Regarding the domoic acid toxicosis event in the Pacific Ocean off [the
coast of] California this past spring, to date we have been unable to
verify classic histopathologic lesions of domoic acid toxicosis in dolphin
brains.
In most mammals, the hippocampus is fairly large and well-developed. The
classic lesion of domoic acid (and other excitotoxins) toxicosis is
necrosis of the hippocampal neurons. We have seen this lesion many times
before in affected sea lions, as well as in experimental studies on rodents.
In dolphins and other cetaceans, the hippocampus is extremely small (less
than 1/4 cm in diameter in dolphins) and poorly developed. The distinctive
histologic layered arrangement is indistinct, so it is very hard to locate
and identify on histologic sections. It lies at the base of the brain, near
the ventral surface of the temporal lobes, just lateral to the optic
chiasm, in an area that is easily damaged if the brain is autolysed or is
not removed with the utmost care. Most of the dolphin brain tissues
submitted to us during this outbreak were either small pieces of brain
tissue, or whole brains cut with multiple parallel longitudinal cuts
("bread loafed"), making the location of this small structure impossible.
In our search for the hippocampus, we have enlisted the help of Department
of Neuropathology and the curator of the Yakovlev-Haleem Comparative Brain
Anatomy collection, with its fine-quality archived dolphin brain
specimens. After several months of effort and many, many slides, last week
we finally got good sections of hippocampus from 2 brains, one domoic acid
suspect (no neuronal necrosis found), and one control animal from the
Atlantic ocean.
Although reports from California indicate this outbreak seems to be over,
it will be important in any future events to submit any dolphin brains
intact, fixed whole by immersion in formalin, with no "breadloafing." Any
sectioning of the brain before fixation severely hampers our ability to
locate the hippocampus.
Domoic acid may have caused the deaths of these animals. Multiple dolphins
had high levels of domoic acid in the urine, stomach contents, or amniotic
fluid, and some have been observed seizuring. However, we have found some
nonspecific inflammation in a few of these brains, which may have caused
the seizures, and the mere presence of a toxin does not necessarily mean it
is causing disease. Without the classic demonstrable brain lesion, we have
so far been unable to support the presumptive diagnosis.
----
Brett Saladino, DVM, Diplomate ACVP
Department of Veterinary Pathology
Armed Forces Institute of Pathology
Washington, DC
<saladino@afip.osd.mil>