Tocque K, Tinsley R C
School of Biological Sciences, Queen Mary and Westfield College, University of London.
Parasitology. 1994 Feb;108 ( Pt 2):175-83. doi: 10.1017/s003118200006827x.
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 degrees C) there was no density-dependent effect on host fat body weight, and at a diurnal temperature cycle of 20-34 degrees 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 degrees 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 fab 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.
先前的一项研究表明，在自然条件下，沙漠蟾蜍（Scaphiopus couchii）的能量储备与美洲伪双睾吸虫（Pseudodiplorchis americanus）的感染密度呈负相关。然而，这主要基于从繁殖群体中采集的活动动物，不可避免地选择了状况良好的蟾蜍。这种寄生虫是一种吸血单殖吸虫，每个宿主身上的虫数可达30条（平均感染强度为6条/宿主），由于宿主在长达10个月的冬眠期间不进食，这对储备能量造成了显著消耗。野外研究无法排除由于寄生虫导致宿主死亡而在自然界中未观察到更高虫密度的可能性。本研究是在对美洲伪双睾吸虫在受控实验室条件下的发育和存活情况进行调查期间进行的，利用实验感染的宿主创造出比自然种群中观察到的更高的虫密度。在所有温度条件下，受感染动物的脂肪体比未受感染的动物小，但由于个体差异较大，差异通常无统计学意义，推测这是由过去摄食效率的差异导致的。在低温（15 - 20摄氏度）下，对宿主脂肪体重量没有密度依赖性影响，在20 - 34摄氏度的昼夜温度循环（模拟宿主和寄生虫在夏季经历的温度）下，由于蟾蜍代谢率增加，高温的影响大于感染的影响。美洲伪双睾吸虫最显著的影响出现在冬眠开始时状况相对较差且冬眠期间经历中等温度（25摄氏度）的宿主身上。蟾蜍通常即使脂肪体重量较低也能维持血细胞比容（PCV）水平，但受感染动物的PCV较低，与脂肪体水平无关。在野外采集后未进食的动物中，未受感染的蟾蜍PCV降低，受感染动物的PCV更低。尽管感染非常严重的蟾蜍（每个宿主有35 - 95条虫）通常比未受感染的蟾蜍状况更差，但它们仍能在极端营养压力下长期冬眠存活。因此，本研究证实了野外研究中的观察结果，即宿主的存活前景与美洲伪双睾吸虫感染之间存在密度依赖性关系。然而，鉴于个体蟾蜍在摄食效率和储存资源方面存在很大差异，没有证据表明由于寄生虫导致的死亡，感染最严重的蟾蜍在野外样本中没有代表性。
