Effects of host condition on susceptibility to infection, parasite developmental rate, and parasite transmission in a snail–trematode interaction


  • Present address: Jukka Jokela, Department of Biology, University of Oulu, Oulu, Finland.

  • Jürgen Wiehn, Orion Corporation, Clinical Development, Turku, Finland.

Amy Krist, Department of Biology, Phillips Hall 330, University of Wisconsin – Eau Claire, Eau Claire, WI 54701, USA.
Tel: 715 836 5386; fax: 715 836 5089;
e-mail: kristac@uwec.edu


Whether or not organisms become infected by parasites is likely to be a complex interplay between host and parasite genotypes, as well as the physiological condition of both species. Details of this interplay are very important because physiology-driven susceptibility has the potential to confound genetic coevolutionary responses. Here we concentrate on how physiological aspects of infection may interfere with genetic-based infectivity in a snail–trematode (Potamopyrgus antipodarum/Microphallus sp.) interaction by asking: (1) how does host condition affect susceptibility to infection? and (2) how does host condition affect the survival of infected individuals? We manipulated host condition by experimentally varying resources. Contrary to our expectation, host condition did not affect susceptibility to infection, suggesting that genetics are more important than physiology in this regard. However, hosts in poor condition had higher parasite-induced mortality than hosts in good condition. Taken together, these results suggest that coevolutionary interactions with parasites may depend on host condition, not by altering susceptibility, but rather by affecting the likelihood of parasite transmission.


Much of the recent work in host–parasite interactions has been focused on the genetics of susceptibility-infectivity. This attention seems justified, given the critical nature of the underlying genetics to coevolutionary theories such as the Red Queen hypothesis (Frank, 1994, 1996; Parker, 1994, 1996; Agrawal & Lively, 2002, 2003). Nonetheless, the genetics of susceptibility to infection explain only part (perhaps even a small part) of the prevalence and intensity of infections in natural populations. Exposure to parasites occurs in organisms living in different environments, with different physiological conditions, and at different stages of their life histories. Hence, ecological conditions may also play a large role in explaining local levels of infection. For example, habitat structure (Sousa & Grosholz, 1991), life-history stage of the host (Richards, 1977; Altaif et al., 1989) and resource levels (Agnew & Koella, 1999) all affect the likelihood of successful infection. In fact, ecology may be more powerful in explaining local levels of infection than the frequencies of various host/parasite genotypes. Specifically, hosts in poor condition would have fewer resources to allocate to both specific and nonspecific defense, which would make them appear more susceptible than their genotypes might otherwise reveal (Jokela et al., 2000). Such a situation could dramatically reduce the ability of parasites to coevolve with their local host populations and consequently reduce selection for outcrossing as envisioned under the Red Queen Hypothesis (Jaenike, 1978; Hamilton, 1980, 1982; Bell, 1982).

In the present study of a snail-trematode interaction, we experimentally evaluated the effects of host condition on susceptibility to infection and survival. We predicted that hosts reared in low-resource environments would be more susceptible to infection. The results were decisively inconsistent with our prediction. Resource-limited hosts were not more susceptible to infection; however, they were more likely to die. Infected hosts reared in low-resource environments suffered much higher mortality than did infected hosts in good physiological condition.

Materials and methods

Host–parasite system

Potamopyrgus antipodarum (Gray; Family Hydrobiidae) is endemic to New Zealand lakes and streams. Reproduction is either parthenogenetic or sexual, where the sexual populations are gonochoric (i.e. they have separate sexes; Winterbourn, 1970a). This snail is the first intermediate host for over a dozen species of digenetic trematodes (Winterbourn, 1970b), the most common of which is an undescribed species of Microphallus sp. (Digenea: Microphallidae). Population genetic data from Microphallus collected from lakes across the South Island of New Zealand confirms that this trematode is a single species (Dybdahl & Lively, 1996), which is hereafter referred to as Microphallus. The definitive hosts for Microphallus include common gray ducks (Anas superciliosa) and mallards (Anas platyrhynchos).

