Update on Toxoplasmosis

The concept of parasites seems far removed from our modern world. It’s the kind of thing one gets eating undercooked food while on an exotic vacation, or that one might read about in the tabloids or see on a “National Geographic” special on TV. And yet, more than 800,000 people will contract toxoplasmosis — a disease caused by the single-cell parasite Toxoplasma gondii — this year in the United States alone.¹ Of those infected, many will be blinded, several hundred will die and thousands will require hospitalization.

Particularly at risk are two populations: those with weakened immune systems and unborn babies. Among the immunocompromised, encephalitis is the greatest risk, along with an unchecked spread of the disease throughout the body — which can be fatal. Unborn babies can get toxoplasmosis from their mothers (there are approximately 4,000 cases of this per year in the United States). Toxoplasmosis can lead to miscarriage, blindness, epilepsy and learning disabilities.¹

There is presently no cure for toxoplasmosis. Once infected, a person is infected for life. Still, the vast majority of people who contract it will have only a brief, mild flu-like reaction during the initial infection. And, human beings cannot pass it on (with the exception of mothers passing it to their unborn children).

What Is Toxoplasma gondii?

Toxoplasma gondii is a single-cell organism in the Sarcocystidae family. Each one is roughly 10 micrometers in length, about the same size as a typical bacteria. However, unlike bacteria, Toxoplasma gondii are eukaryotes that have nuclei, mitochondrion, ribosomes, Golgi complex and other complex organelles universal to eukaryotes.²

While Toxoplasma gondii is a near-universal parasite able to infect almost all mammals and birds, it can only sexually reproduce in cats, which are its primary host.³ All other hosts are considered secondary hosts, since they cannot reinfect others (except for a mother passing it on to her unborn child).

After Toxoplasma gondii sexually reproduces inside a cat’s body, it creates a resistant stage (known as an oocyst), which is released in the feces; when infected feces gets into soil or onto plants, it can stay there for months until it is ingested by another host.³

When a mammal or bird ingests an oocyst, the Toxoplasma gondii can reproduce asexually in two different forms: 1) tachyzoite, in which it reproduces rapidly before spreading throughout the host, or 2) bradyzoite, a dormant form that develops inside cysts in the host after the tachyzoite stage, and reproduces much more slowly.^4,5

How Is Toxoplasmosis Spread?

As mentioned, the primary method that Toxoplasma gondii spreads to new hosts is through oocysts. It can be transmitted to humans when they work in soil that has been contaminated, or by eating fruits or vegetables that have come in contact with infected soil.¹ Cleaning a pet cat’s litter box can also transmit the disease.

However, there are instances in which the tachyzoite and bradyzoite cysts can transmit the disease even without the sexual reproduction cycle limited to infection in a cat.

Unfortunately, as stated at above, Toxoplasma gondii is able to cross the placental barrier between a mother and her unborn child.⁶ It is not fully understood how the parasite breaches this protective barrier, but research is ongoing.

People can also contract the disease when they eat meat from infected livestock or poultry without fully cooking it (which kills off the Toxoplasma gondii). This is perhaps the most common way humans contract the disease, which makes transmission wholly preventable. (Cats become infected when they catch and eat a secondarily infected rodent or bird.)

Another way it can be transmitted without involving the oocysts is via organ transplant or blood transfusion from an infected donor.

Symptoms and Diagnosis

Many, perhaps most, cases of toxoplasmosis will be asymptomatic, and patients may go through life never knowing they have it. For patients who do exhibit symptoms, common ones are:⁷

• Fever
• Headache
• Body or muscle aches
• Rash
• Swollen lymph nodes

If the Toxoplasma gondii has infected the eye, symptoms may include:

• Pain in the eye
• Worsened vision
• Floaters

For patients with weakened immune systems, the infection may be more severe, and symptoms will depend on where in the body the parasite is growing. In addition, as long as a patient is immunocompromised, there is a significant risk that a latent infection can reactivate. A recurring infection can cause organ damage as it spreads throughout the body.⁸

Toxoplasmosis in the lungs can lead to breathing difficulties or cough. Toxoplasmosis that has invaded the brain can lead to encephalitis, which may manifest as confusion, reduced coordination, seizures, brain fog or muscle weakness.⁷

Any time a physician suspects exposure to Toxoplasma gondii, a blood test for specific antibodies can confirm or rule out that diagnosis. There are two antibody tests: one for an active infection and one that indicates a previous infection.

Treatment

While there is no cure for toxoplasmosis, most people will recover from their symptoms with no treatment beyond getting extra rest and drinking plenty of fluids. High-risk patients and their physicians also have several treatment options to help the body control the infection.

