Primary infection
A primary infection refers to the initial occurrence of a new illness in an individual. This type of infection can affect people across all age groups, including fetuses, who may contract infections before developing their own antibodies. The immune system responds to primary infections by producing specific antibodies, which can provide lifelong immunity against certain illnesses, though some infections may lead to recurrent or chronic conditions. The risk of transmitting an illness is highest during this initial phase, particularly for individuals with weakened immune systems, who may experience more severe symptoms. Common examples of primary infections include herpes simplex and varicellovirus infections, with the former leading to recurrent symptoms and the latter causing chickenpox but potentially resulting in shingles later in life. In pregnant women, distinguishing between primary and recurrent infections is crucial, as primary infections can pose risks to the fetus. Diagnosis typically involves lab tests, but there may be delays in detecting the infection due to the incubation period. Timely diagnosis and treatment are essential for effective management, as primary infections continue to pose significant public health challenges.
Primary infection
Definition
A primary infection is the initial manifestation of a new illness. Primary infections affect people of all ages and can occur in the perinatal period, during which the fetus has not yet formed antibodies and can thus acquire infection. The human immune system responds to infection by building antibodies to a specific illness. These antibodies remain in the person’s system.
A primary infection also can cause a cascade of secondary infections, as a new infection predisposes a person to acquiring other illnesses because of that person’s now-weakened immune system. Antibiotics and antiviral medications are often prescribed to treat the primary illness; but they are also prescribed to prevent the development of further illness or secondary infection.
Symptoms
The risk of transmitting an illness to other persons is highest in the period of primary infection. Although everyone is susceptible to new viral and bacterial illnesses, persons at greatest risk for developing a new infection are those who have a compromised immune system. Many primary infections are asymptomatic, but they typically cause more severe symptoms in immunocompromised persons.
Primary infections may lead to lifelong immunity against a particular illness, may predispose a person to future recurrences, or may cause chronic infections. An example of a resulting chronic infection is one caused by the human immunodeficiency virus (HIV). Primary infection of HIV is defined as the first phase of the illness and may last for a few weeks or months. During this phase, persons are either asymptomatic or develop a rash or flu-like symptoms. The infection will eventually enter a different phase, which results in the further progression of disease symptoms.
In contrast, one primary infection that results in recurrent episodes is herpesvirus infection, which includes herpes simplex and varicellovirus. Herpes simplex viruses will recur at sporadic times with symptoms that are similar to those of the initial infection, including oral and genital herpes. Primary varicellovirus infection causes chickenpox, to which the body then develops lifelong immunity. A recurrence, however, can lead to shingles. A primary infection may present differently than recurrent or chronic infections, with some illnesses remaining stable and some becoming progressively more severe.
Primary Infections in the Perinatal Period
It is clinically important to distinguish between a primary infection and a recurrent or reactivated infection during pregnancy, as certain viral infections pose a risk to the fetus for congenital anomalies and also cause adverse pregnancy outcomes. Examples of viral infections include cytomegalovirus and toxoplasmosis. Pregnant women who have a past infection with these viruses are at a much lower risk of transmitting the illness to the fetus than are those who acquire a primary infection during pregnancy. Women who contract a primary infection during the first trimester of pregnancy are at low risk to transmit the illness to the fetus, but if the fetus is affected, it has a high likelihood of showing significant abnormal findings.
Newborns may also be diagnosed with a primary infection, either during labor and delivery or shortly after delivery. Such an illness is defined as a primary infection of the newborn. With neonatal herpes infection, for example, the risk to the newborn is greatest when a pregnant woman gives birth with a primary infection and during an outbreak of that infection during vaginal delivery.
Diagnosis
Most infections are diagnosed by routine laboratory tests, including blood work, urinalysis, and cultures. However, the primary infection may not be detected immediately after infection occurs because of an incubation period and a conversion period, the time after the initial infection in which the immune system begins responding but has not yet made the antibodies that would be detected through laboratory analysis. For example, a primary infection with HIV leads to rapid replication of the virus and the immune system’s subsequent response of a decreased white cell count. However, the HIV antibodies will not be detectable until one to three months after infection. Hence, a delayed detection time might consequently hinder the time to treat, which can be critical.
Serology screening of at-risk pregnant women may be performed to distinguish between a past or primary infection. Additionally, avidity testing may help distinguish a primary from a nonprimary infection for some viruses. Avidity testing measures the length of time passed since a person was first infected.
If a new infection is detected, prenatal diagnosis by amniocentesis is available. This invasive procedure can detect certain fetal infections by polymerase chain reaction on amniotic fluid.
Impact
The ability to rapidly diagnose and treat a primary infection is of great clinical importance. As the illness is most contagious during this time, proper education and treatment will allow for infection control. Although public health guidelines have set forth measures for prevention, such as using good hygiene techniques, practicing safer sex or abstinence, and encouraging rapid medical care, the amount of primary infections in children and adults remains high and poses a significant health care and economic burden.
Bibliography
Boskey, Elizabeth. "What Is a Secondary Infection?" VeryWell Health, 24 Aug. 2023, www.verywellhealth.com/what-is-a-secondary-infection-3132823. Accessed 4 Feb. 2025.
Crucerescu, Elena, and Diana Rodica Lovin. “Study on Specific IgG Avidity as a Tool for Recent Primary Toxoplasma gondii Infection Diagnosis.” Journal of Preventive Medicine 10 (2002): 56-62.
Khare, Manjiri. “Infectious Disease in Pregnancy.” Current Obstetrics and Gynaecology 15 (2005): 149-156.
Mandell, Gerald L., John E. Bennett, and Raphael Dolin, eds. Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases. 7th ed. New York: Churchill Livingstone/Elsevier, 2010.
Pass, Robert, et al. “Congenital Cytomegalovirus Infection Following First Trimester Maternal Infection: Symptoms at Birth and Outcome.” Journal of Clinical Virology 35 (2006): 216-220.