Brain Lesions and Clinical Signs in Two Pygmy Killer Whales (Feresa attenuata) Associated with Nasitrema Sp.
IAAAM 1999
Howard L. Rhinehart1; Charles A. Manire1; Fred W. Klutzow1; Gregory D. Bossart2
1Dolphin and Whale Hospital, Mote Marine Laboratory, Sarasota, FL, USA; 2University of Miami, School of Medicine, Division of Comparative Pathology, Miami, FL, USA


Nasitrema spp. are common trematode parasites which normally inhabit the pterygoid sinuses and tympanic cavities of many species of small cetaceans. However, ova and occasionally adults of these parasites have also been found damaging the 8th cranial nerve and in the brain and meninges of several cetacean species (Table 1). It is speculated that in these cases, Nasitrema may play a significant role in the morbidity/mortality of affected dolphins due to central nervous system dysfunction and disruption of equilibrium or acoustic abilities.

Staff at Mote Marine Laboratory's Dolphin and Whale Hospital have treated three live stranded specimens of the pygmy killer whale, Feresa attenuata, in the past 5 years. Histological examination of brain tissue from two of the three demonstrated numerous trematode ova in the brain and meninges of MML 9313, and the meninges of MML 9805, both consistent with Nasitrema sp.

Table 1. Species with reported cranial nerve damage/brain lesions associated with Nasitrema spp.


Common Name


Delphinus delphis

Common dolphin

Cowan et al., 1986

Feresa attenuata

Pygmy killer whale

Schwab, 1985

Globicephala macrorhynchus

Short finned pilot whale

Morimitsu et al., 1987

Grampus griseus

Risso's dolphin

Morimitsu et al., 1992

Lagenorhynchus obliquidens

Pacific white-sided dolphin

Lewis and Berry, 1988

Lissodelphis borealis

Northern right whale dolphin

Cowan et al., 1986

Peponocephala electra

Melon-headed whale

Morimitsu et al., 1986

Phocoenoides dalli

Dall's porpoise

Cowan et al., 1986

Pseudorca crassidens

False killer whale

Morimitsu et al., 1987

Stenella attenuata

Pan tropical spotted dolphin

Forrester, 1991

Stenella coeruleoalba

Striped dolphin

O'Shea et al., 1991

Steno bredanensis

Rough toothed dolphin

Forrester, 1991

Description of Brain Pathology

 MML 9313 ("Lunar")--A 232 cm. male which stranded on 16 Nov. 1993 at Marco Island, FL. Nasitrema-like ova were associated with focal lesions of the frontal, parietal, and occipital lobes consisting of cortical depletion, cavitation, and liquefaction necrosis of affected regions. Lesions also involved the subarachnoid space. Parasite ova but not adults were found.

 MML 9805 ("Pasco")--A male, 214 cm. in length which stranded at New Port Richey, FL on 22 Aug 1998. The cerebrum contained subdural aggregates of trematode ova consistent with Nasitrema. The extensive lesions seen in the previous case were not seen. No adult parasites were seen.


If present in sputum, gastric, or fecal samples, Nasitrema ova are readily discernible microscopically. The operculated, golden brown ova measure ~45 microns x ~80 microns and are triangular in cross section. In these two cases very few Nasitrema ova were seen in multiple blowhole, gastric, and fecal samples. Therefore, relative abundance may not correlate well with severity of infection. In additional cases involving Tursiops truncatus, blowhole exudates often have a distinctive frothy, mucoid appearance, red-brown in color. This was not observed in the cases reported here.

The premortem diagnosis of brain or auditory involvement associated with these parasites is much more difficult. Magnetic resonance imaging (MRI) may prove fruitful if the machine can accommodate the animal and motion can be controlled. MRI was attempted with MML 9805. Head motion, however, caused results of inadequate quality for a diagnosis.

Eosinophilia, which may indicate parasitic infestation, is often offset by stress-induced eosinopenia in stranded cetaceans. In the two cases mentioned here absolute eosinophil counts were not elevated, ranging from 70-1400.

Therefore, the clinician may need to rely on clinical signs for a tentative diagnosis.

