Abstract

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.

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.

Diagnosis

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.

Treatment

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.

Conclusions

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.

Acknowledgements

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

References

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