by wayne persky
Founder and President of the Microscopic Colitis Foundation
Many of us have pets. And many of us are getting old enough, and have accumulated enough health issues, that we're inclined to be more cautious about health threats than we were when we were young, and lived as though we were invincible.
A recent Medical Xpress article caught my eye, for that very reason.1 Toxoplasma gondii is a parasite commonly found in cats, although any warm-blooded animal or bird (including humans) can be infected. The parasite causes the disease known as toxoplasmosis. You may have heard about it at some point in the past. Cats are the definitive host, meaning that cats are the host in which the parasite sexually reproduces.
Cats are ubiquitous (they're very common on farms and ranches, in cities, and in most wild areas). Many cats are pets, many are wild, and many are feral. A feral cat looks like a house cat, but has either never had any physical contact with humans, or has returned to the wild, and avoids human contact). According to Wikipedia, over two thirds of the cats in the world are feral.
How do cats become infected?
According to the Centers for Disease Control and Prevention (CDC), here's how cats become infected with toxoplasmosis (Staff, 2021, June 11):2
Cats become infected by eating infected mice and rats, birds, small animals, or by eating anything that has been contaminated by feces from another infected cat that happens to be shedding the parasite in its feces. A newly infected cat can shed the parasite (in its feces) for up to two weeks. The parasite can become infective one to five days after being passed by the cat. And the parasite can survive in the environment for many months, during which it can contaminate soil, water, fruits, vegetables, sandboxes, grass where animals graze, litter boxes, or anywhere else an infected cat may have defecated.
Note that the CDC's explanation omits certain technical details.
Although the CDC refers to cats shedding the parasite, while this is technically correct, it fails to mention certain important details, as far as the risk of infection is concerned. The actual problem lies in the fact that during that two-week window, cats shed the parasite's oocysts, which are egg-like organisms. The reason the parasite becomes infective after one to five days, is because that's the incubation period of the oocyists.
The parasite is common almost everywhere.
Because the world has a significantly large population of cats almost everywhere, means that T. gondii is present almost everywhere on the planet. The fact that cats only shed the parasite's ooncysts for a couple of weeks after becoming infected, gives the impression that an infection risk for other species is low. However, the ability of the parasite to survive and remain infectious for months, means that many species may become infected by an exposure to the cat's scat during that period., as it deteriorates, and often becomes scattered over wider areas, and contaminates grass, growing vegetables, water, and various other locations.
All warm-blooded species can become infected.
Even farm animals raised for slaughter can be infected by the parasite. It's assumed that humans can be infected by eating the meat of farm raised animals (and presumably the meat of wild animals) that have become infected (Stelzer, et al., 2019).3 Therefore, it's possible that many people may be carrying a T. gondii infection even though they've never been around a cat. These infections tend to last for a lifetime. And of course, contaminated water and litter boxes used by pet cats in homes are common sources of infection, not only for humans, but other animals and birds as well (especially if that cat litter and water are not properly disposed of).
The CDC says this is how humans become infected with toxoplasmosis (Staff, 2021, June 11).
We can become infected if any food or water (or anything else) that we ingest has been contaminated with feces from an infected cat.
We can become infected by eating raw or undercooked meat from any animal infected with Toxoplasma. Undercooked meat from pigs, lambs, and wild game probably carries the most risk.
An unborn child can become infected directly from its mother if she becomes infected before or during pregnancy.
Most cases in humans are benign.
The CDC points out that for individuals who have healthy immune systems, a Toxoplasma infection may cause no symptoms for most people, so they will be unaware that they even have an infection. If symptoms do appear, they are usually mild, similar to the symptoms of the flu. Lymph glands may be swollen and an infected individual may have muscle aches and pains that last for several weeks or so. In a few cases, eye disease (such as posterior uveitis of infectious origin) may occur (Desmettre, 2020).4
But as the CDC points out,individuals who have compromised immune systems may experience severe symptoms, the most common of which occurs in people who have an HIV infection. They may experience a headache, confusion, fever, and in more severe cases, seizures, poor coordination, and nausea with or without vomiting.
An infection causes behavior changes.
