台灣外來種及本土種兩棲動物感染蛙壺菌之概況

Abstract

全球性兩棲動物族群數量呈現明顯下降趨勢，蛙壺菌(Batrachochytrium dendrobatidis)感染可能是主要元兇之一，此病原主要感染有尾目及無尾目，目前發現有14科250種以上兩棲動物感染蛙壺菌症，帶原者有蝌蚪、美國牛蛙 (Rana catesbeiana)、非洲爪蟾 (Xenopus laevis)、蔗蟾 (Bufo marinus) 及虎斑鈍口螈 (Ambystoma tigrinum)。受到蛙壺菌感染而帶有病原，但卻無明顯的症狀，並可能將病原傳給其他兩棲動物，這些動物稱之為帶原者 (carrier)。本研究之目的為以組織病理學及PCR診斷技術針對回溯性及持續收集之病例進行檢測，以了解蛙壺菌症是否入侵台灣及其發生的情形。回溯性以組織病理學檢查1993到2007年，台北市立動物園送檢的64隻蛙類及36隻蠑螈，發現於2003年12月21日送檢的迷彩箭毒蛙 (Dendrobates auratus) 為第1例檢出有蛙壺菌感染症之病例。現況調查分為三大類，(ㄧ)外來種寵物蛙：總數278隻，檢體包括新鮮皮膚檢體14個、皮膚棉花拭子109個、蛙類飼養箱的水樣132個及石蠟包埋組織塊99個，經由PCR檢測及組織病理學診斷，共有17個陽性病例，其中含4隻角蛙 (Ceratophrys sp.)、4隻南美角蛙 (Ceratophrys crawelli)、3隻黃金角蛙 (Ceratophrys cranwelli, alphosis)、2隻鐘角蛙 (Ceratophrys ornate)、2隻猴樹蛙 (Phyllomedusa hypochondrialis) 及2隻白化非洲爪蟾 (Xenopus laveis)，陽性病例主要以角花蟾屬的蛙類為主，佔總陽性病例76.4％ (13/17)；分別以各種蛙類的樣本總數估算陽性率，角花蟾蛙類的平均檢出陽性率為12.9% (13/101)， 白化非洲爪蟾 (Xenopus laveis ) 為5.7% (2/35)，猴樹蛙 (Phyllomedusa hypochondrialis) 為33.3% (2/6)。(二)食用蛙：總數113隻，檢體包括新鮮皮膚35個、皮膚棉花拭子36個、蛙類飼養池的水樣36個及石蠟包埋組織蠟塊50個，PCR及組織病理學檢測結果皆為陰性。(三)野外蛙隻：總數為443隻，檢體包括石蠟包埋組織蠟塊393個、新鮮皮膚檢體9個、皮膚棉花拭子39個、蝌蚪口器棉花拭子2個及蛙類飼養池的水樣32個；PCR檢測及組織病理學檢測結果皆為陰性。研究結果顯示，蛙壺菌症已藉由外來種寵物蛙的輸入而入侵台灣，惟目前尚無證據顯示本病已散播至食用蛙及野外蛙隻。

Batrachochytrium dendrobatidis is the cause of chytridiomycosis, a potentially fatal epidermal infection in amphibian. The fungus infects 2 amphibian orders (Anura and Caudata), 14 families, and at least 250 species. Bull frog (Rana catesbeiana), Xenopus laevis, tadpoles, tiger salamander (Ambystoma tigrinum), and cane toad (Bufo marinus) are known carriers. To determine whether chytridiomycosis has intruded Taiwan, a survey was carried to examine the infecion by histopathology and/or polymerase chain reaction (PCR) in the imported pet frogs, salamanders, meat frogs and field frogs. Retrospectively, a total of 64 frogs and 36 salamanders dying spontaneously and necropsied routinely at Taipei Zoo during the years of 1993 and 2007 were subjected to evaluation. During this period, only one Dendrobates auratus died in December of 2003, which was diagnosed as a mild infection of chytrid fungus histopathologically. For imported pet frogs, a total of 278 frogs were examined by light microscopy, hostopathology and /or PCR, specimens including 14 fresh skin samples, 109 skin swabs, 132 bath water samples and 99 paraffin-embedded skin tissues. Seventeen positive cases were diagnosed, including 4 Ceratophrys sp., 4 Ceratophrys crawelli, 3 Ceratophrys cranwelli, alphosis, 2 Ceratophrys ornate, 2 Phyllomedusa hypochondrialis and 2 Xenopus laveis. Ceratophrys sp. seemed to be the major species affected by chytridiomycosis with a positive rate of 76.4％ (13/17), and the overall positive rates of Ceratophrysl, Xenopus laveis, Phyllomedusa hypochondrialis were 12.9% (13/101), 5.7% (2/35), and 33.3% (2/6), respectively. For meat frogs, a total of 35 fresh skin samples, 35 skin swabs, 36 bath water samples, and 50 paraffin-embedded tissues were collected from 113 frogs. For field frogs, a total of 9 fresh skin sampels, 39 skin swabs, 2 mouthpart swab, 32 bath water samples, and 393 paraffin-embedded tissues were collected from 443 frogs. However, all specimens from meat frogs and field frogs were negative by light microscopy, histopathology and/or PCR. The results of the investigation indicate that chytridiomycosis has indeed been introduced into Taiwan possibly via the importation of pet frogs. Utile now, no evidence of spreading chytridiomycosis to meat frogs and field frogs has been revealed.