Foul-smelling diarrhea is always a cause of concern because it can signify underlying health conditions, including diseases like parvovirus and giardia, infected anal sacs, intestinal parasites, or dietary issues. Always take puppies and dogs to the vet for an examination, especially because some diseases causing diarrhea can be fatal.
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Infected anal sacs
If your dog has diarrhea and smells like fish, it's possible that he is having problems using his anal sac when he poops. Dogs have anal sacs (also known as anal glands) on either side of their anus. These anal sacs naturally produce smells that dogs use to mark their territory, but these smells can start smelling fishy when there is a health problem present. Anal sacs can become infected or impacted (when the anal sacs don't fully empty and fluid dries within them), or they can have tumors.
If your dog has diarrhea and smells like a fish, ask a veterinarian about expressing (emptying) his anal sacs. Veterinarians and groomers know how to express anal sacs, and veterinarians can determine the exact problem.
Your dog's stomach is more sensitive than your own, and dietary changes can lead to foul-smelling results. Dogs may have diarrhea when switching kibble brands or in response to eating something they shouldn't.
A lot of human foods can cause gastrointestinal problems in dogs since they are difficult to digest, so don't feed your dog table scraps. Unfortunately, dogs love to fetch their own "treats" out of the trash and consume whatever foods they can get, so try to keep food out of their reach. Eating foreign objects can also lead to a case of diarrhea.
Parvovirus in dogs
A dog infected with parvovirus will have chronic, smelly diarrhea that may have blood or mucus in it. Parvo is a disease that's mostly passed from the poop of infected dogs or items contaminated with the poop (like shoes, feet, and clothing). Unvaccinated dogs under a year old are the most at risk for parvovirus, but puppies under the age of 5 months are the most affected and the hardest to treat.
Most dogs with parvovirus will recover as long as they get treatment. In addition to diarrhea, dogs infected with parvovirus may also vomit, lack appetite, be restless, and have a fever. Take your dog to the veterinarian if you notice that she has diarrhea that's smelly. Dog breeds including Rottweilers, Doberman pinschers, and English springer spaniels have a higher risk of fatality. There's no cure for parvo, but dogs can survive with veterinary care.
Giardia in dogs
While most dogs with giardia don't have diarrhea, this parasite can cause older adult dogs and puppies to have watery diarrhea. Dogs can become infected with this parasite by drinking contaminated, stagnant water — like from a puddle or a shared dog bowl outside a restaurant.
Dogs with giardia may have persistent or intermittent diarrhea that smells horrible because of the parasites that are attached to their intestinal walls. Their poop will be soft and watery, may contain blood, and may look green. Giardiasis isn't typically fatal except in dogs with immunocompromised immune systems, but it does need to be treated by a veterinarian.
Preventing smelly dog diarrhea
To protect puppies from parvovirus, make sure that they are up to date on their vaccinations. During the time that puppies aren't fully vaccinated, be careful when taking them to public areas. Additionally, never allow your dog to drink from puddles and avoid communal dog bowls by having a fresh source of water on hand. Never allow your dog to touch other dogs' poop, as it can contain all kinds of diseases and intestinal parasites, like hookworms, roundworms, and parvovirus.
When to see a veterinarian
Whenever diarrhea is smelly, dogs need to be seen by a veterinarian. Diarrhea is a symptom of many illnesses and should always be taken seriously. Diarrhea can become a serious issue even in mild cases because it can lead to severe dehydration if fluids aren't replenished. If your dog's diarrhea is bloody or is accompanied by other symptoms, like vomiting, a lack of appetite, fever, abdominal pain, weakness, and other health issues, speak with your veterinarian.
Veterinarians may ask for a sample, so collect fresh diarrhea with a poop bag before your appointment. Veterinarians will work to determine what's causing diarrhea, so they may run fecal tests, blood tests, and X-rays to make a diagnosis. Treatment will depend on the underlying issue but may include medication, dietary changes, and intravenous fluids if your dog is dehydrated.