Why are most antibiotics contraindicated in rabbits and hamsters?

The use of most antibiotics in rabbits and hamsters is contraindicated primarily because they can disrupt the gram-positive gut flora. These small mammals rely heavily on a balanced gut microbiome to aid in digestion, particularly since their digestive systems are designed to ferment plant material. Gram-positive bacteria play a crucial role in the normal functioning of their gut.

When antibiotics that target a broad spectrum of bacteria are administered, they can inadvertently kill off these beneficial gut bacteria, leading to dysbiosis. This disruption can result in gastrointestinal issues such as diarrhea, which can be life-threatening in these species. Managing their gut flora is imperative; thus, using antibiotics without a clear indication and careful consideration can have serious consequences.

The other options, while related to risks associated with antibiotics, do not accurately capture the primary concern regarding rabbits and hamsters. For instance, while the permeability of the blood-brain barrier is relevant to specific medications affecting the central nervous system, it's not the primary issue with antibiotic use in these species. Similarly, the heart and renal toxicity considerations, although important in pharmacology and veterinary care, are not the key factors leading to the contraindication in the context of the gastrointestinal health of rabbits and hamsters.