Toxoplasma gondii is a parasite that impacts human and animal health worldwide. Infection in humans is usually very mild, however, there can be severe or life-threatening disease in immune compromised people or pregnant women. Toxoplasma also impacts the livestock sector where it can cause abortion in sheep and goats. Foodborne transmission of T. gondii is thought to be a major source of infection in people, particularly the consumption of raw or undercooked meat. However, there is a significant lack of data on the role of retail meat in the transmission of this parasite.
We aimed to address this knowledge gap by investigating the presence of T. gondii in meat products purchased from retail outlets in Scotland, as part of work funded through the Scottish Government Strategic Research Programme (2016-2022).
Stage
Work CompletedDirectory of Expertise
Purpose
Toxoplasma gondii is a globally important parasite and the causative agent of toxoplasmosis, recognised by the World Health Organisation as one of the most important foodborne diseases worldwide. Cats shed the infectious stage of the parasite (oocysts) in their faeces after a primary infection, causing widespread environmental contamination. Meat-producing animals can become infected with T. gondii when they ingest oocysts on pasture or in contaminated feed and water, and the parasite develops into cysts in their tissues. These tissue cysts can last for the lifetime of the animal and pose a risk to humans if the meat is consumed raw or undercooked.
Eating undercooked meat is a common way to get infected with Toxoplasma and is thought to account for around half of toxoplasmosis cases. Most people with T. gondii infection will not have symptoms and may not even realise they are infected. The biggest health impact is on those with weakened immune systems or those infected from birth. For them, the infection can cause severe or life-threatening illness. Over the last 5 years, an average of 35 clinical cases of toxoplasmosis were diagnosed annually in Scotland.
Currently, the role of foodborne transmission in T. gondii infections in the UK is unknown. Therefore, as part of our work under the Scottish Government Strategic Research Programme, we have investigated the presence of T. gondii in different meat products available for human consumption from retail outlets in Scotland. We also determined the viability of parasites isolated from specific meat products to investigate the risk of infection to consumers if the meat was consumed undercooked.
Results
In total, across three individual studies, we collected 390 fresh meat products from farm shops, supermarkets, and butcher shops across Scotland. Meat types included beef, chicken, lamb, pork, and venison, and we included different cuts of meat such as steak, ground meat (e.g. sausages and burgers), breast, and chops (as well as others). A portion of each meat sample was processed and screened for T. gondii using molecular methods (PCR).
Results from the first two studies revealed that T. gondii DNA was present in 32% of venison samples, 7% of lamb samples, 5% of chicken samples, 4% of pork samples and 0% of beef samples (Figure 1).
Figure 1: Detection of Toxoplasma gondii in different meat samples purchased from retail outlets in Scotland. Parasite DNA was detected in all meat samples tested, except beef.
Of note, was the incidence of T. gondii in venison products which was much higher than any of the other meat types tested. Given the tendency for people to eat this meat undercooked, we conducted a further study to determine whether parasites were viable in this meat type and capable of causing infection.
We purchased 23 fresh venison products and detected T. gondii DNA in 5 of the products (21.7%). To assess whether the parasites we found were alive and capable of causing infection, we conducted an experiment using mice, as this is currently the only reliable method for determining parasite viability. We inoculated mice with the T. gondii-positive meat product to observe if the parasites within the meat could infect and replicate within the mice, which would indicate that they were viable and potentially harmful. This experiment was conducted in accordance with the Animals (Scientific Procedures) Act 1986 and was approved by the Moredun Research Institute's Animal and Welfare Ethical Review Body. Results of the experiment showed that the mice became infected with T. gondii (Figure 2), meaning that the parasites present in the meat products had been viable.
Figure 2: Viable Toxoplasma gondii isolated from a venison product. Parasites (arrows) were isolated from mice inoculated with venison products and cultured in cells in the laboratory.
These published results highlight the potentially important role of retail meat, specifically venison, in foodborne transmission of T. gondii.
Since it is difficult to prevent livestock consuming T. gondii oocysts on pasture, and tissue cysts of T. gondii cannot be detected at the abattoir, the main control option to reduce foodborne toxoplasmosis is the thorough cooking (65°C to 74°C) or freezing (-12°C to -20°C) of meat before consumption. However, the current guidelines can be variable and are based on old studies done with pork. Therefore, it is important that we continue this research and investigate appropriate cooking and freezing temperatures in other meat types, including venison, to make sure we can fully inform control and prevention guidelines needed. We are addressing this as part of our ongoing work in the Strategic Research Programme 2022-2027.
Benefits
Results from this work have highlighted the potential for foodborne transmission of T. gondii from retail meat and further raises awareness about the potential risks associated with consuming undercooked meat. Our work could help inform healthcare professionals on the potential sources of toxoplasmosis infection and how they consider it in their diagnosis and treatment of patients presenting with related symptoms.
The identification of viable T. gondii in venison could lead to changes in guidelines for meat consumption to mitigate the risk of toxoplasmosis, especially in vulnerable groups (e.g. pregnant women, immune compromised people). Although venison is not consumed as often as more popular meat types such as chicken or beef, it is commonly eaten undercooked potentially presenting more of a risk of infection if the meat contains T. gondii. Market research indicates that 30% of venison in the UK is eaten by individuals aged 65 and older - an age group that might have weakened immune systems, which could raise the chance of developing a severe T. gondii infection. More awareness of the risks associated with undercooked meat may help to reduce foodborne toxoplasmosis.
Documents
Related Links
Research Papers
- Detection of Toxoplasma gondii in retail meat samples in Scotland
- Detection of viable Toxoplasma gondii in retail venison