Infectious Mononucleosis in Middle Age

A previously healthy 42-year-old man developed temperatures to 38.8°C, muscle and joint aches, fatigue, and loss of appetite. Within several days, he also had diarrhea and emesis. On the fourth day of his illness, he saw his physician, who made a diagnosis of gastroenteritis.

When the fever and fatigue had continued for 2 weeks, the patient went to an urgent care center. By this time, his enteric symptoms had ceased. He had no complaints of headache, sore throat, or rash. He was described as looking tired. He had a temperature of 38.2°C, blood pressure of 126/70 mm Hg, pulse of 94/min, and respirations of 18/min. He had no icterus. His oropharynx showed no injection or exudate. The examining physician did not report lymphadenopathy and judged his liver and spleen to be of normal size. There were no other notable findings. The patient again left with a diagnosis of gastroenteritis.

The next day, however, his laboratory test results came back documenting a hematocrit value of 0.37, hemoglobin level of 130 g/L, and white blood cell count of 10.0×10⁹/L, with an automated differential showing 0.24 segmented neutrophils, 0.74 lymphocytes, and 0.02 monocytes. The chemistry panel showed a total bilirubin level of 29.07 µmol/L (1.71 mg/dL), conjugated bilirubin of 17.10 µmol/L (1 mg/dL), aspartate aminotransferase of 412 U/L, alanine aminotransferase of 811 U/L, and alkaline phosphatase of 524 U/L. His renal function, cholesterol, total protein, albumin, and glucose levels were normal. Urinalysis showed bilirubin by dipstick. Blood, urine, and stool cultures were negative. A chest radiograph was interpreted as showing a possible left lower lobe infiltrate. Oral clarithromycin was prescribed, and the patient was asked to return to the center for further evaluation of his hepatitis.

Test results were negative for antihepatitis A IgM, hepatitis B surface antigen, and antihepatitis C antibody (third-generation test). A test for hepatitis C virus RNA by polymerase chain reaction was also negative. An antihepatitis B surface antibody test was positive, consistent with the patient's history of immunization for hepatitis B.

His past medical history is notable only for surgical repair of strabismus in childhood and correction of a deviated nasal septum. His only medication was occasional acetaminophen in an attempt to control his fever. He reported no history of significant alcohol use, no tobacco or other drug use, and no blood transfusion. He is single and heterosexual, identifying no risk factors for human immunodeficiency virus (HIV). He works at a hospital, where he has some contact with patients. He has traveled to the Caribbean, Central America, Europe, India, and Australia, but did not leave the United States during the year before his illness. There is no family history of liver disease.

During his third week of illness, the patient returned to the urgent care center because of continued temperatures to 38.5°C. On physical examination, he had a temperature of 37.6°C, mild oropharyngeal erythema, and enlarged tonsils. Crackles were heard at the left lung base. The physician who saw him noted unspecified hepatomegaly but could not feel a splenic edge. Rapid mononucleosis and streptococcal screens were negative. The erythrocyte sedimentation rate was 18 mm/h. The patient continued taking clarithromycin but did not improve.

More than 21 days into his illness, he made a fourth visit to the urgent care center because of persisting fevers and slight hoarseness without sore throat. His temperature was 37.7°C. His throat was again described as red with enlarged tonsils, and now was covered by a white exudate. Careful examination failed to detect significant peripheral lymphadenopathy. His breath sounds were entirely normal. His liver was felt 3 finger breadths below the costal margin; the spleen was still not palpable. Repeated tests showed a hematocrit value of 0.37, hemoglobin level of 127 g/L, and white blood cell count of 9.7×10⁹/L. This time a manual differential was requested; it showed 0.35 segmented neutrophils, 0.52 lymphocytes, 0.10 atypical lymphocytes, and 0.03 monocytes. A repeat chemistry panel revealed levels of total bilirubin of 15.39 µmol/L (0.9 mg/dL), aspartate aminotransferase of 143 U/L, alanine aminotransferase of 508 U/L, and alkaline phosphatase of 407 U/L. Mononucleosis was suspected and confirmed by a positive test result for Epstein-Barr virus (EBV) viral capsid antigen (VCA) IgM and negative test result for cytomegalovirus (CMV) IgM.

After a full month, the fevers disappeared. Six weeks after his illness began, the patient came to me at the Johns Hopkins outpatient clinic, seeking confirmation of his mononucleosis and his recovery. He felt completely restored, with normal stamina and no complaints. He was afebrile and his physical examination and laboratory test results were normal except for an alanine aminotransferase value of 43 U/L. Recent EBV infection was indicated by indirect immunofluorescence titers of anti-EBV VCA IgM greater than 1:400 (positive, >1:10), anti-EBV VCA IgG greater than 1:400 (positive, >1:10), anti-EBV early antigen (EA) IgG of 1:10 (positive, >1:10), and anti-EBV nuclear antigen (EBNA) IgG less than 1:5 (positive, >1:5).

