Tomorrow, November 3rd, is One Health Day, an annual international campaign to recognize the importance of human, animal, and environmental health – all of which are interconnected. Human medicine as well as veterinary medicine often overlap, as various diseases may be zoonotic, meaning they can be transmitted between humans and animals.
Toxoplasmosis is one of these diseases. Primarily a parasitic disease of cats, humans can also be affected, especially significant to pregnant women. The Meowing Vet explains the potential health effects of Toxoplasmosis and how you can protect yourself and family from this parasitic infection.
Learn more below… 🤰🐈
What is Toxoplasmosis?
Toxoplasmosis (or “Toxo”) is a parasitic infection caused by the protozoan organism Toxoplasma gondii. Domestic cats and wild cats serve as the definitive host for this parasite, meaning it can only complete its life cycle and reproduce within a feline host. However, Toxoplasma can also infect intermediate hosts for parts of its life cycle, forming cysts within their tissues. Such intermediate hosts include birds and other mammal species, primarily mice and other rodents but also livestock, dogs, and even humans. Because Toxoplasmosis is a potentially zoonotic infection (meaning it can be transmitted to humans), people (particularly cat owners) should understand the life cycle of this parasitic disease and learn how to minimize their chances of becoming infected.
Life Cycle of Toxoplasma gondii
(image courtesy of the Centers for Disease Control and Prevention, CDC)
As explained, Toxoplasma gondii primarily affects felids, both domesticated and wild cats, and can only reproduce sexually within a feline host, resulting in oocyst production. Oocysts, similar to fertilized eggs, are passed in the infected cat’s stool for 1-2 weeks post-infection, entering and contaminating the environment (such as in soil or litter). These oocysts are initially unsporulated, meaning they’re not infective at this stage and cannot yet cause disease if consumed by an intermediate host. Several days (typically 1-5 days) after the oocyst is passed, it begins to develop into a sporulated oocyst, at which time it becomes infective if consumed by another cat or by an intermediate host which may be foraging the contaminated soil or nearby vegetation for food. The most common intermediate hosts are rodents, especially mice. Once inside the intermediate host, tachyzoites (the active feeding form of Toxoplasma) emerge from the swallowed oocyst to then form cysts in the host’s body; the parasite then enters into a dormant stage called a bradyzoite. Though the parasite cannot finish its life cycle within these intermediate hosts, it hides out in this dormant stage within the intermediate host’s tissues (primarily muscle and neural tissue), patiently biding its time until this new generation of Toxoplasma can enter another definitive cat host. But how does the parasite hope to find a new cat host?
Research studies of mice and rats infected with Toxoplasma gondii have revealed evidence that the parasite alters the behavior of these intermediate rodent hosts. Most mice and rats generally avoid potential predators, such as cats. However, rodents infected with Toxoplasma actually seek out areas marked by feline odor, increasing the likelihood that they will be captured and their infected tissues eaten by a feline host, thereby granting the mind-controlling parasite a chance to complete its life cycle within the cat. The mechanisms by which Toxoplasma alters intermediate host behavior is not fully understood but may be linked to brain lesions and inflammation in the host, by increasing levels of dopamine in the brain, or by blocking N-methyl-D-aspartic acid (NMDA) receptors in the brain which normally function to trigger feelings of fear and anxiety. Without the typical feelings of fear which develop in normal mice upon encountering the scent of a nearby cat, these infected mice do not receive a warning signal in the brain to avoid such predators and instead go seek them out, where a cat may be waiting to pounce.
Sometimes, an unsuspecting bird or mammal other than a rodent accidentally ingests contaminated soil or drinking water which contains oocysts shed into the environment from infected cat feces. Such hosts may include wild animals, farm animals, dogs, or humans. The Toxoplasma parasite also forms tissue cysts in these intermediate hosts (as it does in rodents). Because livestock, canines, and people are less likely to be eaten by a feline predator than are rodent hosts, it’s usually end of the road for these parasites. They cannot complete their life cycle and reproduce in these hosts, and they cannot be passed as oocysts in the feces of such hosts. They may eventually be destroyed by the host’s immune system or else persist as tissue cysts in a type of “suspended animation” state for the lifetime of the host. In some cases, however, they can cause the intermediate host to become sick.
