Thequahog parasite unknown, orQPX, is a single-celledprotist parasite in the classLabyrinthulomycota that affectshard clams (Mercenaria mercenaria; quahogs),[1] both cultured and wild.[2]
Parasites similar to QPX were first observed inNew Brunswick, Canada, in 1959, when a mass death of hard clams was observed. Outbreaks have also occurred inNova Scotia,Prince Edward Island, New York, New Jersey, Massachusetts, and Virginia, always only in quahog clams.[1][3]
Symptoms of QPX include chipping of the shells, mantle swelling, stunted shell growth and the development of nodules.[3][2]
The life stages of thalli,endospores, andsporangia have been observed.[3] It somewhat resembles the unrelated protist haplosporidian parasiteHaplosporidium nelsoni (MPX), a pathogen of theeastern oyster.[2]
QPX was originally considered achytrid, but has now been provisionally assigned to the phylumLabyrinthulomycota in either the families Thraustochytriidae or Labyrinthulidae, based on the organism having a "uninucleate biflagellatezoospore stage, a loose multilaminar cell wall, and particularly an intracellular sagenogen-like structure".[4] However, much about it is still unknown.[1] QPX-like organisms also may not be all of the same species.[2]
Molecular phylogenetic on theSSU rRNA of QPX was able to produce the same phylum-level identification, and more precisely down to the family Thraustochytriidae next toThraustochytrium pachydermum. APCR primer targeting this region that detects QPX infection has been constructed.[5]
QPX can be cultured in laboratories, with the optimal temperature being 20 to 23 °C. It produces amucus that causes inflammation in the clam and shields itself from clam immune response.[6]
The partialgenome of QPX has been sequenced by theshotgun method in 2013, along with thetranscriptome to identify genes important in stress response (mainly temperature). A few interesting proteins and a wide range of genetic polymorphisms have been identified. It expresses some genes involved in the synthesis ofantibiotics that are also toxic to clams.[6] Further experimental characterisation in 2018 of thesesingle-nucleotide polymorphisms shows variations in genes related to virulence and environmental (salinity and temperature) adaptation.[7]