Introduction

Trichinosis (or Trichinellosis) is a disease caused by parasites from the Trichinella genus (e.g. Trichinella spiralis).¹ It is estimated that approximately 10,000 cases of Trichinellosis occur worldwide every year.¹ One of the main concerns surrounding this disease is in relation to meat, specifically how it’s been cooked.⁵ It has been well established that the main cause of the disease is the consumption of meat and meat products from animals that were hosts to the parasite.^2,6 For example, one documented case involved the consumption of a meal including black bear meat served rare to eight individuals. Of the eight, six developed trichinellosis, and two of them had only eaten the vegetables that the meat had been grilled with. For several weeks following the meal, the remaining frozen bear meat was found to still contain traces of the parasites.⁷

Trichinosis is a zoonotic disease, meaning it is transferred from animals to humans or vice versa.^2,3 In this case, the disease is first caused by roundworms (parasites), which are then transferred to humans by reservoir animals.² Some examples of roundworms that cause Trichinosis include:

• Trichinella spiralis²
• Trichinella nativa²
• Trichinella britovi²
• Trichinella pseudospiralis²

The aim of this article is to give you an understanding of Trichinosis, including its dangers and symptoms, and to explain how it can be avoided through various cooking methods and precautions.

Understanding trichinella and the risks

Reservoir animals host the pathogen (in this case, the parasite), maintaining it until it can be transmitted to another host.⁴ Here are some examples of reservoir animals that could transfer this disease to humans:

• Domestic pigs⁵
• Wild boars⁵
• Bears⁵
• Rodents⁵
• Horses⁵

To cause  Trichinosis in humans, larvae residing inside these reservoir animals would have to be transferred through eating meat. These larvae then grow into adult worms within the intestines, reproducing. The newly formed larvae penetrate through the epithelial lining of the small intestine, reaching the body’s muscles via the bloodstream (circulatory system), moving from the intestines to embed within the striated muscle tissue.^8,9,17 Once in the muscles, they become encysted. The encysted larvae can remain viable for between 5 and 10 years.¹⁷ The parasite, however, is not transferred from person to person. Outbreaks, therefore, may refer to a situation in which a group of people have all consumed Trichinella-infected undercooked meat products and all became infected themselves.⁵

As previously mentioned, there are different types of Trichinella worms, which are each found in different kinds of meat. For example, Trichinella spiralis is found globally in a range of animals, whilst T. pseudospiralis, T. nativa and T. papuae are found in birds, Arctic bears and domestic pigs, respectively. They also differ in different parts of the world, for example, from Papua New Guinea to predators and scavengers in Africa.¹⁰

The infection doesn’t always present symptoms if it is mild.¹⁰ Usually, 1-2 days following the ingestion of infected meat, symptoms begin to show.⁶ The invasion of the parasite into the intestines could cause gastrointestinal symptoms, such as diarrhoea.¹⁰ Following migration of larvae via the bloodstream into the muscle tissue, you could start feeling feverish and develop rashes.¹⁰ The migration can also lead to:

• Myalgias¹⁰ (muscle aches and pains)¹¹
• Conjunctivitis¹⁰ (also known as pink eye)¹²
• Splinter haemorrhages¹⁰ (longitudinal haemorrhages beneath the nail)¹³
• Facial and periorbital edema¹⁰ (fluid accumulation causing swelling)^14,15
• Peripheral eosinophilia¹⁰ (increased peripheral blood count)¹⁶

Other, more serious symptoms could include:

• Myocarditis¹⁰ (heart muscle inflammation)¹⁸
• Complications related to the central nervous system¹⁰
• Pneumonitis¹⁰ (lung tissue inflammation)¹⁹

Safe cooking temperatures

First of all, to simply break down the key points regarding safe cooking temperatures, consider the following:

• Check internal meat temperatures using a meat thermometer²⁰
• Meat should be cooked to a minimum of 165°F (roughly 74°C) for wild game, and ground pork meat and sausage to at least 160°F  (71°C)^20,22 
• Bear in mind that temperatures differ for different types of meat, and it’s important to research the type that is being cooked. For example, pork steak, roasts and chops require a lower temperature (145°F or 63°C), but also require a 3-minute rest time. The rest time helps to ensure that the meat juices and the inside of the meat itself are fully cooked²²
• Be careful of judging the meat by colour instead of by temperature. Keeping track of the temperature will not only help prevent overcooking (so you don’t end up with very dry meat in an attempt to kill Trichinella larvae), but it will also help prevent undercooking due to misjudgment. Cooking pork has been a topic of fear, particularly regarding the risk of Trichinosis. However, over time, it has become clear that pork doesn’t have to be overcooked to be safe. But don’t forget the importance of balance²³
• For a list of different meats and their appropriate cooking temperatures, see the foodsafety.gov table here. Consider saving it to refer to later²²

Organisations such as the World Health Organisation (WHO) and Centers for Disease Control and Prevention (CDC) collectively outline some key dos and don’ts regarding cooking potentially Trichinella-infected meat.^5,10

Alternative methods to eliminate trichinella

• It is possible to kill the Trichinella larvae by freezing cuts of pork that are less than 6 inches thick. The meat should be frozen at 5°F (-15°C) or colder for 20 days or more
• Note that freezing is not a foolproof way of killing the larvae.²⁰ Larvae found in wild game may be freeze-resistant.²¹
• The following methods do not effectively kill Trichinella larvae:
  • Curing²⁰
  • Drying²⁰
  • Smoking²⁰
  • Microwaving²⁰

The effect of curing, drying and smoking (food preservation techniques) on inactivating the Trichinella parasites is inconsistent; therefore, they are not considered reliable ways to kill the worms.²⁴

Best cooking practices

For this section, we will list some of the most important things to keep in mind whilst cooking meat, to help avoid infection with the Trichinella parasite.

• Keep raw and cooked meat separated²⁰
• Thoroughly clean any equipment used to handle/prepare meat between uses²¹

These two points will help prevent cross-contamination between meats. Remember the example from the introduction? Also, keep accompanying foods, such as vegetables, apart from uncooked meat at all times.

• When using a thermometer, insert it into the meat itself, avoiding the bone. Hit the thickest part of the meat as the centre of this will most likely take a longer time to heat up than thinner sections. Insert the thermometer through the side of the meat if the cut (e.g. pork slice) is thinner than around 2cm²⁵
• A digital thermometer is recommended.²⁵ It provides a clear picture of the temperature, so that you can read more accurately when the meat is ready (in comparison to an analogue thermometer)²⁵
• Using a thermometer is a nice, simple way to ensure the meat is cooked thoroughly. However, you might not always have one to hand. These are some visual cues that can help:
  • Check that the juices run clear - cut into the thickest part of the meat and check²⁶
  • Check the colour - the meat should not be pink and fleshy²⁶
  • Check for steam - when the meat is cut open, steam should escape if it is steaming hot²⁶

Summary

Trichinosis is now actually quite a rare disease. Regarding meat produced in the UK, there have been no human cases for 30 years.²⁷ The United States have had approximately 15 cases per year, and there have been around 10,000 cases a year globally.²⁸ Whilst uncommon, implementing safe cooking practices will be hugely beneficial to hygiene, and is good for the continued prevention of parasite infection. Most Trichinella infections are linked to undercooked wild game meat, so take extra care with meat sourced from wild animals.²⁹ It is also important for pork to be properly produced to avoid consumers getting the disease.²⁹ The Agriculture and Horticulture Development Board (AHDB) lists important risk management necessities, such as the traceability of the pigs, and the correct treatment/production and storage of feed supplies. You can read their full list here.²⁷