After the patient's illness, the laboratory switched to enzyme-linked immunosorbent assay (ELISA) tests for EBV serologies. At follow-up 14 months after he recovered, repeat serologies showed a negative EBV capsid IgM (17.0 optical density [OD]; positive, >20.0 OD), positive EBV capsid IgG (369.6 OD; positive, >20.0 OD), positive EA IgG (137.5 OD; positive, >110.0 OD), and positive EBNA IgG (23.9 OD; positive, >20.0 OD). He also had negative test results for CMV IgG, toxoplasma IgG, and HIV by ELISA.

This patient illustrates some of the challenges in diagnosing a common disorder that presents in a less than common way. A completely healthy man develops a prolonged and perplexing illness marked by fever, fatigue, diarrhea, hepatitis, and pneumonitis. Only when pharyngeal exudates and atypical lymphocytes are found late in the illness is infectious mononucleosis considered. However, for older adults with primary EBV infection, this "unusual" presentation is much more typical than the symptoms usually seen in adolescents and young adults. Its protean manifestations can cause this usually pedestrian illness to be misdiagnosed.

The first genuine clinical description of infectious mononucleosis was by pathologists Sprunt and Evans in 1920.¹ They described 6 patients at The Johns Hopkins Hospital who had fevers, sore throat, and lymphadenopathy and were believed to have leukemia because of abnormal circulating lymphocytes. When the patients recovered spontaneously, however, the authors concluded that they had had an acute infection. In 1923, Downey and McKinlay² described the atypical lymphocytes in such patients, thereby helping to differentiate the condition from acute leukemia.

The next milestone was Paul and Bunnell's³ 1932 description of heterophile antibodies agglutinating sheep red blood cells; this finding firmly established infectious mononucleosis as a clinical entity, making serologic diagnosis possible. Davidsohn⁴ further refined this work by introducing guinea pig kidney adsorption of serum to distinguish mononucleosis from serum sickness. Despite a reasonably precise laboratory aid, however, the exact cause of infectious mononucleosis remained uncertain.

A clue came only in 1964, after the first human gammaherpesvirus, EBV, was isolated from African children with Burkitt lymphoma.⁵ Although later serologic studies showed that 90% of American adults had evidence of EBV infection, only in 1968 was EBV fortuitously found to be the cause of infectious mononucleosis when a previously seronegative laboratory worker handling EBV developed typical illness and then high EBV-specific antibody titers to the virus.⁶

Besides infectious mononucleosis and Burkitt lymphoma, EBV has been linked to nasopharyngeal carcinoma, a subset of Hodgkin lymphomas, HIV-related lymphoma, T-cell lymphoma, gastric carcinoma, and some smooth-muscle tumors (in immunosuppressed children). Lymphoproliferation induced by EBV can prove fatal to patients with profound immune suppression, such as people requiring potent immune suppressives for organ transplants and boys with immunodeficiency from X-linked lymphoproliferative disorder. Oral hairy leukoplakia from lytic-phase EBV infection afflicts patients with HIV infection, producing characteristic white lesions on the lateral margins of the tongue.

Two features contribute to the widespread EBV infection of populations: frequent subclinical infection (only a minority of those infected ever develop recognized symptoms of primary infection) and viral latency, with intermittent shedding of virus into oropharyngeal secretions. People may have low titers of EBV in throat washings up to 18 months after recovering from infectious mononucleosis; as many as 20% of healthy, previously infected adults also intermittently shed virus.⁷

The virus is thought to spread mostly by exchange of saliva, as might occur in kissing, but also appears to be transmitted among siblings in families.⁸ Mature adults contract EBV in the same ways as children, with about the same susceptibility. Today's patient may have been infected by his girlfriend.

Although evidence of EBV has been found in up to 90% of most populations studied, geographic and socioeconomic factors explain a wide variation in the age at which people acquire the infection.^{9 - 10} Among large, closely knit extended families in developing countries, infection may be universal before age 4 years. In the United States, antibody prevalence rates among socially disadvantaged groups are 25% to 50% by age 4 years, yet 30% to 50% of freshmen entering college are still susceptible to infection.

