Toxocara canis

       Hello everyone! Sorry for the radio silence.  I’ve been very busy with getting married, moving to a new city, and starting a PhD program.  But excuses aside, I’m back! This week’s parasite was chosen because it is one that I’m hoping to find evidence of in some of the coprolites that I’m examining for Dr. Reinhard (my new advisor). 

Toxocara canis larva emerging from its egg.

Toxocara canis is found all over the world and is most prevalent in areas populated by domesticated dogs and other canids.  This is because dogs are the definitive hosts for this parasite. In the U.S., about 98% of puppies and about 20% of adult dogs are infected. Pigs, mice, foxes, and even birds can serve as paratenic hosts for Toxocara canis. They live in and feed on intestinal contents when in the definitive host.  *Fun side note*: The adults undergo a specialized type of anaerobic metabolism that results in the production of an extra ATP!!!

Posterior end of a male T. canis.

These squirmy little dudes are dioecious (meaning that they have separate male and female individuals) with the males being smaller than the females and possessing a ventrally curved posterior end sporting simple spicules for a direct transfer of sperm.  The females possess large, extensive ovaries and can hold up to 27 million eggs at a time in their uteri.  *Another fun side note*: nematode sperm are not flagellated like human sperm, they are actually amoeboid.

Anterior end of a female T. canis.

Taxonomy

            Toxocara canis is a nematode belonging in the class secernentea.  Members of this class have caudal papillae as well as lateral canals in the excretory system.  This class includes the harmless nematode Caenorhabditis elegans, which is commonly used in genetic and soil ecology studies.  Toxocara canis also belongs in the order ascaridida, which includes many families of roundworms with three “lips” on the anterior portion of the body.  In fact, this species is one of the smallest members of this group. It also belongs to the family toxocaridae. Members of this family infect mostly felids and canids. 

Anterior end of a T. canis specimen.

Life Cycle

            When an infected dog defecates, the eggs of T. canis can live in the feces for up to three weeks as they become embryonated with larvae.  If a new host ingests eggs that have developed into the L3 stage (after second molt, but still inside the egg), the larvae break out in the gut and move to the lungs through the circulatory system. Then they migrate to the bronchial tubes and on into the esophagus. Eventually they make their way back to the gut. When the larvae make their way to the lumen of the intestine they develop into adults, mate, and lay eggs that exit the body via the feces. Eggs don’t start appearing in the feces until about 5 days post-infection. Studies have shown that eggs can survive in the external environment for 10-20 days. One British study demonstrated that under the right climatic conditions, some eggs can persist in the soil for up to three years!!! Also, pregnant dogs that are infected can pass these parasites on to their offspring.

Life Cycle of T. canis

Toxocariasis

Dogs infected with these parasites are said to have toxocariasis. Symptoms may include emaciation, anemia, diarrhea, constipation, roughness of the pelage, and/or pale mucous membrane. Dogs tend to be reluctant to moving or being moved and may display either a pot-bellied appearance or a tucked abdomen appearance due to the damage caused to the intestinal lining. There will often be vomiting, coughing, nasal discharge, and noisy breathing in puppies as they worms migrate through the respiratory tract.  Puppies may also develop nervous system problems known collectively as “ascaris toxaemia”.  If the infection is severe enough for long enough, death can be caused due to intestinal obstruction, ulceration, or perforation of the intestinal wall.

If the parasites enter a non-canid host, such as a human, they will wander throughout the body.  These wandering larvae are termed larva migrans.  If they wander to the lungs, brain, heart, muscles, liver, or other organs, they are called visceral larva migrans.  This wandering causes inflammation and can result in a myriad of syptoms such as fatigue, anorexia, pneumonia, fever, coughing, abdominal pain, headaches, rashes, and even occasionally seizures.

A child with ocularis larva migrans.

If the parasites wander to the eyes, the resulting infection is called ocularis larva migrans.  This tends to happen in older children and young adults.  Symptoms for this form include red eyes, decreased visual clarity, a whitening of the pupil (leukokoria), granulomas, chorioretinitis (inflammation of the choriod and the retina), and even retinal detachment.

Children can also develop a form known as covert toxocariasis, which can result in coughing, abdominal pain, difficulty sleeping, wheezing, or headaches.  This form develops due to chronic exposure.

Because infection is so prevalent in canids in the U.S., human exposure risks are very high here.  Most cases go unrecognized or unreported.  However, about 10,000 cases occur annually.  Somewhere between 4% and 8% of children test positive in serological tests for toxocariasis. Toxocariasis is most prevalent in children ages 1-3, and boys are at a higher risk of infection that girls. (Probably due to an increased probability of fecal-oral contamination since kids this age aren’t exactly well-known for their impeccable hygiene.)

Diagnosis, Treatment, and Prevention of Toxocariasis

            Diagnosis includes identifying clinical symptoms and demonstrating the presence of adult worms in feces or vomit.  Often times, a fecal examination is needed to confirm a diagnosis in dogs.  For humans, ELISA, PCR, and serological tests are most commonly used for diagnostics.

Ivermectin in paste form.

            Dogs are treated using an antihelminthic drug such as piperazine adipate, diethyl carbamazine, pyrantel, benzimidazole compounds, or ivermectin.

Human infections typically resolve themselves, but where treatment is required, it typically includes the administration of antihelminthic drugs such as albendazole or mebendazole.  In severe cases, corticosteroids may be given or surgery may be required.

Infection can be prevented by giving pets regular de-wormers such as fenbendazole, dichlorvos, and piperazine. (Good for your pooch, and reduces your risk of infecting yourself or your dirty children.)  This is especially important in females because dormant larvae may persist in the tissues for years and can be passed on to pups. Outdoor kennels should have floors that are impervious and easy to clean. (In other words, no dirt floors.) Keep food/water containers and bedding clean.  Make sure that the rodent populations are controlled as rodents can be paratenic hosts for the parasites.  Finally, try to keep your dog from eating feces or soil that may have been contaminated with feces.  Infections can also be prevented by owners who take the initiative to clean up after their pets, and by always washing hands thoroughly with soap and water following the handling of dog feces…especially before handling food.  Also, try not to let your children eat dog poop or dirt that could have come into contact with dog poop.

Moral of the Story

            Don’t be the d-bag who is too good to clean up after his/her Toxocara-infested mutt! Keep your pups and kiddos from eating poop and/or dirt and keep your kennels cleaned! (Dog and kid-kennels alike! J/k…Scrubs reference? Anyone?) Also keep Fluffy up to date with de-wormers, and wash your hands if you are playing with his poop. So, watch your children, watch your pets, and wash, wash, wash your hands!