**2. Biological cycle**

Embryonated *T. canis* eggs are shed in the feces of puppies. In the environment, a first-stage larva develops inside the egg, which molts twice until it becomes larva 3 (L3). Larvated eggs (passive L3 inside) are the infective stage. Depending on humidity and temperature, the development of the infective stage requires 2–5 weeks in the environment. In susceptible hosts after ingestion of the infective stage, L3 hatches (active L3) in the duodenum and traverses the intestinal wall; the larvae pass into the lymphatic flow or blood capillaries. From this moment on, the development and migration of the larvae vary depending on whether the host is a young dog (<3 months), an adult dog, a pregnant bitch, or a paratenic host (rats, mice, birds, and humans, among others) [4].

In puppies, L3 migrate via blood or lymph to the liver, where they remain for 1 to 2 days. Subsequently, they migrate through blood, pass through the lumen of the atrium and right ventricle of the heart and via the pulmonary artery, reach the lungs, and cross the capillaries to reach the alveoli. The larvae migrate through the lumen of the bronchioles, bronchi, trachea, larynx, and pharynx (tracheal migration), where they are swallowed; during this tracheal migration, the larvae molt to L4. The larvae remain in the stomach for some time (up to Day 10 postinfection), return to the duodenum, and molt to L5 or preadult to finally become adults (19–27 days postinfection). The prepatent period is 4–5 weeks [4].

In paratenic hosts and adult dogs, L3 larvae migrate through the blood and are distributed throughout the body, mainly to the striated muscle, liver, lungs, kidneys, and brain, where they remain for years in a state of latency or dormancy as infective somatic larvae (dormant larvae) until they die and calcify.

In pregnant bitches, on approximately Day 20 of gestation, many of their dormant larvae are reactivated by the influence of progesterone. Between Days 43 and 47 of gestation, under the influence of progesterone and prolactin, the larvae cross the placenta and infect the fetuses. The larvae remain in the fetal liver until birth; later, by blood, they migrate to the lungs where they remain during the first week of life, molting to L4 occurs during this stage or later when the larva arrives in the stomach by tracheal migration. By the end of the third week, the larvae molt at L5 and develop rapidly into adult worms. After copulation, the females produce eggs that are passed in the feces of the pups at 15 days of age. In recently delivered bitches, some reactivated larvae arrive by the influence of prolactin on the mammary gland and are excreted in the colostrum and milk to be ingested by the puppies, constituting another important source of infection for the litter. The larvae ingested in this way molt at L4 and L5 in the intestinal lumen, where they develop into adult worms without tracheal migration [5].

*Toxocariosis: From a One Health Perspective DOI: http://dx.doi.org/10.5772/intechopen.104508*

In recently delivered bitches, some larvae may reactivate during gestation migrate to the intestine, molt to L4 and L5 and become adult worms. Bitches can remain up to 60 days passing eggs in feces until the adult worms are eliminated spontaneously. This is one of the ways adult worms can develop in adult dogs [1].

Dormant larvae in the tissues of paratenic hosts can be reactivated when they are predated. If the predator is another paratenic host, the ingested reactivated larvae undergo a new somatic migration and become dormant in this new host. On the other hand, if the predator is an adult dog, the ingested reactivated larvae molt at L4 and L5 and develop into adult worms in the lumen of the small intestine without further somatic migration. In this way, dogs can spend a short time excreting eggs in the feces until the adult worms are eliminated spontaneously. This is another way that adult worms can develop in adult dogs [1].

The life cycle of *T. cati* is similar to that of *T. canis* except that prenatal transplacental infection in this parasite does not occur [6].