Toxoplasmosis in Humans
The Centers for Disease Control and Prevention (CDC) estimates that over 40 million humans in the United States harbor Toxoplasma. Many more are affected worldwide. Most infections in otherwise healthy humans go unnoticed. The majority of infected individuals are unaware of the parasite which may cause no signs at all or only mild flu-like symptoms, including muscle aches and swollen lymph nodes which may persist for several weeks or months.
Others may not be so lucky, particularly children, the elderly, and immunocompromised people. People may be immunocompromised (meaning their immune system is not functioning normally, leaving them susceptible to infections) due to a co-existing illness (such as AIDS or certain cancers) and the use of immunosuppressive drugs (such as chemotherapy agents or medications to prevent organ transplant rejection). Toxoplasmosis infection can result in severe disease in these individuals, primarily affecting neural tissue of the brain and eyes. Signs of severe infection include ophthalmic changes, resulting in eye redness, lacrimation (excessive tear production), blurred vision, and ocular pain (particularly around bright light). If the brain is affected, behavioral changes can occur. Certain forms of schizophrenia and dementia have even been theorized to have a link to Toxoplasmosis infection. Humans with high levels of Toxoplasma gondii antibodies may also be at increased risk for developing gliomas, a type of brain tumor. Though not common, death by Toxoplasmosis can occur in some individuals. Not only is acquiring a new or acute Toxoplasma infection of concern in immunocompromised individuals, but reactivation of dormant infections that had been acquired previously (perhaps while the person was still healthy) can suddenly spring to life, causing severe signs of disease when active tachyzoites emerge from their slumberous state within tissue cysts.
Furthermore, pregnant women who acquire a new (acute) Toxoplasma infection during or just prior to pregnancy can pass the parasite to her unborn fetus, causing potentially severe problems, including miscarriage and birth defects. The mother herself may exhibit no signs of illness. Therefore, expectant mothers must take great precautions to prevent a Toxoplasmosis infection during pregnancy. The majority of newborns who became infected with Toxoplasma during gestation exhibit no clinical signs of illness initially, yet they may form signs of the disease later on. Other babies may be born with severe brain or eye problems. Mothers who were infected with Toxoplasma earlier in life are less likely to pass on the parasite to their offspring, though transmission is still possible in some cases.
How Is Toxoplasmosis Transmitted to Humans?
Cat owners and those who reside or work around felines are not the only individuals at risk of Toxoplasma infection. Because the parasitic organism may infect intermediate hosts such as food animals, the consumption of undercooked meat that contains Toxoplasma cysts in its tissue can cause infection in humans. Other modes of acquiring Toxoplasmosis are listed below.
Consuming contaminated food and water: Toxoplasmosis is considered a potentially deadly foodborne illness, and consuming contaminated meat or vegetables is the most common route for Toxoplasmosis infection, far outweighing the likelihood of becoming infected from your cat.
- Eating undercooked meat of infected animals (primarily lamb, pork, and venison)
- Drinking contaminated unpasteurized/raw milk (primarily goat milk)
- Drinking contaminated water
- Accidentally ingesting items that have become contaminated with infected cat feces (i.e. fecal-oral route of contamination): This can occur by…
- Bringing your hands to your mouth after failing to wash them thoroughly after cleaning your cat’s litterbox or gardening in soil in which your cat has defecated
- Eating vegetables without properly washing them first to remove contaminated soil
- Congenital transmission via pregnancy: expectant mothers can transmit the parasite to her unborn fetus when parasitic tachyzoites travel through the placenta
- Via infected organ transplantation or blood transfusions (rare)
How Can I Reduce My Risk of Toxoplasmosis Infection?