Potamopyrgus antipodarum become infected by Microphallus after ingesting the embryonated eggs that are defecated into the environment by ducks. The egg hatches inside the snail, develops and successful infections eventually produce hundreds of metacercariae by asexual reproduction. The metacercariae are the larval stage that are transmitted to the definitive host and hereafter are referred to as the transmission stage. Development of the parasite castrates both sexes of these snails. Microphallus infections are detectable at approximately 9 weeks after the snails are exposed to the infection (e.g. Krist & Lively, 1998), and development of Microphallus is complete after approximately 33 weeks under laboratory conditions (e.g. Krist et al., 2000).

Experimental methods

In January 2000, we collected the snails from Lake Alexandrina on the South Island of New Zealand. We collected snails using nets and snorkeling equipment at three different sites (Campsite, JMS and West Bay, see Fig. 1 in Jokela & Lively, 1995 for a map) from midwater littoral (depth 1–3 m) macrophytes (Isoetes kirkii). Only adult snails were used in these experiments (mean size = 3.37 mm); juvenile snails were removed with a sieve. The snails were transported to the Edward Percival Field Station in Kaikoura, New Zealand and used in one of two related experiments.

Figure 1.

Comparison among snails exposed to the trematode parasite Microphallus (white) and unexposed snails (gray) among three food treatments. Means (with ± SE) are shown for the no-food, 0.018 g day−1 (intermediate food) and 0.055 g day−1 (high-food) treatments for survival (a), prevalence of Microphallus infection (b), and total ‘mortality’ (c). Total ‘mortality’ was measured by adding the number of snails that died during the experiment to the number of snails that were infected by Microphallus in the end of the experiment for each experimental container. The numbers below each bar refer to the number of replicates present in each group.

Experiment 1

In the first experiment, we manipulated the condition of the snails to determine the effects on susceptibility to infection, and survival of infected snails. The overall design of the experiment consisted of three levels of food treatments and two parasite treatments (exposed and control snails). We used five replicates for every combination of food level and parasite treatment (a total of 30 experimental units). Each experimental unit contained 150 randomly selected snails. The three levels of food treatment were: no food, intermediate levels of food (0.018 g of Spirulina) and high levels of food (0.055 g of Spirulina). The intermediate and high-food levels were given food everyday. The experimental containers were washed weekly to eliminate any algae or bacteria that ‘volunteered’ in the containers and could have been a source of food. These treatments were applied for a period of 65 days, 22 days before and 43 days after exposure to the parasite treatment. The source of parasites for experiments 1 and 2 were the same. A description of the methods used for infecting snails for both experiments follows a description of experiment 2.

Experiment 2

In the second experiment, we again manipulated the condition of the snail. However, in contrast to experiment 1 where the food level was manipulated, in experiment 2, snails were either fed a constant level of food or starved before or after exposure to Microphallus. Therefore, from experiment 2, we were able to determine whether host condition prior to or after exposure affected prevalence at the end of the experiment. There were four food treatments: (i) starved before and after exposure (starved/starved), (ii) fed before and after exposure (fed/fed), (iii) starved before exposure and fed after exposure (starved/fed) or (iv) fed before exposure and starved after exposure (fed/starved). For each of these treatments there were 10 replicates. In addition to the food treatment, there was also a parasite treatment. All four food treatments were exposed to Microphallus (total of 40 exposed units). In addition, there were seven control replicates, which were fed ad libitum and not exposed to the parasite. Each experimental unit contained 100 snails. The fed snails were given (0.036 g of Spirulina) daily. These treatments were applied for a period of 68 days, 24 days before and 44 days after exposure to the parasite treatment. The experimental containers were washed every other day.

