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Salmon poisoning

Posted: December 2, 2011 - 9:07am

The first time I heard the term "salmon poisoning," I thought it meant salmon were being poisoned. It turned out to be the infections fishermen get when they have an open wound and handle fish.

The next time I ran across salmon poisoning was when a dog died after eating raw salmon. Someone told me worms had worked their way into the dog's brain and killed it.

Years earlier, I'd had a dog that loved salmon. Whenever I filleted fish, "Bill" was right there. A piece of salmon flipped toward him never reached the ground. So when I heard that raw salmon could kill a dog, I wondered why it hadn't killed mine. I was told that dogs didn't get salmon poisoning in Alaska.

I had forgotten about salmon poisoning until the other day, when I came across it on the Internet. Salmon poisoning disease (SPD) is interesting. First off, the "poison" in the name is misleading. The culprits ultimately responsible for the disease are bacteria (Neorickettsia helminthoeca), not a toxin.

How these bacteria get into dogs is both complicated and fascinating. It starts with a snail (Oxytrema silicula) whose habitat is brackish fresh water along the coastal Northwest, from Northern California to southern British Columbia. Some waters have it, some don't. Acting as an intermediate host, the snail carries a fluke, or parasitic worm (Nanophyetus salmincola). This parasite is the carrier, or vector, of the bacteria that cause salmon poisoning disease.

The infected flukes leave the snails and live in the water for a while in their larval stage. When they come into contact with fish -- most commonly salmon and trout -- they burrow through the skin and wind up in the kidneys, muscles and fins.

When dogs eat raw trout, salmon and other fish that contain these infected parasites, the flukes invade the dog's small intestine and lymph nodes. In the process, they release the N. helminthoeca bacteria into the dog's intestinal tissues and bloodstream. The bacteria multiply. Within a week or two, the dog is very sick. If untreated, SPD in domestic dogs is nearly always fatal.

The life cycle of the snail, the fluke and the bacteria is maintained when the eggs of infected flukes are passed in the dog's feces. The eggs infect more snails. Infected fluke larvae leave the snail and burrow into more fish. Dogs eat the fish. Mink, skunks and raccoons, though unaffected, help spread the fluke by eating fish and defecating the fluke eggs in or near the water.

I called a few veterinarian clinics, asking if they'd had any dogs with SPD. Of the four clinics I called in Washington, all had treated dogs for SPD. In Ketchikan, I got an "It's here." A call to Juneau got an "I don't think so." In two calls to clinics in the Kenai-Soldotna area, I was told "no cases that we know of." Maybe Southcentral Alaska is "SPD-free." Maybe. "Feeder" king salmon that originated in Washington and Oregon are commonly caught in Southcentral waters.

The best cure for SPD is prevention. Don't let a dog eat raw fish. The hazard in the SPD "hot zone" is greatest in fall and winter, when salmon are spawning and dying. Infected fish have been found in the ocean from San Francisco to Southeast Alaska. Dogs have become infected by getting into garbage.

Symptoms of SPD include fever, lethargy, loss of appetite, vomiting and diarrhea. Treatment can best be administered in a pet hospital. Untreated dogs can die in 4 to 10 days.

Humans who eat raw fish infected with the parasite (N. salmincola) can be infected, but suffer only mild gastrointestinal disturbance.  Thorough freezing or cooking fish eliminates the risk of SPD to both humans and dogs.

On the subject of thoroughly freezing or cooking, doing so eliminates the possibility that you'll play host to roundworms (nematodes) or tapeworms. Both parasites are commonly found in fish all along the Pacific Coast, including Kenai Peninsula waters.

As for cooking, the Seafood Network Information Center advises: "Cooking fish to an internal temperature of 140*F will kill all fish nematodes and tapeworms. Normal cooking procedures generally exceed this temperature."

About freezing, the information center advises: "Parasites become a concern when consumers eat raw or lightly preserved fish such as sashimi, sushi, ceviche, and gravlax. When preparing these products, use commercially frozen fish. Alternatively, freeze the fish to an internal temperature of -4*F for at least 7 days to kill any parasites that may be present. Home freezers may not be cold enough to kill the parasites."

I have to go now. I have an irresistible urge to check the temperature in my freezer.

Les Palmer can be reached at les.palmer@rocketmail.com.

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