Invasive Colonic Entamoebiasis in Wild Cane Toads, Australia

Do changes in body mass alter white blood cell profiles and immune function in Australian cane toads (Rhinella marina)?

Variation in food resources can result in dramatic fluctuations in the body condition of animals dependent on those resources. Decreases in body mass can disrupt patterns of energy allocation and impose stress, thereby altering immune function. In this study, we investigated links between changes in body mass of captive cane toads (Rhinella marina), their circulating white blood cell populations, and their performance in immune assays. Captive toads that lost weight over a three-month period had increased levels of monocytes and heterophils and reduced levels of eosinophils. Basophil and lymphocyte levels were unrelated to changes in mass. Because individuals that lost mass had higher heterophil levels but stable lymphocyte levels, the ratio of these cell types was also higher, partially consistent with a stress response. Phagocytic ability of whole blood was higher in toads that lost mass, owing to increased circulating levels of phagocytic cells. Other measures of immune performance were unrelated to mass change. These results highlight the challenges faced by invasive species as they expand their range into novel environments which may impose substantial seasonal changes in food availability that were not present in the native range. Individuals facing energy restrictions may shift their immune function towards more economical and general avenues of combating pathogens. This article is part of the theme issue 'Amphibian immunity: stress, disease and ecoimmunology'.

First Evidence of Entamoeba Parasites in Australian Wild Deer and Assessment of Transmission to Cattle

Australian wild deer populations have significantly expanded in size and distribution in recent decades. Due to their role in pathogen transmission, these deer populations pose a biosecurity risk to the livestock industry. However, little is known about the infection status of wild deer in Australia. The intestinal parasite Entamoeba bovis has been previously detected in farm and wild ruminants worldwide, but its epidemiology and distribution in wild ruminants remain largely unexplored. To investigate this knowledge gap, faecal samples of wild deer and domestic cattle from south-eastern Australia were collected and analysed for the presence of Entamoeba spp. using PCR and phylogenetic analysis of the conserved 18S rRNA gene. E. bovis parasites were detected at high prevalence in cattle and wild deer hosts, and two distinct Entamoeba ribosomal lineages (RLs), RL1 and RL8, were identified in wild deer. Phylogenetic analysis further revealed the existance of a novel Entamoeba species in sambar deer and a novel Entamoeba RL in fallow deer. While we anticipated cross-species transmission of E. bovis between wild deer and cattle, the data generated in this study demonstrated transmission is yet to occur in Australia. Overall, this study has identified novel variants of Entamoeba and constitutes the first report of Entamoeba in fallow deer and sambar deer, expanding the host range of this parasite. Epidemiological investigations and continued surveillance of Entamoeba parasites in farm ruminants and wild animals will be required to evaluate pathogen emergence and transmission to livestock.

A RETROSPECTIVE ANALYSIS OF AMOEBIASIS IN REPTILES IN A ZOOLOGICAL INSTITUTION

Amoebiasis is a significant protozoal disease of reptiles causing nonspecific clinical signs including diarrhea, anorexia, and lethargy. It frequently results in acute death. Investigation of the pathophysiology of amoebiasis in reptiles has been hampered by the inability to accurately identify amoeba to the species level using conventional techniques. This study reviewed reptile medical records from the Wildlife Conservation Society's archives from 1998 to 2017. Amoebae were identified histologically in 54 cases in 31 different species. Of these, amoebiasis was the cause of death in 32 (18 chelonians, 7 lizards, and 7 snakes), a significant co-morbidity in 14 (six chelonians, two lizards, and six snakes), and seen incidentally in eight cases (one chelonian, six lizards, and one snake). Relocation from one enclosure to another was also evaluated and 65% of cases had been moved within 180 days of death (median 46 days). Frozen tissue samples from 19 of these cases were tested via an Entamoeba (genus-specific) polymerase chain reaction (PCR) assay. PCR products were sequenced and Entamoeba species were identified. Six individuals were positive for Entamoeba invadens (three chelonians, two snakes, one lizard), two for Entamoeba ranarum (both snakes), and one for Entamoeba terrapinae (chelonian); the other 10 cases were negative via PCR. Entamoeba ranarum has typically been considered a disease of amphibians with only one report of disease in a snake. Entamoeba terrapinae has only been reported without associated disease in chelonians. These results suggest that amoebiasis is a complicated and nuanced disease of reptiles, and warrants additional study.

