Dynamics of an Emerging Infectious Disease of Amphibians: From Individuals to Communities

Chytridiomycosis is an infectious disease of amphibians caused by the fungus Batrachochytrium dendrobatidis (Bd), and it has played an important role in the worldwide decline of amphibians. My dissertation research examined the consequences of Bdinfections on tadpole feeding biomechanics and activity, pathogen transmission, and host immunology. The keratinized labial teeth of Bd-infected Fowler\u27s Toad (Anaxyrus [=Bufo] fowleri) and Grey Treefrog (Hyla versicolor) tadpoles slipped off of surfaces on which they grazed and were in contact with an algal-covered surface for a shorter duration of time during each feeding cycle. During feeding trials, Bd-infected A. fowleri and H. versicolor tadpoles obtained significantly less food and were less active during feeding relative to non-infected tadpoles. Collectively, these data show that Bd-infected tadpoles are less efficient and less active while feeding and provide a potential mechanism for reduced growth and development in Bd-infected tadpoles of these species. In artificial ponds, A. fowleri tadpoles raised in the presence of Bd aggregated significantly more relative to controls, whereas H. versicolor aggregated significantly less. In addition, ponds with A. fowleri tadpoles supported higher Bd prevalences and infection intensities relative to ponds with H. versicolor, suggesting that aggregation behavior may impact intraspecific Bd transmission. Independent of species, tadpoles raised in the presence of Bd were smaller and less developed than tadpoles raised in disease-free conditions, even when Bd prevalence was low. Although A. fowleri tadpoles seem more susceptible to Bd and carry heavier infections, our results suggest that Bd can negatively impact larval life history traits associated with fitness. To test possible mechanisms related to differential Bd susceptibility, I raised Southern Leopard Frog (Lithobates sphenocephalus) tadpoles on diets that differed in their protein content. Tadpoles fed a low-protein diet had less effective immune responses (PHA-induced skin-swelling resonse and the ability of tadpole blood to kill E. coli), increased susceptibility to Bd, and were less developed relative to tadpoles fed a high-protein diet. However, the immune responses of tadpoles infected with Bd were similar, suggesting that neither T cell recruitment nor cytotoxicity of tadpole blood (i.e., PHA and bacterial killing ability, respectively) specifically inhibit Bd infections

Does the thermal mismatch hypothesis predict disease outcomes in different morphs of a terrestrial salamander?

Many aspects of ectotherm physiology are temperature‐dependent. The immune system of temperate‐dwelling ectothermic host species is no exception and their immune function is often downregulated in cold temperatures. Likewise, species of ectothermic pathogens experience temperature‐mediated effects on rates of transmission and/or virulence. Although seemingly straightforward, predicting the outcomes of ectothermic host−pathogen interactions is quite challenging. A recent hypothesis termed the thermal mismatch hypothesis posits that cool‐adapted host species should be most susceptible to pathogen infection during warm temperature periods whereas warm‐adapted host species should be most susceptible to pathogens during periods of cool temperatures. We explore this hypothesis using two ecologically and physiologically differentiated color morphs of the Eastern Red‐backed Salamander (Plethodon cinereus) and a pathogenic chytrid fungus (Batrachochytrium dendrobatidis; hereafter Bd ) using a fully factorial laboratory experiment. At cool temperatures, unstriped salamanders (i.e., those that are tolerant of warm temperatures) had a significantly higher probability of Bd infection compared with cool‐tolerant striped salamanders, consistent with the thermal mismatch hypothesis. However, we found no support for this hypothesis when salamanders were exposed to Bd at warm temperatures: the probability of Bd infection in the cool‐tolerant striped salamanders was nearly identical in both cool and warm temperatures, opposite the predictions of the thermal mismatch hypothesis. Our results are most consistent with the fact that Bd grows poorly at warm temperatures. Alternatively, our data could indicate that the two color morphs do not differ in their tolerance to warm temperatures but that striped salamanders are more tolerant to cool temperatures than unstriped salamanders. Research Highlights: In a test of the thermal mismatch hypothesis, we found that in cool temperatures, warm‐tolerant salamanders had higher parasitism compared with cool‐tolerant salamanders. There was no difference in parasitism for salamanders in warm temperatures

Female Salamanders Experience Higher Parasitism Compared to Males: A Cost of Female Reproduction?

