Pseudodiplorchis americanus has an extremely brief opportunity for transmission, restricted to less than 24 h in the year when the host (Scaphiopus couchii) enters water to breed. This strict annual cycle means that invading worms have 1 year to complete reproductive development. Despite this, a large proportion of the parasite suprapopulation is not prepared for transmission at the time of host breeding. The present study correlates detailed soil temperatures for one field site in S.E. Arizona, spanning 3·5 years, with laboratory data on parasite growth and reproductive development at a series of controlled temperatures. Development is totally inhibited at 16°C. Optimal growth and development occurs at 25°C and is slower at both 20°C and a diurnal cycle fluctuating between 20 and 34°C (mean 27°C) (simulating summer temperatures experienced by host and parasite). The effective period for accumulation of infective stages by P. americanus in any one season varies according to the timing of the summer rains in consecutive years. Between 1985 and 1988, the interval between host breedings varied from 11 to 13 months and the period when temperatures were above 20°C varied from 4 to 5·5 months per year. Since the first month post-infection (p.i.) consists of pre-reproductive development, first-year worms have only 3–4·5 months to produce infective larvae. In the shortest seasons, these worms may not be able to complete reproductive preparation in time for the first opportunity for transmission. In different parts of the geographical distribution of S. couchii, different cycles of temperature, rainfall and host breeding occur; varying temperature regimes represent an important abiotic control of P. americanus reproductive biology and transmission.

Pseudodiplorchis americanus infects the urinary bladder of a desert toad and feeds exclusively on blood, producing the iron-rich waste product haematin. The host, Scaphiopus couchii, retains dilute urine within the urinary bladder as a water store throughout hibernation and parasite waste will therefore not be eliminated. This study utilized atomic absorption spectrophotometry to quantify the iron within the urinary bladders of infected and uninfected toads after different periods of hibernation. During the first 4 months hibernation, no detectable iron, above that of the controls, was found in the urine and bladder tissue of infected animals, but after 5–6 months hibernation there was a small but not significant increase. The iron contained within most individual parasites was greater than that detected in the urine and bladder tissue of the host, and the total iron in each parasite infrapopulation was 3–83 times greater than the urinary bladder iron. This suggests that the parasites do not regurgitate their gut contents during host hibernation. The amount of iron in individual parasites increased with time after migration to the bladder and this was used to estimate the amount of blood ingested by P. americanus. At a controlled temperature of 25 °C, the estimated rate of ingestion increased from 0·33 μl blood/parasite/week by worms 2 weeks post-migration to a maximum of around 1·6 μl by worms 5 months post-migration. The rate of blood ingestion by older worms was also calculated as a function of the hibernation period (since this did not correspond to worm age) assuming complete gut evacuation at the start of hibernation. This provided maximum estimates of ingestion rate of 1·9–5·3 μl blood/parasite/week.

Pseudodiplorchis americanus accumulates its entire reproductive output in utero during hibernation of its host, Scaphiopus couchii. Infective larvae are released only during the toads' brief entry into water for breeding. In one host population studied prior to transmission, in S.E. Arizona, adult worms contained 0–326 infective stages; 19% of 178 worms produced 51% of the total larvae. Reproductive output was positively correlated with body length which was considered to reflect worm age; first-, second- and third-year worms produced means of 5, 43 and 96 larvae, respectively. Within second- and third-year cohorts, there was a density-dependent reduction in offspring numbers. Under controlled laboratory conditions, the mean rate of offspring accumulation during the first 6 months of development was significantly reduced only in burdens in excess of those found naturally (36–68 worms). In older parasites, a density-dependent reduction in the mean accumulation rate occurred in burdens of 20–32 worms, where 88% of parasites produced offspring at less than half the maximum rate. The asymmetry in individual reproductive output increased with competition and there were significant reductions in the median rates of offspring accumulation at smaller burdens (17–25 and 5–14 worms, in experimental and natural infections, respectively) than evident from decreases in the mean. Thus, individual variation is a more sensitive measure of competition than is the mean.

Populations of Pseudodiplorchis americanus infecting the desert toad, Scaphiopus couchii, have previously been shown to be remarkably stable from year to year, despite wide variation in conditions promoting invasion. The present study aimed to document the survival of both first-year and pre-existing adults under controlled laboratory temperatures. First-year worm survival in experimentally infected toads was shown to be very high for the first 5 months after migration to the definitive site, and there was no difference in survival at 25 °C or 15–20 °C. There was also no density-dependent survival during the same period: 20% of worms were recovered from the host urinary bladder, irrespective of initial intensities. After the first 5 months, there was a progressive loss of worms in toads maintained at 25 °C but not at 15–20 °C. Pre-existing adult populations were shown to be virtually identical to those under natural conditions for the first 4 months after toad capture. Following this, at 25 °C, populations declined and no pre-existing adults were recovered after 11–14 months. There was no such loss of pre-existing adults at 15–20 °C for up to 14 months. A diurnal temperature cycle of 20–34 °C (simulating temperatures during the desert summer) did not lead to a significant loss of worms. These observations suggest that during one annual cycle in the desert, when temperatures remain above 20 °C for less than 6 months, most adult worms established in the host urinary bladder will survive to the next transmission opportunity. However, the similar longevity of first-year and pre-existing adults in laboratory maintained toads shows that loss of worms cannot be due solely to parasite ageing. Temperature-dependent survival of P. americanus is suggestive of a host immune response. Low temperatures, which inhibit parasite growth and development, are essential for the survival of P. americanus.

A previous study has shown that, under natural conditions, energy reserves of the desert toad, Scaphiopus couchii, are negatively related to the density of infection by Pseudodiplorchis americanus. However, this was based predominantly on collections of active animals from breeding congregations and inevitably selected toads which were in good condition. The parasite, a blood-feeding monogenean, occurs in burdens of up to 30 worms/host (mean intensity 6 worms/host) and represents a significant drain on reserves because the host does not feed during a 10-month hibernation. Field studies cannot resolve the possibility that larger worm densities are not observed in nature due to parasite-induced host mortality. The present study was conducted during investigations of P. americanus development and survival under controlled laboratory conditions, utilizing experimentally infected hosts which created worm densities larger than those observed in natural populations. At all temperature regimes, infected animals had smaller fat bodies than those uninfected but differences were generally not statistically significant due to large individual variations, presumably resulting from variations in past feeding efficiency. At cool temperatures (15–20 °C) there was no density-dependent effect on host fat body weight, and at a diurnal temperature cycle of 20–34 °C (simulating that experienced by host and parasite during the summer months), the effects of high temperatures were greater than the effects of infection, due to increased toad metabolic rates. The most significant effects of P. americanus were observed in hosts that began hibernation in relatively poor condition and experienced moderate temperatures (25 °C) during hibernation. The toads generally maintained packed blood cell levels (PCV) levels even when fat body weights were low, but infected animals had a lower PCV irrespective of fat body levels. In animals unfed after field collection, PCV was reduced in uninfected toads and was even lower in infected animals. Although very heavily infected toads (burdens of 35–95 worms/host) were generally in poorer condition than uninfected toads they still survived long-term hibernation under extreme nutritional stress. This study therefore confirmed observations made in field studies that there is a density-dependent relationship between the hosts' survival prospects and P. americanus infection. However, given the large variability in feeding efficiency and stored resources between individual toads, there is no evidence that the most heavily infected toads would have been unrepresented in field samples due to parasite-induced mortality.