The parasites for experiments 1 and 2 were obtained by infecting mice with Microphallus metacercariae and collecting feces (containing parasite eggs) from the infected mice. Six mice were infected with Microphallus on 2 February 2000. Each mouse received the metacercariae from 25 infected snails from Lake Alexandrina (from snails living in willow roots along the shallow shore bank). In addition, four mice were maintained as controls that did not receive Microphallus metacercariae. Because studies have shown that mice infected with Microphallus will defecate parasite eggs between 2 and 5 days post-infection (Lively & McKenzie, 1991) fecal pellets were collected three times daily from 4 February to 7 February, 2000. Immediately following collection, the feces from the infected mice were evenly distributed among 55 2-L containers (15 for experiment 1 and 40 for experiment 2) and the feces from the control mice were evenly distributed among 22 2-L containers (15 for experiment 1, and seven for experiment 2). An equal number of feces were distributed to the containers from the control mice as from the infected mice. To prevent fouling, we changed the water daily in all containers.

On 8 February 2000, we exposed the snails to Microphallus eggs by emptying the contents of each container of feces into a different container of snails such that there was one container of feces for every container of snails. For experiment 1, the containers held 150 snails; and for experiment 2, the containers held 100 snails. During the 13 days that the snails were exposed to the inoculum, the food treatments in both experiments 1 and 2 were suspended so that the snails would consume the fecal material.

In late February, 13 days after exposing the snails to Microphallus eggs, we transported experiment 1 to Indiana University, Bloomington, Indiana and experiment 2 to ETH Zürich, Switzerland. Experiment 1 was maintained at 19 °C. The experimental containers were washed weekly until the end of the experiment. After the food treatment was complete (65 days into the experiment) all snails were fed 0.055 g of Spirulina (high-food level). Experiment 2 was maintained at 20 °C. The experimental containers were washed daily until the food treatment was ended and weekly after that. Snails were fed ad libitum after the food treatment was completed (68 days into the experiment). For both experiments, the food treatments were stopped because we wanted to be certain that some snails survived in the no-food treatment.

In May 2000, 86–97 days post-exposure for experiment 1 and 97–100 days post-exposure for experiment 2, all of the snails were destructively sampled. Shell length, infection status (uninfected, or infected with Microphallus or infected with other species of trematode), gender, and the number of surviving snails were recorded for each experimental unit. For individuals infected with Microphallus, we also determined the developmental stage of the infection (described in Dybdahl & Lively, 1998).

Statistical analyses

We used food treatment and parasite treatment as fixed factors in the anova to assess the variation in survival, prevalence, and development rate of Microphallus infection. Additionally, we calculated a composite score of the full impact of exposure during the experiment by calculating the total number of individuals that died or were castrated by the parasite during the experiment. This variable, total ‘mortality’, illustrates the stress caused by exposure to parasitism and shortage of resources on the snail hosts. Although the snails in the control group were not exposed, total ‘mortality’ for these animals included all individuals that died or were infected from the onset of the experiment [note: One experimental container was omitted from the analysis of experiment 1, and another one from experiment 2. In the first occasion the reason for omission was an accident during maintenance of the experiment. In the second case the reason was very high and sudden mortality in the container suggesting that an unknown disease killed the snails in the container. This reduced the sample size in one experimental group from five to four (in experiment 1), and from 10 to nine (in experiment 2)]. Sample sizes are indicated in all figures. All analyses were conducted with SPSS 10 software. Assumptions of homogenous variances and normally distributed residuals were met for all anovas.


Experiment 1

Size of uninfected snails at the end of the experiment increased with food level, indicating that we were able to create a food gradient during the experiment [3.55 ± 0.049, 3.60 ± 0.037 and 3.93 ± 0.035 mm (mean ± SE), respectively for the 0, 0.018 and 0.055 g day−1 food treatments]. Snail size also varied by replicate containers, but neither the parasite treatment nor the parasite-treatment-by-food treatment interaction had an effect on snail size (Table 1).

Table 1.  Nested anova for the length of uninfected female snails at the end of experiment 1 by parasite (P) and food treatment (F).
  1. Superscript letter indicates the error term used in each F-test. Mean squares are denoted by MS.

