Cell mediated immunity may also be impaired in tadpoles. In mammals the antiviral response relies on cytotoxic T lymphocytes and these are Major Histocompatibility Complex Class I (MHC class I) restricted. Larval Xenopus reportedly lack MHC classical class I expression [33] suggesting the antiviral response in larvae may be compromised. Studies of the adaptive immune response in Xenopus adults and larvae to frog virus 3 (FV-3), the type virus of the ranavirus genus, indicate this may be so. While adult Xenopus cleared an initial infection and showed an accelerated response to a second injection, tadpoles were much more susceptible, suffering a high mortality rate and a reduced ability to clear the infection compared with infected adults [34] . In spite of lacking MHC class I, tadpoles do have CD8 T cells [35] and so the role played by the lack of MHC class I in the poor antiviral response is unknown. While most of these studies have been carried out in Xenopus, limited studies indicate bullfrogs are capable of mounting a detectable antibody response to an antigen, but apparently not to influenza virus despite repeated inoculations and a substantial and anamnestic response to bacteriophage T7 [36] . Our own studies indicate that B. marinus tadpoles did not mount a detectable antibody response to a widely used and well characterised immunogen, ovalbumin, to which adult B. marinus did respond.