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Lungworms of the cosmopolitan genus Rhabdias are among the most common parasites of amphibians and squamate reptiles. The present study used experimental infections, field studies, and a molecular phylogeny to determine the host specificity of 6 Rhabdias spp. that infect snakes and anurans from North America. The molecular phylogeny suggests Rhabdias ranae from Nebraska and Mississippi may represent separate, cryptic species. In addition, the phylogeny strongly supports separate clades for anuran and snake lungworms. Field studies and experimental infections indicate that snake lungworms are generalist snake parasites; however, laboratory experiments also suggest that lizards can be infected under some environmental conditions. Lungworms from anurans were found not to infect salamanders or reptiles, in nature or in the laboratory; anuran lungworm species ranged from strict host specificity, e.g., R. ranae from Nebraska, to relative generalist, e.g., Rhabdias joaquinensis from Nebraska. Overall, host specificity for species of Rhabdias does not provide support for the evolution of progressive specialization over time. For most species of lungworms, host specificity in nature appears to be limited by both ecological and physiological factors, which vary between species and their hosts. Furthermore, some lungworms, e.g., Rhabdias bakeri from Missouri, appear to be tracking host resources instead of host phylogenies, an example of ecological fitting.

Experimental infections and field-collected lizards were used to investigate issues of transmission, host specificity, and seasonal occurrence in the nematode Cyrtosomum penneri (Cosmocercoidea: Atractidae). Anolis sagrei (87 males, 42 females) were captured from the Florida Southern College campus, Polk County, Florida, from October 2010 to September 2011, and 8,803 C. penneri were collected from their intestines. During the breeding season all sexually mature (SVL ≥ 34 mm) A. sagrei were infected, whereas juvenile lizards (SVL <34 mm) were never infected. Experimental infections, using A. sagrei , found that worms were transferred to new hosts venereally, but not during oral exposures. Mating trials confirmed that worms were consistently transferred between hosts during copulation under natural conditions. Experimental exposures found that land snails and crickets do not serve as transport or intermediate hosts, which supports the idea that C. penneri is transferred only during host copulation. Experimental infections to test host specificity in C. penneri successfully infected A. sagrei , Hemidactylus turcicus , and Sceloporus undulatus , but not Anolis carolinensis or Plestiodon inexpectatus. Overall, this is the first study to fully elucidate the life cycle of any atractid nematode, and we suggest a venereal route of transmission for all atractid worms that infect reptilian hosts. Our findings also have implications for the host's reproductive and behavioral biology, e.g., support for covert or satellite males in the A. sagrei mating system.

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Given their ubiquitous nature, it is surprising that more oligochaete annelid worms (Annelida: Clitellata) have not adopted an endoparasitic lifestyle. Exceptions, however, are the understudied members of the genus Dero (Allodero) that parasitize the ureters of tree frogs and toads. This study experimentally explores the life cycle and host specificity of Allodero hylae, the worm's use of chemical cues in host searching, and its seasonal prevalence and abundance over a year-long collection period on the Florida Southern College campus. A total of 2,005 A. hylae was collected from the ureter, urinary bladder, or expressed urine of wild Osteopilus septentrionalis ; a significant positive correlation was found between host snout-vent length and parasite intensity for female but not male hosts. Monthly prevalence of A. hylae reached a peak of 58% in April, but never dropped below 20% in any month; mean abundance peaked March-May, whereas few worms were recovered in December and January. Confirming a parasitic lifestyle, wild-collected hosts with intense infections, typically >40 worms, showed obvious dilatation of the ureter wall, and some young-of-the-year O. septentrionalis exposed to A. hylae in the laboratory were killed by the apparent rupture of the host's ureter. The worm has a direct life cycle: worms expelled in the host's urine are capable of locating and re-infecting other hosts within aquatic microhabitats such as bromeliad tanks, and worms can survive for weeks in a free-living environment, even undergoing a morphological change. Further, chemotaxis assays found a positive response to a tree frog attractant for worms recently removed from hosts. Overall, this study provides the first multifaceted investigation on the life history and ecology of any Allodero spp., which offers new insights into an understudied endoparasitic oligochaete.

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The lungworm Rhabdias joaquinensis (Nematoda: Rhabdiasidae) is a common parasite of anurans in eastern Nebraska, yet absent from the same hosts in western Nebraska. This study investigated the ecology of the lungworm's free-living stages that reside in host feces and surrounding soils to establish the worm's free-living niche in eastern Nebraska. Using a comparative method, this study also investigated the absence of lungworms in western Nebraska's Sandhills. Soil composition, moisture, and temperature were experimentally varied in the laboratory to assess their effects on juvenile development and survival. Field mesocosm and host defecation experiments were used to determine where in nature lungworms survive and eventually infect frogs and toads and to discover if vegetation facilitates lungworm transmission to hosts. The results found loam soils were amenable to lungworm development, whereas soils with high clay or sand content produced few infective lungworms. Soil moisture <50 % did not support lungworm development. Infective juveniles successfully developed between 5 and 35 °C, albeit at different rates, whereas only a limited number of non-infective worms developed at 40 °C. Field studies found that shoreline environments supported lungworm development, and the majority of lungworm infections appear to occur within a zone of infection adjacent to shorelines in eastern Nebraska. The prevalence in vegetation mesocosms was 100 %, and a significantly higher mean abundance was found in toads from containers with vegetation than without. Overall, these experiments suggest that the spatial distribution of R. joaquinensis in Nebraska is constrained by the worm's ability to survive and reproduce in a soil matrix.