Salmon poisoning is the common name for an important disease of domestic and wild dogs that is caused by the tiny bacterial microorganism, Neorickettsia helminthoeca. It requires the involvement of a number of other animals, including flukes, snails, fish and mammals. Salmon poisoning occurs almost exclusively in the northwestern United States and along the Pacific coast of Canada. Dogs, foxes and wolves get this disease when they eat fresh raw fish – usually salmon, trout or steelhead – that are contaminated with the infective bacterial parasite and its vector, which is a fluke called Nanophyteus salmincola. This disease can be fatal in a very short period of time, unless the infected animal is treated aggressively.
Causes of Salmon Poisoning Disease
Neorickettsia helminthoeca are the bacteria that ultimately are responsible for causing salmon poisoning disease in dogs. How dogs actually become infected with these parasites is complicated, but the process is fascinating. Tiny N. helminthoeca bacteria live naturally inside of larger organisms known as Nanophyteus salmincola. These are parasitic worms, or trematodes, more commonly called "flukes." Flukes are thick, fleshy, flat, leaf-like creatures that take in their nourishment through one or more suckers that they attach to the inside of their host animals. Nanophyteus salmincola flukes are the carriers, or vectors, of the bacterial parasites that cause salmon poisoning disease in our domestic dogs.
How dogs get salmon poisoning is even a bit more complicated. The flukes that carry the infective bacteria have their own intermediate host, which is a snail called Oxytrema silicula. "Intermediate hosts" are animals that other organisms, usually parasites, live inside of and go through some transitional stage of development while they are living inside of their host. For example, the parasite may hatch from an egg into larvae in its intermediate host. The intermediate host may or may not also act as the vector that transmits the parasite to its ultimate victim. In their capacity as intermediate hosts for Nanophyteus salmincola flukes, Oxytrema silicula snails simply carry the flukes passively while they mature. The actual bacterial infection is spread directly from infected flukes to fish to dogs. However, the snail intermediates are an essential part of the life cycle of the flukes that transmit the parasites causing salmon poisoning to domestic dogs.
The natural habitat for these particular snails is the northern Pacific rim of the United States and coastal western Canada. That is why salmon poisoning is almost always only diagnosed in dogs in British Columbia, Oregon, Washington and Northern California, and occasionally in other areas where fish infected by flukes that carry the bacteria migrate or are transported. The infected flukes leave their snail intermediate hosts and live for a short time in water in their larval stage, where they eventually come into contact with salmon or other aquatic creatures. They get inside of those creatures by burrowing through their skin, tending to lodge in their kidneys.
Dogs become infected when they eat raw freshwater or ocean salmon, trout, steelhead, giant Pacific salamanders or other fish that contain the larval stage of infected Nanophyteus salmincola flukes. Once the flukes get into a dog's gastrointestinal tract, they mature and invade its small intestine and lymph nodes. There, they latch on and feed, causing a significant inflammatory reaction. The fluke infection itself usually doesn't cause any measurable disease. However, during this process, the flukes release the infective Neorickettsia helminthoeca bacteria into the dog's intestinal tissues and bloodstream, where they start to replicate. It only takes about one to two weeks for the dog to become sick. The disease usually becomes systemic, or body-wide, in short order. If not treated, salmon poisoning disease is almost always fatal in domestic dogs.
The life cycle of the organisms that cause salmon poisoning disease is maintained when the eggs of infected flukes are passed in the feces of infected dogs. The intermediate snail hosts become infected from those hatching eggs free in the environment. The infected fluke larvae eventually leave the snail and penetrate the skin of the salmon, trout or salamander. Dogs eat these fish. Then, the cycle begins again.
Prevention of Salmon Poisoning Disease
Dogs living in or visiting the Pacific northwestern United States or western coastal Canada should not be permitted to eat raw fish - especially live, dead or dying salmon, steelhead, trout or similar freshwater fish from rivers, streams or lakes. If they do, their owners should take them to the closest veterinary clinic for treatment as quickly as possible. Salmon poisoning can be fatal in a very short time. Fish that are cooked thoroughly or frozen for more than 24 hours are usually not infective, because the flukes and infective bacteria can be killed by temperature extremes.
Salmon poisoning disease is an important illness in dogs living in or traveling to the Pacific coast of Canada or to the northwestern United States. Infected fish are found from the San Francisco area to the southern coast of Alaska, in the Pacific Ocean. They are most common from northern California to Washington's Puget Sound, and in rivers coming from those areas that support fish migration and spawning. Infected dogs need to be treated quickly and aggressively to prevent them from dying from this disease.
