Hard to say just what these things are without a better description
than "balls" or without a picture. However, this is also reminiscent
of metacercariae in the brain. Metacercariae, the larval stage
following cercariae in digenes (Platyhelminthes: Digenea), are commonly
found in fishes, both marine and freshwater. They are also fairly well
known to encyst in fish brains. There, they do not cause any gross
pathology (metacercariae are a "resting stage"), may reach very high
intensities, and may cause some behavioural alterations in the host.
While dissecting the fish, some may be observed as "balls" while others
may be seen crawling about. There are 2 possible causes for this: 1)
mechanical disturbance breaks the metacercaria's cyst and 2) some
chemical cue (from tearing apart host tissues, or, more likely,
digestive chemicals) causes the metacercaria to excyst (it "thinks" it
is in the final host's stomach). What crawls out of the metacercarial
cyst is, in essence, an small adult platyhelminth (put it on a slide and
compare to pictures in any invertebrate zoology textbook).
A recent, and very good, paper on metacercariae in fish brains is:
Lafferty, K. D., and Morris, A. K. 1996. Altered behaviour of
parasitized killifish increases susceptibility to predation by bird
final hosts. Ecology 77: 1390-1397.
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