Monday, March 30, 2015

Toxoplasmosis in cats: background and an unusual case

A Toxoplasmosis organism in fluid from the respiratory tract

What is toxoplasmosis?

Toxoplasmosis is a disease caused by a one-celled parasite called Toxoplasma gondii, hereafter called the "toxo organism."  In humans, it may affect many different organs of the body, causing many different types of clinical signs.  The respiratory system is commonly involved and pneumonia may result.  The most common finding is a mild, flu-like illness that lasts a few days.  Most people recover uneventfully.  Even if the patient sees a physician, the illness may still be attributed to the flu unless special blood tests are run. There have been some recent studies of populations with a high rate of Toxoplasmosis infection that suggests that infection with the toxo organism may cause humans to be more likely to develop "guilt prone" neuroses and that there may be a link between Toxo and certain mental disorders such as OCD and schizophrenia. Many of the stories about this study have strongly exaggerated the strength of this link, and while there appears to be a correlation between the two, there is no evident cause.

Dirty Carrots - Image courtesy of dragonariaes
Additionally, Toxoplasmosis infection is much more likely to occur in humans due to ingestion of undercooked meat or eating or drinking substances contaminated with Toxo organisms, gardening, or eating unwashed fruits or vegetables from a garden than from infection due to sharing a home with a

How does it relate to pregnant women?

If a pregnant woman contracts toxoplasmosis just before or during pregnancy, it is possible for the toxo organism to affect the unborn baby.  It is this form of the disease that has the most dire consequences because the baby may be affected for life. Infections with Toxoplasmosis earlier in life should not cause problems with pregnancy, but any concern about this matter should be discussed with an obstetrician.

Pregnant women should not clean the litterbox if at all possible, but if they must, should wear gloves and a mask. If the litterbox is kept clean daily, the risk is lower as the Toxoplasmosis oocysts do not become infective for 1-5 days after being passed in the stool. Avoid feeding your cat raw meats, and keep your cat indoors to minimize exposure. While you do not have to get rid of your cat because you are pregnant, pregnant women should not bring new cats into the home or handle stray cats or kittens. Outdoor sandboxes should be kept covered, and any gardening should be done while wearing gloves. Avoid undercooked meats.
  • From the CDC website, the definition of "cooked meats":
  • For Whole Cuts of Meat (excluding poultry)
    • Cook to at least 145° F (63° C) as measured with a food thermometer placed in the thickest part of the meat, then allow the meat to rest* for three minutes before carving or consuming.
  • For Ground Meat (excluding poultry)
    • Cook to at least 160° F (71° C); ground meats do not require a rest* time.
  • For All Poultry (whole cuts and ground)
    • Cook to at least 165° F (74° C), and for whole poultry allow the meat to rest* for three minutes before carving or consuming.

How common is toxoplasmosis in adults?  And how common in cats?

Labwork helps diagnose Toxoplasmosis in cats and people
Exposure to the toxo organism will result in the production of antibodies.  Antibodies are the defense   The presence of antibodies means that the person or cat has been exposed; it does not necessarily mean that any disease occurred.  There are some estimates that about 50% of domestic cats in the United States have been exposed to toxoplasmosis. This does not mean that 50% of cats have an active infection with Toxoplasma. The presence of antibodies only means that exposure to the toxo organism has occurred in the past. However, the Companion Animal Parasite Council reports that the  prevalence of oocysts (infective shedding) in cats in the United States at a rate of approximately 1% In six surveys from different states in which more than 10 cats were included in all studies, oocyst shedding ranged from 0.0 to 6.6% (mean of 0.7%). In a study of over 200 cats, approximately 50 cats were infected and none of them were actively shedding infective Toxoplasmosis organisms. Positive antibody titers in cats in the U.S. range anywhere from 14% to 100% - these cats have been exposed but are non-infective.
agents of the immune system and are produced in response to immune system stimulation.