Males tend to experience higher rates of parasitism compared to females, a phenomenon associated with ecological factors, the fact that males engage in risky behaviors, and because testosterone is known to be immunosuppressive. However, females could experience higher rates of parasitism if energy is allocated from costly immune responses towards producing eggs. We used pooled data sets from laboratory experiments to investigate sex-specific differences in salamander (Plethodon cinereus) resistance to the emerging fungal pathogen Batrachochytrium dendrobatidis (‘‘Bd’’). Contrary to our predictions, we found that female salamanders had a higher prevalence of infection (~56%) and carried a higher Bd infection burden (455 zoospores equivalents per sample) compared to male salamanders (which had a Bd infection prevalence of ~24% and an average infection burden of 58 zoospore equivalents per sample). We also found that female reproductive investment (i.e., mass of eggs) positively correlated with Bd infection burden, suggesting that females who previously invested more into reproduction carried a higher Bd infection burden. Collectively, our findings might indicate that female salamanders experience a cost of reproduction in the form of decreased disease resistance

Taking tadpole biology into the 21st century: a consensus paper from the First Tadpoles International Workshop

Although tadpoles are common, frequently encountered vertebrates, their natural history is poorly known. Despite the fact that Brazil harbors the highest anuran diversity in the world, with almost a thousand known species (Segalla et  al., 2014), the larvae of many species remain undescribed. The problem is not unique to Brazil. At the turn of the century, approximately ⅔ of almost 3,300 anuran species with a larval stage lacked tadpole descriptions (McDiarmid & Altig, 1999). The proportion of neotropical anurans with free-swimming larvae that are not described is estimated to be around 40% (Provete et al., 2012).Fil: Rossa Feres, Denise de C.. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Venesky, Matthew. Allegheny College; Estados UnidosFil: Nomura, Fausto. Universidade Federal de Goias; BrasilFil: Eterovick, Paula C.. Pontifícia Universidade Católica de Minas Gerais; BrasilFil: Vera Candioti, María Florencia. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Menin, Marcelo. Universidade Federal do Amazonas; BrasilFil: Juncá, Flora A.. Universidade Estadual de Feira de Santana; BrasilFil: Schiesari, Luis C.. Universidade de Sao Paulo; BrasilFil: Haddad, Celio F. B.. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Garey, Michel V.. Universidade Federal da Integração Latino-Americana; BrasilFil: dos Anjos, Luciano A.. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Wasserug, Richard. Dalhousie University Halifax; Canadá. University Of British Columbia; Canad

Intraspecific variation in life history traits among two forms of ambystoma barbouri larvae

Variation in available resources (e.g.. environment or food) can influence life history variation. Populations of the streamside salamander (Ambystoma barbouri) breed in both streams and ponds and larvae exhibit diverse life history patterns related to the environment within which they develop. Previous experiments demonstrated that environmental factors are largely responsible for differences in larval period. mass and size at metamorphosis between the two forms of A. barbouri; however. it is unclear which environmental factors influence the variation. Under controlled laboratory conditions. we tested if hydrological differences between lotic and lentic habitats contribute to life history divergence in A. barbouri. We reared laboratory born larvae from stream- and pond-collected eggs in laboratory environments with hydrological patterns similar to their natal environments. Ambystoma barbouri stream larvae metamorphosed faster and were smaller in mass and size compared to pond larvae. Although we did not test whether genetic or environmental factors influence larval life history traits. our data provide support that hydrological variation alone may contribute to the life history variation in the two forms of larval A. barbouri

Impacts of Batrachochytrium dendrobatidis infection on tadpole foraging performance

Pathogen-induced modifications in host behavior, including alterations in foraging behavior or foraging efficiency, can compromise host fitness by reducing growth and development. Chytridiomycosis is an infectious disease of amphibians caused by the fungus Batrachochytrium dendrobatidis (Bd), and it has played an important role in the worldwide decline of amphibians. In larval anurans, Bd infections commonly result in reduced developmental rates, however, the mechanism(s) responsible are untested. We conducted laboratory experiments to test whether Bd infections reduced foraging performance of Grey Treefrog (Hyla chrysoscelis) and Fowler\u27s Toad (Anaxyrus [= Bufo fowleri) tadpoles. In the first experiment, we observed foraging behavior of Bd-infected and uninfected tadpoles to test for differences in foraging activity. In a second experiment, we tested for differences in the ingestion rates of tadpoles by examining the amount of food in their alimentary track after a 3-hour foraging period. We hypothesized that Bd-infected tadpoles would forage less often and less efficiently than uninfected tadpoles. As predicted, Bd-infected larvae forage less often and were less efficient at obtaining food than uninfected larvae. Our results show that Bd infections reduce foraging efficiency in Anaxyrus and Hyla tadpoles, and that Bd differentially affects foraging behavior in these species. Thus, our results provide a potential mechanism of decreased developmental rates of Bd-infected tadpoles. © 2010 International Association for Ecology and Health

Fungal pathogen changes the feeding kinematics of larval anurans

Pathogens can alter host life-history traits by affecting host feeding activities. In anuran tadpoles, keratinized mouthparts (teeth and jaw sheaths) are essential for feeding. Batrachochytrium dendrobatidis (Bd) is a pathogenic fungus of amphibians that can infect these mouthparts and reduce tadpole survival. However, the precise way that Bd-induced changes in tadpole mouthparts impact tadpole feeding is unknown. We use high-speed (500 frames/sec) videography to study how Bd-induced mouthpart deformities affect the feeding kinematics of Fowler\u27s toad (Anaxyrus [= Bufo ]fowleri) and grey tree frog (Hyla chrysoscelis) tadpoles. We tested for species-specific patterns of Bd-induced mouthpart deformities to assess how deformations to specific areas of tadpole mouthparts alter feeding kinematics. The teeth of tadpoles from the Bd-exposed treatment slipped off of surfaces on which tadpoles graze and were in contact with an algal-covered substratum for a shorter duration in each gape cycle compared to teeth of control tadpoles. We also found that the jaw sheaths had significantly more deformations than labial teeth; however, how this relates to feeding kinematics is unclear. Our data show explicitly how Bd infection reduces foraging efficiency of anuran tadpoles by altering feeding kinematics and elucidate a mechanistic link between the pathogen infection and reduced host fitness. © 2010 American Society of Parasitologists

