Parasites in healthy marine mammals

Harbour seals at Belle Chain Islet, Strait of Georgia. June 2012.

I am a recently graduated veterinarian and the first ‘official’ student of the Marine Ecosystem Health Network.  I will begin my PhD this fall, in a joint program between the National Institutes of Health (USA) and the University of British Columbia’s Marine Mammal Research Unit (Canada).  In the meantime I’m collaborating with a fellow marine mammal researcher to collect scats from populations of harbour seals in the Strait of Georgia, which is the body of water that separates the southern end of Vancouver Island from Vancouver and the mainland.

Collecting scat at Norris Rocks, near Hornby Island, May 2012.


The Steller Quest, the UBC boat we use to collect scat.

So, why would a veterinarian be working with a foraging ecologist to collect scat from wild populations of harbour seals?  To date, we know very little about the natural cycle of parasites in wild, healthy populations of marine mammals.  What little we do know has come from stranded, sick animals at rehabilitation facilities or from dead, stranded animals that are necropsied by a veterinary pathologist.  Thus, what we know is a very limited, biased perspective of parasites seen in unhealthy marine mammals.

Parasites are part of the natural ecosystem and often do not cause clinical disease in their host, so long as their host is immunocompetent and generally healthy.  It is completely natural for a wild animal (and even domestic animals) to have a parasite burden.  Animals (and humans!) evolved with parasites: the parasites that are natural to their host will not kill them.  What good would it do a dependent organism, such as a parasite, to kill its host?  Instead, through evolutionary forces, parasites have co-evolved with specific, natural host species.  The health danger arises when a parasite that co-evolved with a specific host (e.g., a cat) ends up in a different host (e.g., a sea otter).  This is another topic for another blog, but the short of it is that when a cat-specific parasite (such as Toxoplasma gondii) infects a sea otter, death is almost certain.

That said, I return to the fact that very little is known about the natural (normal) parasites found in healthy marine mammals.  This type of information is extremely important as “baseline health data”.  If the parasites that should be present in wild, healthy populations of marine mammals are known then it will be easier to identify those that should not be present.  For example, parasites of known terrestrial origin are now found in marine mammals.  Two prominent examples: Toxoplasma gondii (mentioned above), and Giardia duodenalis, canine genotype.  Both dogs and cats can transmit their parasites to marine mammals—the likelihood is that all terrestrial animals, especially those that live in dense numbers (think cattle, chickens, pigs) can transmit their parasites to marine mammals.  This is a reminder that what happens on land is ultimately connected to what happens in the ocean.

So, my study is focusing on characterizing and identifying the “normal”, baseline parasite burden in healthy populations of harbour seals.  To do this, I have teamed up with a marine mammal foraging ecologist who collects their scat to identify what they have been eating (through various techniques including bone identification and molecular analysis).  I take a small subsample of the feces to conduct a fecal float.  This is a technique commonly used by veterinarians to concentrate any parasite eggs that are present in the fecal sample.  The sample is then viewed under a microscope to identify parasite eggs and worms.

Field set-up: centrifuge and microscope to process harbour seal scats for parasite identification.

To date I’ve found numerous types of parasites in the scat samples I’ve collected.  Since so little is known about parasites in marine mammals I can only identify most of these to their specific Phylum; if I’m really lucky, I may get to the level of genus.  This fall I will implement molecular techniques to identify, to the species level, the parasites I have seen on the microscope.

Coccidian eggs (likely Eimeria) found in harbour seal scat.


Ascarid (Nematode) egg found in harbour seal scat.Interestingly, there does appear to be a unique geographic distribution of parasite type in harbour seals throughout the Strait of Georgia.  There is also a temporal relationship between season and the density of parasites observed on fecal float.  This is all very preliminary, as I’ve only collected samples in May and June.  I expect that as the season progresses and the seals’ diet changes that their parasite burden will also change and become more pronounced.

Eventually, I will be able to work with the foraging ecologist to link the specific parasites with what the individual seals are eating.  This will help fill in a lot of informational gaps and we might, one day, be able to understand the life cycle of natural parasites in harbour seals!  Why is this important?  Well, by knowing what parasites naturally occur in the marine environment, and their normal life cycle, it will become easier to identify the occurrence, and life cycle, of ‘invasive’ parasites that have very harmful health effects on marine mammals.

(very pregnant) Harbour seal, Belle Chain Islets, May 2012.