Pinworms that infect humans are not carried in cow manure. Horses sometimes host another type of pinworm, but these also do not infect humans. It is likely the tiny white worms in the manure are enchytraeid worms (pronounced en-ke-treed). They are sometimes called potworms because they often are found in moist containers filled with peaty, acidic soil.

In the garden, enchytraeid worms are beneficials, rather like midget cousins to earthworms. They eat fungi and tiny bits of organic matter, and their castings are rich in nitrogen and other minor nutrients. They are a prized food for tropical fish. Potworms often are used as biological indicators when soils are tested for heavy metal contamination because they avoid copper, lead and other such contaminants.

Moisture and acidic conditions are all that are needed to get them going, so I think the manure you bought had been allowed to get quite wet, making it an enchytraeid nursery. However, because these worms often indicate a low (acidic) pH, you probably should check the soil’s pH and add lime to bring it closer to the neutral range. Most garden centers sell simple pH test kits for a dollar or two, and lime also is inexpensive.