Since their first detection and description, intracellular protistan parasites of the genus Bonamia have been recognized as serious pathogens of oysters, severely impairing the health of oyster populations and the production of these economically important molluscs.
The appearance of B. ostreae in the 1970s in Europe caused mass mortality in its host Ostrea edulis and hastened the decline of this species (Grizel 1985, Montes 1990, Hudson & Hill 1991, McArdle et al. 1991, Van Banning 1991).
This was a dramatic impact that may have reflected the encounter between an introduced pathogen (Elston et al. 1986) and a host with no natural resistance to it. The effects of B. exitiosa infection of O. chilensis in New Zealand have been similarly severe, reducing the population to 9 per cent of the pre-disease level, although this host− parasite relationship may be long-standing (Doonan et al. 1994, Hine & Jones 1994).
The impact of B. exitiosa on European oyster populations since its discovery in Europe is less clear. The extensive nature of the oyster production process and limited options for disease control in cultured stocks in open water surrounded by wild oyster populations hampers effective management of disease outbreaks.
Both B. ostreae and B. exitiosa are pathogens listed by and notifiable to the World Organisation for Animal Health (OIE) and the European Union (EU). With the advances in research on Bonamia species over the years, knowledge on their global spread has been extended, as additional species have been discovered and assigned to the genus Bonamia (Carnegie et al. 2006).
Hosts are now known to include not only ostreid oysters but also Crassostrea species. Recently, mechanisms of interaction between Bonamia spp. and their hosts, including mechanisms involved in the resistance to the disease, have been investigated in more depth at cellular and molecular levels (Morga et al. 2009, 2011, 2012).
Furthermore, it seems that in some cases, after their introduction, the parasites are only present at a low prevalence and with presumably limited effects on the population (Culloty & Mulcahy 2007). In other populations exposed to the parasites over a long period of time, there are indications that a certain level of resistance is acquired (Culloty et al. 2004).
In this review, we focus on the advances in research on Bonamia over the last decade in phylogenetics, geographical distribution, susceptible host species, diagnostics, epizootiology, host−parasite interactions, and disease resistance and control.
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