172
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
Farley’s intermediate infection stage either follows the initial infection
stage or occurs as ‘‘ the result of relapse of chronic infection.” Intermediate infections that follow initial infections occur most frequently
during July through September while relapses occur most commonly
during June. This stage is characterized by localized lesions, with
accompanying host response, in the gills, gonad, digestive diverticula,
and Leydig tissues adjacent to the gut and mantle epithelia. Furthermore, recessive shell growth and shell-pustule formation may occur.
According to Farley, the intermediate infection phase resuIting from
relapse is characterized by the occurrence of lesions in the diverticula
and connective tissue while that resulting from a continuation of the
initial infection phase is characterized by the appearance of lesions in
the gill, gonad, and gut epithelium.
The third phase, termed the advanced infection stage, is reportedly
most prevalent from September through December. It is characterized
by massive infection and host response throughout the oyster, resorption of gametes, shrinkage of gonadal tubules, and a marked tendency
toward recessive shell growth. This is followed by the terminal infection
stage which occurs in the fall and winter. Oysters at this stage include
massive invasion of tissues ” by the parasite but, surprisingly,
Farley has stated that “ host response is usually light or absent ” but
pycnosis of host and parasite cells often occur. Furthermore, recessive
shell growth is usually present. Death of oysters generally follows the
terminal infection stage.
I n some instances, Farley is of the opinion that recovery, the fifth
stage, may occur. Oysters a t this stage are recognized by the deposition
of yellowish brown conchiolin-covered shell pustules which is preceded
by the formation of localized fibrotic lesions in the tissues situated near
or comprising the mantle epithelium, Moribund M . nelsoni plasmodia
are said to occur in both the lesions and shell pustules but rarely
elsewhere within the host’s body. Recovery, if it occurs, takes place
most commonly in the summer and fall.
Farley’s progressive stages of infection may represent a natural
sequence of events in parasitized oysters; however, since thesc stages
have been described from naturally infected oysters rather than at
time intervals during experimental infections, it may be questioned
whether these actually form a sequence or merely reflect different
intensities of infection. This doubt becomes even more serious, especially
that pertaining to recovery, when one considers the report by Haskin
et al. (1966) that the onset of oyster mortalities occurs within three
weeks after the initial infection and the ‘‘ incidence of gapers (recently
dead oysters) during the epizootics is commonly 100 per cent.”
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MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
Farley’s intermediate infection stage either follows the initial infection
stage or occurs as ‘‘ the result of relapse of chronic infection.” Intermediate infections that follow initial infections occur most frequently
during July through September while relapses occur most commonly
during June. This stage is characterized by localized lesions, with
accompanying host response, in the gills, gonad, digestive diverticula,
and Leydig tissues adjacent to the gut and mantle epithelia. Furthermore, recessive shell growth and shell-pustule formation may occur.
According to Farley, the intermediate infection phase resuIting from
relapse is characterized by the occurrence of lesions in the diverticula
and connective tissue while that resulting from a continuation of the
initial infection phase is characterized by the appearance of lesions in
the gill, gonad, and gut epithelium.
The third phase, termed the advanced infection stage, is reportedly
most prevalent from September through December. It is characterized
by massive infection and host response throughout the oyster, resorption of gametes, shrinkage of gonadal tubules, and a marked tendency
toward recessive shell growth. This is followed by the terminal infection
stage which occurs in the fall and winter. Oysters at this stage include
massive invasion of tissues ” by the parasite but, surprisingly,
Farley has stated that “ host response is usually light or absent ” but
pycnosis of host and parasite cells often occur. Furthermore, recessive
shell growth is usually present. Death of oysters generally follows the
terminal infection stage.
I n some instances, Farley is of the opinion that recovery, the fifth
stage, may occur. Oysters a t this stage are recognized by the deposition
of yellowish brown conchiolin-covered shell pustules which is preceded
by the formation of localized fibrotic lesions in the tissues situated near
or comprising the mantle epithelium, Moribund M . nelsoni plasmodia
are said to occur in both the lesions and shell pustules but rarely
elsewhere within the host’s body. Recovery, if it occurs, takes place
most commonly in the summer and fall.
Farley’s progressive stages of infection may represent a natural
sequence of events in parasitized oysters; however, since thesc stages
have been described from naturally infected oysters rather than at
time intervals during experimental infections, it may be questioned
whether these actually form a sequence or merely reflect different
intensities of infection. This doubt becomes even more serious, especially
that pertaining to recovery, when one considers the report by Haskin
et al. (1966) that the onset of oyster mortalities occurs within three
weeks after the initial infection and the ‘‘ incidence of gapers (recently
dead oysters) during the epizootics is commonly 100 per cent.”
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