Gastrointestinal entamoebiasis in captive anurans in North America

Infections with Entamoeba spp. are recognized as a cause of clinical disease in many species including humans and reptiles; however, cases in amphibians are under-reported. Investigation of a mortality event among a captive population of Cranwell's horned frogs Ceratophrys cranwelli at a production facility in Florida, USA, revealed that deaths were due to the newly described Entamoeba species CT1. Infection caused severe necroulcerative gastroenterocolitis with a predilection for the colon. To date, this Entamoeba species has only been described in invasive cane toads Rhinella marina in Australia. Retrospective screening of archived anuran cases from a zoological pathology service identified 8 cases from captive populations that had histological evidence of gastrointestinal entamoebiasis. Molecular characterization was positive in 3 cases. Two cases, 1 in a Puerto Rican crested toad Peltophryne lemur and 1 in an Amazon milk frog Trachycephalus resinifictrix, showed 100% homology to E. ranarum and 1 case in a White's tree frog Litoria caerulea showed 100% homology to Entamoeba sp. CT1. This is the first report of novel Entamoeba sp. CT1 being associated with clinical disease in anurans within North America and also the first report of this Entamoeba species causing disease within managed collections as far back as 2003.

Apparent lack of spill-over of parasites from an invasive anuran: PCR detects Entamoeba in cane toads (Rhinella marina) but not in sympatric Australian native frogs

The recent detection of a novel amoebozoan parasite (Entamoeba sp. CT1) killing invasive cane toads (Rhinella marina) in tropical Australia raises concerns of potential spill-over into native anuran populations. Considering the vulnerability of anuran communities globally, Entamoeba sp. CT1 may pose a serious threat to anuran biodiversity. Through PCR-based detection and molecular identification, we investigated the prevalence of Entamoeba spp. in the faeces and colon tissue of cane toads (Rhinella marina) and eleven native Australian frog species from a single locality in the Northern Territory. No Entamoeba DNA was detected in samples of native frog faeces (N = 57) or colons (N = 17). Entamoeba DNA was detected in 24% of 45 cane toads (95%CI 14.08–38.82). Both E. ranarum and Entamoeba sp. CT1 were present in cane toads. The failure of faecal samples to indicate Entamoeba spp. in infected cane toads may be due to cysts in faeces being shed intermittently, degraded before analysis, or impervious to lysis prior to DNA isolation. Our results suggest that native frogs do not carry the pathogen in an area where 20–30% of cane toads are infected with Entamoeba sp. CT1. We demonstrate the importance of recognising PCR inhibition prior to molecular diagnostics, and the apparent inadequacy of faecal samples for the detection of Entamoeba spp. in anurans.

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Entamoeba spp. were detected in cane toads, but not in faeces or colons of native frogs.

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PCR inhibition of anuran faecal samples.

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Additional purification, the presence of inhibition was reduced.

The cost of chemical defence: the impact of toxin depletion on growth and behaviour of cane toads ( Rhinella marina)