The CDC estimates that about
22.5% of the U.S. population 12 years and older have been infected with Toxoplasma. The Companion Animal Parasite Council reports that 11% of the U.S. population between the ages of 6 and 49 are seropositive (have antibodies to Toxoplasmosis). In France and Germany, approximately 80% of the population have been infected with Toxoplasmosis. In some regions of the world, particularly where the climate is hot and humid, up to 95% of the population has been infected with Toxoplasma.

How common is it in babies?

The disease toxoplasmosis occurs in about 400 to 4000 births per year in the US. About 4 million babies are born, each year - a rate of 0.01% (as few as 1-10 births out of 10,000 will be infected). Although this is indeed a real disease with dire consequences, it should be noted that its incidence is very small, especially in light of how many people have Toxoplasma antibodies.

How is it transmitted?

Although several species may develop the disease toxoplasmosis, including humans and dogs, the organism can only complete its life cycle in the domestic cat.  This means that the cat may be infected with the toxo organism and transmit it to other cats or to other species, including humans.  However, in order for this to occur the following must happen:
Toxoplasmosis life cycle

1.        The cat must be infected with the toxo organism, and most cats are not.  In order for this to occur, the cat must eat something infected with it.  It is most commonly available to the cat by ingestion of infected mice or infected raw or undercooked meats, especially pork or mutton.

2.        The cat must be shedding the toxo organism in its feces.  This occurs for only about a 10 day period.  It usually only occurs once in the cat's lifetime.  (In a few situations, the cat may shed the organism again; however, if that occurs, the number of organisms that are shed are so small that transmission is very unlikely.)

3.        The toxo organism must "incubate" in the cat's feces for 1-5 days before it is infective to humans.  This "incubation" must occur after the feces leaves the cat's body and have access to oxygen (i.e. in the litter box or in soil).

4.        The toxo organism must be swallowed by the person being infected.  It is not spread to humans through the air.

Raw or undercooked meat is a common source of Toxoplasmosis infection
The toxo organism may also be transmitted to humans by eating raw or undercooked meats, especially pork or mutton.  Since many hamburgers from fast-food restaurants are made of beef diluted with pork, most authorities feel that human infection occurs much more frequently by this method than by association with cats.  The incidence of toxo antibodies in U.S. veterinarians is not different than that of the rest of the population. None of our staff members who have been tested for Toxoplasmosis have tested positive, and we probably clean more litterboxes than all of our clients, combined!

What is involved in testing for toxoplasmosis?

We are frequently asked to test a cat that belongs to a pregnant woman for toxoplasmosis.  Pregnant women should know the following concerning toxoplasmosis testing.

1.         A screening test for toxo antibodies can be performed on both the pregnant woman and the cat.  A negative result means that the woman (and/or the cat) has not been exposed to the toxo organism.  However, it does not infer that either the woman or the cat has any immunity to toxoplasmosis in the event of a future exposure.  In fact, it means just the opposite.  Both are susceptible to infection.

2.        A single antibody titer that is positive, performed on the woman and/or the cat, means that there has been exposure to the toxo organism in the past or that there is an active infection of toxoplasmosis in progress.  In order to know which situation exists, a second test must be run 2-4 weeks later. 

a.        If the two tests give similar results, there has been an infection in the past and a certain degree of immunity exists. 

b.       If the second test is significantly higher that the first, there is a strong possibility that an active case of toxoplasmosis is in progress. 

c.        It is very important that both tests be performed by the same testing laboratory in order to properly compare results.

3. The direct means of documenting the possibility of transmission of toxoplasmosis requires that we microscopically examine a fecal sample from the cat looking for the oocysts (eggs) of the toxo organism.  Because these oocysts are very tiny (even under a microscope) and because the cat may not be shedding oocysts today but may do so in the future, multiple examinations must be done during the course of pregnancy, preferably once weekly.  This is not a very high yield procedure, meaning that it can be difficult to detect the parasites, and they can be missed by this diagnostic technique.

