Toxoplasmosis is an infection caused by the microscopic parasite Toxoplasma gondii. Transmission often occurs through the ingestion of undercooked meat or contact with contaminated soil or water. The IgG test detects Immunoglobulin G antibodies against this parasite. A positive result confirms that an individual has been exposed to T. gondii at some point in their life.
Decoding the IgG Positive Result
The presence of Immunoglobulin G (IgG) antibodies signifies that the body’s immune system has successfully mounted a defense against a past Toxoplasma gondii infection. These antibodies are produced several weeks following initial exposure. They remain detectable in the bloodstream for the remainder of a person’s life, representing immunological memory.
For the majority of otherwise healthy, non-pregnant adults, a positive IgG result is generally interpreted as an indication of long-term protection. The infection has entered a dormant, or latent, phase where the parasite exists as tissue cysts, typically in muscle or brain tissue. The immune system keeps these cysts inactive, preventing the parasite from causing active disease.
This latent infection rarely causes symptoms in an individual with a functioning immune system. The positive IgG result means the person is protected from acquiring a new, acute infection, a status often referred to as being “immune”. No specific medical treatment is usually required.
When a Positive IgG Result Requires Action
While a positive IgG result is reassuring for most people, it can become a cause for concern in specific situations where the body’s ability to manage the latent infection is compromised. The two primary groups for whom a positive IgG result necessitates immediate clinical attention are pregnant women and individuals with a compromised immune system. In these cases, the status of immunity shifts into a potential risk.
For a pregnant individual, a positive IgG result is usually protective, but if the infection occurred recently, it poses a risk to the fetus. The parasite is transmitted to the fetus only when the mother acquires a primary infection during gestation. Congenital toxoplasmosis can lead to severe developmental issues, including neurological damage and vision loss in the child.
Immunocompromised individuals, such as patients with advanced Human Immunodeficiency Virus (HIV) or organ transplant recipients, are also at high risk. Immune suppression can cause the dormant parasite cysts to reactivate. This reactivation can lead to severe, life-threatening conditions, most commonly toxoplasmic encephalitis. Patients with HIV whose CD4 cell count drops below 100–200 cells/μL are at an increased risk for this reactivation.
Determining the Timing of Infection
When a positive IgG result is detected in a high-risk patient, especially a pregnant woman, the most important subsequent step is to determine the approximate timing of the original infection. This process is crucial because the risk of fetal transmission and the severity of congenital disease are directly related to the gestational age when the mother was infected.
To address this timing, doctors often order additional serological tests, starting with Immunoglobulin M (IgM) antibodies. IgM antibodies appear rapidly, often within one to two weeks after the initial infection, and are considered a marker for recent or acute disease. However, IgM can sometimes persist for months or even years after the acute phase, which complicates the interpretation.
The most definitive method for differentiating between a very recent and an older infection is the IgG avidity test. Avidity measures the binding strength between the IgG antibodies and the parasite’s antigens. Initially, the antibodies have a low binding strength, or low avidity, but this strength matures over a period of about three to six months.
A high avidity result suggests the infection occurred more than three to five months prior to testing, ruling out a recent infection in the first trimester of pregnancy. Conversely, a low avidity result indicates a probable recent infection, which warrants further monitoring and possible intervention. This combination of IgG, IgM, and avidity testing provides a serological timeline for the patient’s exposure.
Next Steps After Diagnosis
Following the full serological workup, the next steps are guided by the patient’s risk profile and the estimated time of infection. For immunocompetent individuals with a positive IgG result and no symptoms, no treatment is usually necessary, as the infection is latent.
If an acute infection is confirmed or highly suspected in a pregnant woman, treatment is initiated to reduce the risk of transmission to the fetus. In the first and early second trimesters, the antibiotic spiramycin is often used to prevent the parasite from crossing the placenta. If the infection is confirmed in the fetus or occurs later in the pregnancy, a regimen of pyrimethamine and sulfadiazine, along with folinic acid to offset side effects, may be prescribed.
Immunocompromised patients require close monitoring because of the risk of reactivation. For patients with advanced HIV, prophylactic treatment, often with trimethoprim-sulfamethoxazole, is used to prevent the dormant infection from reactivating and causing severe disease. If a full-blown reactivation occurs, the standard treatment typically involves the combination of pyrimethamine and sulfadiazine.