Yes, humans can contract roundworm from dogs, an infection known as Toxocariasis, primarily caused by the dog roundworm Toxocara canis. The dog is the definitive host, meaning the parasite reaches maturity and reproduces in the canine intestine. Humans are considered accidental hosts because the parasite cannot complete its full life cycle within the human body. The infection is entirely preventable through proper pet care and hygiene practices.
The Transmission Pathway to Humans
Humans become infected not by the adult worms, but by ingesting the microscopic, infective eggs of the Toxocara canis parasite. The adult worms live in the dog’s small intestine and shed unembryonated eggs into the environment through the dog’s feces. These fresh eggs are not immediately infectious.
The eggs must mature, or embryonate, in the soil for approximately two to four weeks before they become capable of causing infection. Environmental contamination, particularly where dogs defecate, is the primary source of human exposure. The hardy eggs can survive in contaminated soil, sand, and dirt for months or even years.
Infection occurs when a person accidentally consumes these embryonated eggs, most commonly through the fecal-oral route. This often happens after touching contaminated soil, sandboxes, or objects and then putting hands to the mouth. Young children are at the highest risk because they frequently play in dirt and may engage in pica, the habit of eating soil. Once ingested, the infective egg hatches into a larva within the human small intestine, initiating the disease process.
Health Effects in Humans
After the infective egg hatches in the human gut, the released larva penetrates the intestinal wall and enters the bloodstream. Because humans are accidental hosts, the larvae cannot mature into adult worms. Instead, they migrate through various tissues and organs, a condition referred to as Larva Migrans. The severity of the resulting illness, Toxocariasis, depends on the number of larvae ingested and where they settle.
The migrating larvae cause an inflammatory reaction in the organs they invade, leading to two main clinical syndromes. Visceral Larva Migrans (VLM) occurs when the larvae travel to organs like the liver, lungs, heart, or brain. Symptoms are non-specific and can include fever, fatigue, coughing or wheezing, and an enlarged liver.
Ocular Larva Migrans (OLM) is a more serious condition that occurs when a larva migrates to the eye, typically affecting one eye. This migration causes inflammation of the retina and surrounding tissues, potentially leading to a whitish granuloma or mass in the back of the eye. OLM can result in significant visual impairment, including unilateral vision loss, and is most often seen in older children and young adults.
Protecting Your Family and Pets
Preventing human infection relies on a two-pronged approach focusing on pet care and strict personal hygiene. Consistent deworming of dogs is the most effective way to prevent environmental contamination with Toxocara eggs. Puppies are typically born with the infection or acquire it through nursing, requiring a rigorous deworming schedule starting as early as two weeks of age.
Veterinarians generally recommend treating puppies every two weeks until two weeks past weaning, followed by monthly treatments up to six months of age. Adult dogs should be treated regularly, with monthly year-round parasite prevention being the standard recommendation for high-risk households, such as those with young children. Pregnant dogs can be treated with specific protocols during late gestation to minimize transmission to their unborn puppies.
Environmental and personal hygiene measures are equally important in mitigating risk. Promptly removing dog feces from the yard, parks, and other outdoor areas is necessary to prevent the eggs from having the two-to-four-week period required to become infective. Feces should be disposed of in sealed bags to avoid contamination.
Strict handwashing with soap and water after handling pets, gardening, or any activity involving contact with soil is a simple but effective barrier. Sandboxes should be covered tightly when not in use to prevent dogs and other animals from using them as a latrine, a common source of infection for children. Parents should discourage children from consuming dirt or sand.