A Tale of Raptors, Cats, and People
Watching a hawk soar through a cloudless blue sky or listening in the dark of night to the distant call of an owl are not usually triggers for thinking about cat parasites, but that is the funny thing about nature—everything really is connected. It is commonly known that small mammals and songbirds serve as food sources for both raptors and free-roaming domestic cats. But perhaps it is less well known that small mammals and songbirds can also play an important role as intermediate hosts in the life cycles of parasites.
It is difficult to prevent infection without understanding a parasite’s life cycle and how it is transmitted, which includes knowing where it is present in the environment. Since some parasites are microscopic, an indirect approach for figuring out where they are (and therefore how much risk they might pose) can be accomplished by looking for the parasites in their hosts. And finding hosts is relatively easy in the case of Toxoplasma gondii because, in addition to small mammals and songbirds, this protozoan has been found to infect almost every species of mammal and bird that researchers have examined for the parasite. So let us circle back to the raptors.
New research indicates that, because of their feeding habits and broad geographic distribution, raptors may serve as good indicators of how broadly contaminated the environment is with the Toxoplasma gondii protozoan, the parasite that causes toxoplasmosis. Although infection with Toxoplasma gondii in raptors has been reported all over the world, raptors are considered largely resistant to toxoplasmosis. In this new study, conducted from 2016 to 2018, researchers collected 155 deceased carnivorous birds including various species of hawks, owls, and vultures and tested them for exposure to Toxoplasma gondii. They found that about 75 percent of the owls examined contained antibodies associated with exposure to the protozoan, 45 percent of the hawks had antibodies, and 3.8 percent of the vultures contained antibodies. Not surprisingly, none of the birds sampled in the study had clinical signs of toxoplasmosis.
What was surprising, and what captured my attention about this study, was the difference between the levels of antibodies detected between the owls, hawks, and vultures. Why were the numbers so much lower in the vultures? The majority of the birds sampled were seemingly healthy black vultures taken from the wild during nuisance wildlife control efforts, and these birds had the lowest levels of antibodies. This may have been because vultures have unique intestinal microbes that help protect them from pathogens present in the carcasses that they scavenge. However, the researchers noted that the other birds, the hawks and owls, were all taken from exotics clinics or rehabilitation centers. They speculated that the hawks and owls exposed to Toxoplasma gondii may have been more susceptible to other parasites or diseases which further weakened them and caused them to suffer various traumas, like being hit by vehicles—which was the reason that some of the sampled birds were admitted to the rehabilitation clinics. Which begs the question of whether hawks and owls in the wild would show similar or lower levels of exposure to Toxoplasma gondii if they were tested?
But what does all of this have to do with cats? As it turns out, felines, including domestic cats, are Toxoplasma gondii’s only known definitive hosts. This means that felines are the only hosts that allow the parasite to complete its life cycle through sexual reproduction. This gets a little complicated, but bear with me. There are three stages that the parasite goes through: oocysts, tachyzoites, and bradyzoites. Oocysts are produced by sexual reproduction and transmitted to the environment through cat feces—this is the resistant stage. The oocysts contaminate soil, plants, or water which is then consumed by birds or mammals. If ingested, the oocysts develop into tachyzoites which are the rapidly dividing tissue stages found in all vertebrate hosts—this is the stage that causes acute infection. The last stage is cysts containing slowly dividing bradyzoites which are found in the tissues of infected warm-blooded vertebrate hosts—this stage causes chronic infection.
A simplified example might help. A pet cat goes outside and kills and eats a mouse infected with the Toxoplasma gondii parasite, and the cat becomes infected. The cat then sheds the parasite through its feces. The parasite-laden feces then contaminate vegetation or water that another mouse or bird consumes. You can see how this cycle will continue to perpetuate itself. Now if a deer, chicken, or pig eats plants, soil, or water contaminated with the infected cat feces, then those animals can also become infected with the parasite. And if a person consumes the undercooked meat of any of those infected animals, then the person can become infected. Infection can also occur through cleaning a cat’s contaminated litterbox or by accidentally consuming contaminated soil, which is why it is always a good idea to wash fruits and vegetables before eating them and to wear gloves while gardening.
And that brings us to the people part of this tale. According to the U.S. Center for Disease Control, toxoplasmosis is considered to be a leading cause of death attributed to foodborne illness in the United States. It is estimated that more than 40 million people in the U.S. have been infected with the Toxoplasma gondii parasite. Fortunately, very few of those infected will have any symptoms or suffer any illness because their immune systems can defend them from the parasite. However, for the babies of women who become newly infected with the parasite shortly before or during pregnancy and for anyone with a compromised immune system, toxoplasmosis can have severe consequences.
So, what can be done? Besides taking the precautions mentioned above, an easy method to reduce the risk of Toxoplasma gondii infection is to simply keep those cute kitties inside. Free-ranging domestic cats, including unsupervised pet cats that are allowed outside, barn cats, “community” cats, and feral cats, kill millions of small mammals and birds each year. Keeping domestic cats inside will reduce the opportunities for the parasite to complete its life cycle and will help protect cats, wildlife, and people.
Laura Kammin is an Educational Programming Specialist with Lewis and Clark Community College. She formerly held positions at Illinois-Indiana Sea Grant, University of Illinois Extension, Prairie Rivers Network and the Illinois Natural History Survey. She received her master’s degree in wildlife ecology from the University of Illinois, Urbana-Champaign.