I would say that the frog & crayfish deaths are just a sad coincidence. There are very very very few diseases transmissable from reptiles to fowl....highly unlikely.
Mareks....I may be wrong but doesn't sound like it.
As to the chickens...this is what I found (below) in reference to the new chicks with white diarrhea.
On the sick hens....is it extremely hot where you are? Heat stress??? They will stretch out this way on a cool patch of ground when hot...they are eating, drinking, & seem normal otherwise???
If these symptoms look like what your chickens may have or if they develop white diarrhea, you must contact your state vet ASAP for testing.
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Pullorum
Synonyms: bacillary white diarrhea, BWD
Species affected: Chickens and turkeys are most susceptible, although other species of birds can become infected. Pullorum has never been a problem in commercially grown game birds such as pheasant, chukar partridge and quail. Infection in mammals is rare.
Clinical signs: Clinical signs including huddling, droopiness, diarrhea, weakness, pasted vent, gasping, and chalk-white feces, sometimes stained with green bile. Affected birds are unthrifty and stunted because they do not eat (see Table 3 ). Survivors become asymptomatic carriers with localized infection in the ovary.
Transmission: Pullorum is spread primarily through the egg, from hen to chick. It can spread further by contaminated incubators, hatchers, chick boxes, houses, equipment, poultry by-product feedstuffs and carrier birds.
Treatment: Treatment is for flock salvage only. Several sulfonamides, antibiotics, and antibacterials are effective in reducing mortality, but none eradicates the disease from the flock. Pullorum eradication is required by law . Eradication requires destroying the entire flock.
Prevention: Pullorum outbreaks are handled, on an eradication basis, by state/federal regulatory agencies. As part of the National Poultry Improvement Program, breeder replacement flocks are tested before onset of production to assure pullorum-free status. This mandatory law includes chickens, turkeys, show birds, waterfowl, game birds, and guinea fowl. In Florida, a negative pullorum test or certification that the bird originated from a pullorum-free flock is required for admission for exhibit at shows and fairs. Such requirements have been beneficial in locating pullorum-infected flocks of hobby chickens.
