While I won't begrudge anyone who wants to be more careful about food safety and manure, I really don't think it should be a problem for @hu-randomnumber. Food safety guidelines are generally about using 'fresh' (not well composted) manure, and IIRC are generally minimum recommended in the range of 4 to 6 months before harvesting. Horse manure that's been outside for two years or more should be perfectly fine. (General 'wash carefully' guidelines apply anyway - even compost from kitchen scraps or just soil have bacteria). Also, if you look around, manure application guidelines are mostly about cow manure - because it's most often from large dairy operations (where the average size in USA is something like 4000 head). That's where the commercial scale manure is. Horse manure is (in my opinion) less of an issue - digestive systems are completely different - more 'roughage' and more air in horse manure, mixed with drier bedding, and much more likely to compost well. And horse operations are smaller and manure much less likely to be held in a slurry pond or wet pile with no air. At any rate, again, to each his own, but personally I'd have zero specific concerns about well-aged horse manure as described in a vegetable garden, for me or my children.

That depends on what the dewormer is and the dose. If more is given than is recommended more will be excreted in the manure.

OMG cat litter!!!! Truly, this familiarity with the end results of digestive systems speaks of a deeper (more insane) love of roses than I am prepared to engage with. For a fastidious nation (regarding shower usage and toiletry sales in the US), you sure are happy to grub about in poo!! I have used manure in the past (and definitely never younger than 6 months on an open heap) but usually as a tea since the practice of putting anything around roses, including proprietry fertilisers has always been something of a conundrum because I do not have any visible space. I do not love the practice of heavy mulching since I do not wish to see soil, mulch, hay, bark, and definitely not poo in any form.......I want plants...and more plants....not spaces for fertiliser which can be either dug in at the first or watered on throughout a plant's life. But that's just me. On a less personal note, using manure of any type has become a complete no-no here in the UK unless you are absolutely sure of what pasturage has been used at every stage of a horses (or cows) life and can guarantee no broad leaf weed killers have been deployed. In the UK, we certainly cannot and since most horse-owners have to buy hay at least some of the year, only a fool would risk putting the resulting manure anywhere near their plants.....unless they are completely OK with manure contaminated with clopyralid/amypyralid or any of the later broad-leaf weed killers after 2.4.D. Of course, if you have access to your own horses and own grazing meadows which are unsprayed, then fair enough, but sadly, manure, in whatever form, has become a thing of the past for may gardeners in the UK since we neither trust nor believe farmers to read the tiny print on the labels stating not to be used when hay is going for silage.

The reason it must go into containers is because I live in the burbs and do not have any room to let it sit out and compost. Also, I will be getting at least 1 cubic yard of the manure, so I plan to apply it over the period of a week or two. Thus, placing it in the containers makes it easier for me to run a few wheel barrows of it to various locations per day. Since we are in our rainy season now, I did not want the containers with the manure to fill up with water. Perhaps that is not a bad idea though. In any case, I will step up my application program, so that I use it up within one week if possible, time allowed.