Rhabdias, Stiles & Hassall, 1905

Lettoof, Damian C., Greenlees, Matthew J., Stockwell, Michelle & Shine, Richard, 2013, Do invasive cane toads affect the parasite burdens of native Australian frogs?, International Journal for Parasitology: Parasites and Wildlife 2 (1), pp. 155-164 : 161

4.1. Rhabdias

Rhabdias lungworms were found in all of the anuran species that we examined, with cane toads containing a significantly high- er intensity overall ( Fig. 1 ; Table 3). Native frogs from toad-inhabited areas had fewer Rhabdias than did frogs from areas without toads, suggesting that the presence of cane toads has reduced rather than increased rates of Rhabdias infection in frogs ( Fig. 2 ). In fact, the toads and frogs likely have different species of Rhabdias that can only be distinguished using genetic methods ( Dubey and Shine, 2008). Although the Rhabdias species infecting wild cane toads was previously identified as R. hylae ( Johnston and Simpson, 1942; Barton, 1998), a parasite native to Australian frogs, genetic tests indicate that cane toads are infected with R. pseudosphaerocephala ( Dubey and Shine, 2008) . Most native frogs from tropical Australia that were experimentally infected with R. pseudosphaerocephala larvae survived, and did not sustain the infection ( Pizzatto and Shine, 2011). However, certain species ( Litoria caerulea , Litoria dahlii , and Cyclorana longipes ) were able to retain even more lungworms than did cane toads. These results suggest that, under the right conditions, native frogs may be susceptible to R. pseudosphaerocephala infection ( Pizzatto et al., 2012). However, vulnerability differs even among closely-related frog species. In our own study, we found similar lungworm prevalence and intensity in L. peronii and L. latopalmata , and a reduced prevalence (and thus, overall parasite burden) associated with cane toad presence.

Counter to our initial predictions, these two species of native frogs not only are not at risk from invading cane toads introducing parasites; the toads may actually reduce rather than increase the parasite burdens of native frogs. The mechanisms underlying this unanticipated effect warrant further research. Plausibly, cane toads might selectively consume insects that otherwise would transmit parasites to native frogs; or cane toad parasites might trigger changes to frog immune systems that make the frog more capable of dealing with infective larvae of native parasite taxa. More likely, many native parasites are taken up by cane toads but fail to survive because of the host’s ability to mount an effective immune defence. Experimental studies show that native frogs can deal effectively with the toad’s lungworm species in this way ( Pizzatto et al., 2010), suggesting that cane toads may be similarly capable of dealing with the parasites of native frogs. Keesing et al. (2006) suggested that higher host diversity actually reduces disease risk, especially when competent hosts are abundant. The invasion of cane toads into native ecosystems may reflect a similar scenario as has been documented with introduced fish ( Kelly et al., 2009), whereby the invaders over- procure parasites that then fail to develop within the introduced host, consequently reducing infection rates of native fauna. Analogously, Freeland et al. (1986) suggested that cane toad invasion has imperilled a tapeworm of native frogs, by breaking its usual transmission to higher-order vertebrate predators.

The prevalence of R. pseudosphaerocephala was as high (70%) in cane toads at the southern invasion front as in well-established cane toad populations in Queensland (80%; Barton, 1998). Studies of the cane toad invasion in tropical Australia indicate that cane toads move almost three times as fast as they do in subtropical populations ( Phillips et al., 2006; Urban et al., 2007), and these toads are parasite free for at least a year or two after initial colonisation. Our results support the prediction from Phillips et al. (2010) that parasitic lungworms will be present in the cane toad’s southern invasion front. The slow speed of the subtropical cane toad invasion in north-eastern NSW compared to the tropical cane toad invasion allows toads to establish denser populations before spreading, and thus offers parasites a greater host density at the range-edge. Kelehear et al. (2012) found that R. pseudosphaerocephala near the range-edge of toads had evolved significant shifts in life-history traits (such as increased body size, faster maturation and smaller quantities of larger eggs) in response to low host-densities (and thus, longer exposure times in harsh external environments prior to finding a new host). However, this may not be the case with the southern cane toad invasion. Nematodes are particularly vulnerable to desiccation ( Perry, 1999), a major threat to exposed free-living lungworm larvae in tropical Australia. North-eastern NSW offers lower ambient temperatures and more consistent rainfall than do the tropics of the Northern Territory ( Bureau of Meteorology, 2012), potentially enhancing the survivorship of R. pseudosphaerocephala in the external environment.

Figures

Fig. 1. (A) Prevalence (% of anurans infected) and (B) intensity (mean number of worms per infected host) of parasitic lungworms in cane toads and native anuran from northern NSW. Bars represent standard errors.

Fig. 2. (A) Prevalence (% of anurans infected) and (B) intensity (mean number of worms per infected host) of parasitic lungworms in anurans from cane toadpresent, and cane toad-absent areas in northern NSW. Bars represent standard errors.