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. cyclobutane–pyrimidine 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.     

Variation in UV sensitivity among common frog Rana temporaria populations along an altitudinal gradient

Solar ultraviolet-B (UV-B) radiation can be harmful for developing amphibians. As the UV-B dose increases with altitude, it has been suggested that high-altitude populations may have an increased tolerance to high levels of UV-B radiation as compared to lowland populations. We tested this hypothesis with the common frog (Rana temporaria) by comparing populations from nine altitudes (from 333 to 2450m above sea level). Eggs collected in the field were used for laboratory experiments, i.e., exposed to high levels of artificial UV-B radiation. Eggs were reared at 14+/-2 degrees C and exposed to UV treatments until hatching. Embryonic developmental rates increased strongly and linearly with increasing altitude, suggesting a genetic capacity for faster development in highland than lowland eggs. Body length at hatching varied significantly with UV-B treatments, being lower when eggs developed under direct UV-B exposure. Body length at hatching also increased as the altitude of populations increased, but UV-B exposure times were shorter as altitude of population increased. However, the body length difference between exposed and non-exposed individuals in each population decreased as altitude of populations increased, suggesting a costly effect of UV exposure on growth. Type of UV exposure did not influence the mean rates of embryonic mortality and deformity, but both mortality and deformity rates increased as the altitude of populations increased (while UV-B exposure duration decreased). The effect of UV-B on body length at hatching, mortality, and deformities suggests that the sensitivity to UV-B varied among populations along the altitudinal gradient. These results are discussed in evolutionary terms, specifically the potential of R. temporaria high-altitude populations to develop local genetic adaptation to high levels of UV-B.     

Immunological properties of Fc receptor on lymphocytes. 4. Fc receptor of Con A induced suppressor and helper T cells.

The effects of a variety of exposure regimens were assessed for their ability to induce optimal, immunologically mediated resistance against subsequent homologous reinfection by Schistosoma mansoni. Following exposure, Fischer rats modulated their subsequent worm burden, through as yet undefined internal mechanisms, and in a manner which resulted ultimately in a relatively consistent worm burden. The magnitude and rapidity of development and the longevity of maintenance of resistance was related to the intensity and pattern of exposure to normal cercariae. The modulatory effects tended to favor the development of optimal resistance over a wide variety of exposure patterns. Optimal resistance was most efficiently stimulated and maintained by immunogens which were associated with early stages of infection. Antihelminthic drug therapy adversely affected both the development and maintenance of resistance. Rats were also exposed to irradiated cercariae and soluble surface membrane preparations obtained from cercariae, adult worms, or eggs. Animals exposed to irradiated cercariae or soluble cercarial immunogen developed resistance to approximately 80 and 35%, respectively, of the levels shown by the naturally infected animals. The mechanism of this variable resistance was investigated using a number of in vivo and in vitro assessments and is the subject of a subsequent report.     

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.     

Long-term dose response of trabecular bone in mice to proton radiation

Astronauts on exploratory missions will experience a complex environment, including microgravity and radiation. While the deleterious effects of unloading on bone are well established, fewer studies have focused on the effects of radiation. We previously demonstrated that 2 Gy of ionizing radiation has deleterious effects on trabecular bone in mice 4 months after exposure. The present study investigated the skeletal response after total doses of proton radiation that astronauts may be exposed to during a solar particle event. We exposed mice to 0.5, 1 or 2 Gy of whole-body proton radiation and killed them humanely 117 days later. Tibiae and femora were analyzed using microcomputed tomography, mechanical testing, mineral composition and quantitative histomorphometry. Relative to control mice, mice exposed to 2 Gy had significant differences in trabecular bone volume fraction (-20%), trabecular separation (+11%), and trabecular volumetric bone mineral density (-19%). Exposure to 1 Gy radiation induced a nonsignificant trend in trabecular bone volume fraction (-13%), while exposure to 0.5 Gy resulted in no differences. No response was detected in cortical bone. Further analysis of the 1-Gy mice using synchrotron microCT revealed a significantly lower trabecular bone volume fraction (-13%) than in control mice. Trabecular bone loss 4 months after exposure to 1 Gy highlights the importance of further examination of how space radiation affects bone.     