Pathogenicity of Batrachochytrium dendrobatidis in larval ambystomatid salamanders

Chytridiomycosis is a disease of amphibians caused by the fungus Batrachochytrium dendrobatidis (Bd), which colonizes keratinized tissues in adult and larval amphibians. Considerable progress has been made in understanding the host-pathogen ecology of Bd in larval anurans, yet little is known about how Bd affects larval salamanders. Because the structure of keratinized jaw sheaths in Ambystoma larvae have not been thoroughly documented, we first described the structure in three species of larval Ambystoma. We then conducted a laboratory experiment to test if Bd affects growth and developmental rates of larval Marbled Salamanders (Ambystoma opacum). We observed keratinized jaw sheaths in all three species of Ambystoma, but the sheath was not present in all individuals. In our exposure experiment, none of the A. opacum, whose mouthparts were screened for Bd, tested positive, nor was there an effect of Bd on larval life-history responses. A cautionary note, however, is that although our method of Bd infection has been successful in other amphibian-Bd experiments in our laboratory, our exposure experiment did not include a positive control of other taxa known to become infected with Bd. We are uncertain why none of the larval A. opacum became infected with Bd, given that we observed keratinized jaw sheaths in this species. Two possible explanations are the keratinized jaw sheaths of larval Ambystoma differ among species in structure or keratin type so that Bd may not be able to successfully infect them or, A. opacum larvae may have cleared low intensity Bd infections prior to metamorphosis

The impact of variation in labial tooth number on the feeding kinematics of tadpoles of Southern Leopard Frog (Lithobates sphenocephalus)

The keratinized mouthparts of tadpoles are complex structures that play a significant role in feeding. Recent evidence has shown that the keratinized labial teeth function in two ways: to anchor the oral disc to a substrate and to rake material from it. Reports of tadpoles with missing or deformed keratinized oral structures have increased. Yet, the impact missing teeth has on feeding remains unexplored. Here we use high speed videography (500 frames per second) of tadpoles of Southern Leopard Frog (Lithobates sphenocephalus [=Rana sphenocephala]) to study how missing teeth affect feeding. Specifically, we test the hypothesis that the number of teeth present correlates with two metrics of the effectiveness of a tadpole to rake material from a substrate. Those are the duration of the full gape cycle (opening and closing of the jaws) and of the closing phase (closing of the jaws once attached to a substrate). We found a significant positive relationship between duration of time of the closing phase of the gape cycle and the number of labial teeth present. Thus, tadpoles with fewer teeth were in contact with a planar algal-covered substrate for a shorter duration than tadpoles with more teeth. Given the reports of tadpole mouthpart abnormalities, data relating feeding kinematics of tadpoles with missing labial teeth are relevant in assessing the impact these abnormalities have on larval performance. Our data indicate that tadpoles missing labial teeth forage less effectively than tadpoles with undamaged dentition. This may help explain the pattern of reduced growth and developmental rates commonly observed in tadpoles raised in polluted environments or infected with pathogens that attack keratinized oral structures. © 2010 by the American Society of Ichthyologists and Herpetologists

How does a change in labial tooth row number affect feeding kinematics and foraging performance of a ranid tadpole (Lithobates sphenocephalus)?

Recent studies have explored feeding kinematics in tadpoles with intact labial teeth; however, it is unknown how missing teeth impacts foraging. We explored the impact of missing labial teeth on the feeding mechanics and foraging performance of Southern leopard frog (Lithobates sphenocephalus [= Rana sphenocephala]) tadpoles by controlling the pattern of labial tooth loss; that is, by surgically removing one row of labial teeth. We then used high-speed (500 frames/second) videography to test the hypothesis that tooth loss reduces the time that tadpoles attach to and graze upon an algal-covered substrate. We next conducted trials of foraging efficiency and foraging activity to test the hypothesis that tadpoles with fewer teeth forage less effectively than control tadpoles. The teeth of tadpoles from the surgery treatment slipped while closing and were in contact with an algal-covered substrate for a shorter duration compared to control tadpoles. Surprisingly, tadpoles with missing labial teeth obtained similar amounts of food and were as active as tadpoles with intact mouthparts. However, tadpoles with missing teeth completed about 25% more gape cycles per unit time than control tadpoles. Our data suggest that tadpoles with missing teeth compensate for inferior feeding kinematics during mouth closing in each gape cycle by increasing the number of gape cycles per unit time. © 2010 Marine Biological Laboratory