Many veterinarians warn against serving dogs and cats raw or undercooked pork because of Trichinella spiralis, an intestinal roundworm parasite that causes trichinosis (aka trichinellosis). Recently, we have heard that Trichinella spiralis has been virtually eradicated – or effectively diminished – from the United States pork industry. We had specific questions about these claims and so looked into the facts.
What is Trichinella spiralis?
As mentioned above, Trichinella spiralis is an intestinal roundworm (nematode).
Where is Trichinella spiralis found?
Trichinella spiralis can be found in any meat-eating mammal. Humans typically can become infected by eating infected pork or horsemeat, and wild carnivores such as foxes, cats, or bears can become infected too.
How is Trichinella spiralis spread?
Trichinella spiralis is spread by eating raw or undercooked meat that is already infected with the Trichinella cysts. Many references point to swine or wild game such as bear as potential culprits.
When ingested, stomach acid dissolves the hard shell around the Trichinella spiralis cyst and releases the worms, which then mature and reproduce. Eventually, the worms are carried to the muscles where they curl up and become enclosed in capsules (encysted). The cycle continues when these worms are consumed.
Additionally, if the larvae pass through the intestine and are excreted in feces before they undergo maturation, they may cause infection to other animals.
What symptoms are caused by Trichinella spiralis?
Trichinosis is the disease that the Trichinella spiralis parasite causes.
In dogs, symptoms are non-specific and include:
- Gastrointestinal upset
- Inflammation of the muscles
- Muscle pain
- Muscle stiffness
Humans, too, are prone to infection. Human symptoms are also non-specific and can vary depending on the parasite load.
- Eye swelling
- Aching joints
- Muscle pain
- Itchy skin
- Heart problems
- Breathing difficulties
How did the U.S. commercial pork industry eradicate, or effectively and significantly reduce the incidence of Trichinella spiralis?
Interestingly, Trichinella spiralis was not curbed in the U.S. pork industry with the use of antiparasitic drugs.
The United States Department of Agriculture (USDA) attributes the elimination of Trichinella spiralis from the pork food chain by switching hogs to grain-fed diets. However, some pork feeds in the commercial industry also contain other animal parts such as fish meal, meat meal, bone meal, animal plasma and animal fat. The U.S. Pork Center for Excellence states, “Currently, while ruminant diets can not legally contain beef or poultry by-products, there is no such restriction for swine diets. Pigs can be fed diets containing meat by-products from pigs, cattle, poultry, etc. Meat meal and meat and bone meal are made from the trimmings at harvest. These include bone, tendons, ligaments, inedible organs, cleaned entrails, and some carcass trimmings. These differ from tankage in that they do not include dried blood and are produced by a different cooking method.” Whether or not the meat meal contains muscle – where Trichinella spiralis resides in animals – is just unknown. It is probably safe to say that the muscle most likely would have been treated to kill the parasite – cooked or frozen – before being added to feed.
It should be noted that the U.S. Congress passed the Federal Swine Health Protection Act in 1980, which prohibits feeding potentially contaminated garbage to swine.
The National Pork Board mentions improved feeding practices as one reason for the reduction of Trichinella spiralis in the food chain, but also points to biosecurity. Examples of biosecurity at a commercial pork farm may include:
- Concrete floors
- One hog per pen
- Employees shower before entering and when leaving the facility
- Rodent control
The USDA does provide services in the areas of inspection, education, data collection, research and security at federal, state and local levels. An additional level of safety is a voluntary program for farmers called the U.S. Trichinae Certification Program run by the agency.
What about pastured pigs or pigs that have access to the outdoors?
One can imagine that many commercially raised pigs do not live in optimum humane conditions. Commercial pork – as defined above – probably could not be sold for example at Whole Foods, the largest natural food store in the U.S. For a product to be considered for entry into the grocery store chain, it must adhere to at least Step 1 of a 5 Step Animal Welfare Policy. Step 1 is, “No cages, no crates, no crowding.” We do know that many of pork farmers meet Step 3, “Enhanced outdoor access.”
Furthermore, grocers like Whole Foods have additional standards for pork products:
- No antibiotics — ever
- No animal byproducts in feed (vegetarian diets)
- No gestation or farrowing (birthing) crates
- Bedding required in the housing
The USDA is concerned about outdoor access since a pig has the increased potential to run into wild animals or rodents. So, the agency has established guidelines that it would like farmers and others along the food supply chain to follow.
The non-profit, Animal Welfare Approved, rebutted a preliminary study that was funded by the National Pork Board. The 2008 study looked at the incidence of trichinella in 600 free-range and commercially farmed pigs in North Carolina, Ohio and Wisconsin. Two free-range pigs were found to be carrying antibodies against Trichinella, while none of the indoor pigs tested were carrying the same antibodies. The results simply mean the free-range pigs had been exposed to the parasite presently or in the past, but did not mean the pigs definitively had a current Trichinella infection.
Animal Welfare Approved offers the following guidelines for pig farmers:
- Don’t feed uncooked waste products, table scraps or animal carcasses to pigs. This is particularly important in the case of carcasses from hunted or trapped wildlife.
- Eliminate or minimize exposure of pigs to living wildlife. In particular, create barriers which are effective in separating pigs from skunks, raccoons and other small mammals.
- Implement and maintain an effective rodent control program. Biosecurity, maintaining perimeters, baiting and trapping are all part of rodent control.
- Maintain good hygiene. Remove dead pigs as soon as they are found. Keep barns free from clutter to reduce havens for rodents, and store feed securely to prevent rodent access.
Can Trichinella spiralis be further mitigated?
To ensure meat is free of Trichinella spiralis, it should be cooked to an internal temperature of at least 145°F (63°C).
Freezing pork – but not any other type of meat – is also a viable and effective option so long as the requisite number of days and temperatures are adhered to:
- 5°F (-15°C) – 20 days
- -9.4°F (-23°C) – 10 days, or
- -22°F (-30°C) for 6 days
What are the number of reported incidents of trichinosis in the U.S?
According to the Centers for Disease Control and Prevention (CDC), reported cases of trichinosis are far less common now than in previous years. In fact, an average of 400 cases were recorded each year In the United States during the late 1940’s. Nowadays, the average is less than 20 and the source is generally wild game meat.
Number of trichinellosis cases, by source of infection and year, 2011–2015, as reported by health department
|U.S. Commercial Pork||0||1||0||0||1||2|
|Pork – Foreign Travel||1||1||1||0||1||4|
|Pork – Unspecified||2||4||1||1||1||9|
|Pork and Deer||1||0||0||0||0||1|
What about imports of swine and pork products?
U.S. imports are minimal and primarily come from Canada and the European Union, both of which have their own standards and procedures to combat Trichinella spiralis.
In 2013, the U.S. produced 118,853,000 swine compared to importing 4,957,000. For pork, the U.S. produced 105,008 metric tons of pork and only imported 389 metric tons.