I am sorry to hear about poor wee Oz.
Johnes is a disease that can cause chronic wasting and diarrhea in goats, that can lead to secondary circulatory issues and death in some cases. It can be quite insidious and it is possible for some animals to carry the bacterial infection with no signs of disease, while others succumb to the bacterial infection. If there was Johnes in your herd, it would be more likely that the bacteria didn't start with Oz. Rather his parents and any in contact animals would have to be under suspicion as possible carriers who could have infected Oz.
That said, there can be other causes for chronic diarrhea that causes wasting, anemia, and lead to the death of the animal. This can include viral disease, parasites (ie coccidia, giardia, fluke, PGE, etc), bacteria (ie salmonella, clostridia), and even toxins (perhaps less likely here).
In regards XXXXX XXXXX if Johnes is an issue for Oz, his father and the flock, you can consider testing for this bacteria. There are 3 common ways to test a herd for Johnes disease. You can send a sample of feces for culture (you can collect it from either or even from both and pool it together for testing) to isolate bacterium, direct PCR of feces (again an option in live and recently deceased animals) or blood testing antibodies by the animal in response to Johnes' infection (obviously only his father or the remaining goats could be tested this way). In this case, the fecal culture or PCR would be the best option to test (since positives tell you Johnes is there, where antibodies only tells us that they have been exposed and may not be infected).
As well, since Oz has just passed on and we do not know if what caused his demise poses a risk to other goats in your herd, I would strongly recommend that you consider submitting him for post mortem. If you speak to the vet, they may be able to perform the autopsy in the practice. Alternatively, if you have a vet school, agricultural college, or vet lab nearby, they may be able to do this for you as well. If they examine him grossly and cannot find an obvious cause of his death, they can collect samples to submit to the lab for the pathologists to evaluate.
The pathologists will be able to examine the tissues (grossly if they do the PM and) under the microscope to determine the causative agent that lead to death. They will be able to evaluate the GI for worm burdens and the liver for acute fluke infestation. They will be able to analyze tissue samples to determine if there is any issue with mineral imbalance. As well, if bacterial or viral causes are suspected, these can be cultured to determine what is to blame. And at this stage, they can also test Oz's feces for the Johnes bacteria if the changes to the intestine (which has a characteristic thickening) are suspicious.
Depending on their findings from the autopsy and culture, you will know what caused his passing and you will be able to take the necessary steps to defend the herd from this appropriately. If Johnes is found, then it is a reportable disease (though not notifiable), and your local vet will be able to guide you on how to do this and whether it would be necessary since these animals are not being bred or sold for meat/milk (since pets might be excluded from this). But the first step here would be determining what has caused Oz's signs before panicking about a single bacterial cause.
I hope this information is helpful.
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