An unusual case of Toxoplasmosis

 In most cases of Toxoplasmosis, the cat is mildly ill and may not even produce symptoms worrisome enough for an owner to call the veterinarian. A few years ago, however, we saw a very unusual case involving Toxoplasmosis. A previously healthy, 4 year old male cat came to see us 9 months after he had a wellness exam with no concerns. He was an indoor-outdoor cat, and had previously had roundworms, so his owners brought in stool samples for checking very regularly. Most of the time, they were negative. In November, they brought T.C. to us as an emergency. He had been eating poorly and had been lethargic for a few days, but they had just found him collapsed and struggling to breathe. When they arrived at the hospital, he was unable to stand and his color was blue-gray instead of a healthy pink. His chest sounded as though it were full of fluid. He was immediately placed in an oxygen cage. He recovered enough for us to take him to the x-ray room and take one x-ray before he needed to return to the oxygen cage. His x-ray showed fluid in the chest, an enlarged heart and
Toxoplasmosis organisms inside a cell
other signs similar to congestive heart failure. Since his breathing was so labored, we were limited as to what we could do to him - he was too critical to remove from the oxygen cage for very long, even when offered an oxygen mask. Placing an oxygen mask caused him to struggle and breathe with even more difficulty. We gave him medications to try to ease his breathing and eliminate the fluid from his chest, however despite treatment, two hours after he entered the hospital, he stopped breathing and could not be resuscitated. Upon post-mortem examination, we found that he had Toxoplasmosis organisms infesting many of his major organs - heart, lungs, and kidneys. His stool sample was negative for Toxoplasmosis.

How can toxoplasmosis be prevented?

There are several practical means of preventing the transmission of toxoplasmosis. To review:

1.         Do not allow your cat to eat mice or poorly-cooked meat.  Feeding a commercial cat food and not allowing your cat outdoors virtually eliminates any possibility of the cat becoming infected.

2.         Clean all feces from your cat's litter box daily.  Even if the cat's feces is infected with toxo oocysts, they must incubate for 1-5 days before becoming infectious.  To be extra safe, do not let a pregnant woman clean the litter box.

3.         When working in soil (flower beds) that cats might use for defecation, wear gloves to keep from getting oocysts on your hands.

4.         Avoid eating raw or poorly-cooked meats.  Be especially careful of fast-food hamburgers.  Since this is probably more of a threat to your baby than your cat, special attention should be paid here.

5.         Keep children's sandboxes covered.  Outdoor cats will frequently use the sandbox for defecation.  Even if the feces are scooped out, the sandbox may remain contaminated with parasites.


1.        Toxoplasmosis that affects babies is quite rare. (Incidence in the United States is 0.028% of all births.)   It is frequently referred to as "A Ladies Home Journal Disease."   (This magazine was the first widely-read publication to link toxoplasmosis and cats.)

2.        Feeding commercial cat food and keeping your cat indoors so it cannot catch mice will prevent spread of toxoplasmosis by your cat.

3.        Having someone other than a pregnant woman clean out the litter box daily will prevent spread of toxoplasmosis by your cat.

4.        Transmission from your cat to you requires that you swallow the toxo oocysts that have incubated in your cat's feces for 1-5 days.  Reasonable personal hygiene should be adequate to prevent that from occurring.

5.        Toxoplasmosis is transmitted more commonly in the United States via poorly cooked meat than by cats.

6.        Testing your cat's blood for toxoplasma antibodies is only meaningful if a positive test is followed 2-4 weeks later with another test.

Weekly testing of your cat's feces will more directly detect a cat that is capable of transmitting toxoplasmosis.

Additional resources:
Cat Parasite Affects Human Culture 
Latent Toxoplasmosis and Human 
Prevalence of Toxoplasma gondii Infection in Feral Cats in Seoul, Korea 
Companion Animal Parasite Council Recommendations for Toxoplasma  
Toxoplasmosis in the Fetus and Newborn
CDC information on Toxoplasmosis and Pregnancy
Preventing Congenital Toxoplasmosis
Cornell Feline Health Center Information on Toxoplasmosis

International Cat Care Information on Toxoplasmosi