Although most people are infected during childhood, fewer than 10% of children with primary EBV infection develop syndromes resembling infectious mononucleosis. Therefore, typical childhood infection is subclinical. The percentage of patients with symptomatic disease increases to 20% to 70% during the teenage and young adult years. The percentage of older adults who develop infectious mononucleosis from primary infection is unknown. Only 3% to 10% of adults older than 40 years have never been infected and are therefore susceptible. In a 1969 study, only 23 of 3043 patients with heterophile-positive infectious mononucleosis were older than 40 years.¹¹

In industrialized countries, incidence rates peak in 15- to 19-year-olds (345-671 per 100,000) and fall substantially after 35 years of age (2-4 per 100,000).^{12 - 13} In the United States, rates of infectious mononucleosis can be 30 times higher among white than black children, reflecting the greater frequency of subclinical infection in black children.¹⁴ Because illness is more frequent in higher socioeconomic groups in developed countries, infectious mononucleosis can be described as a disease of the affluent 20th century. Theories for the emergence of mononucleosis include better personal hygiene and improved public sanitation, leading to exposures later in life.⁹

The biology of primary EBV infection helps explain why some people develop symptomatic disease and others do not. With a narrow cellular tropism, the virus appears first to infect nasopharyngeal epithelial cells, and, later, B cells by way of CD21 (complement-receptor type 2, CR2), which acts as a receptor for EBV. Initial infection may activate B cells, leading to proliferation, stimulation, and then production of immunoglobulin.¹⁵ Cells infected with EBV produce a polyclonal array of antibodies, which may include classic heterophile antibodies and, more rarely, clinically important antineutrophil, anti–red blood cell, and antiplatelet antibodies. Infection of B cells by EBV can also cause transformation or immortalization of the cells. Infection of B cells lets the virus pass unceasingly to progeny cells over years, evading immune clearance. The viral genome becomes an extrachromosomal plasmid replicating with each cell division.

The individual immune response to primary EBV infection is unpredictable, and probably determines whether the infection becomes symptomatic. The tendency toward symptomatic infection is thought to be explained by vigorous immune responses, which are more common in older people than in infants and children.¹⁶ The virus appears capable of replicating even in the presence of neutralizing antibodies; hence, controlling the primary infection and maintaining viral latency afterward depend critically on cell-mediated immunity.¹⁷ Abundance of atypical lymphocytes during initial EBV infection reflects the activation and clonal expansion of T cells, including cytotoxic phenotypes, rather than infection of B cells, as was once believed.^{18 - 20} Indeed, lack of significant T-cell response to EBV over the long term may beget EBV-related lymphoproliferation or tumors.

The cardinal symptoms of infectious mononucleosis are the well-known triad of fever, pharyngitis, and peripheral lymphadenopathy, especially involving the posterior cervical chains.²¹ The pharyngitis can be severe. Enlarged tonsils with exudates can obstruct the airway.

A minority of patients have splenomegaly, gastrointestinal symptoms, rash, and headache.²¹ About 5% to 10% of patients present with a macular, urticarial, or petechial rash; a few patients present with erythema nodosum. Ampicillin, often given after a mistaken diagnosis of streptococcal pharyngitis, predictably yields a luxuriant maculopapular rash in 95% to 100% of patients with EBV infectious mononucleosis.²²

Diagnostic confusion is worsened in some patients by a myriad of complications from primary EBV infection. These include hemolytic anemia, thrombocytopenia, neutropenia, and aplastic anemia, all from EBV-driven immunoglobulins; the hemophagocytic syndrome; pneumonitis, as in our patient today; and electrocardiographic abnormalities. Neurologic presentations, such as encephalitis, meningitis, cranial nerve palsies, seizures, myelitis, optic neuritis, and Guillain-Barré syndrome, can make EBV infection especially difficult to diagnose. The neurologic manifestation can be the sole presentation of illness; patients may have none of the typical clinical features of infectious mononucleosis. Finally, classic symptoms of infectious mononucleosis may be mistaken for other common diseases such as bacterial pharyngitis or tonsillitis, influenza, and gastroenteritis.²³

Deaths from primary EBV infection are rare. Most deaths result from infection in immunodeficient patients or from airway obstruction, splenic rupture, or neurologic complications.

Our patient today had no peripheral lymphadenopathy, and he developed evidence of pharyngitis only late in his illness. He had more prolonged fever, fatigue, and liver involvement than do most patients with infectious mononucleosis. How common is this sort of presentation, especially one that first looks like infectious hepatitis?

