Parasite Profile: Nanophyetus salmincola

I've learned since starting this blog that any topics I write about are ones that I'm more likely to remember later on a test. Since parasitology is proving to be largely memorization, I thought doing some blogging about it might help. It doesn't hurt that I find this stuff absolutely fascinating, so hopefully I'll be able to continue with these posts.

So, Nanophyetus salmincola, also known as salmon posioning of dogs (SPD).

This one has a pretty awesome life cycle, and is interesting in that it really only seems to affect domestic canines, and only ones with little or no previous exposure. Salmon poisoning is caused by a rickettsia called Neorickettsia helminthoeca that is carried by the Nanophyetus salmincola fluke. The fluke itself produces no real disease, but the rickettsia causes classic symptoms which include vomitting, diarrhea, enlarged lymph nodes, depression, anorexia, high fever, and death within two weeks if left untreated. Understandably, this is cause for a little concern.

Trials on wild coyotes showed that being infected with Nanophyetus salmincola produced virtually no symptoms, and certainly didn't negatively impact the animals, and these seems to be the case in all the other wild animals who host this parasite. These hosts can be racoons, bears, foxes, coyotes, dogs, etc. The life cycle of this parasite starts when a definitive host passes ova in feces near to fresh water. The ova develop in the water for 3 months and then hatch into miracidium that find and burrow into an Oxytrema snail which only lives in the rapids of fresh water streams. While in this intermediate host the fluke develops into a sporocyst and then to a redia and asexually forms cercaria which swim off and penetrate any salmonid fish that happens to be in that fresh water. Once they've found a salmonid they change to a metacercaria and become encysted in the fish. One of the definitive hosts then catches and eats said fish, and tada! Salmon Poisoning!

The neat part about this parasite is that it can remain viable in the salmonid for years, even making the transition from fresh to salt water and back again. The fluke is also quite difficult to diagnose definitivly based on fecal tests because the ova don't float (therefore making a fecal flotation useless and a direct fecal necessary), the large amounts of stool produced by a sick host can mask the presence of the ova, and the flukes don't produce huge numbers of ova in a host who is very sick (which defines most dogs with salmon poisoning by the time they make it to the vet clinic). So, diagnoses is usually by clinical signs and response to therapy (ie, put it on tetracycline...does it improve, or does it die?)



I'm also fascinated by the ease with which domestic dogs can be innoculated against this rickettsia. I'm sure a lot of people who have or have had dogs in the past are thinking "but my dog got into raw fish all the time and never got sick....". Our parasitology instructor recommends feeding dogs at high risk of infection small amounts of raw salmon, and then taking their temperature twice daily. When the temperature spikes, start them on tetracycline. This gives their immune system a chance to build up some antibodies and the next time they eat raw fish, they'll do just fine. In dogs where exposure is minimal, he recommends doing this once a year, but most dogs with any access to raw fish will quite happily re-innoculate themselves any chance they get.

This disease is also limited to a very small area. The Oxytrema snail that acts as an intermediate host only exists in the Pacific Northwest, ranging from Southern B.C. to Northern California. I was a bit surprised since I'd never heard this even being mentioned back home (and yet I know there isn't a lack of raw fish for dogs to get into...it's an island for pete's sake). This doesn't mean that dogs in other locations can't become infected though, it just means that veterinarians in other places don't tend to know how to recognize it. Dogs have been infected by stealing the fish that was thawing on the counter for dinner, or by being fed a treat of smoked salmon (smoking and drying doesn't always kill it - only cooking). Toy poodles have been infected by drinking water from the pan of a flower pot that the owner used organic raw fish meal on, and many many dogs have been infected by licking up the water left on the lawn after a fisher washes off his or her catch.

So, there you have it. Nanophyetus salmincola; don't feed your dogs raw fish.


photo credit here.


(and yes, I'm still going to the all-you-can-eat sushi buffet for my birthday. I'm not a domestic dog, and good sushi is totally worth potential parasites - so there!)