Deworming horses has changed in the last few years as more parasites are becoming resistant to current deworming drugs. Rotational deworming was necessary before broad spectrum drugs were available, that was a long time ago.  Broad spectrum drugs include  ivermectin,  and moxidectin.  So all this time, around 33 years, rotational deworming has really been unnecessary. By rotating drugs, parasites get a little taste of this, a little taste of that until they become immune. This has already happened in the goat, and llama industry.  Making it necessary to double and triple doses to rid their animals of parasites. Some horses are very sensitive to even a normal dose of drug, let alone a double or triple dose.  Drug companies currently are NOT developing any new drugs. This is why it is so important to test first. Low egg counts do not require deworming. A small colony maintains the immune system to keep numbers down.  Most adult horses only require a once or twice a year deworming.  Another interesting fact is when spreading manure it must be hot at least 80 degrees and no rain in the forecast to kill the eggs. If the weather is not hot and dry, all you are doing is spreading parasites.