Although infectious mononucleosis is easy to diagnose when patients have the typical clinical features, primary EBV infection in adults older than 40 years is often a diagnostic challenge. These patients are much less likely than younger people to have lymphadenopathy and pharyngitis (Table 1). Almost all older patients have fevers, often persisting beyond 2 weeks. Many studies have also reported hepatitis, cholestasis, and hepatomegaly in substantial numbers of older patients.^{24 - 25} Infection with EBV has been reported to cause fulminant liver failure in adults.²⁶ The initial differential diagnosis may focus on fever of unknown origin, jaundice, hepatitis, lymphoma, leukemia, or neurologic disorders.²⁷ Older patients are more likely to be hospitalized for evaluation of these conditions, and they may undergo unnecessary tests.²⁷

Liver function abnormalities are a frequent source of misdirection, as they were in today's patient. Prominent fever, hepatomegaly, abdominal pain, and cholestasis may suggest extrahepatic biliary obstruction.²⁸ Hyperbilirubinemia affects fewer than 9% of children and adolescents with infectious mononucleosis, while up to 30% of adults older than 40 years have total bilirubin values higher than 34 µmol/L (2.0 mg/dL).^{25 ,29} However, as many as 80% of young adults with infectious mononucleosis have mild liver function abnormalities, which can help distinguish their illness from acute bacterial pharyngitis and tonsillitis.

As with our gentleman today, patients without jaundice can have transaminitis with an especially high alkaline phosphatase level. One study reports that older patients have significantly higher aspartate aminotransferase and γ-glutamyltransferase values than younger people; this may explain why so many older patients are first thought to have hepatitis A, B, or C.²⁴ Other studies suggest that values of serum transaminases and alkaline phosphatase are similar across age groups.²⁵

During primary EBV infection, the portal region of the liver has an inflammatory infiltrate containing mostly lymphocytes and immunoblasts. Little distinguishes the histopathologic changes of EBV from those of typical viral hepatitis. Few patients who present with predominant liver manifestations undergo liver biopsy because physicians order appropriate diagnostic serologic tests. However, patients who present with enigmatic hepatitis or fever of unknown origin may undergo liver biopsy, which can reveal specific evidence of EBV through in situ hybridization or polymerase chain reaction. These methods are more typically used to diagnose reactivated disease in immunodeficient patients with EBV-related lymphoproliferative syndromes.³⁰ Serologic testing remains the easiest and surest way to diagnose infectious mononucleosis.

Atypical lymphocytes in the peripheral blood are often the diagnostic tip-off to EBV infection. Figure 1 illustrates some of the morphologic features. From 10% to more than 30% of all circulating lymphocytes may be atypical, and bizarre forms can be mistaken for leukemic cells. However, atypical lymphocytes are not pathognomonic for mononucleosis. They may be found in toxoplasmosis, rubella, roseola, viral hepatitis, mumps, and CMV or acute HIV infection and in patients with drug reactions.

More than 70% of patients with infectious mononucleosis have an absolute lymphocytosis, with values peaking during the second and third weeks of illness. Total white blood cell counts average 12×10⁹/L to 18×10⁹/L, but some patients have more than 30×10⁹/L. Typically, 60% to 70% of the white blood cells are lymphocytes and monocytes.

Some patients with infectious mononucleosis, particularly older people, have few atypical lymphocytes. Older adults also have less significant lymphocytosis. In a review of EBV in older adults, 16% of patients older than 40 years had fewer than 5% atypical lymphocytes, and 23% had lymphocyte counts below 50%.²⁵

The diagnosis in today's patient may have been further delayed by the use of an automated counter and electronic differential for the complete blood cell count. These methods may not identify atypical lymphocytes. Unless an abnormality such as an elevated white blood cell count triggers a manual inspection of a blood smear, atypical cells go undetected. The physician may have to request a manual differential, especially for older patients who do not have leukocytosis or lymphocytosis detected by automated differential.

Most cases of infectious mononucleosis are confirmed by demonstration of heterophile antibodies. A heterophile antibody recognizes cells from a different species. With EBV, the heterophile antibodies were classically described as human IgM antibodies agglutinating sheep red blood cells. These antibodies do not recognize viral epitopes and appear to arise from the immunologic chaos caused by viral infection of B lymphocytes.

Heterophile antibodies usually appear within 1 week after infectious mononucleosis begins and are generally short-lived. They peak during weeks 2 to 5 but can persist at low levels for up to a year. Rather than the traditional Paul-Bunnell-Davidsohn test, which is based on differential absorption, most laboratories now use rapid tests like the Monospot, which is a latex agglutination assay using horse red blood cells as a substrate. These newer methods, using either latex agglutination or ELISA, have a sensitivity of 70% to 92% and a specificity of 96% to 100%.³¹

Even when heterophile status is determined, diagnosis of EBV may still be stymied because assays are negative in 10% of patients with infectious mononucleosis, including our patient today. Repeated testing later in the disease course sometimes yields a positive result. Most heterophile-negative patients are young children with EBV, but they may be of any age. The differential diagnosis of heterophile-negative mononucleosis includes toxoplasmosis and viral infections such as CMV, viral hepatitis, rubella, and acute HIV infection. False-positive tests are rare, occurring in patients with lymphoma, hepatitis, or autoimmune disease. For most patients with compatible clinical features, primary EBV infection can be reliably confirmed by a positive result of a Monospot or similar rapid test that correlates with classic agglutination study results. No further tests are needed.

