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In the spring of 2021, the Marine Mammal Alliance Nantucket hotline received a telephone call from a concerned citizen about a dolphin floundering in the surf near Nobadeer Beach. When the rescue team arrived they found a Short Beaked Common Atlantic Dolphin, disorientated and struggling in the wash. The dolphin was alone and seemed to be in good body condition with no external injuries or signs of what was causing his distress. A decision was made to relocate him to calmer waters. From there he was taken to Jetties beach and released, only to make a feeble attempt to swim away, then quickly reversing course back toward shore where he died.
Our stranding permit legally allows us to deal with marine mammals in distress and to investigate the possible reasons for a dolphin’s stranding. The best way to proceed in this sad case was to perform a necropsy (an animal autopsy) looking for the cause of this animal’s demise.
During a necropsy, the dolphin’s body was examined both externally and internally for diagnostic clues. Our external exam showed a dolphin in good physical condition with no reasons as to why he stranded. Internal organs were then examined with the naked eye and tissue samples were collected for a pathologist to examine under a microscope.
The internal exam was unremarkable until an examination of the brain was begun. External examination of the brain revealed an indented lesion in the left cerebral hemisphere. On cut section of the brain there was an obvious cavitated area underneath this lesion and in other areas of the brain that should not have been there. Microscopically, the pathologist examined these lesions and found associated with them the eggs of an air sinus parasite called Nasitrema.
Normally the Nasitrema parasite might cause a mild inflammation of the nasal air sinuses where it normally resides, but when it gets into an abnormal location like the brain, serious damage can occur.
So how did this parasite egg find its way into the dolphin’s brain? In order for the egg to be found there, the adult trematode (fluke) that lays the egg had to migrate from its normal air sinus location into the brain. In medicine this is called aberrant (abnormal) migration and the damage caused was the reason for the stranding and the dolphin’s death. By misbehaving, this parasite killed its host.
Nasitrema eggs deposited in the air sinuses prior to the aberrant migration were also aspirated into the lungs causing a mild pneumonia, but the neurological damage was the main cause of death.
P.O. Box 3625
Nantucket, MA 02584
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