Many animals capable of deploying chemical defences are reluctant to use them, suggesting that synthesis of toxins imposes a substantial cost. Typically, such costs have been quantified by measuring the elevation in metabolic rate induced by toxin depletion (i.e. during replenishment of toxin stores). More generally, we might expect that toxin depletion will induce shifts in a broad suite of fitness-relevant traits. In cane toads ( Rhinella marina), toxic compounds that protect against predators and pathogens are stored in large parotoid (shoulder) glands. We used correlational and experimental approaches in field and laboratory settings to investigate impacts of toxin depletion on growth rate and behaviour in cane toads. In free-ranging toads, larger toxin stores were associated with smaller gonads and livers, suggesting energetic trade-offs between toxin production and both reproduction and energy metabolism. Experimental removal of toxin (by manually squeezing parotoid glands) reduced rates of growth in body mass in both captive and free-ranging toads. Radio tracking demonstrated that de-toxined toads dispersed more slowly than did control toads. Given that toxin stores in cane toads take several months to fully replenish, deploying toxin to repel a predator may impose a substantial cost, explaining why toads use toxin only as a final line of defence.

Diurnal activity in cane toads (Rhinella marina) is geographically widespread

Although adult cane toads (Rhinella marina) are generally active only at night, a recent study reported that individuals of this species switched to diurnal activity in response to encountering a novel habitat type (deeply shaded gorges) in the course of their Australian invasion. Our sampling over a broader geographic scale challenges the idea that this behaviour is novel; we documented diurnal behaviour both in the species’ native range and in several sites within the invaded range, in multiple habitat types. Diurnal activity was most common in the tropics and in areas where toads attain high population densities and are in poor body condition, suggesting that the expansion of activity times may be induced by intraspecific competition for food.

Entamoeba chiangraiensis n. sp. (Amoebozoa: Entamoebidae) isolated from the gut of Asian swamp eel (Monopterus albus) in northern Thailand

The genus Entamoeba comprises mostly gut parasites and commensals of invertebrate and vertebrate animals including humans. Herein, we report a new species of Entamoeba isolated from the gut of Asian swamp eels (Monopterus albus) in northern Thailand. Morphologically, the trophozoite is elongated and has a single prominent pseudopodium with no clear uroid. The trophozoite is actively motile, 30-50 µm in length and 9-13 µm in width. Observed cysts were uninucleate, ranging in size from 10 to 17.5 µm in diameter. Chromatin forms a fine, even lining along the inner nuclear membrane. Fine radial spokes join the karyosome to peripheral chromatin. Size, host and nucleus morphology set our organism apart from other members of the genus reported from fish. The SSU rRNA gene sequences of the new isolates are the first molecular data of an Entamoeba species from fish. Phylogenetic analysis places the new organism as sister to Entamoeba invadens. Based on the distinct morphology and SSU rRNA gene sequence we describe it as a new species, Entamoeba chiangraiensis.

Pathology Associated With an Outbreak of Entamoebiasis in Wild Cane Toads (Rhinella marina) in Tropical Australia

Infection due to Entamoeba spp. is known to cause serious disease in primates (Entamoeba histolytica) and snakes (Entamoeba invadens), but there are no detailed descriptions of the pathology associated with Entamoeba spp. infection in amphibians. In 2014, an outbreak of entamoebiasis associated with a novel species of Entamoeba induced clinical illness and poor body condition in free-ranging cane toads in Australia's Northern Territory. Here, we describe the gross pathology, histology, and clinical pathology linked to the outbreak. The study compared 25 toads with invasive entamoebiasis, defined as histologically visible amoebas within tissue, and 12 toads without invasive entamoebiasis. Grossly, affected toads had mild to marked congestion of colonic serosal vasculature, with variable thickening of the intestinal wall and serosanguineous to hemorrhagic colonic content. Histologically, invasive entamoebiasis manifested primarily as moderate to severe, variably hyperplastic to ulcerative colitis. The small intestine was affected in 10 of 25 toads, and 5 of 25 toads also had gastric lesions. Amoebas consistent in morphology with Entamoeba sp. were commonly intermingled with mucosal epithelium, frequently along the basement membrane, with deeper invasion into the superficial lamina propria in only 5 toads. Toads with invasive entamoebiasis had neutrophilia, monocytosis, and lymphopenia, and thus elevated neutrophil to lymphocyte ratios, suggestive of an inflammatory and/or stress leukogram.