Thrush occurs when dirt and manure packed into the hoof contains which bacterium?

• A. Bacteroides fragilis
• B. Fusobacterium necrophorum
• C. Candida albicans
• D. Pseudomonas aeruginosa

Answer: (B)

Thrush is an infection of the frog area that develops when dirt and manure trap moisture and create a low-oxygen environment under the hoof. In this setting, Fusobacterium necrophorum thrives and is the organism most commonly linked to the condition, driving the tissue decay and foul smell you often see.

Fusobacterium necrophorum is an anaerobic bacterium that naturally exists in the environment and can colonize the hoof when conditions are damp and dirty. That makes it the best fit for explaining thrush in horses, because the problem centers on this specific organism’s tendency to flourish in the anaerobic, contaminated hoof tissue.

The other options don’t fit as well. Bacteroides fragilis is also anaerobic, but it’s more associated with gut or sterile-site infections in humans and animals rather than the classic hoof thrush. Candida albicans is a fungus, not a bacterium, so it’s not the typical cause of thrush in the hoof. Pseudomonas aeruginosa can cause various hoof problems in wet environments, but it’s not the organism most characteristically linked to thrush.

So, the best answer is Fusobacterium necrophorum because it best explains the bacterial etiology of thrush under dirty, damp, anaerobic hoof conditions.