Toxoplasmosis (Toxoplasma gondii)

Toxoplasmosis under a microscope and a flow chart showing the infection process

Description

Toxoplasmosis is a protozoal disease caused by the parasite Toxoplasma gondii. It can be found in most warm-blooded animals, including humans, but does not usually cause disease. However, it can cause severe symptoms for animals and people with weakened immune systems or newborns if the mother was infected prior to or during pregnancy. 

Distribution

Toxoplasmosis is known to be one of the most common parasitic infections worldwide. It can infect mammals, birds, sea life such as dolphins and sea otters, and humans. The Centers for Disease Control and Prevention (CDC) estimates that more than 40 million people in the U.S. carry the parasite. Infections are more prominent in countries where eating raw meat is common.

In Michigan, toxoplasmosis has only been confirmed as a primary diagnosis and cause of death in eastern fox squirrels. However, the Michigan DNR Wildlife Disease Lab has seen it in other mammals including white-tailed deer, red fox, raccoons, skunks, porcupines, and muskrats.

Transmission and Development

Definitive hosts of the parasite (hosts that support sexual reproduction of the parasite) are restricted to the Felidae family (domestic and wild cats). Once sexual reproduction of the parasite has occurred, oocysts are shed in cat feces and become infective 1-5 days later. Intermediate hosts, such as birds and rodents, become infected from soil, vegetation, food, water, etc. in the environment that are contaminated with the oocysts. Once the oocysts are ingested, they develop into tachyzoites, which rapidly multiply and enter neural and muscle tissue to become tissue cysts known as bradyzoites. These tissue cysts can vary in size and can contain a few to hundreds of organisms. Cats then eat the intermediate hosts containing these tissue cysts and become infected, completing the life cycle.

Humans can be infected in several ways: by eating undercooked meat from animals that are infected with tissue cysts, via the fecal-oral route, transplacentally from pregnant mother to fetus, and by blood transfusions from infected persons.

Clinical Signs and Pathology

Most definitive hosts and intermediate hosts of the parasite do not exhibit any symptoms. If they do, it is likely that their immune system was compromised prior to being infected. An example of this would be young animals lacking fully formed immune systems or animals that were immunocompromised from another disease or infection. If symptoms do appear, they are typically specific to the site where tissue cysts accumulate, and can consist of fever, lethargy, cough, diarrhea, jaundice, difficulty breathing, blindness or eye problems, pneumonia, seizures, and death. It is also known to cause fetal death or abortion in sheep and goats.

People infected with Toxoplasma gondii are usually asymptomatic as well. However, infection can cause problems in pregnant women and people with compromised immune systems. Minor symptoms include flu-like symptoms, such as aches and pains, and swollen lymph nodes. Immunocompromised individuals may experience symptoms of severe toxoplasmosis, which can cause damage to the brain, eyes, and other organs. Ocular toxoplasmosis can happen in both healthy individuals and people with weakened immune systems. Symptoms of ocular toxoplasmosis include blurred or reduced vision, red eyes, and pain when light changes occur.

When women become infected with Toxoplasma gondii during pregnancy, depending on the time of infection, serious outcomes can result. Infections of the mother in the first trimester uncommonly cause fetal infections, but when these occur, they are extremely serious, and spontaneous abortion is common. The greatest risk to the fetus occurs when the mother is infected in the second trimester. Severe infections not ending in abortion may result in infants with severe birth defects such as hydrocephalus, retinal damage, small heads, neurodevelopmental deficits, seizures, and deafness. These may be immediate or develop later in life. The severity of symptoms depends on the number of bradyzoites found in tissues throughout the body. Maternal infection in the third trimester is more likely to result in fetal infections, but their course is usually inapparent clinically.

Gross pathological findings of toxoplasmosis in wildlife are restricted to pulmonary congestion and consolidation.

Histopathological findings consist of pneumonia, pulmonary congestion and edema, and necrotizing lesions of the lungs, liver, heart, and occasionally the spleen and brain. 

Diagnosis

Toxoplasmosis is primarily diagnosed by serological testing. This method detects toxoplasma antibodies in the blood, which indicates an infection occurred at one time, but the animal may not be currently infected. Postmortem diagnosis occurs through immunohistochemistry (IHC) staining of tissues from different organs (lung, heart, liver, brain, etc.) to look for the presence of antigen at the site of the tissue cysts of Toxoplasma gondii.

Treatment and Control

There are no feasible treatments for the control of Toxoplasma gondii in wildlife, other than reducing populations of feral cats to reduce the amount of infectious material in the environment. Additionally, treatment of wildlife is not necessary as the parasite has not been seen to cause die-offs in any species in Michigan and most hosts do not exhibit any symptoms of infection.

For healthy people, treatment is not usually necessary. However, for people with weakened immune systems or people suffering from symptoms, medications are available. People can avoid becoming infected by making sure meat is cooked thoroughly and hands are washed after touching raw meat. People can also avoid toxoplasmosis by keeping their cats indoors, not feeding them raw meat, and cleaning litter boxes daily as the oocysts take 1-5 days to become infectious. Pregnant women should not clean litter boxes and should avoid contact with cat feces.

Significance

In Michigan, there have not been any significant losses of wildlife species from toxoplasmosis. However, other species from around the world have been affected. For example, monk seals, an endangered species, have been known to have die-offs caused by the parasite, as have sea otters. Farmers can also suffer losses of newborn sheep and goats from toxoplasmosis.

For humans, toxoplasmosis is considered to be the number one foodborne illness in the U.S. and can cause complications for people with compromised immune systems and pregnant women and their fetuses. It is also considered to be a neglected parasitic infection by the CDC because little is done for education, surveillance, and prevention of the disease.