No lengthening of life span in mice continuously exposed to gamma rays at very low dose rates

Late effects of continuous exposure to ionizing radiation are potential hazards to workers in radiation facilities as well as to the general public. Recently, low-dose-rate and low-dose effects have become a serious concern. Using a total of 4000 mice, we studied the late biological effects of chronic exposure to low-dose-rate radiation as assayed by life span. Two thousand male and 2000 female 8-week-old specific-pathogen-free (SPF) B6C3F1 mice were randomly divided into four groups (one nonirradiated control and three irradiated). Irradiation was carried out for approximately 400 days using (137)Cs gamma rays at dose rates of 21 mGy day(-1), 1.1 mGy day(-1) and 0.05 mGy day(-1) with total doses equivalent to 8000 mGy, 400 mGy and 20 mGy, respectively. All mice were kept under SPF conditions until they died spontaneously. Statistical analyses showed that the life spans of mice of both sexes irradiated with 21 mGy day(-1) (P < 0.0001) and of females irradiated with 1.1 mGy day(-1) (P < 0.05) were significantly shorter than those of the control group. Our results show no evidence of lengthened life span in mice continuously exposed to very low dose rates of gamma rays.     

低温对沙葱萤叶甲越冬卵存活和发育的影响

【目的】沙葱萤叶甲Galeruca daurica(Joannis)是近年来在内蒙古草原暴发成灾的一种新害虫,以卵在牛粪、石块及草丛下越冬,了解其越冬卵的抗寒能力有助于预测其分布范围及种群数量动态。【方法】在室内测定了沙葱萤叶甲越冬卵在不同低温条件下(-18~-39℃)暴露12和24 h及在-30℃低温条件下暴露不同时间(0~60 d)的存活率以及存活卵的发育历期。【结果】低温强度和暴露时间对沙葱萤叶甲越冬卵的存活率有显著影响,随着温度的降低和暴露时间的延长,越冬卵的存活率降低。当温度≤-33℃暴露12 h或≤-30℃暴露24 h,越冬卵存活率显著低于其相应的对照(25℃)。越冬卵低温暴露12和24 h的致死中温度(LT50)分别为-33.08和-32.13℃,在-30℃下的致死中时间(Lt50)为33.33 d。经-36℃低温暴露12 h或≤-33℃低温暴露24 h后,存活的越冬卵发育历期显著延长,而-30℃低温暴露30 d内差异不显著。【结论】沙葱萤叶甲越冬卵抗寒能力强,冬季低温通常不会造成越冬卵的大量死亡。     

Histopathology of experimental Schistosoma incognitum infection in mice following exposure to normal and irradiated cercariae.

The tissue reactions in mice, experimentally infected with normal and irradiated cercariae of S. incognitum were studied. The lesions observed in the skin, liver, lungs and the intestine of mice infected with normal cercariae are briefly described, and compared with those observed with cercariae irradiated at 3000 r of gamma rays. In general, the reactions in mice exposed to normal cercariae were more intense than in those infected with irradiated cercariae. The severity of the reactions appeared largely due to the deposition of eggs in the tissues of the mice infected with normal cercariae. The experimental evidence suggested that most of the flukes from the irradiated cercariae are destroyed in the liver by tissue reaction.     

Experimental Infection of Danish Landrace/Yorkshire Crossbred Pigs with Schistosoma japonicum from the People’s Republic of China

This study was undertaken to assess the suitability of Danish Landrace/Yorkshire (L/Y) crossbred pigs as experimental hosts of a Chinese mainland strain of Schistosoma japonicum. Pigs were exposed to 200, 500 or 1000 cercariae and parasite burdens were determined by perfusion after either 8 or 11 weeks. All pigs became infected with onset of faecal egg excretion 6 to 7 weeks following exposure to cercariae. The pattern of faecal egg excretion differed markedly among the individual animals. Gross hepatic pathological lesions of varying degrees were noted in all of the pigs. Schistosome worm recoveries ranged from 1.5–23.4% of the cercarial exposure dose. Most schistosome eggs recovered from the tissues, expressed as eggs/g tissue, were found in the rectum (91%), caecum (3.1%) and liver (5.1%). The results show that Danish L/Y pigs may serve as appropriate experimental final hosts of the Chinese mainland strain of S. japonicum.     

Cercarial shedding of Echinostoma cinetorchis and experimental infection of the cercariae to several kinds of snails

The development of Echinostoma cinetorchis in several snail species reared in laboratory aquaria was observed. The eggs from adult flukes collected from the intestine of rats were cultivated to miracidia, and exposed to Hippeutis sp. snails. Observations were made for cercarial shedding from the exposed snails. The cercariae shed from the snails were again exposed to several species of fresh water snails in order to observe metacercarial formation in the snails and their infectivity to final hosts. The results obtained in this study were as follows: 1. Twenty miracidia were exposed to each snail of Hippeutis sp. About 58.3% of the above snails (7 out of 12) were dead before shedding the cercariae, and the remainder shed the cercariae for a period of 7 to 9 days before death. 2. Cercarial shedding from the infected snails started from the 25th day after the exposure to miracidia, and the total number of cercariae shed per snail was 684 in average (range; 482-904). 3. The size of rediae developed in the infected Hippeutis sp. snails was 1,242 x 214 microns in average, and the number of rediae per snail was 350 in average (range; 120-510). 4. About 40 to 50 cercariae shed from the Hippeutis sp. snails were each exposed to several species of snails reared in the laboratory. The metacercarial formation was confirmed by dissecting the infected snails, 12 to 16 days after the infection.(ABSTRACT TRUNCATED AT 250 WORDS)     

