Diagnosing Salmonella
Recent Salmonella recalls have led to some questions about the best way to diagnose salmonellosis in dogs and cats.
The first issue is when to test. In general, there is no indication to test healthy animals. Testing should be reserved for animals with diarrhea or other signs of salmonellosis (e.g. fever).
When testing is indicated, a few things should be considered:
The Sample
- A fresh sample is best. It should be submitted for testing as soon as possible, but it can be kept cool (i.e. refrigeration temperature) for a day or more if it can't be submitted right away. (Do NOT keep a fecal sample in your fridge at home! Take it to your local vet clinic and they will keep it in a designated specimen fridge.)
- More is better. A reasonable volume of stool (e.g. a tablespoon or two) is preferred to something like a rectal swab. Testing can be performed on rectal swabs but they are lower yield because they contain a lot less stool. See image right: standard-size 30 mL fecal sample containers (click for source).
The Test
There are two tests used to detect Salmonella in feces: culture and PCR.
Culture is used to grow and isolate the Salmonella bacterium. Usually, enrichment culture is used, whereby the sample is first cultured in a selective broth culture medium, then put on culture plates. This increases the recovery rate but takes more time.
Advantages of culture are:
- A positive is definitive - the bacterium is definitely there and alive.
- An isolate is available for subsequent testing such as determining the susceptibility to antibiotics and typing it to see what strain is involved.
Disadvantages of culture:
- Salmonella can be hard to grow for labs that don't have a lot of experience and good protocols.
- A few days are required to obtain results, particularly if proper enrichment methods are used.
PCR (polymerase chain reaction) is a molecular diagnostic test that looks for DNA from a particular organism (in this case, Salmonella).
Advantages of PCR:
- Speed. Results may be available within 24 hours.
Disadvantages of PCR:
- Tests validated for dogs and cats are not usually available.
- The test detects both live and dead bacteria, so a positive result could theoretically be from ingestion of dead (and therefore irrelevant) bacteria.
- False negative results can occur from low levels of Salmonella or substances in the stool sample that inhibit the test.
- Lab quality control is critical but not always good.
Current recommendations are to base diagnosis on culture. PCR can be used as a faster presumptive test, but culture should be performed to confirm the diagnosis and get a bacterial isolate that can be further tested for antibiotic sensitivity and typed to see what strain is involved.
The things my dogs eats...
Yesterday morning, I found part of a goose carcass in the backyard.
Yesterday afternoon, I found more of it... in a pile of dog vomit on the dining room floor. (Yes, my dog's an idiot, but that's a separate story).
Between the vomiting and my dog's rather solid gut, there hasn't been any diarrhea, at least not yet, but it not an uncommon problem in most dogs after an episode like this.
Diarrhea after dietary indiscretion can be caused by a number of different factors, including ingestion of disease-causing bacteria such as Salmonella. It can also simply be from eating too much, overloading the gut with a new substance, effects of fermenting material on the intestinal tract, and other physiological causes.
The best way to prevent this kind of diarrhea is to make sure dogs don't eat anything unusual, but that may be easier said than done. If a dog does get into something it shouldn't, odds are it's not going to be a major problem in terms of infectious diseases. Most of these become typical cases of "garbage gut," the results of which are the dog vomits and maybe has some diarrhea, but is otherwise fine and gets over it quickly. Sometimes they puke once and never look back. Other times, they need to be kept off feed or be fed a bland diet for a couple days to let their system get back to normal. During this period you should be aware that, while the cause is unlikely to be infectious, there's still certainly a possibility that the dog is shedding more harmful bacteria than normal. Vomit isn't that big of a risk, particularly compared to diarrhea, but you should clean it up promptly and wash your hands thoroughly afterwards. Cleaning up diarrhea takes a little more care, as I've previously discussed.
Garbage gut is usually more of a nuisance than a major health problem. You don't necessarily need to go running to your vet every time your dog throws up after eating something, but you also need to make sure you don't miss a potentially serious problem such as ingestion of a foreign body (e.g. bones, indigestible garbage), severe intestinal infection or pancreatitis. You should go to your vet anytime you have concerns. It's particularly important to get your dog evaluated if it has persistent vomiting or diarrhea, signs of abdominal pain (e.g. hunched up stance, crying, moving around like it hurts), doesn't want to eat or drink or is lethargic. It's also wise to get to a vet quicker if your dog has underlying health problems, has a history of pancreatitis or is very young (i.e. a puppy) or very old.
Over-indulgence: Canine-style
As we approach the holidays, a lot of people are going to eat and/or drink too much, and suffer the consequences. The same can happen with dogs, and sometimes both the dog and their owner pay the price. Dogs get into things they shouldn't all the time... too much food, garbage, dead critters and various other "dietary indiscretions" can easily lead to diarrhea (and sometimes more serious problems). During the holidays, there's often a greater opportunity for dogs to steal food or to be fed too many treats or leftovers. Sometimes it's dramatic - like a 60 pound Lab eating a 15 pound turkey - but often the first sign of a problem is the pile of diarrhea on the floor (usually at 3 AM, in the case of my dog).