Parasite (P)11.211.19b0.285
Food (F)215.5015.37b<0.001
Replicate (P × F)b241.011.85a0.008
P × F20.140.14b0.868

The analysis of survival provided further support that food level affected host condition in experiment 1. Food treatment had an effect on survival of snails during the experiment such that survivorship was highest in the high-food group and lowest in the no-food group (Table 2a; Fig. 1a). Furthermore, survival of snails that were exposed to parasites was clearly lower than that of the unexposed snails, indicating that parasites induced host mortality during the experiment (Table 2a, Fig. 1a). Interestingly, the interaction between the food and the parasite treatments was statistically significant, indicating that the relative survival of exposed and unexposed snails depended on the food treatment. More specifically, relatively more snails died when exposed to Microphallus at the low-food level than at the high-food levels (Fig. 1a). These results suggest that parasites induced higher mortality among hosts in poor condition.

Table 2.  Two-way anova for the number of (a) surviving snails, (b) prevalence of Microphallus{Arcsin[Sqrt(x)]} and (c) total ‘mortality’ (sum of dead snails and the number of snails infected with Microphallus) by parasite (P) and food (F) treatments in experiment 1.
(a) SurvivalParasite (P) 18169 99.62<0.001
Food (F) 24673 56.99<0.001
P × F 2464  5.66 0.010
(b) PrevalenceParasite (P) 125 545654.06<0.001
Food (F) 2375  9.62 0.001
P × F 2399 10.22 0.001
(c) Total ‘mortality’Parasite (P) 124 964400.86<0.001
Food (F) 21521 24.43<0.001
P × F 274  1.180.324

The prevalence of Microphallus infection was much higher in the exposed containers than in the unexposed ones (Table 2b, Fig. 1b), indicating that the infection protocol was successful. Furthermore, the infections in the exposed group were on average at a much earlier developmental stage (younger) than the infections in the unexposed group [Stage: 2.81 ± 0.04 vs. 4.24 ±0.14 (mean ± SE) for exposed and unexposed groups, respectively. Developmental stage indicates the age of the parasite on a scale from 1–5, see Krist et al. (2000)]. Thus, the later developmental stages in the unexposed groups indicate the presence of snails that were infected prior to collection. Also, there was no difference in development stage of Microphallus between snails in the food treatments (Nested anova of parasite development stage among food treatments of infected individuals in the parasite treatment, F = 0.363, d.f. = 2, P = 0.703).

One of the key results of this experiment was that manipulation of host condition had an effect on Microphallus prevalence (Table 2b). Contrary to our expectations that susceptibility to infection, and therefore prevalence, would increase when hosts are in poor condition, the prevalence of Microphallus was highest in the high-food treatment and lowest in the no-food treatment (Fig. 1b). The significant interaction between the parasite exposure and food treatments for prevalence of infection was due to an increase in prevalence by food treatment in the exposed groups, whereas food treatment had no effect on infection levels in the nonexposed groups (Table 2b, Fig. 1b). This result suggests that infections that already were established in the beginning of the experiment were unaffected by the food treatment, whereas those that were initiated in the laboratory had a higher likelihood of developing to the transmission stage when the hosts were in good condition.

As parasite-induced host mortality was higher and prevalence of infection was lower in the low-food group (Fig. 1a,b), we assessed the overall effect of infection and parasite-induced mortality. We measured total ‘genetic mortality’ by adding the number of snails that died during the experiment to the number of snails that were infected/castrated by Microphallus in the end of the experiment for each experimental container. We used this sum to indicate the full impact of exposure on the host population under variable resource and parasite stress. We named the variable total ‘mortality’ because death and infection by Microphallus are equivalent from an evolutionary perspective; Microphallus infections always sterilize the snail host. The results of this analysis indicated that exposure to Microphallus leads to clear negative effects on performance of the host population (Table 2c). Also, as expected, hosts experiencing low-food conditions suffered more than the hosts in good condition. Interestingly, there was no significant interaction between the parasite-exposure and food level treatments, suggesting that the relative magnitude of the negative effects of parasite exposure were independent of food level (Table 2c, Fig. 1c). Although we did not directly measure susceptibility, it is best viewed as the difference between the gray and white bars in Fig. 1c (the difference in total genetic mortality between the exposed and control snails). Hence, the absence of an interaction between parasite-exposure and food-level treatments for total mortality (Fig. 1c) suggests that variation in host condition did not affect overall susceptibility, but exposure to Microphallus led more frequently to host death when hosts were in poor condition (significant interaction, Fig. 1a) and more frequently to fully developed infections when hosts were in good condition (Fig. 1b).

