Polinski Mark P, Meyer Gary R, Lowe Geoffrey J, Abbott Cathryn L
Fisheries and Oceans Canada, Pacific Biological Station, 3190 Hammond Bay Road, Nanaimo, BC V9T 6N7, Canada.
Dis Aquat Organ. 2017 Oct 18;126(2):143-153. doi: 10.3354/dao03167.
Mikrocytos mackini is an intracellular parasite of oysters and causative agent of Denman Island disease in Pacific oysters Crassostrea gigas. Although M. mackini has been investigated for decades, its natural mode of transmission, mechanism for host entry, and environmental stability are largely unknown. We explored these biological characteristics of M. mackini using a recently described quantitative PCR (qPCR) assay. We detected M. mackini in the flow-through tank water of experimentally infected oysters and during disease remission in host tissues following 6 wk of elevated water temperature. Waterborne exposure of oysters to M. mackini further confirmed the potential for extracellular seawater transmission of this parasite and also identified host gill to have the highest early and continued prevalence for M. mackini DNA compared to stomach, mantle, labial palps, or adductor muscle samples. However, infections following waterborne challenge were slow to develop despite a substantial exposure (>106 M. mackini l-1 for 24 h), and further investigation demonstrated that M. mackini occurrence and infectivity severely declined following extracellular seawater incubation of more than 24 h. This study demonstrates a potential for using qPCR to monitor M. mackini in wild or farmed oyster populations during periods of disease remission or from environmental seawater samples. This work also suggests that gill tissues may provide a primary site for waterborne entry and possibly shedding of M. mackini in oysters. Further, although extracellular seawater transmission of M. mackini was possible, poor environmental stability and infection efficiency likely restricts the geographic transmission of M. mackini between oysters in natural environs and may help to explain localized areas of infection.
麦氏微胞藻是牡蛎的一种细胞内寄生虫,也是太平洋牡蛎(Crassostrea gigas)丹曼岛病的病原体。尽管对麦氏微胞藻的研究已有数十年,但对其天然传播方式、进入宿主的机制以及环境稳定性仍知之甚少。我们使用最近描述的定量聚合酶链反应(qPCR)检测方法,探究了麦氏微胞藻的这些生物学特性。我们在实验感染牡蛎的流水养殖池水中以及水温升高6周后宿主组织疾病缓解期间检测到了麦氏微胞藻。牡蛎通过水体接触麦氏微胞藻,进一步证实了这种寄生虫通过细胞外海水传播的可能性,同时也发现,与胃、外套膜、唇瓣或闭壳肌样本相比,宿主鳃中麦氏微胞藻DNA的早期和持续检出率最高。然而,尽管有大量接触(>106个麦氏微胞藻/升,持续24小时),但经水体感染后的发病过程较慢,进一步研究表明,在细胞外海水中孵育超过24小时后,麦氏微胞藻的出现率和感染力会大幅下降。这项研究表明,在疾病缓解期或从环境海水样本中,利用qPCR监测野生或养殖牡蛎种群中的麦氏微胞藻具有可能性。这项工作还表明,鳃组织可能是麦氏微胞藻通过水体进入牡蛎以及可能从中排出的主要部位。此外,尽管麦氏微胞藻通过细胞外海水传播是可能的,但较差的环境稳定性和感染效率可能限制了其在自然环境中牡蛎之间的地理传播,这或许有助于解释感染的局部区域。