Which diseases can actually pass from a dog to a person?

The CDC lists roughly a dozen pathogens with documented dog-to-human transmission, but a handful account for nearly all real-world cases: rabies virus, Campylobacter and Salmonella bacteria from feces, roundworms and hookworms (Toxocara canis and Ancylostoma species), ringworm (a fungus despite the name), Capnocytophaga canimorsus from bite wounds, and leptospirosis from urine-contaminated water (CDC, 2024). Rabies is nearly always fatal once symptoms start but is preventable with vaccination, and the United States sees only one to three human rabies deaths a year, most linked to bats rather than dogs (CDC, 2023). Ringworm is the most common of the group in practical terms, spreading easily between a dog's coat and a child's skin during normal play.

Not every organism on a dog spreads disease, and dogs are not major reservoirs for anything close to the scale of, say, backyard poultry and Salmonella. The AVMA notes that most documented zoonotic transmissions from pet dogs involve puppies, strays, or dogs with diarrhea rather than healthy adult pets on a normal deworming and vaccination schedule (AVMA, 2023). That distinction matters for risk assessment: a well-cared-for adult dog on preventives is a low-risk companion, while an unvaccinated stray or a puppy with loose stool carries meaningfully higher odds of shedding something transmissible.

What are the first signs a dog might be carrying something transmissible?

Diarrhea is the single most useful warning sign, since Campylobacter, Salmonella, Giardia, and several intestinal parasites all show up in loose or bloody stool before they show up anywhere else. The Cornell Riney Canine Health Center notes that puppies under six months are far more likely than adult dogs to test positive for Giardia and roundworms on a fecal exam, which is why most veterinary schedules call for deworming every two to three weeks until 12 weeks of age (Cornell Riney Canine Health Center, 2023). Circular, scaly, hairless patches on the dog's skin point toward ringworm, while a dog licking or scratching one ear repeatedly with a foul odor can flag a bacterial or yeast issue that, while not zoonotic itself, signals a dog whose overall health and hygiene need attention.

A dog that seems perfectly healthy can still shed Salmonella or carry hookworm larvae in its stool without any visible symptoms, which is part of why routine fecal testing matters more than watching for obvious illness. The CDC recommends a fecal exam at every puppy visit and at least once or twice yearly for adult dogs, since asymptomatic shedding is common enough that visual inspection alone misses most at-risk animals (CDC, 2024).

Why do some dogs carry these germs and others don't?

Age, environment, and preventive care history explain most of the variation. Puppies, dogs from shelters or rescue pipelines, and dogs that eat raw diets or scavenge outdoors carry substantially higher rates of Salmonella and Campylobacter shedding than adult dogs on commercial diets with regular veterinary care, according to AVMA guidance on raw pet food risks (AVMA, 2023). Dogs that drink from stagnant ponds, ditches, or floodwater are at elevated risk for leptospirosis, a bacterium spread through the urine of infected wildlife like raccoons and rodents that contaminates standing water; the ACVIM notes leptospirosis cases in dogs have been rising in parts of the U.S. over the past two decades as suburban development pushes into wildlife habitat (ACVIM, 2022).

Close, frequent contact style also matters independent of the dog's health status. Letting a dog lick an open cut, sharing a food plate, or allowing face-licking raises exposure to Capnocytophaga and oral bacteria that live normally in a dog's mouth but can cause serious infection in a person with a compromised immune system, even from a lick rather than a bite (CDC, 2024).

How do doctors and vets confirm one of these infections is actually zoonotic transmission?

Diagnosis on the human side usually starts with a standard workup for the symptom — a stool culture for gastrointestinal illness, a skin scraping and fungal culture for a suspected ringworm lesion, or a blood culture for suspected Capnocytophaga after a bite — and confirmation of a shared source requires matching the pathogen strain in both the person and the dog. The CDC's One Health framework specifically supports coordinated testing between physicians and veterinarians when a household cluster of illness points toward a shared animal source (CDC, 2024).

On the veterinary side, a fecal flotation test identifies parasite eggs, a fecal PCR panel can detect Salmonella, Campylobacter, and Giardia DNA even when a standard culture is negative, and a dermatophyte culture or PCR test confirms ringworm species. Cornell's veterinary diagnostic lab notes that PCR testing has largely replaced older culture-only methods for ringworm because it returns results in days rather than the two to three weeks a fungal culture can take (Cornell University College of Veterinary Medicine, 2023).

What treatment does a dog need if it tests positive for something transmissible?

Treatment tracks the specific organism. Roundworms and hookworms respond to standard dewormers like fenbendazole or pyrantel pamoate, typically dosed every two weeks over a puppy's first three months per Companion Animal Parasite Council guidelines (CAPC, 2023). Giardia usually clears with a course of fenbendazole or metronidazole combined with a thorough bath to remove cysts from the coat, since reinfection from licking a contaminated coat is common. Ringworm in dogs is treated with topical antifungal shampoos containing miconazole or chlorhexidine plus an oral antifungal such as itraconazole for four to six weeks, and the household environment needs cleaning with a diluted bleach solution because spores survive on bedding and carpet for months (AAHA, 2023).

Bacterial infections like Salmonella and Campylobacter in dogs with diarrhea often resolve without antibiotics in otherwise healthy adult dogs, and veterinarians frequently withhold antibiotic treatment specifically to avoid selecting for resistant strains, instead managing hydration and diet while the infection runs its course (AVMA, 2023). Leptospirosis is the exception requiring immediate aggressive treatment, since it can cause acute kidney and liver failure in dogs within days and is treated with intravenous fluids and antibiotics like doxycycline in a hospital setting.

What can owners actually do at home to reduce the risk of catching something?

Hand washing after handling feces, litter, or a dog's mouth area is the single highest-yield habit, and the CDC specifically recommends washing hands with soap and water for at least 20 seconds after cleaning up after a dog, before eating, and after playing outdoors where a dog has been (CDC, 2024). Picking up feces promptly, ideally within a day, limits the time roundworm and hookworm eggs have to become infective in soil, since Toxocara eggs typically need two to four weeks in the environment before they become capable of infecting a person who ingests contaminated soil.

Keeping dogs on a year-round veterinary-prescribed parasite preventive, scheduling fecal exams at annual wellness visits, and avoiding raw or undercooked diets meaningfully lowers the odds a dog is shedding something in the first place. The CDC also advises against letting dogs lick open wounds, the face, or a baby's skin, and recommends immunocompromised individuals avoid cleaning litter boxes or cages and let a healthy household member handle stool cleanup instead (CDC, 2024).

Are some people more at risk from their dog than others?

Yes, and the CDC groups these as higher-risk populations: children under 5, adults over 65, pregnant people, and anyone with HIV, undergoing chemotherapy, or taking immunosuppressive medication after an organ transplant (CDC, 2024). Young children are at particular risk for roundworm infection because of hand-to-mouth behavior and playing on the ground, and the CDC estimates thousands of cases of toxocariasis occur annually in the U.S., with some causing partial vision loss when larvae migrate to the eye (CDC, 2023).

For these groups, the AVMA recommends specific precautions beyond general hygiene: avoiding contact with reptiles and very young puppies, having someone else handle feces cleanup, and asking a veterinarian to prioritize fecal testing and deworming schedules for any pet in the household (AVMA, 2023). None of this means avoiding pet ownership; it means shifting a few specific tasks to a lower-risk household member.