Previous studies had already established that a T. gondii infection is associated with risk-taking behavior and mental illness in humans, even including behavior such as road rage. (Desmettre, 2020) Implications regarding additional human health associations have recently been investigated by scientists in University of Colorado Boulder, University of Maryland School of Medicine, and the University of A Coruña in Spain. Their study, published in The Journals of Gerontology: Series A, positively associates a T. gondii infection with frailty in older adults (Mohyuddin, et al., 2023).5
In the study, frailty was defined as the presence of 3 or more of these symptoms — “unintentional weight loss, exhaustion, low physical activity, slowness, and weakness”And obviously, none of us are looking forward to increased frailty as we get older. Interestingly, those are also symptoms of untreated microscopic colitis (MC).
Toxoplasmosis may contribute to inflammageing.
The study also found that toxaplosmisis may be associated with inflammageing, which is defined as an age-related increase in pro-inflammatory markers, but it did not absolutely prove a connection. Could the background inflammation associated with toxoplasmosis, or a similar infection, be a possible reason why some MC patients are unable to achieve and maintain complete remission from MC, despite carefully following a non-inflammatory diet? Inflammageing is also a strong risk factor for multimorbidity (multiple diseases).
Regarding risk-taking behavior,
according to the Medical Xpress article cited above, research shows that individuals who have a T. gondii infection tend to be more likely to take chances. They may act impulsively, they are more likely to be entrepreneurial, and they're more likely to be involved in a car accident.
Even animals develop risky behavior. Rodents that are infected with T. gondii often lose their fear of cats (because they apparently become attracted to cat urine), making it easier for cats to catch them, and thereby become infected (if they aren't already infected). Leopards are a primary predator of chimpanzees, but scientists have discovered that in the wild, infected chimpanzees have been observed to become attracted to the smell of leopard urine.
Human infection rates vary.
In the U.S., roughly 11 to 15% of the population has been infected at some point or another, and research shows that infection rates increase with increasing age. Infection rates in some countries are much higher, however. In a study that involved 601 Spanish and Portuguese adults over the age of 65, an almost unbelievable 67% of the subjects in the study were found to be infected with T. gondii.
The CDC makes these suggestions for protecting ourselves from toxoplasmosis (Staff, 2021, June 11):
Change cat litter boxes daily. Toxoplasma takes more than one day to become infectious. If you are pregnant or have a weakened immune system, ask someone else to change the litter box. If this is not possible, wear disposable gloves and wash your hands thoroughly with soap and water afterwards.
Cover any outdoor sandboxes when not in use to keep cats from defecating in them.
Avoid adopting stray cats, especially kittens. Younger cats are more like to be releasing Toxoplasma in their feces.
Do not eat undercooked meat. Cook whole cuts of meat to at least 145°F (63° C) with a 3 minute rest, and ground meat and wild game to at least 160°F (71º C).
Wash all kitchen supplies (such as knives and cutting boards) that have been in contact with raw meat.
If you have a weakened immune system, it is important to talk to your health care provider about getting a blood test to determine if you have been infected with Toxoplasma.
The CDC also suggests that we can protect a pet cat from infection (and thereby reduce our own risk) by (Staff, 2021, June 11):
Feeding our cat commercial dry or canned cat food.
Never feeding our cat raw meat, because of the infection risk.
Never letting an indoor cat roam outdoors, to prevent them from eating infected small animals and birds.
So whats the bottom line, regarding this risk?
While all the foregoing tends to give the impression that escaping a toxoplasmosis infection is about as likely as winning the lottery, in reality, the risk may not be nearly that high for most of us. For one thing, a cat has to have Toxoplasma before they can pass the infection to humans, and many pet cats are never exposed to Toxoplasma, so they never develop the disease (unless they eat raw meat). But even if a cat becomes infected, it will only shed the parasite's oocysts (egg like structures) in its feces for approximately 10 days to maybe two weeks after it initially develops the disease. After that, it develops a resistance to the disease, and no longer sheds the oocysts, so the cat is no longer infectious (unless eaten).
And hopefully, none of us are eating raw ( or undercooked) meat. Toxoplasmosis is not the only disease that's capable of being transmitted when raw or undercooked meat is eaten.
Even when a cat is shedding the parasite's oocysts, so that it's infectious, touching or handling the cat normally doesn't carry a risk of infection, since the parasite's oocysts are not normally on the fur According to the Merck Veterinary Manual (Dubey, 2022, October):6
Cats generally develop immunity after the initial infection; therefore, they shed oocysts only once in their lifetime (from approximately 3 days after infection and for about 20 days thereafter).
