Somewhat as in human donation around the world, blood donors for veterinary transfusions of cats and dogs are both volunteers and professionals. (Of course all these donors are themselves dogs and cats.)
Here's the NYT on the story:
The Pet ‘Superheroes’ Who Donate Their Blood.
Transfusions have become an important part of veterinary medicine, but cat and dog blood is not always easy to come by. By Emily Anthes
"All kinds of ailments — including injuries, infectious diseases, immune conditions and cancer — can leave a pet in desperate need of blood, and transfusion has become an increasingly routine part of veterinary care.
“It is just as important a part of veterinary medicine as it is for human medicine,” said Dr. Dana LeVine, a small-animal internist at Auburn University and the president of the Association of Veterinary Hematology and Transfusion Medicine.
...
"There is no canine Red Cross. Instead, there are hospitals with in-house blood donation colonies, veterinary clinics with a roster of ad hoc donors on call and a small number of commercial blood banks, with wait lists that can stretch for months. There is also a growing community of pet owners who are signing their animals up to provide blood for other pets in need
...
"Commercial blood banks for animals began emerging in the 1980s. Some rely on “closed colonies,” a group of cats or dogs that live on site, providing blood for several years before they are put up for adoption.
"Closed colonies have been a critical source of animal blood and can be run humanely, experts said. “I know many places that have fabulous cat rooms for cat donors,” said Dr. LeVine, who adopted her previous cat, Salt, from a blood donation colony.
"But animal rights activists have also exposed mistreatment and abuse at some commercial blood banks with closed colonies, and demand far outstrips the volume of blood they can provide.
"These factors have helped fuel interest in an alternate model, which recruits local pets to become regular donors. At DoveLewis, roughly 90 dogs and 40 cats serve as regular donors, or what the hospital calls “superheroes.”
...
"Community blood banks don’t pay pet owners for blood, but they do offer other perks, which often include free veterinary exams, blood work and flea and tick preventatives. The animals are rewarded, too. At DoveLewis, donor dogs get a jar of chicken or beef baby food. “It’s just the perfect size jar of smelly meat,” said Kelsey Reinauer, the blood bank director. “And then they get to pick out a toy from our toy bucket.”
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See also
California Animal Blood Banks Program
“Historically, California required commercial blood banks for animals to be closed-colony establishments. On January 1, 2022, Assembly Bill 1282, the California Pet Blood Bank Modernization Act, went into effect. This law aims to address the shortage of animal blood available for veterinary transfusion medicine in California and transition the state from closed-colony blood banks to community blood banks.”