Ron Hines DVM PhD

Toxoplasmosis is is a very common organism – but thankfully, one that rarely causes symptoms or threatens the health of cats or humans. But when the organism that causes toxoplasmosis does produce disease, the effect can be quite devastating. All type of animals, birds and humans are susceptible.

Toxoplasmosis is caused by a single-celled microscopic protozoan parasite called Toxoplasma  gondii. It has a special relationship with cats. The majority of animals that are exposed to the organism develop a life-long immunity that keeps the parasite walled off in the body, incapable of reproducing and without the ability to ever cause visible disease. The same thing is thought to occur in humans. 

What is This Parasite’s Life Cycle?

Wild and domestic felines (cats) are the only animals in which Toxoplasma can fully live out its life cycle and in which the sexually mature forms of the parasite occur. (read here) Because of this, cats are called the definitive host of Toxoplasma. Members of the cat family are also the only true reservoir of the adult parasite in the wild. Cats play an important roll in transmitting Toxoplasma to other species including humans. But we do not know what percentage of humans obtain their exposure from their household or neighborhood cats and what percentage obtain it from exposure to raw meat, fresh vegetables and other products contaminated with the organism. In humans and all species other than cats, only non-sexual (asexual) stages of Toxoplasma occur. Raw or under-cooked, never-frozen, meat is thought to be a common way for the infection to be passed from animal to animal or animal to man.

Toxoplasma is killed quickly by heating meat and other foods to an internal temperature of 152 F/67 C) or freezing at 12 F/- 12 C. A few days in an ordinary refrigerator freezer (set at 0 F/18 C or less) is sufficient.  

The Toxoplasma organism has three life stages, tachyzoite, oocyst and cyst. (read here) After a cat eats any one of these stages that is present in an infected small mammal or bird, the cat’s intestinal lining becomes infected. Cats are unique in that they are the only species of animal that develop this intra-intestinal form of the parasite. The oocysts that are later shed from these cells pass out of the cat with its feces. This active infection usually persists only for a few weeks in the cat before it itself becomes immune to the disease. (so your vet is unlikely to find them in your cat’s stool specimen) After being passed in the feces, these oocysts mature (sporulate over 1-3 days) and become infectious for any human or animal that accidentally eats them. The sporulated oocysts are tightly encapsulated (covered) in an protective membrane that resists drying and heat. These stages survive for up to a year in damp, shaded soil.

Although it rarely happens, in an occasional cat that is host to this parasite, some of the Toxoplasma oocysts within its intestine burrow deeper within the lining where they divide into tachyzoites. These tachyzoites can then leave the cat’s intestine, spreading throughout its body, multiplying, and causing the serious, systemic or extra-intestinal phase of the disease. What is more likely to occur is a situation where the cat’s immune system produces sufficient antibody against toxoplasma so that the organisms are walled off as dormant (“sleeping”) oocysts in muscle and nerve tissue throughout its body. A few cats never produce sufficient antibody and so, chronically shed infective oocysts into their environment with their stools. Encysted (dormant but still living) parasites are thought to remain in the host cat for the remainder of that cat’s life. In all other species of animal, the intestinal, reproductive, stage is never formed. These other animals and man share only the extra-intestinal portion of the disease and, if they survive (as they usually do) , they harbor dormant cysts in their nerve tissue and muscle for the rest of their lives. In sheep, goats and cattle, toxoplasma is a significant cause of abortion and weak offspring. 

We believe that most cats become infected by eating creatures that contain these dormant toxoplasma cysts and that most other animals become infected by ingesting contaminated under-cooked meat or material contaminated with cat feces. In a few cases infection is passed from animal to animal and cat to cat, through the womb (read here) This is called congenital infection and it is quite uncommon. Even unpasteurized mother’s milk can occasionally spreads the toxoplasma organism. (read here)

What Would Be The Symptoms of Toxoplasmosis In My Cat?