Analysis of the anti-Schistosoma mansoni surface antibody response during murine infection and its potential contribution to protective immunity

Absorption of serum from chronically infected mice with homogenized schistosome eggs reduced antibody binding to the schistosomulum surface by 94%, indicating that almost all schistosomulum surface recognition during chronic infection is due to epitopes shared with the egg. Absorption of the serum with egg homogenate from which protein antigens had been removed by boiling and digestion with proteinase K resulted in a similar reduction of antisurface antibody demonstrating that all the shared epitopes that are recognized are carbohydrate in nature. Analysis of the time course of anticarbohydrate antibody production and the levels of antibody in mice infected with a single sex of schistosome indicated that eggs directly stimulated this response. Mouse mAb were identified that bound at very high levels to the schistosomulum surface and that recognized carbohydrate epitopes shared with the egg. Three of these had previously been demonstrated to passively transfer resistance, indicating that these surface carbohydrates are potential targets of protective immunity in the mouse. All the anticarbohydrate mAb also bound to the surface of schistosomula of other schistosome species. Thus, the strong immune response against these epitopes in chronic infection could account for the cross-specific immunity observed. Mice vaccinated with irradiated cercariae lacked high levels of anticarbohydrate antibodies and their recognition of the surface was largely due to antibody to species-specific polypeptide epitopes. With respect to the Mr greater than 200,000 and 38,000 antigens, it was demonstrated that these epitopes were present on the same antigens that bear the carbohydrate moieties recognized by antibodies from chronically infected mice. This specific polypeptide recognition is also reflected in the immunity generated by exposure to irradiated cercariae.     

Penetration and maturation of Schistosoma mansoni in suckling and adult Swiss Webster and DBA/2 mice

Murine schistosomosis is a widely used experimental model of the human disease. Different methods have been employed to infect mice with Schistosoma mansoni cercariae, such as subcutaneous or intraperitoneal injections, the tail immersion technique, and the use of a metal ring placed on the abdominal skin of anaesthetized animals. An alternative method of infection that requires no anaesthesia and no restraint is to place suckling mice (10 days old) on a Petri dish with a small volume of water containing cercariae. In this study, we compared the penetration and maturation of S. mansoni in mice infected by this alternative method with those noted in mice infected at an older age (45 days) by the tail immersion technique. Besides evaluating the effects of age and method of infection, we also compared the susceptibility of two strains of mice (Swiss Webster (SW) and DBA/2). Mice were exposed to 100 cercariae and worms (by portal-hepatic perfusion) as well as eggs were recovered in the liver and intestines on postinfection (PI) days 35, 55 and 90. Skin penetration was very efficient (about 100%) irrespective of the mouse strain, sex, age and method of infection. Worm and egg recoveries were higher in SW mice at any PI interval, but strain differences tended to be less pronounced on PI day 90. In both strains, recoveries of worms and eggs were clearly higher in mice infected at a younger age (10 days old). This study thus suggests that infection of free-moving suckling mice is a suitable alternative to other methods of infection with S. mansoni.     

Giant clams in shallow reefs: UV-resistance mechanisms of Tridacninae in the Red Sea

The photosymbiosis of tropical giant clams (subfamily Tridacninae) with unicellular algae (Symbiodiniaceae) restricts their distribution to the sunlit, shallow waters of the euphotic zone where organisms are additionally exposed to potentially damaging levels of solar UV radiation. Metabolic and physiological responses of Red Sea Tridacna maxima clams, including net calcification and primary production, as well as valvometry (i.e., shell gaping behavior) were assessed when exposed to simulated high radiation levels received at 3 and 5 m underwater. The two levels of radiation included exposure treatments to photosynthetically active radiation (PAR; 400–700 nm) alone and to both, PAR and ultraviolet-B radiation (UV-B; 280–315 nm). The valvometry data obtained using flexible magnetic sensors indicated that specimens under PAR + UV-B exposure significantly reduced the proportion of their exposed mantle area, a potential photo-protective mechanism which, however, reduces the overall amount of PAR received by the algal symbionts. Consequently, specimens under PAR + UV-B displayed a slight, although non-significant, reduction in primary production rates but no signs of additional oxidative stress, changes in symbiont densities, chlorophyll content, or levels of mycosporine-like amino acids. Net calcification rates of T. maxima were not affected by exposure to UV-B; however, calcification was positively correlated with incident PAR levels. UV-B exposure changes the valvometry, reducing the exposed mantle area which consequently diminishes the available PAR for the photosymbionts. Still, T. maxima maintains high rates of primary production and net calcification, even under high levels of UV-B. This provides experimental support for a recently described, effective UV-defensive mechanism in Tridacninae, in which the photonic cooperation of the associated algal symbionts and giant clam iridocytes is assumed to establish optimal conditions for the photosynthetic performance of the clams’ symbionts.