Also, it's worth noting that cats are usually fastidious about where and how they defecate. Even in the wild, they almost always make a conscientious effort to dig a hole for their scat, and then cover it after defecating. So unless a cat is a bad shot, or it's scared off before completing the job, or the scat is disrupted later, it may decompose in the soil, and the ooncysts won't be able to cause any infections.
This dainty habit of cats probably helps to protect most farm and ranch animals raised for slaughter from becoming infected. And earthworms are cold-blooded, so they can't become infected and subsequently infect earthworm-foraging animals and birds. So hopefully we won't all catch toxoplasmosis and become as risk-taking, and frail in our old age as the research suggests.
References
1. Marshall, L. (2023, November 7). Common cat-borne parasite is positively associated with frailty in older adults. Medical Xpress, Retrieved from https://medicalxpress.com/news/2023-11-common-cat-borne-parasite-positively-frailty.html?utm_source=nwletter&utm_medium=email&utm_campaign=weekly-nwletter
2. Staff. (2021, June 11). Toxoplasmosis: An Important Message for Cat Owners. Centers for Disease Control and Prevention, retrieved from https://www.cdc.gov/parasites/toxoplasmosis/toxoplasmosis_catowners.html
3. Stelzer, S., Basso, W., Benavides Silván, J., Ortega-Mora, L. M., Maksimov, P., Gethmann, J., . . . Schares G. (2019). Toxoplasma gondii infection and toxoplasmosis in farm animals: Risk factors and economic impact. Food and Waterborne Parasitology, 15, e00037. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7033994/
4. Desmettre, T. (2020). Toxoplasmosis and behavioural changes. Journal Français d'Ophtalmologie, 43(3), e89–e93. Retrieved from https://pubmed.ncbi.nlm.nih.gov/31980266/
5. Mohyuddin, H., Laffon, B., Teixeira, J. P., Costa, S., Teixeira-Gomes, A., Pásaro, E., . . . Postolache, T. T. (2023). Toxoplasma gondii IgG Serointensity Is Positively Associated With Frailty, The Journals of Gerontology: Series A, glad228, Retrieved from https://academic.oup.com/biomedgerontology/advance-article/doi/10.1093/gerona/glad228/7334598?login=false#
6. Dubey, J. P. (2022, October). Toxoplasmosis in Cats. Merck Manual - Veterinary Manual, Retrieved from https://www.merckvetmanual.com/cat-owners/disorders-affecting-multiple-body-systems-of-cats/toxoplasmosis-in-cats.
Cats are ubiquitous (they're very common on farms and ranches, in cities, and in most wild areas). Many cats are pets, many are wild, and many are feral. A feral cat looks like a house cat, but has either never had any physical contact with humans, or has returned to the wild, and avoids human contact). According to Wikipedia, over two thirds of the cats in the world are feral.
How do cats become infected?
According to the Centers for Disease Control and Prevention (CDC), here's how cats become infected with toxoplasmosis (Staff, 2021, June 11):2
Cats become infected by eating infected mice and rats, birds, small animals, or by eating anything that has been contaminated by feces from another infected cat that happens to be shedding the parasite in its feces. A newly infected cat can shed the parasite (in its feces) for up to two weeks. The parasite can become infective one to five days after being passed by the cat. And the parasite can survive in the environment for many months, during which it can contaminate soil, water, fruits, vegetables, sandboxes, grass where animals graze, litter boxes, or anywhere else an infected cat may have defecated.
Note that the CDC's explanation omits certain technical details.
Although the CDC refers to cats shedding the parasite, while this is technically correct, it fails to mention certain important details, as far as the risk of infection is concerned. The actual problem lies in the fact that during that two-week window, cats shed the parasite's oocysts, which are egg-like organisms. The reason the parasite becomes infective after one to five days, is because that's the incubation period of the oocyists.
The parasite is common almost everywhere.
Because the world has a significantly large population of cats almost everywhere, means that T. gondii is present almost everywhere on the planet. The fact that cats only shed the parasite's ooncysts for a couple of weeks after becoming infected, gives the impression that an infection risk for other species is low. However, the ability of the parasite to survive and remain infectious for months, means that many species may become infected by an exposure to the cat's scat during that period., as it deteriorates, and often becomes scattered over wider areas, and contaminates grass, growing vegetables, water, and various other locations.
All warm-blooded species can become infected.
