Horse slaughter is the final common pathway for a number of illegal, shady and/or underground activities taking place in the US and around the world. The meat of slaughtered US horses directly enters the food supply, and because these horses have not been raised to be safe for human or animal consumption, and have not passed any US food safety standards or inspections, horse slaughter has created a grave global and US public health threat. The use of international meat distributors, widespread global food fraud, lack of traceability of meats distributed throughout the world and huge imports of meat from Mexico (where studies have shown "beef" to contain as much as 100% horse meat) to the US, has put Americans at serious risk of unknowingly consuming potentially toxic and illegal horse meat. This is particularly concerning from a public health and safety perspective, because if the consumer does not realize their food is contaminated with illegal horse meat, they will not be able to determine what caused any serious side effects they experience as a result of eating toxic horse meat.
How has this situation been allowed to develop? It has evolved as a giant loophole in food safety practices and laws that has been "grandfathered" in over the last century.
Because it is very lucrative to sell horse meat that has no costly safety regulations around it as food, horse slaughter has become nearly impossible to eliminate. Demands by the public to end horse slaughter are persistently portrayed by legislators, as well as the horse and agricultural industries as being based on "humane" concerns rather than the legitimate public health concerns they represent. This "misdirection" has allowed unethical lawmakers and the horse and agricultural industry special interests to turn a blind eye to the threat that horse slaughter poses to human health and safety.
Horse slaughter as a commercial activity has been taking place since the early 1900s when US horse meat was used for dog food. During WWII, when food was scarce, Europeans (and a few Americans) consumed horse meat. In the early 1900s, very few medications or chemicals were given to horses, and food safety laws were non-existent, so this practice was not questioned. However, over the years, increasing numbers of medications began to be administered to horses, because horses are used for performance or as companions, and not intended as food. So horse meat now potentially contains over 300 drugs and chemicals banned by FDA for use in animals raised for food. In addition, horse meat may contain illegal chemicals, substances and serious diseases that there is no way of knowing about, and that are not tested for before it enters the food supply.
Deaths of dogs due to horse meat in pet food ended the sale of horse meat for pet consumption in the US in the 1970s. Today in 2016, the sale of horse meat in the US is illegal, because it does not meet any modern US food safety standards. However, the slaughter of horses in the US is not illegal, and up until 2007, US horses were slaughtered in the US and this unsafe meat was sold to foreign markets.