     

Schistosoma mansoni: salicylanilides as topical prophylactic against cercarial penetration of mice

Salicylanilide and 37 of its analogs were applied topically to mice as candidate chemoprophylactic agents against Schistosoma mansoni cercarial penetration. The compounds were solubilized in absolute methanol, dimethyl sulfoxide, or isopropanol at concentrations not exceeding 1.25% W/V. The tails of the mice of each experimental group were treated by immersion for 5 min in the test compound solution or in the vehicle. The treated tails of 5 mice from each group were washed for 30 min in flowing tap water 3-4 hr after compound application. Tails of all mice were then exposed to approximately 100 cercariae by tail immersion for 1 hr, 24 hr after treatment. The portal veins were perfused 49 days after exposure and worm burdens were determined. The protective capacity of each compound was calculated by comparing the reduction of the mean worm burdens of the compound treated mice to the worm burdens of those treated with only the vehicle and expressing the resulting value as percentage protection. Of the 38 compounds tested, 20 provided 98% or better protection if the treated tails were not washed before exposure to cercariae. Of these 20 active compounds, 16 provided 98% or better protection from infection by S. mansoni cercariae even after the mouse tails were subjected to the 30 min wash.     

Schistosoma mansoni: Vaccination of mice with highly X-irradiated cercariae

Vaccination against schistosomiasis with highly X-irradiated Schistosoma mansoni cercariae was studied in mice. The optimum dose of X radiation for the attenuation of cercariae was in the range of 24–48 krad. In selecting the optimum dose, lesions caused by migrating schistosomula in the lungs of the immunized host were considered. Cercariae exposed to 48 krad caused fewer lesions than those exposed to 24 krad but still effected a comparable worm reduction. The percentages of worm reduction in mice immunized with 48-krad X-irradiated cercariae increased with the number of immunizations up to the fifth immunization and then fluctuated in the sixth, seventh, and eighth days without increase. The optimum dose of immunizing cercariae was 500, and the optimum time interval for successive immunizations was 4 weeks. There was no significant difference in susceptibility to infection in the adult mice 161 to 694 days of age. The duration of acquired immunity in immunized mice is long, still evident 545 days from the last immunization. The present studies clearly showed that with the bioengineering method, the worm reduction in the immunized mice reached 91.1%, the effect of immunization was stronger in mice immunized with the highly X-irradiated cercariae than with the low X-irradiated cercariae, and X-irradiated cercariae were demonstrated to be a strong inducing agent for immunity in mice.     

Long-term exposure of E-mu-Pim1 transgenic mice to 898.4 MHz microwaves does not increase lymphoma incidence

A total of 120 E mu-Pim1 heterozygous mice and 120 wild-type mice were exposed for 1 h/day 5 days/week at each of the four exposure levels in "Ferris-wheel" exposure systems for up to 104 weeks to GSM-modulated 898.4 MHz radiation at SARs of 0.25, 1.0, 2.0 and 4.0 W/kg. In addition, 120 heterozygous and 120 wild-type mice were sham-exposed; there was also an unrestrained negative control group. Four exposure levels were used to investigate whether a dose-response effect could be detected. Independent verification confirmed that the exposures in the current study were nonthermal. There was no significant difference in the incidence of lymphomas between exposed and sham-exposed groups at any of the exposure levels. A dose-response effect was not detected. The findings showed that long-term exposures of lymphoma-prone mice to 898.4 MHz GSM radiofrequency (RF) radiation at SARs of 0.25, 1.0, 2.0 and 4.0 W/kg had no significant effects when compared to sham-irradiated animals. A previous study (Repacholi et al., Radiat. Res. 147, 631-640, 1997) reported that long-term exposure of lymphoma-prone mice to one exposure level of 900 MHz RF radiation significantly increased the incidence of non-lymphoblastic lymphomas when compared to sham-irradiated animals.     

