Gross Findings: On the right rostral cerebrum was a focal 3 x 2.5 cm area of hemorrhage and cavitation (Fig. 1A). There were two erosions on the palate (35 x 7.5 mm and 45 x 7.5 mm).  There were multiple healed shark bites on the ventral and dorsal aspects of the peduncle, the right lateral dorsum, and around the dorsal fin. The forestomach contained copious fluid and scant otoliths. There were no other significant findings.

Histopathological Findings: Focally, there is marked cavitation, expansion, and replacement of the cerebral meninges by abundant hemorrhage, fibrin, and neutrophils, with many intralesional ~30 µm diameter triangular to ovoid trematode eggs with a thick yellow shell (Fig. 1B, 1C). Within the underlying cortex, there is multifocal to coalescing hemorrhage (Fig. 1D), occasionally accompanied by neuronal necrosis and perivascular neutrophils. Throughout the remainder of the section, there is widespread hemorrhage within both gray and white matter and moderate white matter vacuolization with spheroid formation (axonal degeneration). Elsewhere, cerebral meninges are expanded by moderate numbers of mononuclear cells with areas of suspect fibrosis and edema. Other findings included marked portal fibrosis and mild to moderate pancreatic and renal interstitial fibrosis.

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Morphologic Diagnosis/es:

1. Brain: Meningeal hemorrhage and necrosis, focally extensive, acute, marked, with intralesional trematode eggs
2. Liver: Portal fibrosis, marked

Disease: Cerebral trematodiasis

Etiology: Typically caused by aberrant migration of Nasitrema sp. trematodes, including N. globicephalae. Other trematodes that may be associated with encephalitis in cetaceans include Campula spp. and Hunterotrema spp.  The nematode Crassicauda sp. has also been associated with encephalitis due to aberrant migration.

Distribution: Worldwide; in North America, reports of cerebral nasitremiasis have been most common in southern California

Host range: Odontocetes

Pathogenesis: Nasitrema spp. are common inhabitants of the pterygoid sinuses and tympanic cavities of odontocetes and are thought to infect the host through consumption of fish infected with larval trematodes. On aberrant migration, adult Nasitrema spp. can cause sinusitis, otitis media, neuritis of the eighth cranial nerve, and hemorrhage, necrosis, and cavitating migration tracts through the meninges and brain.

Clinical signs: Stranding, difficulty maintaining equilibrium, dyspnea

Pathology: Migration tracts in the brain are characterized by areas of cavitation, necrosis, hemorrhage, edema, and variable numbers of mixed inflammatory cells including neutrophils, mononuclear cells, and Gitter cells. Naisitrema spp. eggs are triangular and ~50 µm diameter. Adult trematodes and black iron-porphyrin “fluke pigment” are less often found. Adjacent areas of brain may exhibit inflammation, hemorrhage, spongiosis, and gliosis. Other species of trematodes with the potential to undergo aberrant migration through the brain, including Campula spp., may also be associated with fibrosis in the liver and pancreas, as was seen in this case.

Diagnosis: Typical gross and microscopic lesions in a susceptible host; PCR can be used to assist in identification of the trematodes.

Public health concerns: None

Wildlife population impacts: Unknown. Cerebral nasitremiasis was historically thought to be a common cause of marine mammal strandings in California, although reported prevalence has decreased since the 2000’s.

Management: None available

References:

• Joint Pathology Center. 2020. N-P13 (NP). https://www.askjpc.org/vspo/show_page.php?id=Qm41QjdORWxKYWtDSExmWW9tcmtJUT09. Accessed August 2022.
• Morimitsu T, Kawano H, Torihara K, Kato E, Koono M. 1992. Histopathology of eighth cranial nerve of mass stranded dolphins at Goto Islands, Japan. J Wildl Dis. 28(4):656-8. https://doi.org/10.7589/0090-3558-28.4.656.
• Morimitsu T, Nagai T, Ide M, Kawano H, Naichuu A, Koono M, Ishii A. 1987. Mass stranding of Odontoceti caused by parasitogenic eighth cranial neuropathy. J Wildl Dis. 23(4):586-90. https://doi.org/10.7589/0090-3558-23.4.586
• O'Shea TJ, Homer BL, Greiner EC, Layton AW. 1991. Nasitrema sp.-associated encephalitis in a striped dolphin (Stenella coeruleoalba) stranded in the Gulf of Mexico. J Wildl Dis. 27(4):706-9. https://doi.org/10.7589/0090-3558-27.4.706.
• Ridgway SH. Cerebral and cerebellar involvement of trematode parasites in dolphins and their possible role in stranding. 1972. J Wildl Dis. 8(1):33-43. https://doi.org/10.7589/0090-3558-8.1.33.
• St. Leger J, Raverty S, Mena A. Cetacea. In: Pathology of Wildlife and Zoo Animals. Terio KA, McAloose D, St. Leger J, eds. San Diego, CA: Elsevier; 2018: 562-563, 564.e6-7.