Clinical Signs

 Head/Tail Arching (opisthotonus). While being supported in the water, the whale would strongly dorsiflex into a "U" shape, holding its head and tail upwards for a few seconds, then relaxing to a normal posture. This behavior may be repeated every few minutes. In cetaceans that are out of water, "arching" is generally considered a sign of severe stress, which may rapidly lead to death. This was not the case in these two patients, both of whom exhibited this behavior repeatedly.

 Head Tilt. Pasco often swam at the pool's surface with his neck flexed upward at a slight angle.

 Disorientation/Loss of Equilibrium. Pasco, when released to evaluate his swimming ability, would often roll from one side to the other. There was no consistency to which side he would fall. There was no evidence of pulmonary pathology to explain this behavior which apparently was due to a loss of equilibrium. It has also been reported that a stranded white-sided dolphin swam in tight circles. This animal too was found to have Nasitrema sp ova in the brain upon postmortem examination3.

 Slowed Respiratory Reflexes. The normal reflex to close the blowhole immediately at the detection of water is of paramount importance to the survival of cetaceans, but was greatly slowed in MML 9805.

Our limited experience with this species indicates that they are voracious eaters. Even so, it is to be expected that the drive to reach the surface to breathe will predominate over the urge to eat. However, on more than one occasion, Pasco continued to make efforts to weakly swim toward food even though he was initiating respiration while still underwater and had to be immediately lifted to the surface.


If parasites have reached the brain, antiparasitic therapy may be a moot point since any CNS dysfunction is likely to be permanent. However, Nasitrema has been treated with praziquantel or bithionol. Because of the suspected CNS pathology, we chose to euthanize MML 9805.


CNS disorders may have many etiologies. However, parasitic migration of Nasitrema sp. should be high on the differential diagnosis list. The CNS symptoms observed may depend on the area of the brain affected. Therefore, the clinical signs described here may not represent a complete listing.

Neural invasion by the trematode Nasitrema is an important morbidity, mortality factor in stranded cetaceans. Gross lesions may or may not be present.

MRI may offer premortem diagnosis if the patient is a suitable candidate and of a size compatible with the machine. Body motion must be controlled, through restraint or sedation.

Nasitrema sp. is commonly found in Tursiops truncatus, although as yet ova have not been reported from brain/nerve tissue. The reason for this is unknown, and merits continued investigation. The life cycle of this parasite remains unknown.


We wish to thank Dr. Robin Overstreet, Gulf Coast Research Laboratory, for help with parasite identity.


1.  Cowan DF, WA Walker, RL Brownell Jr. 1986. Pathology of small cetaceans stranded along southern California beaches. In: Research on Dolphins, MM Bryden and R Harrison (eds.), Oxford Science Publications, Pp.323-368.

2.  Forrester DJ. 1991. Parasites and diseases of wild mammals in Florida. University of Florida Press.

3.  Lewis RJ, K Berry. 1988. Brain lesions in a Pacific white-sided dolphin (Lagenorhynchus obliquidens). Journal of Wildlife Diseases 24:577-581.

4.  Morimitsu T, T Nagai, M Ide, A Ishi, M Koono. 1986. Parasitogenic octavus neuropathy as a cause of mass stranding of Odontoceti. The Journal of Parasitology 72:469-472.

5.  Morimitsu T, T Nagai, M Ide, H Kawano, A Naichuu, M Koono, A Ishii. 1987. Mass stranding of odontoceti caused by parasitogenic eighth cranial neuropathy. Journal of Wildlife Diseases 23:586-590.

6.  Morimitsu T, H Kawano, K Torihara, E Kato, M Koono. 1992. Histopathology of eighth cranial nerve of mass stranded dolphins at Goto Islands, Japan. Journal of Wildlife Diseases 28:656-658.

7.  O'Shea TJ, BL Homer, EC Greiner, AW Layton. 1991. Nasitrema sp.-associated encephalitis in a striped dolphin (Stenella attenuata) stranded in the Gulf of Mexico. Journal of Wildlife Diseases 27:706-709.

8.  Schwab GL. 1985. Live strandings of Feresa attenuata along the Texas Coast. Presentation at IAAAM Conference.

Speaker Information
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Howard L. Rhinehart, CVT
Dolphin and Whale Hospital, Mote Marine Laboratory
Sarasota, FL, USA

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