Toxoplasma: ‘The Cat Parasite’

It’s widespread in Hawai‘i, threatening wildlife, pets, and humans. What can be done?

Odds are, you’re infected with a parasite that causes impaired vision, reduces decision-making, and may lead to premature death. On average, the parasite infects one in three people globally. The U.S. Centers for Disease Control and Prevention (CDC) estimates that 40 million Americans are infected, and lists it as a leading cause of death from foodborne illness in the United States.

It is one of six neglected parasitic diseases currently targeted by the CDC for public health action. Although the Hawaiʻi Department of Health is aware of the parasite and the disease it causes, there are no public health statistics available about its prevalence in Hawaiʻi. The Hawaiʻi Department of Land and Natural Resources reports that the parasite is widespread in all habitats in the main Hawaiian Islands.

Cover design by Jeremy Acpal

A Deadly Parasite

Toxoplasma gondii (T. gondii) is a single-celled protozoan which causes the disease toxoplasmosis. It is one of the most common parasitic diseases worldwide and infects nearly all warm-blooded animals–bird and mammal–including humans. Cats are the definitive (required) host of this parasite, without which it would not be able to reproduce. Cats shed Toxoplasma eggs (oocysts) in their feces. These infect secondary hosts where T. gondii asexually reproduces and—from a parasite’s perspective— hopefully gets eaten by a cat and starts the cycle again. 

An infected cat can shed millions of oocytes during the roughly two week period the infection is active. As cat feces break up, they contaminate soil, sand and water, potentially causing infections far from the original source. Oocytes in the environment can remain infective for up to a year.

Secondary hosts, also known as intermediate hosts, can be infected by direct contact with feces, ingestion of contaminated soil or water, or by eating another infected intermediate host. Intermediate hosts allow T. gondii to asexually reproduce and remain viable, in the form of cysts in host tissue, for as long as the host lives.

Intermediate hosts are more likely to get sick and even die from a T. gondii infection. This is advantageous for the parasite, as a sick bird or rodent is more likely to be caught by a cat. However, these effects are problematic in humans, domestic animals, and in a state where many native animals are already under threat of extinction. In Hawai’i, toxoplasmosis is among the top three causes of death for the endangered ‘llio holo I ka uaua (Hawaiian monk seal). It is also known to cause fatalities in native birds such as the endangered ‘alala (Hawaiian crow), the endangered nēnē (Hawaiian goose), and ʻā (red-footed booby).

Courtesy CDC

Threats to Humans

Toxoplasmosis is a zoonotic disease, meaning an animal disease that can affect people. Most healthy cats infected with T. gondii show no outward signs of disease. In cats (and other organisms, including humans) symptoms are most likely to show up in individuals with suppressed immune systems. For humans, susceptible individuals include premature infants, young children, individuals with HIV, and anyone undergoing treatment that suppresses the immune system—cancer patients, for example. For cats, a similar group is susceptible to the disease: young kittens and cats with feline leukemia virus (FeLV) or feline immunodeficiency virus (FIV). Although less studied, the susceptibility of dogs and livestock appear to follow similar patterns.

For healthy adult humans, the typical symptoms of T. gondii infection are fairly minor: fever, loss of appetite, and lethargy. Which part of the body the parasite infects and the state of your immune system can change symptoms and outcomes. The parasites form tissue cysts, most commonly in muscle, the heart, brain, and eyes. These cysts are long-lasting and may remain in place for life.

Lung infections can result in pneumonia. Eye infections can result in inflammation and scarring of the retina, abnormal pupil size and responsiveness to light, and even blindness. Infection of the nervous system (brain, spine, muscular nerves) can result in loss of coordination, seizures, increased sensitivity to touch, personality changes, increased risk-taking, difficulty chewing and swallowing, and more. Pregnant women, infants, and immune-compromised individuals are at much greater risk of poor outcomes, including death, from toxoplasmosis.  

Infections in women can be passed on to their babies during pregnancy. One in three infants born to mothers infected with toxoplasmosis during pregnancy will also be infected. If parent and child are healthy, these infections are most often asymptomatic. For the child, there is an increased risk of developing symptoms such as loss of hearing and vision, as well as intellectual disability and possibly even death from the disease.

Cats are the required host for toxoplasma. Photo by Dan Collins

Taking Action

Prevention is critical in managing this disease. Despite decades of active research, there is currently no vaccine to protect against toxoplasmosis in cats, humans, or other animals. Once infected, there is also no way to get rid of T. gondii cysts.  

Preventing toxoplasmosis requires action to reduce oocysts entering the environment. Steps are required to reduce exposure. If you are a cat owner, it’s best to keep your cat inside. Feed your pet commercially prepared, cooked foods. Don’t feed your pets uncooked meat or allow them to prey on intermediate hosts, such as rodents or birds. Keep them away from food storage and food-producing livestock.

For humans, the chance of direct exposure from the cats they live with is relatively small, if appropriate precautions are taken. Cat feces are the primary transmission path rather than exposure to cats themselves. If your cat is kept inside and fed cooked meat, it is unlikely that your cat is or will become infected. As it takes one to five days for T. gondii oocysts to become infective (they have to go through a process called sporulation first) frequent cleaning of your cat’s litterbox while wearing gloves and washing your hands should keep you safe from infection. While it was recommended that immunodeficient people and pregnant women avoid cats in the past, this is no longer considered necessary. 

For most people in the U.S., the greatest chance of infection is by eating raw meat and unwashed fruits and vegetables. Prevention here is as simple as following basic food safety recommendations around safe cooking temperatures for meat and carefully washing fruits, veggies, and herbs. Contact with soil that has been contaminated with cat feces is another path of transmission. If you are gardening, wearing gloves and washing your hands afterward will help. Encourage children to wash their hands after playing outdoors, especially in areas where cats are present.  

If your pet, cat or otherwise, does go outside, supervise it so that it doesn’t risk infection from contact with birds and rodents. 

If you don’t have a cat, you can help spread the word about how to avoid this disease and encourage people to follow prevention measures. Cook meat thoroughly before eating it. Wash your veggies. Wash your hands.  

If you want to do more, help keep cats off the street. Spay and neuter your pets. Don’t feed pets you don’t own. Support local conservation and animal welfare organizations that get pets into homes and off the street.

T. gondii is a problem for all vertebrates. People, pets, and native wildlife can all be harmed by toxoplasmosis. We can all play a role in reducing the occurrence and spread of the disease on our island.

More Information

CDC Toxoplasmosis Home Page

Hawaii Disease Control Division,have%20been%20infected%20with%20Toxoplasma.

Toxoplasmosis – HI DOH Fact Sheet

Toxoplasmosis overview – CDC
Fact Sheet

CDC Guidance for Cat Owners

Toxoplasmosis and Monk Seals – NOAA Fisheries Fact Sheet

Adopt a Shelter Animal

4EverPets – A Maui Humane Society Resource for Low Income Pet Owners

Feral Cats HI DLNR Invasive
Species Council 

John Starmer