Your new addition to the family—little Fluffy or Fido—is absolutely perfect! He fits in with the kids, the adult cat that rules the house, sleeps through the night, and is learning his house manners. He’s affectionate, happy, and content—perfect in every way!
That is, until one day he vomits, and hidden within the vomit are six-inch long, fat, wriggling strings now squirming around on the floor! Suddenly, Fluffy or poor Fido has become the anti-Christ, and you swear you’ll never let him lick your face again!
Roundworms are the most common infestation in puppies and kittens. They are typically passed from an infected mother during embryonic development or through nursing. However, pets of all ages can also become infected with roundworms by consuming contaminated soil or eating infected prey, such as rats or squirrels.
There are three types of roundworms commonly found in domestic pets: Toxocara cati (in cats), Toxocara canis (in dogs), and Toxascaris leonina (found in both species). Identification of the specific roundworm species is done through a fecal flotation test, where the eggs are extracted from the feces and examined under a microscope. The same medication kills all three species of worms, but if you have multiple pets in the house, it may be helpful to know which pets are at risk for further infection.
Not all species of roundworms share the same lifecycle. Toxocara canis and Toxocara cati have more complicated life cycles than Toxascaris leonina, and understanding these life cycles is essential for successful treatment.
Stages of Roundworms
Stage One: The eggs are passed in the host’s feces and can be detected during a fecal flotation exam. Once in the environment, the eggs develop into larvae within their egg sacs. This process takes about a month. The larvae are not infectious during this time, meaning roundworms cannot be transmitted through fresh feces. Roundworm eggs can remain dormant for years and survive even the harshest weather conditions within their microscopic egg.
Stage Two: At this stage, Fluffy or Fido may walk through contaminated soil and pick up second-stage larvae on their fur. When they groom themselves, they ingest the larvae, which hatch in the intestinal tract before burrowing out to encyst or attach to other tissues in the body. If it’s not Fluffy or Fido that picks up the larvae, the hatched worm remains encysted until it is eaten by the appropriate species for that particular worm.
Fluffy has it worse than Fido. While Toxocara cati larvae rarely encyst, they instead travel directly to the lungs, reaching them within three days of infection.
Stage Three: The encysted larvae can remain attached for years with few problems for the host. However, if the host is the appropriate species, the larvae eventually excyst and migrate to the lungs, where they burrow into the small airways and travel up toward the throat. A mild infection may cause a cough, while a severe infection can lead to pneumonia. Once in the upper airways, the host will cough them up, swallow them again, and the larvae continue their development.
The only difference in second-stage larvae development occurs if the host is pregnant or nursing. In that case, the larvae travel to the uterus and infect the fetuses or to the mammary tissue, passing through the mother’s milk. In most cases, the puppies or kittens are already infected through the intrauterine cycle.
Stage Four: Once the larvae reach the intestines, they mature and begin to mate within a week. If there are no interruptions in the cycle, the journey from initial infection to laying new eggs takes about four to five weeks.
Roundworm infections can cause a number of health issues in the host. Vomiting or diarrhea is common in young animals, and the worms consume the host’s food, causing nutrient deficiencies and the characteristic “pot-bellied” appearance seen in worm-infested puppies and kittens. Serious infections can cause pneumonia or obstruct the bowel if a significant number of worms infest the intestines.
It’s generally assumed that puppies and kittens are infected, and treatment can begin as early as six weeks old. Fecal testing should be part of the yearly check-up for adult animals, and hunting pets should be treated with a deworming agent every few months.
Your veterinarian will prescribe one of several deworming medications, depending on the age and overall health of your pet. There are also monthly treatments available that are particularly effective at limiting environmental contamination, giving you peace of mind that your pet is well-protected. Dewormers anesthetize the worms, allowing them to detach from the intestinal wall, where they are then passed through the feces or vomited up. Don’t be alarmed if you see live worms in your pet’s feces after treatment—this is normal, and the worms will soon die outside their host.
Because of the multi-stage life cycle of roundworms, one treatment is usually not enough to eliminate the infestation. Typically, three treatments are needed over a span of nine weeks. In serious infestations, your vet may request a fecal sample at the end of the treatment course to ensure no eggs are still present.
During treatment, traditional dewormers only kill the worms already in the intestinal tract—not the encysted larvae. Since second-stage larvae in pregnant or nursing dogs don’t travel to the lungs or intestinal tract, treatment can’t prevent the pups or kittens from becoming infected. However, there are new-generation dewormers available that have been proven safe for preventing this cycle of infection in unborn puppies.
Toxascaris leonina has a simpler lifecycle and does not migrate through the body like the species-specific roundworms. Treatment is the same for Toxascaris leonina.
If you have any concerns about your pet’s health or suspect a roundworm infestation, contact your veterinarian. They will prescribe the appropriate medication for your pet and provide guidance on how to avoid further contamination in your household.
