Blood in a puppy's stool warrants a call to a vet today. The rest of this page explains why the threshold is lower than it would be for an adult dog, and what information makes the call more useful.
Why puppies are treated differently
A puppy has a much smaller total body water volume and far less physiological reserve than an adult. Fluid losses that an adult dog would absorb without difficulty can dehydrate a puppy within hours, and dehydration is what turns most gastrointestinal problems from unpleasant into dangerous.
Puppies also have immature immune systems and, until their vaccination course is complete, no reliable protection against several infections that cause bloody diarrhoea. That combination is why the advice does not include a watch-and-see period.
The causes that come up most
Parasites. Roundworms, hookworms, whipworms and coccidia are all common in puppies and several cause bloody or mucoid stools. Hookworms in particular can cause significant blood loss in a small puppy.
Infections, including parvovirus, which is the reason vets ask immediately about vaccination status. Parvovirus typically presents with severe, often bloody diarrhoea, vomiting, and profound lethargy, and it progresses rapidly.
Dietary problems, including a food changed too abruptly, scavenging, or something eaten that should not have been.
Foreign bodies. Puppies swallow things. Something sharp can injure the gut, and something bulky can obstruct it.
Stress colitis, which is common in the first days after a puppy moves to a new home and is usually the mildest thing on this list.
Emergency rather than same-day
Some presentations need care within hours: large volumes of bloody watery diarrhoea, repeated vomiting alongside it, pale or white gums, lethargy or floppiness, a swollen or painful abdomen, or a puppy that cannot be roused normally.
A puppy that is bright, playful and eating, with one streak of blood on an otherwise normal stool, is at the less urgent end. It still needs a call today.
What to have ready
Vaccination history, including exactly which vaccinations have been given and when. This is the first thing a vet will ask.
Worming history — product and dates.
A sample if you can collect one, in a clean sealed bag or pot. It allows testing for parasites straight away rather than at a second visit.
A photograph, which is more informative than any description.
The timeline — when it started, how many times, and whether the puppy is still eating, drinking and playing.
Anything it might have eaten, including toys, plants, rubbish, and any change of food or treat in the last few days.
What not to do
Do not give human anti-diarrhoeal medicines. Several are unsafe in dogs, and slowing the gut is actively harmful when the cause is an infection or a toxin.
Do not withhold water. Brief food withholding is sometimes appropriate on veterinary advice; water is not, and dehydration is the main risk.
Do not wait to see whether it settles overnight. In an adult dog that is often reasonable. In a puppy it is the decision that turns a treatable problem into an emergency, and it is the single most common regret owners describe afterwards.
The first fortnight in a new home
A large proportion of these calls come in the first two weeks after a puppy moves. Several things change at once — the food, the water, the routine, the people, the noise level — and a puppy's gut responds to all of it.
That is why breeders are usually asked to send a bag of the food the puppy has been eating, and why the standard advice is to change to a new food gradually over a week or more rather than on the first day. It also explains why a puppy that had firm stools at the breeder's can have loose ones on day two without anything being wrong.
Stress colitis of that kind is mild, tends to resolve within a few days, and typically produces mucus and occasional streaks of fresh blood rather than volume. It is still worth a call, because the other causes on this page look similar at the start and only one of them is harmless.
Preventing what can be prevented
Complete the vaccination course on schedule and keep the puppy away from unvaccinated dogs and from places where unknown dogs toilet until it is done. Worm to the schedule your vet sets, which for a puppy is far more frequent than for an adult. Change food gradually. Keep small objects, bones and rubbish out of reach, which in practice means supervising rather than trusting.
None of that eliminates the risk, but between them they remove most of the causes that are avoidable.