Every breeder you talk with will have a different perspective on herd testing for CL, CAE, and Johne’s. I can tell you what I do, why, and what I recommend, but I will also tell you to speak with your veterinarian, do your own research, and make your own decision regarding what is best for your herd.
I highly recommend annual whole herd bio-security testing (ELISA/ blood test for CL, CAE, and Johne's) and Johne's PCR (fecal) testing. Most testing centers have a bio-security option at a reduced rate and have pooled Johne's PCR testing which allows you to test up to 5 goats for one price (unless the test comes back positive then each sample is automatically looked at individually to find the positive result).
Why do I recommend whole herd testing? Because in 2017 I purchased a beautiful doe as a 3 year old. She was tested before I purchased her with negative results; she tested negative on my property in the fall of 2017, she kidded the end of March 2018 and tested positive for Johne's in June 2018 during my annual herd testing. She was the only goat that tested positive then and since.
Due to this experience I do whole herd bio-security testing every year in late summer/early fall, then I do Johne's ELISA and PCR testing in late winter/early spring. All goats that are brought in to NAF are tested before the purchase takes place, and are tested again in quarantine at my place. Then they are added to my testing schedule. I also test all kids as they reach 12 months of age, then they too are added to the regular testing schedule.
Johne's is out there. It doesn't matter if you have only purchased from tested herds, it doesn't matter if you test once with negative results and don't bring any additional goats in - you can still have the disease in your herd. If you purchase hay, if you have deer, sheep, or cows; if you have purchased a goat and brought it in...you can have Johne's pop up in your herd at any time. Remember, the doe I had go positive had tested negative multiple times PRIOR to testing positive. Johne's is mean, and it's sneaky. It will hide and then silently spread. Please whole herd test annually. Be part of the solution to eradicate this disease from the Myo world.