The physician who suspects that a heterophile-negative patient has primary EBV should test for EBV-specific antibodies. Most patients already have these antibodies when they develop their first symptoms of mononucleosis (Figure 2), in part because of the lengthy incubation period of 5 to 7 weeks following exposure. Although the EBV genome can code for up to 100 proteins, only a few antigens have been used for serologic diagnosis: VCA and EA, which are expressed during the lytic phase, and EBNA, which is expressed in transformed or latently infected B cells.

Detection of VCA IgM usually confirms acute EBV infection, as this antibody wanes by 3 months after the illness begins. Although some laboratories use VCA IgM as the sole determinant of primary EBV infection, this is not recommended because other herpesviruses, especially CMV, can elicit strong IgM reactions to the cell lines expressing EBV antigens. Current recommendations suggest combining IgG and IgM VCA assays with tests for EBNA antibodies. The EBNA antibodies tend to appear later, 6 to 12 weeks after initial symptoms; therefore, the finding of this antibody early in an illness excludes a diagnosis of primary EBV infection.³²

Anti-EA antibodies have 2 subsets: diffuse anti-EA antibodies (anti-D) and restricted anti-EA antibodies (anti-R). Anti-D antibodies in infectious mononucleosis are associated with recent infection, and higher titers correlate with more severe illness. However, absence of anti-D does not exclude EBV infection, because 30% of patients never develop the antibody after infectious mononucleosis.³³ Only a rare patient with primary EBV infection has anti-R antibodies, and they have no clinical significance. High titers of anti-D are associated with nasopharyngeal carcinoma, while high titers of anti-R are found in chronic active EBV infection or Burkitt lymphoma.

In the immunocompetent person, VCA and EBNA IgG antibodies persist throughout life and mark prior infection with EBV. They do not indicate chronic active EBV infection. Chronic active EBV infection has been proposed as an explanation for chronic fatigue syndrome; however, neither high antibody titers nor isolation of EBV should suggest chronic fatigue, as both are too common to be of diagnostic value. Routine testing of EBV titers does not help in the evaluation of chronic fatigue syndrome because current data neither support nor completely disprove an association. True chronic infectious mononucleosis, which is a rare and distinct entity not to be confused with chronic fatigue syndrome, is marked by pancytopenia, chronic lymphadenopathy, interstitial pneumonitis, and persistent liver dysfunction.³⁴ The precise role of EBV in these patients remains unclear.

In the vast majority of patients, infectious mononucleosis is self-limited. Most symptoms resolve spontaneously within 3 weeks without specific treatment. The illness tends to be somewhat more protracted in older adults, such as today's patient. In patients of any age, splenomegaly and adenopathy may persist after most other symptoms resolve, but few patients need further workup if the clinical diagnosis seems secure. Patients with splenomegaly should be told to avoid heavy lifting and contact sports for 2 to 3 weeks.

Corticosteroids are generally given to patients with impending airway obstruction, profound thrombocytopenia, or hemolytic anemia. Some practitioners also give glucocorticoids to ameliorate severe constitutional symptoms, but this practice is controversial. It is not known whether corticosteroids hasten the return of normal liver function in older patients.

Acyclovir does not seem to affect either the severity or duration of infectious mononucleosis, but it does reduce oropharyngeal shedding of the virus.³⁵ In a recent study, the combination of acyclovir and prednisolone inhibited oropharyngeal EBV shedding but had no effect on duration of symptoms or development of EBV-specific cellular immunity.³⁶ Acyclovir appears to have little role in the current management of infectious mononucleosis.

Infectious mononucleosis in the older adult behaves quite differently from the typical disease in children and young adults. Many older adults have no pharyngitis or lymphadenopathy but instead display prominent fevers and liver abnormalities. About 10% of all patients with mononucleosis, especially children and older adults, have heterophile-negative disease, which must be confirmed by EBV-specific antibodies. As the 20th century draws to a close, nearly 80 years after the first description of infectious mononucleosis, primary EBV infection can be one of the easiest or hardest diseases to diagnose.