How Salmon Poisoning Disease Affects Dogs
How salmon poisoning affects dogs can vary quite a bit. Most dogs initially get feverish to some degree, but then their fever tends to go away. Their temperature may even drop to below normal. When dogs are severely affected, they usually develop gastrointestinal symptoms, including abdominal pain, vomiting and diarrhea, within one or two weeks after eating raw infected fish. The signs can be virtually indistinguishable from the signs of distemper or parvoviral infection, especially in young animals.
Symptoms of Salmon Poisoning Disease
Dogs with salmon poisoning disease typically become obviously sick within about 5 to 7 days after they eat infected raw fish. This disease can be fatal, especially if it is not caught and treated early enough in its course. Owners of dogs with salmon poisoning may notice one or more of the following symptoms:
Diarrhea (often profuse; may be dark with digested blood or bright red with fresh blood)
Vomiting (usually persistent)
Enlarged lymph nodes (lymphadenopathy; lymphadenomegaly; usually pronounced)
Nasal discharge (abnormal discharge from the nostrils)
Ocular discharge (abnormal discharge from the eyes)
Swollen eyelids (edema)
Fever (often high and severe)
Loss of appetite (inappetence; anorexia; often absolute)
Weight loss (often profound)
Elevated heart rate (tachycardia)
Poor pulse quality (usually only detected by a veterinarian during a physical examination)
Anemia (abnormally low levels of circulating red blood cells; usually only detected by a veterinarian during a physical examination)
Death (unfortunately common in untreated animals)
Dogs at Increased Risk
Salmon poisoning can affect dogs of any age, breed or mixed breed. There is no gender predisposition to this disease. Dogs that have access to raw fish in the Pacific northwestern United States and western coastal Canada, especially if they are exposed to brackish streams, lakes or beaches, have an increased risk of contracting this disease. Hunting dogs and dogs that are allowed to roam freely in the Pacific Northwest have a heightened chance of coming into contact with infected fish. Most cases occur during late summer through the first part of winter, when dogs have the greatest access to dead and dying fish as they spawn up through northwestern rivers and creeks.
How Salmon Poisoning Disease is Diagnosed
Most cases of salmon poisoning disease are presumptively diagnosed based on a history of the dog's recent consumption of raw fish and/or exposure to lakes, rivers or streams in an area that is known to be a natural habitat for Neorickettsia helminthoeca bacteria, Nanophyetus salmincola flukes and Oxytrema silicula snails. In a nutshell, this is the Pacific northwestern part of North America, up to but not including Alaska. Many veterinarians will treat a dog that meets this profile on the presumption that it has eaten infected fish. The dog's positive response to treatment is often the most common way that a diagnosis of salmon poisoning is made.
Routine blood and urine tests performed on dogs with salmon poisoning disease are usually unremarkable, which means that they tend to be pretty normal. X-rays (radiographs) of the dog's chest or belly usually show nothing out of the ordinary, unless something else is going on besides salmon poisoning. Samples taken from enlarged lymph nodes by a procedure known as fine needle aspiration may reveal the infective bacteria. Microscopic examination of fresh fecal samples may also reveal eggs of the fluke vector, Nanophyetus salmincola.
The goals of treating salmon poisoning disease are to stabilize the dog and resolve the shock, symptoms and systemic illness that typically accompany this disease. It is also critical to eliminate the infective parasite from the dog's system.
Dogs that are acutely ill from salmon poisoning probably will be admitted to a veterinary hospital. They often need to be given intravenous fluids, electrolytes and medications to control their nausea, vomiting and diarrhea and to restore them to normal hydration levels. Some of the drugs used to treat infection with Neorickettsia helminthoeca are oxytetracycline, doxycycline, tetracycline, ampicillin, chloramphenicol and praziquantel. There are others. These medications have been used alone and sometimes together, depending upon how severe the infection is in a particular dog. Supportive care, including a warm, clean environment and good nutritional support, is also essential. Sometimes, blood transfusions may be necessary, especially if an infected dog becomes severely anemic.
Without aggressive treatment, dogs that eat fresh raw fish infected with Neorickettsia helminthoeca usually die within one or two weeks. With early diagnosis and treatment, the prognosis can be fair to quite good. Unfortunately, many cases go undiagnosed, especially in hunting dogs or dogs that are allowed to roam freely in contaminated areas.