November 24, 2016

Second Pathogen Discovered in Vector of Salmon Poisoning Disease

Oregon State University researchers have discovered a second pathogen in vector of salmon poisoning disease that allows for dogs to become infected a second time.
By Laurie Anne Walden, DVM, ELS
Salmon poisoning disease is a potentially fatal bacterial infection affecting dogs that eat raw or undercooked fish. The conventional wisdom is that dogs that have recovered from infection are immune to future infection, but evidence suggests that some dogs can become infected a second time, usually with milder signs. Researchers from Oregon State University have discovered a possible explanation: a different pathogen hosted by the same disease vector. The report was recently published in Veterinary Parasitology.
 
Salmon poisoning disease affects dogs in the Pacific Northwest, causing signs similar to those of canine parvovirus infection: fever, vomiting, diarrhea, depression, anorexia, and dehydration. The disease can be treated with antibiotics but may be fatal if untreated. Dogs become infected by eating undercooked or raw fish containing metacercariae of the fluke Nanophyetus salmincola. The fluke itself does not typically cause symptoms in dogs. The disease is actually caused by Neorickettsia helminthoeca, a bacterium that the fluke hosts.
 
The investigators sequenced DNA of fluke metacercariae recovered from the kidneys of Chinook salmon from the Willamette River in Oregon. They also screened blood samples from two Oregon dogs that experienced symptoms months after being treated for salmon poisoning disease.
 
The researchers identified the DNA of another neorickettsial species, Stellanchasmus falcatus (SF) agent, in metacercariae from three of the salmon. SF agent has previously been found to cause mild fever and diarrhea in dogs. However, this is the first report of SF agent being found in Nanophyetus flukes and the first report of it being found in a fish of the family Salmonidae, say the authors.
 
The authors suggest that SF agent might be the true cause of Elokomin fluke fever, another disease caused by a neorickettisial infection vectored by Nanophyetus flukes. Elokomin fluke fever was studied in black bears in 1973, but the exact species responsible for the infection was not clearly identified. It causes severe illness in bears but only mild symptoms in dogs. According to the authors, only two reports of a fluke species hosting more than one species of Neorickettsia have been published: the 1973 report of Elokomin fluke fever and the current study. Because SF agent and Elokomin fluke fever cause similar symptoms in dogs, the authors hypothesize that SF agent causes Elokomin fluke fever.
 
N helminthoeca induces an immune response, and dogs that have recovered from infection should be protected against future infection. However, the authors note anecdotal evidence from Oregon veterinarians that some dogs seem to develop repeat infections. A previously published report of salmon poisoning disease also mentioned possible recurrence of the disease in one dog.
 
The blood samples from the two dogs with apparently recurrent symptoms of salmon poisoning disease were positive for N helminthoeca. The authors suggest two possible explanations: the initial infection could have been a neorickettsial species other than N helminthoeca (possibly SF agent), or the dogs did not develop immunity after an initial N helminthoeca infection. They recommend further testing of dogs that develop symptoms of salmon poisoning disease, especially those that experience a repeat infection, to find out if SF agent is responsible.
 
“Conventional wisdom amongst veterinarians as well as pet owners is that once a dog has survived salmon poisoning, that he’ll have strong protective immunity to getting salmon poisoning again,” said study coauthor Dr Michael Kent, of the Department of Microbiology, Oregon State University, in a video interview. Because salmon flukes can carry more than one neorickettsial species, Dr Kent recommends that dog owners be aware that dogs may experience symptoms of salmon poisoning disease a second time and seek veterinary care if they develop fever or diarrhea.  “[Salmon poisoning disease] could kill a dog if not treated, but if treated there’s a very high rate of success.”
 
The study was partially funded by a grant from the National Institutes of Health.
 
Dr. Laurie Anne Walden received her doctorate in veterinary medicine from North Carolina State University. After an internship in small animal medicine and surgery at Auburn University, she returned to North Carolina, where she has been in small animal primary care practice for over 20 years. Dr. Walden is also a board-certified editor in the life sciences and owner of Walden Medical Writing, LLC. She works as a full-time freelance medical writer and editor and continues to see patients a few days each month.

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