Roundworms (ascarids)
Roundworms are the most common intestinal parasite in dogs. Due to the roundworm’s life cycle, most puppies have them and require treatment to remove them. If not treated, the worms will absorb nutrients from the puppy which can cause weight loss and slow growth, and will interfere with digestion. Roundworms can also cause diarrhea, vomiting, constipation, anemia, dulling of the coat, intestinal damage, and can cause the puppy to form a pot-belly.
Due to migration of larva through the respiratory system, a cough may be observed in some dogs and in young puppies larvae in the lungs may cause pneumonia. In sever cases, roundworms can cause intestinal obstruction.
Roundworms are prevalent throughout North America and infective eggs can be picked-up almost anywhere. It is important for puppies to be tested for roundworms at an early age. If puppies are infected, it will take several treatments to fully rid them of the parasite.
Two types of roundworms affect dogs, the Toxascaris Leonia and Toxocara canis. Both of these worms mature in the small intestine and both can be transported by rodents.
Toxascaris Leonia
T. leonina has the simple life cycle of the two. Infective eggs are ingested from the environment where they hatch into larvae and attach to the walls of the small intestine. Here they mature and adult females produce eggs. These eggs pass through the system and are expelled with the fesses where they can be spread throughout the environment. After 3 to 6 days, the eggs become infective and will infect other dogs who ingest them. An infected dog can pass millions of these eggs per day.
Rodents who consume these eggs become carriers. The eggs hatch inside the rodent where larvae migrate to the tissues. When a carnivore eats the rodent, these larvae are released into the intestines where they develop into adult roundworms.
Toxocara canis
T. canis roundworms have a more complicated life cycle. This life cycle ensures that the next generation of the host will be infected by the worms. T. canis can be contracted in three ways: ingestion of eggs, ingestion of a transport host such as a rodent, or by larvae through the uterus or in milk.
Ingestion of Eggs
After infective eggs are ingested, the larvae hatch and enter the circulatory system through the walls of the small intestine. From here they migrate to the respiratory system and other organs and tissues. In the body tissues they can encyst. They can remain encysted, walled and inactive, for years.
Larva that enters the respiratory system are coughed up and then swallowed. These larvae than attach to the small intestine and mature into adults who then lay eggs which are passes into the fesses. Once expelled, these eggs take 10 to 14 days to become infective.
Ingestion of a Transport Host
Like in the actual host, the transport host also will contain encysted larvae. When eaten, these encysted larvae are released into the intestines where they make their way into the circulatory system in the same was as would larvae ingested as eggs. In the same way, they are coughed up, swallowed, and mature in the intestines.
Through the Uterus
A female who has ever been infected with T. canis roundworms will likely have larvae in her tissues. These larvae can migrate through the uterus and placenta where they infect the fetal puppy. After birth, the larvae migrate to the puppy’s respiratory system, are coughed up, and swallowed where they mature in the puppies intestines.
Through the Milk
In the mother dog, encysted larvae in the mammary tissues can be passed to the puppy through nursing. Again, these larvae make their way to the respiratory system, are coughed up, and mature in the intestines.
Regardless of how the dog is infected, T. canis will take about 4 weeks to mature and start producing millions of eggs. With this diverse life cycle, it is easy to see why most puppies are infected with roundworms and should be tested and treated early to avoid health problems.
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