Vomiting in Dogs and Cats: When to Test for Parasites

Vomiting can be the only clue

A dog or cat that keeps vomiting may have a parasite, even if diarrhea and other typical signs are absent. That possibility is easy to miss: vomiting has many causes, and some parasites are better known for causing diarrhea. Yet fecal testing is often quick, inexpensive, and minimally invasive. For a patient without an obvious emergency such as a foreign body or toxin exposure, it belongs early in the diagnostic workup. Young animals can be especially difficult to assess. Vomiting has many possible causes, and several are more common in dogs under a year old—the same age group in which parasitism is often diagnosed. Parasites capable of causing vomiting include roundworms, tapeworms, flukes, and protozoa. Some cause vomiting only occasionally; for others, it can be the main sign, even when the animal otherwise appears well. Testing takes a fecal sample, sometimes collected over several days when parasites shed intermittently. Some tests can be run in a clinic in under 15 minutes; others require specialized equipment or a reference laboratory. The test has to fit the suspected parasite. Sugar-solution centrifugal flotation works well for many nematode eggs, including hookworm, roundworm, and whipworm, but may miss heavier eggs such as those of

Physaloptera and some flukes. Sedimentation is better suited to those eggs. The practical point is simple: no one fecal test finds every parasite. Choosing the wrong method can leave an infection undetected and send the workup toward more costly or invasive procedures, including endoscopy or exploratory surgery. The diagnostic and treatment options for common parasitic causes are summarized in Table 1.

Parasites where vomiting may stand out

Physaloptera species

Physaloptera rara and Physaloptera praeputialis are stomach-dwelling nematodes. Dogs and cats can become definitive hosts after eating infected arthropods, usually crickets or beetles, or a paratenic host.^1,2 Vomiting is the signature sign. It can persist for a long time, occur with a low worm burden, and appear on its own; affected pets may remain alert and interested in food but be unable to keep it down.^3,4 That pattern deserves attention because the usual fecal flotation may not detect the parasite. Its small, larvated eggs are typically found by fecal sedimentation instead.⁵ Adult worms may also appear in vomit; a cuticular collar and caudal alae help identify them.⁶

Toxocara canis and Toxocara cati

These common roundworms live in the small intestine. Puppies and kittens are frequent hosts, and nearly every litter encounters them early in life. Infection can follow ingestion of environmental eggs or a paratenic host; Toxocara can also pass across the placenta.⁷ Signs can include coughing before the worms reach maturity, vomiting—more often reported in cats than dogs—and abdominal discomfort or a potbelly. Severe disease is more likely in young animals, and untreated heavy infections can form large masses of worms. Sugar-solution centrifugal fecal flotation is the usual in-clinic test; reference laboratories can also test samples by fecal PCR or coproantigen methods.^8,9 Puppies and kittens should be tested repeatedly during their first year.

Exposure history matters

Heterobilharzia americana

This trematode causes canine schistosomiasis. Eggs passed in the feces of an infected dog or other mammal release larvae called miracidia, which infect freshwater snails. The snails release the next infectious stage, cercariae, into the water. Dogs acquire infection when cercariae penetrate their skin while they swim.¹⁰ Chronic enteropathy is a common reason for veterinary presentation, but signs vary. Vomiting may occur alongside diarrhea, weight loss, increased urination or thirst, fever, and dermatitis.^10-12 Diagnosis may combine fecal sedimentation with a complete blood count, serum chemistry, urinalysis, and imaging. Findings can include anemia, eosinophilia, high or low calcium, high globulins, low albumin, and isosthenuria. For fecal sedimentation, saline is required: water causes the eggs to hatch immediately.^12,13

Spirometra species

Adult tapeworms live in the small intestine of dogs and cats. Their eggs enter water and release coracidia, which infect freshwater copepods. In the copepods, the larvae develop into procercoids; a second intermediate host—such as a frog, snake, or other small vertebrate, but not a fish—then carries the plerocercoid stage. Dogs and cats become infected by eating those small vertebrates.¹⁴ Most intestinal infections cause no signs. Some animals, especially cats, develop diarrhea, vomiting, lethargy, loss of appetite, or abdominal pain and distention. Proglottids may appear in the vomit of some cats.^15-17 Recommended tests are fecal centrifugation with zinc sulfate or fecal sedimentation; copro-PCR can help identify the species.^14,17

Protozoa to keep on the list

Cystoisospora species

Animals acquire these protozoa by ingesting sporulated oocysts from the environment, infected paratenic hosts, or insects.^18,19 Coccidiosis is common in puppies and kittens. Diarrhea is the usual sign, though some affected animals also vomit, become lethargic, or show abdominal pain.^18,20,21 Diagnosis rests on finding oocysts in a fecal centrifugal flotation sample and matching that result to the clinical signs.

