To get ourselves oriented appropriately, let's start by looking at the range of hosts known to harbor T. gondii. Though the typical life cycle involves a hungry cat and a brainwashed mouse, T. gondii actually has quite a broad range of hosts. It has been known to infect a wide variety of rodents as well as birds, bats, deer, bears, sea otters, and rabbits. The prevalence of toxoplasmosis in those animals ranges between 50% and 70% in some populations. Domesticated dogs can get toxoplasmosis, but domesticated cats are much more likely to become infected. Livestock such as cattle, goats, pigs, and sheep are also target hosts for this parasite. Infection rates in livestock may have prevalence rates of 50% or higher. Humans can also become infected (and they do!). An estimated 30% of the world population is infected with T. gondii, while the same parasite infects about 22% of the U.S. population.
|Not the best resolution, but you can tell from the pictures that people may become infected in a variety of ways.|
Okay, so now let's focus on the stuff we might be eating. Like I mentioned a moment ago, this parasite can be found in many forms of livestock. I recently read an article about the prevalence of this parasite in chickens as well. That article was comparing prevalence rates of free-range chickens with those of cage-raised chickens. You would think that cage-raised chickens would have a higher prevalence since they live in closer contact with other chickens. While this is certainly true for a number of diseases that plague the poultry industry, this is not the case with toxoplasmosis. Instead, free-ranged chickens had twice the prevalence of cage-raised chickens. The theory is that free-range chickens are more likely to come into contact with wild hosts like field mice, rabbits, etc.
There are also several papers documenting the occurrence of toxoplasmosis in free-range beef, mutton, and especially in pork. Unlike with the chicken article, most of these papers were not comparing free-range animals with their caged counterparts. Instead, the authors of these papers were simply looking at prevalence. Most of the papers were reporting prevalence rates well over 50%. However, the question remains...how much of that tainted meat actually makes it to market?
A survey of supermarkets in Taipei showed the prevalence of Toxoplasma was 10% in pork and other pig products, 4% in mutton, 6% in chicken and chicken products, and 5% in beef. This survey just came out this year. Another study conducted in 2005 concluded that U.S. meat retailers also had low prevalences of viable T. gondii oocysts. (The majority of tainted meats were of pork origin.)
|This graphic came from this article. The graphic shows the relative importance |
of consumed animals in the transmission of toxoplasmosis to humans.
While these numbers aren't staggering, they are still present. Consumers, particularly pregnant or otherwise immunocompromised consumers, should be aware of the fact that they can contract this parasite even from free-range meat sources. Luckily, you can avoid getting the parasite even if you do purchase tainted meat by simply ensuring that your meats are cooked to an internal temperature of 150.8 degrees F (66 degrees C) in order to kill Toxoplasma.
Moral of the Story
While there are lots of benefits to consuming free-range meats, this does not mean that you no longer have to worry about parasites in your food. Always properly cook your burgers, gizzards, and pulled pork...especially if you are prego or if you have AIDS. Just another reason to try cultured beef when it finally hits the shelves here in 10 years or so...petri-dished meat can't contract toxoplasmosis unless some lab tech (who would shortly cease to be employed) purposefully infected a batch. Even then, the unemployed, idiotic lab tech wouldn't get anyone sick as long as consumers cooked the meat long enough!