A patient who is pregnant and has an active toxoplasmosis infection may be prescribed pyrimethamine, sulfadiazine and/or folinic acid, which is administered to reduce the side effects of the pyrimethamine.⁹ (While pregnancy induces some manifestations of immunosuppression, one study indicated that women who have a latent case of toxoplasmosis, in which the Toxoplasma gondii is in the bradyzoite or dormant stage, do not seem statistically inclined to have a reactivation of toxoplasmosis during pregnancy.¹⁰)

After birth, a newborn will likely be treated with the same drug combination as the infected mother for a period of up to two years. Additional therapies may be used to treat any associated birth defects from the toxoplasmosis, including vision problems, physical development issues and learning disabilities.

Those with compromised immune systems will also receive the previously mentioned drugs, but the treatment period will last longer — until the patient’s condition has stabilized or they are no longer immunocompromised. Those with HIV generally will receive treatment for the rest of their lives.¹¹

Notably, there is some evidence that Toxoplasma gondii is developing resistance to pyrimethamine, which is a clear worry for researchers and physicians alike.¹²

Prevention

There is currently no vaccine for toxoplasmosis in humans or cats. (There is, however, a vaccine called Toxovax, which is only available for sheep and goats, both of which suffer high rates of miscarriage during infection.¹³)

Most methods of transmission of Toxoplasma gondii are easily prevented by using the following strategies:

• Washing hands with hot water and soap after gardening or changing a litter box, and wearing protective gloves during both activities.
• Thoroughly cleaning fruits and vegetables before consuming them.
• Cooking meat thoroughly. Using a meat thermometer, the internal temperature should be measured in the thickest part of the cut for at least three minutes as follows: meat to 145 degrees Fahrenheit, ground meat to 160 degrees Fahrenheit and poultry to 165 degrees Fahrenheit.
• Avoiding eating uncooked shellfish, since shellfish can contract Toxoplasma gondii from runoff.

In addition, the Centers for Disease Control and Prevention suggests pregnant women refrain from cleaning the litter box or adopting a new cat. People with compromised immune systems should avoid adopting a new cat as well, and keep existing pet cats indoors to prevent them from contracting ^{Toxoplasma gondii}.

The only transmission vectors that are difficult to control are maternal transmission across the placenta, organ transplant and blood transfusion.

Research

As one of the most prevalent parasitic diseases in the world — up to one-third of all human beings on the globe are infected, plus uncounted livestock and pets — that carries significant health risks for those with weakened immune systems, it might seem that there would be a fairly large number of ongoing clinical trials seeking treatments for toxoplasmosis. And yet, there are fewer than four dozen ongoing or recently completed studies listed on the U.S. Food and Drug Administration’s (FDA’s) clinical trials website.

The largest contingent of these studies were looking at faster, more accurate diagnostic tests or fine-tuning the use of the existing pharmaceutical treatments.

One study conducted nearly eight years ago in Paris examined whether shortening the period of treatment for newborns with asymptomatic congenital toxoplasmosis to three months would have any impact on the disease’s progress.¹⁴

Others were looking at whether prescribing pyrimethamine to HIV-positive patients could help prevent reoccurance of toxoplasmosis.

FDA’s clinical trials website does not track veterinary research. That is overseen by the Agriculture Department’s Center for Veterinary Biologics, which does not have a public database similar to clinicaltrials.gov. There are, however, several public databases similar to clinicaltrials.gov tracking veterinary medicine research, including one operated by the American Veterinary Medical Association (ebusiness.avma.org/aahsd/study_search.aspx¹⁵). A search of these also failed to turn up clinical trials testing a vaccine for cats.

The lack of many ongoing trials into proposed vaccines for humans or cats could be due to the fact that there have been dozens — maybe hundreds — of previous attempts, none of which have yielded success. In testing on mice, live attenuated vaccines have provoked antibody response, but have not prevented infection. MRNA, DNA and other alternative vaccines have proved similarly ineffective to date.¹⁶

Looking Ahead

The ideal development would be an affordable, effective feline vaccine. Preventing cats from contracting Toxoplasma gondii would be a critical breakthrough, because it would seriously diminish the raw numbers of Toxoplasma gondii in the environment. Without feline hosts, Toxoplasma gondii cannot continue to reproduce outside their hosts. However, it seems that researchers are still waiting for some kind of technological or theoretical advance before such a vaccine can be developed and available.

In the meantime, a significant number of people will continue to contract toxoplasmosis. Education about preventing transmission — by properly cooking food, practicing good hygiene after gardening and cleaning up after pets — is currently the most effective means of combating the spread of Toxoplasma gondii.