Are oral deformities in tadpoles accurate indicators of anuran chytridiomycosis?

Abstract

We evaluated the use of oral deformities as reliable proxies for determining Batrachochytrium dendrobatidis (Bd) infection in tadpoles of six anuran species of the Atlantic Forest in southeastern Brazil. We examined oral discs of 2156 tadpoles of six species of anurans collected in 2016: Aplastodiscus albosignatus, Boana albopunctata, Boana faber, Scinax hayii, Crossodactylus caramaschii, and Physalaemus cuvieri. Three oral deformities were recognized: lack of keratinization only in upper and/or lower jaw sheaths, lack of keratinization only in upper or lower tooth rows, and both deformities together. A subsample composed of all the individuals possessing oral deformities (N = 195) plus randomly selected individuals without oral deformities (N = 184) were tested for Bd via qPCR. Oral deformities were observed in all six species, but only five were infected with Bd. Since we found that dekeratinization of tooth rows was not associated with the presence of Bd in any of the studied species we used a new proxy (jaw sheaths dekeratinization with or without dekeratinization in tooth rows: JSD-proxy) for Bd detection. Our results showed a nonrandom relationship between Bd infection and JSD-proxy in three species of the family Hylidae. However, the use of JSD-proxy for Bd detection in these species resulted in up to 30.8% false positives and up to 29.3% false negatives. The use of the JSD-proxy in species for which no relationship was found reached 100% of false positives. We conclude that the use of oral dekeratinization as a generalized proxy for Bd detection in tadpoles should not be used as a single diagnosis technique.

Chytridiomycosis Outbreak in a Chilean Giant Frog (Calyptocephalella gayi) Captive Breeding Program: Genomic Characterization and Pathological Findings

“…Although oral deformations in postmetamorphic amphibians have not been used before as an indication of chytridiomycosis, they might be still important and may indicate an unknown sequela of Bd infection that may have been overlooked previously. This study highlights the need to use accurate diagnostic techniques such as qPCR or histology to be able to complement these observations (57).…”

Bullfrog farms release virulent zoospores of the frog-killing fungus into the natural environment

“…Prevalence of Bd in farmed bullfrog tadpoles approximated or exceeded prevalences found in wild tadpoles in the Brazilian Atlantic Forest and savannah (Cerrado) 34 , the biomes where our sampled farms were located. Although visual inspection can be used as a proxy for Bd diagnosis 34,36,38,41,42 , it is possible that mouthpart dekeratinization may not be observed at early stages of infection 36 ; thus, the prevalence we detected in bullfrog farms may be an underestimate. When we applied the error estimation to our raw data, Bd prevalence increased significantly, making our findings even worse.…”

“…S2). Although visual inspection for mouthpart dekeratinization is used for Bd diagnosis in tadpoles 34,41 , some studies show that tadpoles with early stages of infection may not present mouthpart dekeratinization 36 , and that tadpoles with fully dekeratinized mouthparts no longer have substrate for Bd growth 34 , therefore leading to false positives and negatives 42 . Thus, in order to validate our method, we ran qPCR analyses with a subsample of 51 bullfrog tadpoles with dekeratinized mouthparts from one of the focal bullfrog farms.…”

“…In addition, and to be even more conservative, we included an estimation of false negatives and false positives in our analyses (see below) based on previous studies. We applied as a possible error the largest estimates observed in a recent study 42 : 12.3% for false positives and 18.7% for false negatives (Supplementary Table S1).…”

A new glassfrog species of the genus Centrolene (Amphibia, Anura, Centrolenidae) from Cordillera del Cóndor, southern Ecuador

“…This could be particularly true in our case, as we did not observe oral deformities in the form of dekeratinization (depigmentation) of mouthparts (which are the typical symptoms of Bd infection; Navarro-Lozano et al 2018), just the loss of the tooth rows. Also, it is possible that the loss of tooth was caused by other factors, as there are studies that have indicated that oral deformities are not always related to Bd infection (Rachowicz 2002;Blaustein et al 2005;Navarro-Lozano et al 2018) and could be attributed to other factors, like low temperatures (Rachowicz 2002), water contamination (Rowe et al 1998) or diet (McDiarmid and Altig 1999). In order to resolve this issue, we plan to implement a survey in the near future to confirm or not the pathogen's presence in the reserve and particularly in the threatened amphibian populations.…”