Hookworm biology starts with the route of infection

Hookworm species share a destination, the small intestine, but the routes infective larvae take to get there vary, and those differences shape clinical outcomes, according to Christopher Lee, DVM, MPH, DACVPM, DACVM. In a short educational video on hookworms, Lee urges clinicians to start with how the parasite arrives rather than what it is called.¹

"If you can follow the route, the biology gets much easier," he said.

Four routes, not interchangeable

Depending on the species, infective third-stage larvae (L3) can be ingested, penetrate the skin, arrive inside a paratenic host eaten as prey, or, in the case of Ancylostoma caninum in puppies, pass from the dam through milk. Lee stressed that these routes are species dependent rather than universal.

"Hookworms are route flexible but not route identical," he said.

After skin penetration, A caninum L3 enter the circulation, reach the lungs, travel up the trachea, are swallowed, and arrive in the small intestine. Oral infection is less predictable: Ingested larvae may develop directly in the intestine or penetrate the buccal mucosa and follow the pulmonary pathway.

Arrested larvae and larval leak

Some A caninum L3 enter somatic arrest, especially in skeletal muscle or the gut wall, where they remain viable. Pregnancy can reactivate them, sending larvae to the mammary glands and into milk after whelping, which makes transmammary infection a major route for young puppies. Transplacental transmission is not considered an important route, Lee said.

"Somatic arrest is essentially a parasite layover," he said.

Reactivated larvae can also return to the small intestine, mature, and resume egg production without any new exposure, a phenomenon known as larval leak. Lee offers timing as a practical, if deliberately loose, guide for telling the two apart: Reinfection requires a new exposure, after which egg shedding generally begins 2 to 3 weeks later, so exposure history helps separate reinfection from larval leak.

"Timing is a guide, not a verdict, but it gives you a much better place to start," Lee said.

When reduced anthelmintic efficacy is suspected, a fecal egg count reduction test (FECRT), comparing quantitative fecal egg counts before and 10 to 14 days after treatment, can help clarify the picture.²

How the species compare

A caninum is primarily a canine hookworm, and oral, percutaneous, paratenic, and transmammary routes can all matter. Ancylostoma tubaeforme, a feline hookworm, is acquired orally, including through paratenic hosts such as rodents, and percutaneously; transmammary transmission has not been demonstrated in cats. Uncinaria stenocephala is more common in dogs than cats and spreads mainly through oral and paratenic routes. Its larvae can penetrate skin and cause dermatitis, but percutaneous exposure rarely leads to a mature intestinal infection, and the species is less pathogenic than A caninum. Bunostomum infects ruminants by oral and percutaneous routes.³

Anemic puppies and negative fecals

Immature and adult A caninum attach to the intestinal mucosa and feed on blood. In young puppies, cumulative blood loss can become clinically important quickly. Pale mucous membranes, weakness, poor growth, dark stool, and anemia warrant attention. Because disease can precede patency, a negative fecal flotation result does not rule out hookworm infection in a puppy with a compatible signalment, exposure history, and clinical picture.

"With an anemic puppy, treat the patient, not just the poop cup," Lee said.

Egg morphology has limits, too. Ancylostoma eggs measure roughly 52 to 79 μm long and Uncinaria eggs roughly 71 to 92 μm, and that overlap means eggs can confirm a hookworm but cannot reliably identify genus or species.

"The egg gives you a clue, not an identity," Lee said.

He closes with case examples, among them a 10- to 14-day-old nursing puppy with pale mucous membranes and anemia, which should prompt suspicion of transmammary A caninum even if the fecal examination is negative.

"Start with the path, then name the parasite," he said.

References

1. Lee C. 012 Hookworms 10 min. YouTube. Published October 4, 2026. Accessed October 7, 2026. https://www.youtube.com/watch?v=7mnLlkqqdsc
2. Kaplan RM, Jimenez Castro PD, Marsh AE, Jesudoss Chelladurai JRJ, Gilleard JS. American Association of Veterinary Parasitologists Hookworm Task Force review on best practices for diagnosis and treatment of multidrug-resistant hookworms. J Am Vet Med Assoc. 2026;264(9):1214-1222. doi:10.2460/javma.26.02.0094
3. Hookworms. Companion Animal Parasite Council. Updated [date]. Accessed October 20, 2026. https://capcvet.org/guidelines/hookworms/