The effects of physiological factors and host plant experience on the ovipositional activity of the sweet potato whitefly, Bemisia tabaci

Both egg load and levels of vitellogenin and vitellin increased when Bemisia tabaci Gennadius (Homoptera: Aleyrodidae), were deprived of host plants. These increases resulted in significant increases in rates of oviposition. There was no difference in total fecundity between whiteflies temporarily deprived of hosts and a control treatment that had continuous access to hosts. The results of previous exposure to a preferred host, melon (Cucumis melo), and a non-preferred host, cotton (Gossypium hirsutum), were also examined. B. tabaci deposited significantly more eggs during a 6-h period on melon than on cotton, regardless of host species they were previously confined to. In addition, whiteflies previously exposed to cotton, deposited more eggs than females previously exposed to melon. This higher rate of oviposition resulted in a low egg load as well as lower levels of vitellogenin and vitellin. Conversely, a low rate of oviposition led to high egg load and high levels of vitellogenin and vitellin. Individuals from the same treatment exhibited variation in egg load, which was negatively and significantly correlated with previous oviposition activity. We conclude that the effects of previous host plant exposure on oviposition were, in part, due to behavioral differences that are not related to the reproductive physiology.     

Effects of Ultraviolet Radiation on Locomotion and Orientation in Roughskin Newts (Taricha granulosa)

Environmental changes, including those associated with the atmosphere may significantly affect individual animals and ultimately populations. Ultraviolet (UV) radiation, perhaps increasing due to stratospheric ozone depletion, has been linked to mortality in a number of organisms, including amphibians. The eggs and larvae of certain amphibian species hatch at significantly lower rates when exposed to ambient ultraviolet light. Yet little is known about the sublethal effects of UV radiation. For example, UV radiation may affect specific behaviors of an animal that could alter its ability to survive. To examine if UV radiation affects amphibian behavior, we used roughskin newts ( Taricha granulosa ) as a model. Newts were exposed to low-level doses of UV in the laboratory and then tested in the field to examine if UV-exposed and control (no UV) newts differed in orientation towards water or in locomotor activity levels. UV-exposed and control newts both exhibited a significant orientation towards water in field tests but there was no significant difference in orientation between treatments. However, UV-exposed newts were significantly more active than control newts. Our results suggest that exposure to short-term low levels of UV radiation alters certain behaviors. Environmentally induced changes in behavior may have significant ecological and evolutionary consequences.     

Schistosoma japonicum infection in pregnant mice.

Ten 1-week and ten 2-weeks pregnant female NMRI mice were experimentally exposed to 70 Schistosoma japonicum cercariae. Ten littermice from each group were examined for worms by perfusion 4, 6 and 8 weeks post infection. Although the mothers (n = 15) were found infected with 15.5 +/- 13.4 worms at perfusion 6 and 7 weeks post infection, no worms were found in any of the examined littermice, as well as no detection of faecal or tissue eggs. Litter sizes did not differ from control groups and all littermice were healthy. The present study therefore suggests that congenital infection with S. japonicum does not occur in percutaneously infected mice and that infection of the mother during pregnancy does not seem to affect the offspring.     

Bioelectronic monitoring of parasite-induced stress in brown trout and roach

A non-contact bioelectronic monitoring system was used to record changes in heart rate, ventilation rate and swimming activity in brown trout Salmo trutta and roach Rutilus rutilus , following exposure to two species of cercariae of digenean trematodes: Diplostomum pseudospathaceum which is a common parasite in the lens of fishes and xiphidiocercariae of Plagiorchis elegans , a parasite of anatid birds, both of which have the snail Lymnaea stagnalis as their first intermediate host. Swimming activity increased significantly in roach exposed to Diplostomum cercariae at densities as low as 3.8l⁻¹ and remained elevated for 24–36 h post exposure. Brown trout showed no response when exposed to low densities of Diplostomum (2.9 and 5.7 cercariae 1⁻¹) but there was a significant response at higher cercarial exposure densities (>501⁻¹). Total activity of brown trout increased immediately, peaking after 2 h and returning to pre-exposure levels within 5–6 h. There was a simultaneous increase in heart and ventilation rates which both declined following the reduction in activity of the fish. Heart rate then increased for a second time to a maximum level at 14–16 h and thereafter declined to reach a steady state at 3 days post-exposure. During this period there was no significant change in total activity. The qualitative nature of the heart response was identical in five infected brown trout although the magnitude of the response varied widely among fish. Exposure to cercariae of P. elegans elicited no response in either fish species. (c) 1996 The Fisheries Society of the British Isles