Alternative metastrongyloid parasite transmission routes from gastropods to definitive hosts

Parasites within the superfamily Metastrongyloidea are bursate nematodes that infect various mammals. Generally, members of this superfamily are found within the lungs of their definitive host; however, some can be found in the pulmonary artery and right heart, meningeal tissues, skeletal muscles, or sinuses. Infections caused by metastrongyloids are significant in both human and veterinary medicine, where the effects on the host can range from subclinical infections to sudden death. Most metastrongyloid parasites incorporate a gastropod intermediate host in their life cycle. Transmission between hosts represents critical stages of the parasite’s life cycle. The first aim of this thesis was to determine the infectivity and retention of infectivity of gastropod-shed Angiostrongylus vasorum and Crenosoma vulpis L3 in dogs. Two “seed” dogs were exposed to 100 L3 each, resulting in patent infections. L1 collected from the seed dogs were used to infect naïve laboratory-raised Limax maximus slugs. Larval shedding was induced by anesthetizing the slugs in soda water and transferring them into warm (45 oC) tap water. Four naïve dogs were exposed to 100 gastropod-shed A. vasorum and C. vulpis L3, each stored for 0, 2, 4, and 8 weeks. All four exposures to shed L3 resulted in patent infections. The second aim was to evaluate the terrestrial gastropod fauna on PEI by (1) estimating gastropod population dynamics (species abundance and richness), (2) identifying which gastropod species were commonly infected with metastrongyloid parasites, (3) determining the prevalence, intensity, and metastrongyloid species found within terrestrial gastropods, and (4) identifying temporal trends of metastrongyloid parasites in gastropods on PEI. Terrestrial gastropods were collected and identified from 6 sites on PEI during the summer of 2018 and 5 in 2019. Metastrongyloid L3 were collected from gastropod tissues using artificial digestion. Larvae were identified using DNA sequencing following PCR using species-specific primers. A total of 964 terrestrial gastropods were collected. Eight gastropod species were identified, with the majority identified as Arion fasciatus-complex (67.01%) and Oxyloma decampi (18.5%). Three gastropod species were found to have metastrongyloid infections: A. fasciatus-complex, Cepaea hortensis, and Deroceras laeve. Metastrongyloids were found in 5.9% of collected gastropods. Metastrongyloid species identified included C. vulpis (81.2%), Filaroides martis (12.5%), and Protostrongylus boughtoni (12.5%), with one coinfection of C. vulpis and F. martis noted. July and August were noted to have the highest average L3/gastropod in 2018 and 2019, respectively. A novel finding of infections of F. martis and P. boughtoni in Arion fasciatus-complex slugs was noted in this study. The final aim was to assess the survival and decay rate of metastrongyloid L3 within a decomposing gastropod intermediate host. Four experiments were undertaken to further understand the survival and rate of decay of A. vasorum and C. vulpis L3, either as co-infections or as mono-infections. Experiments 1 and 2 used artificial digestion on dead L. maximus slugs co-infected with A. vasorum and C. vulpis weekly for 4- and 6-weeks, respectively. Experiments 3 and 4 also used artificial digestion on dead L. maximus slugs infected with C. vulpis 3 days/week for 4 weeks. Deceased L. maximus were stored in an environmental chamber set to 16 oC and 75% relative humidity (RH) for experiments 1-3 and at ambient room conditions (22 oC and 68% RH) during experiment 4. Third-stage larval migration into the environment was also assessed by a modified Baermann examination. Third-stage larval survival times in decomposing gastropods were assessed in two successive experiments. In the first and second experiments, 16 and 60 dead slugs were left to decompose at 16 oC and 75% RH, respectively. Four slugs were artificially digested every week in experiment 1, whereas 10 slugs were digested at time-0 and 10 slugs/week thereafter for experiment 2. To assess the rate of decay (loss) of L3, two subsequent experiments (3 & 4) were conducted at 16 oC and 75% RH and 22 oC and 68% RH, respectively. Fifteen slugs were artificially digested at time 0 and then 5 slugs 3 days/week. Living L3 were recovered up to three weeks post-mortem during all four experiments, with survival time exponentially decreasing and a maximum measured survival time of 6 weeks post-mortem. Migration of living L3 from the gastropod into the environment was low, with median counts of 12.5 L3 or less