Even farm animals raised for slaughter can be infected by the parasite. It's assumed that humans can be infected by eating the meat of farm raised animals (and presumably the meat of wild animals) that have become infected (Stelzer, et al., 2019).3 Therefore, it's possible that many people may be carrying a T. gondii infection even though they've never been around a cat. These infections tend to last for a lifetime. And of course, contaminated water and litter boxes used by pet cats in homes are common sources of infection, not only for humans, but other animals and birds as well (especially if that cat litter and water are not properly disposed of).
The CDC says this is how humans become infected with toxoplasmosis (Staff, 2021, June 11).
We can become infected if any food or water (or anything else) that we ingest has been contaminated with feces from an infected cat.
We can become infected by eating raw or undercooked meat from any animal infected with Toxoplasma. Undercooked meat from pigs, lambs, and wild game probably carries the most risk.
An unborn child can become infected directly from its mother if she becomes infected before or during pregnancy.
Most cases in humans are benign.
The CDC points out that for individuals who have healthy immune systems, a Toxoplasma infection may cause no symptoms for most people, so they will be unaware that they even have an infection. If symptoms do appear, they are usually mild, similar to the symptoms of the flu. Lymph glands may be swollen and an infected individual may have muscle aches and pains that last for several weeks or so. In a few cases, eye disease (such as posterior uveitis of infectious origin) may occur (Desmettre, 2020).4
But as the CDC points out,individuals who have compromised immune systems may experience severe symptoms, the most common of which occurs in people who have an HIV infection. They may experience a headache, confusion, fever, and in more severe cases, seizures, poor coordination, and nausea with or without vomiting.
An infection causes behavior changes.
Previous studies had already established that a T. gondii infection is associated with risk-taking behavior and mental illness in humans, even including behavior such as road rage. (Desmettre, 2020) Implications regarding additional human health associations have recently been investigated by scientists in University of Colorado Boulder, University of Maryland School of Medicine, and the University of A Coruña in Spain. Their study, published in The Journals of Gerontology: Series A, positively associates a T. gondii infection with frailty in older adults (Mohyuddin, et al., 2023).5
In the study, frailty was defined as the presence of 3 or more of these symptoms — “unintentional weight loss, exhaustion, low physical activity, slowness, and weakness”And obviously, none of us are looking forward to increased frailty as we get older. Interestingly, those are also symptoms of untreated microscopic colitis (MC).
Toxoplasmosis may contribute to inflammageing.
The study also found that toxaplosmisis may be associated with inflammageing, which is defined as an age-related increase in pro-inflammatory markers, but it did not absolutely prove a connection. Could the background inflammation associated with toxoplasmosis, or a similar infection, be a possible reason why some MC patients are unable to achieve and maintain complete remission from MC, despite carefully following a non-inflammatory diet? Inflammageing is also a strong risk factor for multimorbidity (multiple diseases).
Regarding risk-taking behavior,
according to the Medical Xpress article cited above, research shows that individuals who have a T. gondii infection tend to be more likely to take chances. They may act impulsively, they are more likely to be entrepreneurial, and they're more likely to be involved in a car accident.
Even animals develop risky behavior. Rodents that are infected with T. gondii often lose their fear of cats (because they apparently become attracted to cat urine), making it easier for cats to catch them, and thereby become infected (if they aren't already infected). Leopards are a primary predator of chimpanzees, but scientists have discovered that in the wild, infected chimpanzees have been observed to become attracted to the smell of leopard urine.
Human infection rates vary.
In the U.S., roughly 11 to 15% of the population has been infected at some point or another, and research shows that infection rates increase with increasing age. Infection rates in some countries are much higher, however. In a study that involved 601 Spanish and Portuguese adults over the age of 65, an almost unbelievable 67% of the subjects in the study were found to be infected with T. gondii.
The CDC makes these suggestions for protecting ourselves from toxoplasmosis (Staff, 2021, June 11):
Change cat litter boxes daily. Toxoplasma takes more than one day to become infectious. If you are pregnant or have a weakened immune system, ask someone else to change the litter box. If this is not possible, wear disposable gloves and wash your hands thoroughly with soap and water afterwards.
Cover any outdoor sandboxes when not in use to keep cats from defecating in them.
Avoid adopting stray cats, especially kittens. Younger cats are more like to be releasing Toxoplasma in their feces.
