An ova and parasites test (often called an O&P test) is a lab exam that checks your dog’s stool for intestinal parasites and their eggs. “Ova” simply means eggs. A veterinarian or lab technician places a small fecal sample under a microscope, looking for adult parasites, larvae, eggs, or cysts that indicate an active infection. It’s one of the most routine tests in veterinary medicine and the primary way vets diagnose worm infections and other gut parasites in dogs.
What the Test Actually Detects
The O&P test casts a wide net. It can identify several categories of intestinal parasites, each with a distinct egg or cyst shape that’s visible under magnification. The most common findings in dogs tested across the United States include hookworms (found in about 2.5% of samples in a review of nearly 1.2 million dogs), roundworms (2.2%), whipworms (1.2%), and two single-celled parasites: Giardia (4.0%) and Cystoisospora, sometimes called coccidia (4.4%). Tapeworm segments or eggs can also show up, though they’re sometimes spotted by owners first, appearing as small rice-like pieces near the dog’s rear end.
Puppies are far more likely to test positive than adult dogs. In that same large-scale review, nearly 30% of dogs under six months old had at least one parasite detected, compared to about 6% of dogs over one year old. Geographic location matters too. Hookworms are most common in the southern U.S., while roundworms and Giardia are more frequently found in the West.
How the Test Is Done
The standard method is called fecal flotation. A small amount of stool is mixed into a special solution that’s denser than parasite eggs. When the mixture is spun in a centrifuge (or sometimes just allowed to sit), eggs and cysts float to the surface and stick to a glass coverslip, which the technician then examines under a microscope. This centrifugal flotation method is reliable for most common worms, with sensitivity around 88% for detecting Giardia cysts alone.
For Giardia specifically, vets sometimes add an antigen test, a rapid in-clinic test that detects proteins from the parasite rather than relying on visual identification. These antigen tests can catch infections that flotation misses, and vice versa. When the two methods are used together, accuracy improves significantly. A positive result from the O&P identifies both the type of parasite and gives the vet a rough idea of how heavy the infection is based on the number of eggs or cysts in the sample.
Why False Negatives Happen
A clean O&P result doesn’t always guarantee your dog is parasite-free. Several factors can produce a false negative. Many parasites shed eggs intermittently rather than continuously. Whipworms are a well-known example: they release eggs in low numbers and on an unpredictable schedule, making them easy to miss on a single test. Giardia and Cryptosporidium cysts are also shed on and off, which is why vets sometimes recommend testing stool from three separate days spread over a week or so for better accuracy.
Timing matters in another way too. After a dog is initially infected, there’s a “pre-patent period,” a window of days to weeks before the parasites mature enough to start producing eggs. During that gap, the dog is infected but the test will come back negative because there are simply no eggs to find yet. Sample freshness also plays a role. Hookworm eggs can hatch within 24 hours in warm, humid conditions, and once they hatch into larvae, they’re harder to identify under the microscope. Bringing in a fresh sample (ideally collected the same day) gives the most reliable results.
Signs Your Dog May Have Parasites
Many dogs with intestinal parasites show no symptoms at all, which is one reason routine testing is important. When signs do appear, they typically include loose stool or diarrhea, blood in the stool, weight loss or difficulty gaining weight, and a dull or coarse coat. In heavy infections, especially in puppies, you might see worms visible in the feces. Some dogs vomit or become lethargic. The severity depends on the type of parasite, the number of organisms present, and your dog’s overall health and age.
How Often Dogs Should Be Tested
The Companion Animal Parasite Council recommends fecal exams at least four times during a dog’s first year of life and at least twice a year for adult dogs. Dogs with higher exposure risk (those that spend time at dog parks, boarding facilities, or in areas with wildlife) may benefit from more frequent screening. Even dogs on monthly preventive medications should still be tested regularly, since no preventive covers every parasite and owner compliance with dosing schedules isn’t always perfect.
What Happens After a Positive Result
Treatment depends on which parasite is found. Roundworms and hookworms are typically treated with a broad-spectrum deworming medication that paralyzes or kills the worms so they pass out in the stool. Tapeworms require a different drug, praziquantel, which is effective against the two most common canine tapeworm species. Giardia is treated with a course of medication over several days, sometimes combined with bathing to remove cysts from the coat. Your vet will usually recommend a follow-up fecal test two to four weeks after treatment to confirm the infection has cleared.
For most parasites, treatment is straightforward and resolves the infection quickly. Reinfection is common, though, particularly if environmental contamination isn’t addressed. Picking up feces promptly from your yard, avoiding areas with heavy dog traffic, and keeping your dog on a regular preventive schedule all reduce the risk of a repeat infection.
Parasites That Can Spread to People
Several canine intestinal parasites are zoonotic, meaning they can infect humans. Roundworm larvae, if accidentally ingested (usually by young children who touch contaminated soil and then their mouths), can migrate through human tissues and cause a condition called visceral larval migrans. Hookworm larvae can penetrate bare skin, typically the feet, causing an itchy, winding rash known as cutaneous larval migrans. This happens most often when people walk barefoot in areas contaminated with dog feces. Giardia and Cryptosporidium can also spread to humans through the fecal-oral route, though the strains that infect dogs don’t always infect people efficiently.
Prompt fecal cleanup, regular deworming, and routine O&P testing are the most effective ways to protect both your dog and your household from these parasites.