The following precautions should be taken to help minimize your chances of acquiring Toxoplasmosis:
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(image courtesy of the Centers for Disease Control and Prevention, CDC)
Cook meat thoroughly
- Avoid unpasteurized milk products
- Wash your hands and cooking utensils after handling and cooking raw meat
- Clean kitchen work stations and cutting boards thoroughly after preparing raw meat
- Wash raw vegetables thoroughly before consumption, and wash your hands after gardening
- Wash your hands (or your children’s hands) after petting your cat, handling your cat’s bedding, or cleaning a litterbox
- Clean your cat’s litterbox daily (this decreases the likelihood that oocysts passed in your cat’s stool will be around long enough to sporulate and become infective)
- Prevent cats from defecating in your home vegetable garden or your child’s playground sandbox (Sandboxes look like giant toilets to cats!)
- If you are pregnant or immunocompromised, have another family member or friend clean your cat’s litterbox.
How Is Toxoplasmosis Diagnosed and Treated?
(image courtesy of the Centers for Disease Control and Prevention, CDC)
Because most cases of Toxoplasmosis in humans are so mild, they are not frequently diagnosed or treated. However, if an individual’s flu-like symptoms and eye problems prompt the seeking of medical or ophthalmic attention, his or her doctor may be suspicious of Toxoplasmosis. Toxoplasma gondii is typically diagnosed via serology, in which a blood sample reveals the presence of antibodies against the Toxoplasma organism. A positive serology test may only confirm exposure to the parasite and not necessarily an active infection. However, the higher the titer of the serologic tests is, the more likely that a more recent and active infection is occurring. Other findings, including tissue biopsies that reveal tissue cysts, parasitic identification in cerebrospinal fluid (CSF), and an eye exam suggestive of Toxoplasmosis infection are also useful diagnostic tools. Congenital infections may be detected in unborn fetuses by PCR (polymerase chain reaction) identification of the parasite’s DNA in amniotic fluid.
Cases resulting in mild symptoms in otherwise healthy individuals do not typically require medication, as the person’s own immune system will help clear active illness. However, antiprotozoal drugs which kill the active Toxoplasma organism are often required in severe cases, such as those acquired by immunocompromised individuals, as well as in pregnant women. Unfortunately, this treatment is not completely curative; it is designed to kill active (tachyzoite) forms of the parasite, but these drugs will not destroy the Toxoplasma which are hiding out in cysts within tissue. These cysts do not typically cause symptoms but can potentially emerge at any time and induce another active illness.
Immunocompromised individuals are encouraged to have their Toxoplasma titer checked via serologic testing in order to know their status. Furthermore, women who are pregnant or are planning on becoming pregnant are also recommended to have their titers checked by their healthcare providers. Following a recent infection, women are advised to wait at least 6 months before becoming pregnant to reduce the chances of passing on a congential infection.
Is Toxoplasmosis Dangerous to My Cat or Other Pets?
An estimated 70% of cats worldwide are thought to be infected by Toxoplasma gondii. However, that doesn’t necessarily mean that your cat is infected. Even so, because your cat’s infection status may not be known, following the safety measures discussed previously are recommended just in case.
Most cats that become infected with Toxoplasmosis may exhibit no signs of illness, though they continue to shed the parasite. However, as in immunocompromised humans, cats with poorly functioning immune systems may also develop clinical signs of illness upon Toxoplasma infection, including ocular problems (such as uveitis, chorioretinitis, or optic neuritis) and neurologic impairment (which may manifest as wobbliness and difficulty walking). Such at-risk cats include those with other illnesses that affect the immune system, such as Feline Leukemia Virus (FeLV) or Feline Immunodeficiency Virus (FIV), or cats receiving chemotherapy drugs or other immunosuppressive therapies. In addition to neurologic and ocular signs, these cats may experience lethargy, decreased appetite and weight loss and may develop fever, gastrointestinal problems (such as vomiting and diarrhea), liver disease which can lead to ascites (fluid accumulation in the abdomen) and yellow gums or sclera of the eyes (a.k.a. icterus or jaundice), as well as respiratory problems (including difficulty breathing, or dyspnea). Young kittens are also at higher risk of infection, and pregnant cats may also pass the parasite on to their offspring, potentially leading to stillbirths or abnormalities detected in live kittens. Many kittens infected prior to birth may die prior to weaning.