Experiment 2

The results of experiment 2 showed that starving the snails prior to parasite exposure had a larger effect on the survival of snails than starving them after exposure (Table 3a, Fig. 2a). This result suggests that when hosts in poor condition are exposed to infection, parasite-induced host mortality increases. Although parasite-induced mortality could not be estimated for each food treatment because the unexposed controls were fed ad libitum, survival is lower in the ‘starved/fed’ group than in the ‘fed/starved’ group despite the longer period of starvation in the ‘fed/starved’ group (24 and 44 days of starvation for ‘starved/fed’ and ‘fed/starved’ groups respectively). This result suggests that the timing of exposure to parasites, and not simply starvation, is responsible for the difference in survival between these two groups. When survival of the exposed snails in each treatment was compared with that of the unexposed controls using Dunnett's test, the two groups where host starvation was applied before exposure (‘starved/starved’ and ‘starved/fed’) experienced higher mortality than did the unexposed controls (Fig. 2a, P < 0.001). The other two groups experienced mortality that was not significantly different from the unexposed controls (Fig. 2a, P = 0.293 and P = 0.292, respectively, for ‘fed/starved’ and ‘fed/fed’ treatments). A similar analysis indicated that starving the snails before or after the exposure had no effect on the prevalence of Microphallus infection (Table 3b, Fig. 2b). Taken together, these results reveal that the condition of the host prior to exposure has a greater effect on host survival than on the resulting parasite prevalence.

Table 3.  Two-way anova for (a) host survival and (b) prevalence of Microphallus infection in experiment 2 where snails were starved either before or after the exposure to parasites.
(a) SurvivalStarved before120.910.0160.003
Starved after11.010.4830.492
(b) PrevalenceStarved before122.560.2200.642
Starved after183.210.8110.374
Figure 2.

Comparison among unexposed snails (gray) and snails that were exposed to the trematode parasite Microphallus (white) and were given different food treatments. Snails were either starved before and after exposure to parasites (starved, starved), starved before exposure and fed after exposure (starved, fed), fed before exposure and starved after exposure (fed, starved) or fed for the entire experiment (fed, fed). Mean survival (a) and mean prevalence of Microphallus infections (b) for these groups are shown (with ± SE). The numbers within each bar refer to the number of replicates present in each group.


We predicted that hosts in poor condition would have higher susceptibility (and therefore higher prevalence of infection), because they had fewer resources available for defense (Jokela et al., 2000). In contrast, we found that hosts in poor condition had the lowest prevalence of infection (Fig. 1b), and we found no indication that susceptibility was altered by host condition (Fig. 1c). We did, however, find that the outcome of infection (death or castration) was modified by host condition. In other words, our results suggest that overall susceptibility was not affected by host condition (Fig. 1c), and that hosts in poor condition were less suitable for parasite development than were hosts in good condition. This result indicates that parasites that do develop to the transmission stage, and are thus able to pass on their genes to the next parasite generation, are transmitted through hosts in good condition. If the hosts in good condition are also the best-fit host genotypes, then coevolutionary interactions between the hosts and the parasites may involve only those host genotypes that are best able to extract resources from their environment.