Do not eat undercooked meat. Cook whole cuts of meat to at least 145°F (63° C) with a 3 minute rest, and ground meat and wild game to at least 160°F (71º C).
Wash all kitchen supplies (such as knives and cutting boards) that have been in contact with raw meat.
If you have a weakened immune system, it is important to talk to your health care provider about getting a blood test to determine if you have been infected with Toxoplasma.
The CDC also suggests that we can protect a pet cat from infection (and thereby reduce our own risk) by (Staff, 2021, June 11):
Feeding our cat commercial dry or canned cat food.
Never feeding our cat raw meat, because of the infection risk.
Never letting an indoor cat roam outdoors, to prevent them from eating infected small animals and birds.
So whats the bottom line, regarding this risk?
While all the foregoing tends to give the impression that escaping a toxoplasmosis infection is about as likely as winning the lottery, in reality, the risk may not be nearly that high for most of us. For one thing, a cat has to have Toxoplasma before they can pass the infection to humans, and many pet cats are never exposed to Toxoplasma, so they never develop the disease (unless they eat raw meat). But even if a cat becomes infected, it will only shed the parasite's oocysts (egg like structures) in its feces for approximately 10 days to maybe two weeks after it initially develops the disease. After that, it develops a resistance to the disease, and no longer sheds the oocysts, so the cat is no longer infectious (unless eaten).
And hopefully, none of us are eating raw ( or undercooked) meat. Toxoplasmosis is not the only disease that's capable of being transmitted when raw or undercooked meat is eaten.
Even when a cat is shedding the parasite's oocysts, so that it's infectious, touching or handling the cat normally doesn't carry a risk of infection, since the parasite's oocysts are not normally on the fur According to the Merck Veterinary Manual (Dubey, 2022, October):6
Cats generally develop immunity after the initial infection; therefore, they shed oocysts only once in their lifetime (from approximately 3 days after infection and for about 20 days thereafter).
Also, it's worth noting that cats are usually fastidious about where and how they defecate. Even in the wild, they almost always make a conscientious effort to dig a hole for their scat, and then cover it after defecating. So unless a cat is a bad shot, or it's scared off before completing the job, or the scat is disrupted later, it may decompose in the soil, and the ooncysts won't be able to cause any infections.
This dainty habit of cats probably helps to protect most farm and ranch animals raised for slaughter from becoming infected. And earthworms are cold-blooded, so they can't become infected and subsequently infect earthworm-foraging animals and birds. So hopefully we won't all catch toxoplasmosis and become as risk-taking, and frail in our old age as the research suggests.
References
1. Marshall, L. (2023, November 7). Common cat-borne parasite is positively associated with frailty in older adults. Medical Xpress, Retrieved from https://medicalxpress.com/news/2023-11-common-cat-borne-parasite-positively-frailty.html?utm_source=nwletter&utm_medium=email&utm_campaign=weekly-nwletter
2. Staff. (2021, June 11). Toxoplasmosis: An Important Message for Cat Owners. Centers for Disease Control and Prevention, retrieved from https://www.cdc.gov/parasites/toxoplasmosis/toxoplasmosis_catowners.html
3. Stelzer, S., Basso, W., Benavides Silván, J., Ortega-Mora, L. M., Maksimov, P., Gethmann, J., . . . Schares G. (2019). Toxoplasma gondii infection and toxoplasmosis in farm animals: Risk factors and economic impact. Food and Waterborne Parasitology, 15, e00037. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7033994/
4. Desmettre, T. (2020). Toxoplasmosis and behavioural changes. Journal Français d'Ophtalmologie, 43(3), e89–e93. Retrieved from https://pubmed.ncbi.nlm.nih.gov/31980266/
5. Mohyuddin, H., Laffon, B., Teixeira, J. P., Costa, S., Teixeira-Gomes, A., Pásaro, E., . . . Postolache, T. T. (2023). Toxoplasma gondii IgG Serointensity Is Positively Associated With Frailty, The Journals of Gerontology: Series A, glad228, Retrieved from https://academic.oup.com/biomedgerontology/advance-article/doi/10.1093/gerona/glad228/7334598?login=false#
6. Dubey, J. P. (2022, October). Toxoplasmosis in Cats. Merck Manual - Veterinary Manual, Retrieved from https://www.merckvetmanual.com/cat-owners/disorders-affecting-multiple-body-systems-of-cats/toxoplasmosis-in-cats.
