Ancylostoma caninum is a species of nematode known as a hookworm, which principally infects the small intestine of dogs. The result of A. caninum infection ranges from asymptomatic cases to death of the dog; better nourishment, increasing age, prior A. caninum exposure, or vaccination are all linked to improved survival. Other hosts include carnivores such as wolves, foxes, and cats, with a small number of cases having been reported in humans.
Warm and moist conditions are important to allow survival of A. caninum during the free-living stages of its lifecycle, so it is largely restricted to temperate, tropical, and subtropical regions. In parts of the world where these climatic requirements are met such as Sri Lanka, Southeast Asia, and Malaysia, A. caninum is the main cause of hookworm disease in canines.
Morphology
A. caninum females are typically long and wide, while the males are smaller at in length and in width. Males have a copulatory bursa , which consists of spine-like spicules positioned on three muscular rays that grasp the female during mating. As with other nematodes, the sperm lack flagella. The copulatory bursa is a unique feature of Strongylida members, thus making it a useful means for identifying members of this suborder; it is also used to distinguish members within the suborder due to differences in bursa appearance between species. The vulva of A. caninum females is located at the boundary of the second and final thirds of the body.
The teeth of A. caninum are found in the buccal capsule and divided into three sets. Two ventral sets form a lower-jaw equivalent, while a further set projects from the dorsal side and loosely equates to an upper jaw. Each ventral set has three points, with those furthest to the sides being the largest. While the ventral sets are prominent, the dorsal set is hidden deeper in the buccal capsule.
A. caninum bends its head end upward (dorsally), which has been noted to be a potential source of confusion when determining how the hookworm is oriented. If it has recently ingested blood, A. caninum is red in colour; if not, it appears grey. A. caninum has an alimentary canal made up of an esophagus, intestine, and rectum – the esophagus is highly muscular, reflecting its role in pulling intestinal mucosa into the body when it feeds. Esophageal and anal rings of A. caninum are the source of nerve fibres that extend throughout the body to innervate sensory organs, including amphids and phasmids.
Eggs are laid by the females, typically when at the eight-cell stage. Eggs are 38–43 μm in width, with thin walls.
Distribution
Freezing, temperatures exceeding , drying, or exposing A. caninum to sunlight all give reduced survival of the free-living stage, with rates of infection rising with temperature, provided 37 °C is not exceeded. A. caninum is, therefore, largely restricted to warm, moist climates, though infections are seen in the United States and southern Canada where the temperature is suboptimal. Specific niches are also able to satisfy the environmental requirements of A. caninum, despite not necessarily being in the tropics, such as mines.
Lifecycle
Transmission via the environment
Eggs are excreted from the host in the feces and typically hatch within a day on moist, warm soil into larvae with a non-living cuticle layer. By 4–5 days, the larvae have moulted twice and are now able to infect a host. Migration occurs from the feces into the surrounding soil. Two routes of infection from the environment exist. The first route involves penetration of skin at hair follicles or sweat glands, especially between the footpads where contact with soil is frequent and the skin is thinner than otherwise. Secretion of a protease by A. caninum is thought to aid this process. The larvae then migrate through the dermis of the skin, enter the circulatory system and are carried to the lungs. A. caninum larvae exit the blood at the lungs, move from the alveoli up through the trachea and are swallowed to end up in the intestine.