Cercarial survival in an intertidal trematode: a multifactorial experiment with temperature, salinity and ultraviolet radiation.

Parasite transmission takes place in the context of a multitude of simultaneously fluctuating environmental factors. As a particularly vulnerable step in the transmission, trematode cercariae are directly exposed to ambient conditions during their search for a host. Here, we investigated the survival of cercariae of the intertidal trematode Maritrema novaezealandensis in a multifactorial experiment (2 × 2 × 2 design) with temperature (20 and 30 °C), salinity (35 and 40 practical salinity units (psu)) and ultraviolet radiation (UVR; exposed and not exposed) as main factors. All factors had significant effects, with cercariae dying faster at the higher temperature, increased salinity and when exposed to UVR. Full activity ceased within ~6 h in all treatments at 30 °C, except at 35 psu under no exposure to UV; in all other treatments full activity was maintained for >6 h. Several factor interactions were identified, of which the interactive negative effect of temperature and UVR was the most important. These results imply that conditions during the main transmission window of M. novaezealandensis are highly challenging for cercariae. Our findings highlight the importance of considering multiple environmental factors in the study of parasite transmission to gain a more ecologically relevant understanding of transmission dynamics.

Effects of ultraviolet radiation on an intertidal trematode parasite: an assessment of damage and protection.

Trematode parasites are integral components of intertidal ecosystems which experience high levels of ultraviolet radiation. Although these parasites mostly live within hosts, their life cycle involves free-living larval transmission stages such as cercariae which are directly exposed to ambient conditions. UV has previously been shown to considerably reduce the survival of cercariae. Here, we investigated potential mechanisms of protection and damage related to UV in the intertidal trematode Maritrema novaezealandensis. Firstly, the presence of sunscreen compounds (i.e. mycosporine-like amino acids) was quantified in the parasite tissue producing cercariae within a snail host, as well as in the free-swimming cercariae themselves. Secondly, levels of oxidative stress in cercariae after exposure to UV were investigated (i.e.protein carbonyls, catalase and superoxide dismutase). Thirdly, the DNA damage (i.e. cyclobutane­pyrimidine dimers) was compared between cercariae exposed and not exposed to UV. Lastly, functional aspects(survival and infectivity) of cercariae were assessed, comparing cercariae under light conditions versus dark after exposure to UV. We confirmed the presence of my cosporine-like amino acids in cercariae-producing tissue from within snail hosts, but were unable to do so in cercariae directly. Results further suggested that exposure to UV induced high levels of oxidative stress in cercariae which was accompanied by a reduction in the levels of protective antioxidant enzymes present. We also identified higher levels of DNA damage in cercariae exposed to UV, compared with those not exposed. Moreover, no clear effect of light condition was found on survival and infectivity of cercariae after exposure to UV. We concluded that cercariae are highly susceptible to UV damage and that they have very little scope for protection against or repair of UV-induced damage.

Effects of ultraviolet radiation on the transmission process of an intertidal trematode parasite.

The transmission of parasites takes place under exposure to a range of fluctuating environmental factors, one being the changing levels of solar ultraviolet radiation (UVR). Here, we investigated the effects of ecologically relevant levels of UVR on the transmission of the intertidal trematode Maritrema novaezealandensis from its first intermediate snail host (Zeacumantus subcarinatus) to its second intermediate amphipod host (Paracalliope novizealandiae). We assessed the output of parasite transmission stages (cercariae) from infected snail hosts, the survival and infectivity of cercariae, the susceptibility of amphipod hosts to infection (laboratory experiments) and the survival of infected and uninfected amphipod hosts (outdoor experiment) when exposed to photo-synthetically active radiation only (PAR, 400-700 nm; no UV), PAR+UVA (320-700 nm) or PAR+UVA+UVB (280-700 nm). Survival of cercariae and susceptibility of amphipods to infection were the only two steps significantly affected by UVR. Survival of cercariae decreased strongly in a dose-dependent manner, while susceptibility of amphipods increased after exposure to UVR for a prolonged period. Exposure to UVR thus negatively affects both the parasite and its amphipod host, and should therefore be considered an influential component in parasite transmission and host-parasite interactions in intertidal ecosystems.

Encystment and excystment of the paramphistomid trematode Zygocotyle lunata.

This study reports further observations on encystment and excystment of the paramphistomid trematode Zygocotyle lunata. Of numerous substrates tested in the laboratory for cercarial encystment, i.e. plastic and glass dishes, Styrofoam™, aluminium foil, snail shells, and cheesecloth, all but cheesecloth allowed for 100% encystment within 4 h of cercarial release at 28°C. Numerous physical and chemical factors were tested to kill cysts, and their capacity to do so was determined by the subsequent inability of cysts to excyst within a chemical excystment medium. Vinegar, Italian salad dressing, soy sauce and 10% solutions of sucrose, acetic acid and sodium chloride were the most detrimental. Freezing, boiling and storage of cysts for more than 2 years also killed them. Several marinades such as lemon juice, Worcestershire sauce, and various concentrations of potassium permanganate, did not kill the majority of cysts tested. Since Z. lunata is a potential foodborne trematode for humans and animals, it is important to determine suitable ways to control or kill these cysts.

Radioresistance of Anisakis simplex third-stage larvae and the possible role of superoxide dismutase.

The radioresistance of Anisakis simplex third-stage larvae and the possible role of sublethal radiation on superoxide dismutase (SOD) were investigated. Larvae were isolated from the viscera of the sea eel Anago anago; irradiated with 10, 100, 200, 500, or 1,000 Gy; and then given orally to rats. Worms were recovered at 16 hr postinoculation. Most larvae were found to have invaded the gastric wall, omentum, and abdominal cavity, suggesting that their viability and infectivity were not controlled by irradiation with the doses used. To determine the relationship between SOD activities in parasites and their radiosensitivities, the larvae of A. simplex and the metacercariae of Neodiplostomum seoulense (a radiosensitive control) were irradiated with 0, 30, 100, or 500 Gy, and parasite SOD levels were measured. In nonirradiated A. simplex larvae, the average SOD level was 38.9 U/mg, and this increased to 51.3 U/mg at 500 Gy. However, at all radiation doses applied, SOD activities of N. seoulense metacercariae were significantly (P < 0.05) lower than those of A. simplex larvae. Our results demonstrate that A. simplex third-stage larvae are radioresistant, and suggest that SOD plays a role in this radioresistance.