The effects of host condition on infection have been investigated in several other studies. Similar to the present study, host condition affected the survival of parasitized damselfly hosts (Braune & Rolff, 2001). As in the current study, hosts in poor condition (as estimated by mass at emergence) exhibited greater mortality than did hosts in good condition (Braune & Rolff, 2001). However, Braune & Rolff (2001) found the effect only in infected female damselflies. Host condition has also been manipulated by varying the level of food of the yellow fever mosquito, Aedes aegypti (Agnew & Koella, 1999). In this study both food level and infectious dose of a microsporidian parasite were varied. Similar to the present study, Agnew & Koella (1999) found mortality to increase with decreasing levels of food and increasing doses of parasite. Additionally, Agnew & Koella (1999) found that poor host condition (low food) increased the probability of horizontal relative to vertical transmission. These studies, together with the results reported herein, indicate that host condition can have surprising outcomes on the dynamics of infection. For example, in our study system, the harmful effects of parasites on the host population may be large (i.e. high parasite-induced mortality) even in cases where it appears that only a few parasites manage to develop to the transmission stage (i.e. low prevalence of infection).

In experiment 1, parasite development rate was unaffected by the food treatment. Because this lack of developmental plasticity of parasites in hosts in poor condition resulted in parasite death, it is difficult to understand why Microphallus would not slow its development rate to prevent killing the host. Perhaps Microphallus is unable to detect host condition and therefore is unable to alter its development rate. Although results from the current experiment suggest that development rate may be canalized and proceed at a constant rate regardless of the host environment, previous studies have shown that Microphallus develops at different rates in hosts from different habitats (Krist et al., 2000).

In experiment 2, the significant effect of starvation on host survival before exposure to Microphallus indicates that host condition at exposure had a greater impact on the likelihood of survival than did host condition after exposure. In contrast to the results from experiment 1, the effect of the food treatment on prevalence in experiment 2 is weaker (Fig. 2b; Table 3b). In particular, unlike experiment 1, the starved snails did not have lower prevalence of infection at the end of the experiment than the snails that were fed. Two important differences between the experiments may explain this discrepancy. First, in each experimental container, the density of snails was 50% higher in experiment 1 than in experiment 2. Therefore, the starvation treatment in experiment 1 was much more severe than the starvation treatment (starved, starved) in experiment 2 because more snails were competing for the few bacteria that opportunistically grew in our experimental containers. Note that survival in the no-food group of experiment 1 was half of that observed in the starved-starved group of experiment 2 (Figs 1a and 2a). Infected snails are more likely to die when conditions deteriorate (Jokela et al., 1999). Thus the higher level of severity in experiment 1 may be the reason for the more pronounced effect observed in prevalence of infection. Secondly, we were not able to estimate parasite-induced mortality for each food treatment in experiment 2 because the control treatment was fed ad libitum. As prevalence is based on surviving individuals, it is also possible that differences in parasite-induced mortality between the two experiments explain the smaller effect of food treatment on prevalence in experiment 2.

In summary, our results indicate that susceptibility to infection is not affected by condition. This could increase the parasite-mediated strength of selection against common host genotypes by decreasing the recombinational load. In contrast, consider a case in which susceptibility was affected by host condition and half of all infections that were transmitted to the final host come from a random sample of individuals that were infected because of their poor condition (with a similarly random sample of host susceptibility genotypes). Then random mating by the parasite population could retard the parasite's response to selection to the most common local snail genotypes. On the other hand, as our experiment suggests, if infection is determined more by the genetic interface between host and parasite than by host condition, then random mating in the final host is less likely to retard the parasite's response to selection imposed by common host genotypes. The overall effect may be to increase the efficacy of selection imposed by parasites for sexual reproduction in these snails where sexual and asexual forms coexist.


We thank Maureen Neiman for her help with setting up and maintaining the experiment in Kaikoura. We would also like to thank the faculty and staff of the Zoology Department, University of Canterbury, New Zealand and especially Jan McKenzie, Jack Van Berkel, and Mike Winterbourn. Without their continued support, our work in New Zealand would not be possible. This work was funded by NSF grant DEB-9904840, the Academy of Finland, and Swiss National Science Foundation grant 31-59242.99.