in one of the control pigs were viable while in the other two only degenerated cysts were found, Table 1.
Experiment 2
The two pigs immunized with an antigen made of metacestodes and oncospheres displayed complete immunity to an oncosphere challenge since neither
Discussion
This study showed that antibodies to two T. solium OA with SDS-PAGE molecular weights of 22.5 and 31.3 kDa, are unique to the oncosphere. Also, the two OA band consistently are present in pigs protected from the development of T. solium cysts, in a pig model where viable oncospheres are injected intramuscularly (IMOA). Using the IMOA model, we also showed that T. solium infected pigs that are cured with OFZ, are immune to further infection by IMOA challenge. This finding confirmed a previous
Acknowledgements
This work was partially funded by ICIDR-O1A135894-06, from the NIH an ABC and ITREID grant from Fogarty Foundation and the anonymous RG-ER fund. We also wish to thank J.B. Phu, D. Sarah and G. Sari-Cece for technical assistance.
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