So, after yelling at the dog, blaming someone else for leaving food out, and perhaps cleaning off your foot (depending on where you stepped), how do you clean up this mess without getting sick yourself?
The good news is diarrhea from dietary indiscretions is not usually associated with zoonotic microorganisms like Salmonella or Campylobacter. However, those and other potentially harmful bacteria can be found in any dog feces, and you have to assume that diarrhea is infectious. The risk of infection of people is probably low, but you don't want to take unnecessary chances (especially over the holidays).
First things first: Clean up as much of the diarrhea as possible. Ideally wear gloves, and clean up the diarrhea using paper towels or something else disposable. Don't wander around the house with the diarrhea-soaked items - bring a garbage bag with you to the scene of the "accident".
After the bulk of the mess has been removed, your next step depends on a few things, including the surface, what you have available, and whether any high risk people are in the house (i.e. infants, elderly, people with compromised immune systems).
- Smooth, sealed surfaces (e.g. tile, laminate, sealed wood) are easy to clean and disinfect. A general cleaner can be used to remove traces of diarrhea. If you want to disinfect the area, use a general household disinfectant or dilute bleach solution (1 part bleach to 50 parts water). While general household disinfectants may not kill everything, I'm not sure aggressive disinfection is needed in most households. Thorough cleaning does a very good job, and we aren't trying to make the house sterile. I'd be more concerned about disinfection in a household with high-risk people (particularly infants who may crawl over that part of the floor). If you are concerned about bleach damaging the surface, use something else or test it on an out-of-the-way area.
- Carpet is problematic because it's pretty much impossible to disinfect. After removing as much diarrhea as possible, use of a carpet cleaning spray might be helpful (but it's more effective for removing stains, not pathogens). A few disinfectants can be used on carpets safely. Bleach isn't a good idea unless the carpet is already (or was originally) white. Even with a good disinfectant, you're very unlikely to kill all of the bacteria present, because of the ability of microbes to hide in fabric. Steam cleaning is another option.
Once that's done, don't forget the most important step: wash your hands thoroughly. (The second-most important step might be to cordon the dog off in a more easily cleanable area for the rest of the night in case further accidents occur).
Overall, the risk of getting sick from overindulgence-associated dog diarrhea is pretty low. I focus on cleaning up the mess and don't worry about thorough disinfection. That's probably reasonable in a low risk household, but I'd be more wary around high-risk individuals.
Severe diarrhea outbreak in Florida dogs
Tuttle Animal Medical Center in Florida has reported six dogs with severe bloody diarrhea, vomiting and fever, over the past month. The affected dogs were from the same general area in Sarasota County, and all but one died within 24 hours. However, care must be taken when interpreting information such as this. Apparently, most dogs were owned by people with limited finances, so it's hard to say whether they would have died if aggressive (and expensive) treatment could have been provided. A disease like parvovirus is highly fatal without treatment, but survival rates are excellent if proper treatment is provided.
Initial testing of these dogs to identify the causative agent has been unsuccessful, including a rapid in-clinic test for parvovirus. Because of limited finances, complete diagnostic testing has not been performed, and it's likely that only a very limited range of possible causes have been investigated. That's a problem with a user-pay system such as this. There's no incentive for owners of dead pets to pay for further testing that could help understand the problem and/or help other peoples' pets.
Various experts have been consulted, but it sounds like there is minimal material (e.g. saved fecal samples) to use for further testing. Trying to make a diagnosis based on clinical signs and basic laboratory data collected by the clinic during standard work-up and treatment is essentially impossible. Veterinary infectious disease expert Dr. Cynda Crawford told VIN News Service by e-mail last Wednesday "There is very little case material to work with, so am struggling with meaningful diagnostic approaches,...Everything is basically speculation at this point."
Florida's Division of Animal Industry is apparently "monitoring the situation." At this point, there's nothing that can really be done, but hopefully assistance with testing will be provided should further cases be identified. One official from this agency speculated that E. coli O157 could be the cause. That seems pretty unlikely. This bacterium can cause disease in dogs but it's quite rare. Six apparently unrelated cases due to such a rare cause seems pretty unlikely, although it shouldn't be dismissed.
A general tenet of medicine is "common things occur commonly." Situations like this are most often unusual presentations of a common disease (e.g. parvovirus) rather than a new disease. More aggressive diagnostic testing for known causes of disease, along with additional testing to try to identify new pathogens is needed if further cases are identified. Sometimes apparent outbreaks like this go away on their own without any intervention or diagnosis. Only time will tell whether this is a small local event or the "tip of the iceberg."
