There are no known species of cactus which possess poisonous spines. Irritation, redness and infection caused by cactus spines are normally the result of bacteria or tiny hair-like barbs that remain lodged in the skin after spines are extracted.
In addition to bacterial infections, deep puncture wounds caused by cactus spines carry the risk of tetanus. While poison is not a worry when dealing with cactus thorns, there are many good reasons to remove them quickly. These spines often have barbs that lodge in the skin of animals. Those same barbs easily break off and can lay on a finger or palm until they come in contact with softer tissue, such as the mouth or eye. Cactus barbs can be extremely fine, small and sharp, resulting in inflammation and causing serious risk, especially to eye tissue.
Certain cacti also produce fruit, often containing its seeds. In order to protect those seeds from being eaten, they too are usually covered by spines or fine barbs. If eaten without proper precautions, these fine barbs can become lodged within the throat, stomach lining or intestines. In addition to the toxins which may be contained in the fruit itself, the spines present a secondary danger of infection.