Dogs are not commonly affected by Toxoplasmosis infection, but they can serve as intermediate hosts just like humans can. Most infections go unnoticed as they do not trigger signs of illness, yet some dogs may develop the same neurological side effects as do humans, including encephalitis, meningitis, or myelitis (inflammation in or around the brain or spinal cord). (Ophthalmic consequences of Toxoplasmosis are rare in dogs.) Other organs can also be affected, such as the liver, which results in hepatic impairment. Rarely, Toxoplasmosis can cause disruption of platelets, resulting in immune-mediated thrombocytopenia (IMTP). Pregnant dogs who acquire Toxoplasma infection during gestation can transmit the parasite to their unborn pups, which can result in stillbirths and birth defects. Puppies under 4 months of age that are infected with Toxoplasmosis (or Neospora caninum, a related protozoal parasite) can develop stiff-stifle syndrome, an irreversible condition in which the limbs remain rigidly extended due to joint fusion secondary to neurologic impairment. Dogs may acquire parasitic infection from consuming contaminated feline fecal material, an infected intermediate host (such as a rodent), or contaminated raw or undercooked meat. So avoid feeding your dogs raw meat and prevent access to your cat’s litterbox.
FYI: Livestock can also become ill from Toxoplasma infection, with Toxoplasmosis causing spontaneous abortions (miscarriage) and other problems in pregnant cattle and other ruminants (sheep and goats).
Routine testing for Toxoplasma in otherwise healthy cats and dogs is not typically performed in veterinary medicine, but your vet will run special tests if he or she is suspicious that your pet’s illness may be attributed to Toxoplasmosis. Cats and dogs are diagnosed with Toxoplasmosis via similar tests for humans, the most common being serologic blood testing. Other findings may accompany Toxoplasmosis, including an abnormal eye exam or bloodwork abnormalities, such as anemia (low red blood cells), thrombocytopenia (low platelets), or leukopenia (low white blood cells). In veterinary medicine, the following antiprotozoal drugs may be administered to cats or dogs infected with Toxoplasma gondii who are also displaying severe signs of illness: clindamycin, ponazuril, or pyrimethamine with sulfadiazine. In addition to supportive care (nutrition, fluid therapy to correct dehydration, etc.), these antiprotozoal drugs are administered for at least 2 weeks before the patient is reassessed.
Most newborn kittens and puppies infected with Toxoplasmosis in utero (during gestation) carry a grave prognosis. Adult animals, especially adult canines, tend to recover well. However, even with treatment, dogs and cats with severe muscular or neurologic disease do not typically fare well long-term; though they may survive their initial illness with prompt treatment, their muscle or neurologic damage tends to be permanent.
So how can you help reduce your pet’s chances of acquiring Toxoplasmosis in the first place? As mentioned, avoid feeding your pets raw meat as muscle or organ tissue may contain Toxoplasma cysts. Additionally, dissuade your pets from preying upon rodents and wild birds and prevent a rodent infestation in your home with good hygiene practices and other prevention (but be careful with rat bait and mouse poisons, many of which are toxic to pets and can deadly if accidentally ingested). Furthermore, keep your dogs and cats as healthy as possible to promote a healthy immune system to ward off Toxoplasma infection. This includes proper nutrition, routine canine and feline vaccination to prevent other diseases, and by spaying or neutering to prevent roaming behavior that can expose your pet to contaminated environmental oocytes and infected intermediate hosts.
In most scenarios, you will not need to re-home your cat if you become pregnant or immunocompromised. With just a few simple changes and more vigilant hygiene practices, you and your cat can safely co-exist. Discuss your particular needs with your local veterinarian and personal physician to develop the best possible healthcare safety plan for both yourself and your cat.
– Maranda Elswick, DVM
The Meowing Vet encourages you to speak to your personal healthcare provider for further information on how you can protect yourself and your family against Toxoplasmosis.
