March 2017

N-P13 (NP)


Signalment (JPC #1579146):  Bottlenose dolphin.


HISTORY:  This dolphin stranded on the southern California coast.  The animal was transported to a local aquarium where it had trouble maintaining equilibrium, was dyspneic, and died.


HISTOPATHOLOGIC DESCRIPTION:  Cerebrum:  Multifocally, there is disruption of the gray and white matter architecture with extensive areas of necrosis and cavitation (linear migration tracts), hemorrhage, edema, numerous gitter cells, degenerate neutrophils and eosinophils, lymphocytes, plasma cells, moderate amounts of black birefringent material (fluke pigment), a tangential section of a trematode, and triangular parasite eggs, occasionally surrounded by multinucleated giant cells.  The trematode is 500 um wide with a 15 um thick spiny tegument and contains numerous vitellaria, paired ceca, and testes, all surrounded by parenchyma.  Eggs are 50 um diameter, have a 5 um thick, golden brown, refractile shell, contain granular basophilic material, and are occasionally surrounded and engulfed by multinucleated giant cells.  Within the neural tissue adjacent to the cavitated areas, there is spongiosis and a mild gliosis.  Multifocally the meninges and perivascular spaces are expanded by edema, fibrin, moderate numbers of lymphocytes, plasma cells, eosinophils, fewer neutrophils and hemosiderin-laden macrophages. 


MORPHOLOGIC DIAGNOSIS:  Cerebrum:  Encephalitis, necrotizing and granulomatous, multifocal, with cavitation, hemorrhage, lymphoplasmacytic meningitis, and adult trematodes and eggs, etiology consistent with Nasitrema sp., bottlenose dolphin (Tursiops truncatus), cetacean.


ETIOLOGIC DIAGNOSIS:  Cerebral nasitremiasis


CAUSE:  Nasitrema sp.


ETIOLOGY SYNONYMS:  Air sinus fluke, brain fluke



·         Relatively common trematode found in numerous odontocete cetaceans (whales, porpoises, and dolphins)

·         Reportedly high prevalence of Nasitrema sp. on the southern California coast and is thought to be an important cause of marine mammal strandings

·         Nasitrema sp. causes lesions ranging from mild sinusitis to severe meningoencephalitis; has been associated with abscesses affecting nervous tissue   

·         Adult flukes are commonly found in head sinus mucosa and simple mucosal glands of the pilot whale and porpoises; also present in the internal ear complexes

·         Characteristic golden brown triangular eggs can be found in the brownish exudate at the blow hole of infected animals



·         Normally resides in air sinuses of small cetaceans

·         Aberrant migration to the brain is thought to be via the eustachian tube and the vestibulocochlear nerve (CN VIII)

·         Anatomic characteristic of the ear of odontocetes:  Complete separation of the typano-periotic bone from the skull base:

·         Helps animal avoid the influence of strong ultrasonic sound used for echolocation

·         May facilitate fluke migration by allowing access to CN VIII, which has no bony protective covering

·         Nasitrema sp. may affect equilibrium dysfunction; possible association with odontocete strandings



·         Unknown



·         Disorientation, disequilibrium, echolocative dysfunction, and stranding

·         Anorexia

·         Brown exudate at the blowhole



·         Brain lesions vary from small areas of discoloration to large irregular areas of malacia in the white or gray matter

·         Parasites can be found in the air sinuses, eustachian tubes, and inner ear complexes associated with inflammation



·         Adults seldom found in the brain, but are necessary to confirm genus identity

·         Spiny tegument, absence of a coelom, paired ceca, vitellaria, male and female reproductive organs

·         Eggs often nonviable or ruptured, 50-80 um diameter, and triangular with a golden brown shell

·         Lesions vary from mild sinusitis to severe meningoencephalitis

·         Necrosis, edema, mixed inflammatory infiltrate

·         Gliosis and hemosiderin laden macrophages associated with fluke eggs



Encephalitis due to aberrant migration in cetaceans:

·         Hunterotrema sp. – large fluke that causes mucoid bronchitis and bronchiectasis in Amazon River dolphins; eggs identical to Nasitrema sp.

·         Cyclorchis campula – fluke that causes fibrosing hepatitis and pancreatitis; eggs identical to Nasitrema sp.

·         Crassicauda sp. – nematode located in pterygoid air sinus; causes sinusitis



1.     Degollada E, Andre M, Arbelo M, Fernandez A.  Incidence, pathology and involvement of Nasitrema species in odontocete strandings in the Canary Islands. Vet Rec. 2002;150:81-82.

2.     Howard E, Britt J, Matsumoto GK. Parasitic diseases. In: Howard EB, ed. Pathology of Marine Mammal Diseases. Boca Raton, FL: CRC Press; 1983;126,181-188,225.

3.     Morimitsu T, Kawano H, Torihara K, Kato E, Koono M. Histopathology of eighth cranial nerve of mass stranded dolphins at Goto Islands, Japan. J Wildl Dis 1992;28:656-658.

4.     Morimitsu T, Nagai T, Ide M, Kawano H, Naichuu A, Koono M, Ishii A. Mass stranding of odontoceti caused by parasitogenic eighth cranial neuropathy. J Wildl Dis 1987;23:596-590.

5.     O"Shea T, Homer B, Greiner E, Layton W: Nasitrema sp. associated encephalitis in a Striped dolphin (Stenella coeruleoalba) stranded in the Gulf of Mexico. J Wildl Dis 1991;27:706-709.


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