Cerebrospinal elaphostrongylosis (CSE) in Canadian caribou was most likely introduced via:

• A. Infected reindeer entered Canada and flies mechanically transmitted the parasite from the reindeer to the Canadian caribou
• B. Infected reindeer entered Canada and shed the parasite in their feces, local snails ingested the parasite, and were then ingested by the Canadian caribou
• C. Infected reindeer transmitted the disease via nose to nose contact with the native Canadian caribou
• D. Contaminated boots on herdsmen from Norway allowed Canadian caribou to sniff and lick their boots

Answer: (B)

This scenario tests how a parasite with an intermediate snail host can be introduced into a naïve wildlife population. Cerebrospinal elaphostrongylosis involves a nematode that sheds larvae in the feces of an infected reindeer or caribou. Snails (or slugs) ingest those larvae, the larvae develop inside the snails to an infectious stage, and caribou become infected when they eat those snails or eat prey that has fed on infected snails. So the spreading route hinges on the fecal contamination seeding local snail populations, which then pass the infection to caribou.

The best answer reflects that: infected reindeer entering Canada and shedding the parasite in their feces, local snails ingesting the parasite, and then Canadian caribou ingesting those snails. This matches the indirect life cycle with a snail intermediate host and a fecal-oral transmission step.

Why other routes aren’t fitting: mechanical transmission by flies wouldn’t complete the parasite’s life cycle since the parasite relies on snails to develop to an infectious stage; direct nose-to-nose contact between reindeer and caribou doesn’t move the parasite through its necessary snail host; contamination of boots could spread something environmentally, but it’s not the typical introduction pathway for a snail-borne nematode into wildlife populations.