Most cats that become infected with Toxoplasmosis show no clinical evidence of the disease. Only a few cats, stressed by other causes of poor health, kittens and immature adults do develop transient or even severe disease related to their infection. Cats that are positive for feline feline immunodeficiency virus (FIV) may be at a greater risk for developing toxoplasmosis. (read here)

Early in severe cases of toxoplasmosis, all species of non-cat animal can be listless and depressed. They may refuse to eat and they often run a  fever . As the disease progresses they might develop signs of lung congestion and pneumonia. The toxoplasmosis organism can invade the liver, causing yellowish gums (jaundiced) and sclera, vomiting and diarrhea. Inflammation of other body organs such as the pet’s pancreas and lymphatic tissue might also occur. (evidence of hepatitis, fever, changes in white blood cell count and increased protein in the pet’s urine might bring toxoplasmosis, among other causes, to your vet’s mind) In some cases the nervous system and eyes are attacked causing blindness, aimless meandering, walking in circles, personality changes, incoordination, seizures and loss of urine and bowel control. These central nervous system signs can be mistaken for rabies, distemper, lead or arsenic poisoning. But because it is not a disease that veterinarians often see and because symptoms are so varied, the diagnosis is often made in the laboratory after the pet or other animal has died.

How Do Veterinarians And Physicians Diagnosis Toxo?

Diagnosis can not be based on clinical signs alone. The definitive (definite) diagnosis is based on clinical signs in the presence of high or rising immunity (antibody titer) to Toxoplasma. Antibody titer is determined through laboratory analysis of your pet’s blood serum. When an animal has died from the disease, pathologists often note the microscopic presence of toxoplasma tachyzoites in muscle and nerve – confirming or establishing the diagnosis. Occasionally infected postmortem tissues are injected into white mice to reproduce the disease and confirm the diagnosis. Most recently, the development of a PCR test that zeros in on the presence of the parasites’ DNA, has displaced the more complicated tests. 

A high serum antibody titer in a healthy cat suggests that the cat had an infection at an earlier date, is not now shedding the organism and has fully recovered. Lack of antibody in a healthy cat suggests that the cat has not yet been exposed to the Toxoplasmosis. A titer that changes significantly over time suggests the infection is current or recent.

Can My Cat Or Other Pets Be Treated?

A combination of two drugs, sulfadiazine and pyrimethamine, have traditionally been used to treat Toxoplasmosis (a folic acid supplement is often given with those medications as well) However, a 2012 study which you can read here found that enrofoxacin might be a better or safer antibiotic choice. It has also been found that clindamycin is effective in treating cats with this disease. Clindamycin does not have the high incidence of side effects veterinarians see using pyrimethamine. Recent studies have shown that another antibiotic, Spiramycin, can reduce the likelihood of a fetus being infected with Toxoplasmosis while still in the womb. To the best of my knowledge, spiramycin has not been approved yet for use within the United States. 

Why Is Toxoplasmosis A Threat To Humans?

There are estimates that about half the World’s human population has been exposed to Toxoplasma at some time in their lives. Many of us received our initial exposure from consuming under-cooked meat or became contaminated with cat feces (say from playing in park sandboxes as children). In rare instances it is passed through infected dairy products. I still believe that the encysted parasites in human tissue cause no problems unless the individual becomes immunocompromized. In most of these cases no signs of the disease occur.

A problem arises when a woman becomes initially infected during pregnancy. In those cases, one half to one third of the infants produced by these pregnancies become infected while still within the womb. (Other sources give the figure at 40%). We call this congenital infection. It is particularly severe if it occurs during the first third of the pregnancy (the first trimester). Although transplacental infections (while still in the womb) can occur in these unborn children, few of these women show signs of disease themselves – they simply sero-convert (develop antibody) and trap the organisms in a fibrous capsule within their muscles tissue. Children born in these circumstances may be sick at birth or disease symptoms may occur weeks to years later. Signs of the disease, when they occur in these children, can include mental retardation, eye and nervous system disease, deafness, lung disease, fever, jaundice and rash. Toxoplasmosis during pregnancy can also result in miscarriage. People undergoing cancer chemotherapy or immunosupression subsequent to organ transplants can also break with reactivated toxoplasmosis disease long after the infection occurred. AIDS is a frequent reactivator of the infection. (read here) In these unfortunate people, signs might include heart disease, lung disease, nervous system disturbances and eye disease. Mortality in this form of the disease is high.

Can Toxoplasmosis In Cats Be Prevented?