Giardia species

Infection follows ingestion of cysts, which are infectious as soon as they are shed in feces. Dogs and cats commonly develop diarrhea, dehydration, and weight loss; vomiting is reported occasionally.²² Zinc-sulfate centrifugal flotation can detect cysts. Direct smears may reveal trophozoites, but they are less often detected because trophozoites survive poorly outside the host compared with cysts.

Other parasitic possibilities

Several less common parasites can also enter the differential: - Dirofilaria immitis causes heartworm disease in dogs and heartworm-associated respiratory disease in cats. Mosquitoes transmit the nematode, whose adults live in the pulmonary arteries. Periodic vomiting is reported more often in cats than dogs.²³ - Ollulanus tricuspis lives in the stomachs of cats and, rarely, dogs. Animals become infected by ingesting infective larvae in cat vomit. Chronic vomiting and gastritis are common findings. Baermann testing of vomit or gastric lavage fluid can recover the larvae.^24,25 - Spirocerca lupi forms nodules in dogs’ esophagi. Dogs acquire it by eating intermediate hosts, mainly dung beetles, or paratenic hosts such as rodents, rabbits, or chickens.²⁶ In the United States, reports have mainly involved wild canids in southern states.²⁷ Infected dogs may vomit, lose weight chronically, have difficulty swallowing, or develop complications including esophageal neoplasia.²⁸ Diagnosis combines signs and imaging to find nodules with fecal sedimentation or zinc-sulfate centrifugal flotation to detect larvated eggs. - Oslerus osleri is a canine nematode whose adults live beneath the tracheal mucosa. In the United States, it has been reported mainly in coyotes. Dams can pass the infective first larval stage to puppies through saliva or feces. Coughing and vomiting are among the signs. Baermann testing of feces or transtracheal wash samples may help; bronchoscopy can reveal tracheal nodules, though diagnosis can be difficult.²⁹ - Mesocestoides tapeworms live in the small intestine of dogs and cats and usually cause no signs. Rarely, larvae penetrate the intestinal wall and cause peritoneal larval cestodiasis, a potentially severe condition that can include vomiting.^30,31 - Platynosomum fastosum is a fluke of cats’ bile ducts, gallbladders, and pancreas. Cats acquire it by eating lizards, geckos, or other paratenic hosts. Signs depend on the number of adult parasites and can include vomiting, diarrhea, and jaundice. In the United States, it is more common in the Southeast and Hawaii.^32,33

When fecal findings mislead

Dogs commonly eat feces, so a fecal test may contain parasites from another host. Eimeria species, for example, are often found on fecal centrifugation tests. These spurious parasites are generally nonpathogenic, although coprophagia can sometimes lead to gastrointestinal signs such as vomiting.

Put the inexpensive test early

Vomiting has many possible causes, and parasitism can be overlooked. Still, parasites can make dogs and cats sick across the United States, and a basic fecal workup may help identify them before a patient undergoes a more invasive or costly procedure. The key is to match the diagnostic method to the suspected parasite and interpret results alongside the animal’s history and signs. Case example Toby, a three-year-old neutered male Chihuahua, had vomited consistently for a week and couldn’t keep food down. He was bright and responsive, behaved and drank normally, and passed formed stools. Bloodwork showed no abnormalities. A barium study was normal, but no fecal parasite tests were performed. Symptoms continued through 10 days of supportive care, after which Toby’s owners chose exploratory surgery. The duodenum was inflamed, and several worms about one centimeter long were attached to the inside of the stomach. A reference laboratory identified them as Physaloptera, a stomach parasite that can cause vomiting on its own. Physaloptera should have been high on the differential list given Toby’s age and signs. Heterobilharzia, Spirometra, and Giardia were other possibilities; a more detailed history could have helped assess them. Fecal tests would have been fast and inexpensive to run before surgery. If Physaloptera was suspected, sedimentation alongside centrifugal flotation would have been appropriate. Would a fecal test during Toby’s illness have guaranteed a diagnosis? No. A routine passive or centrifugal flotation would likely have missed Physaloptera eggs; sedimentation is the better choice. Even that test can be negative when infection is prepatent: immature worms may irritate the stomach and cause signs before they produce eggs. Vomiting: Sometimes It Is Parasites first appeared in Today's Veterinary Practice.