There is no vaccine on the market to immunize cats, humans or other animals against Toxoplasmosis; although there is hope that one might eventually be developed. (read here)  You can guard against infection by cooking meat and animal products well, wearing latex gloves when handling your cat’s litter boxes, and avoiding or sanitizing areas with large cat populations. Most sources say heating to a temperature of 150° F (66° C) held for thirty minutes kills Toxoplasma oocysts in meat as will freezing (about -18°C or 0°F). But suggestions vary (read here)

If you keep cats, keep them indoors, do not let them eat wildlife or raw meat. Do not feed them unpasteurized milk or milk products. Very little information exists on how common toxoplasmosis-carrier cats are in North America. In the Netherlands, cats that are allowed to hunt rodents have the highest incidence of toxoplasmosis (35%) ; in Egypt, close to 100%.

When a woman is pregnant, special added precautions to prevent contact with toxoplasmosis oocysts might include:

1) Keeping cats out of garden soil, children’s sandboxes, flower beds and moist shady areas of your yard.

2) Wearing gloves when handling anything that may be contaminated with cat feces. Wash your hands thoroughly with soap and water when there was any possibility contamination occurred.

3) Cover children’s sand boxes and sand piles with a plastic or canvas tarpaulin when they are not being used.

4) Change your cat’s litter box frequently (daily) and let a non-pregnant member of the family do it.

5) Use boiling water and bleach to sanitize litter boxes. Remember, however, that bleach does not always kill the tough oocysts.

6) Do not serve rare or under-cooked meats or unpasteurized dairy products. Poorly cooked pork, lamb and venison seem to possess the most risk.

7) Have the woman and the cat’s Toxoplasma antibody titer checked before pregnancy. Positive steady-level titer in a healthy animal or human on two subsequent occasions suggests that they can not contract the active disease now because they did so in the past. Antibody negative cats and women can still become infected during pregnancy and so are at greater risk.

8) Do not acquire new cats, volunteer at animal shelters or attend to feral cat colonies during pregnancy.

9) Minimize the contact of pregnant women with cats.

10) Wear gloves when working in the garden or with soils.

11) Wash your hands frequently with soap and water throughout the day.

12) If pregnant in the 1st third of pregnancy and frightened by the thought of this disease, consider the PCR test mentioned earlier.

13) Over the years, I fussed over my younger female vet techs allot about this and x-ray exposure – But you need to get your personal health advice from your physician, not a veterinarian like me.

 

I Read A Popular Press Article That Claimed That There Is A Link Between  Owning A Cat, Toxoplasmosis And  Suicide – Is That True? 

I modified this article in mid 2024. The link between cat ownership and mental issues now appears to be a real – but it is highly complex and very rare. It is not even clear that toxoplasmosis has anything to do with this problem. (ask me for McGrath2024Toxocplasmosis1 & 2) R.S.H

No,  I think the authors of that particular study drew a false conclusion from the data they had collected.You read a lot of startling eye-catching reports of scientific research in the popular press. Often, a few years later, the data gets re-evaluated by another team of scientists who reach the opposite conclusion. Commonly statistical methods in the initial study were faulty, control and exposure subjects were not equal in one factor or another or an data-influencing factor was not considered.  

The toxoplasmosis organism is one of the most common and successful parasites in the World. So it was quite likely that its traces would be found in human patients no matter what was being studied. The authors of the more disturbing suicide study concluded that women with Toxoplasma infections were 54% more likely to attempt suicide than those with no exposure to toxoplasmosis. They based that conclusion on the prevalence of antibodies to toxoplasma in these women’s newborn infants. Some have logically extended that relationship to the ownership of cats. Some studies have even gone as far as to conclude that those who harbor a history of exposure to toxoplasma have more car crashes ! 

What none of these studies considered was that the psychological problems and the emotional stress that accompany depression and stress of all kinds significantly increase the risk of a toxoplasmosis relapse (stress recrudescense) or a short term rise in antibodies against toxoplasmosis even without relapse. This same phenomena can occur in relapses of malaria, a close cousin of toxoplasma. That is because of the chemical surges sudden emotional and physical stress often cause. We know that immunosupression from diseases like AIDS will reactivate Toxoplasmosis as well. As a result, antibody levels against the parasite in the women studied would have been expected to  rise. But I believe that the rise in toxoplasmosis antibody was the result of the stress – not the other way around as the authors supposed.

So I think the authors of the study that linked toxoplasmosis antibody level to self-directed destructive behavior got it backwards  – it was the stress that caused their antibodies against toxoplasmosis to rise, not the toxoplasmosis that caused the stress and self-directed trauma. I am not the only one to questions these conclusions drawn between toxoplasmosis and mental states.