About IM
Introduction
Infectious mononucleosis
(IM) was first described in 1889 by Pfeiffer, when it was known as
glandular fever1. The disease is characterised by a
triad of symptoms; fever, pharyngitis and cervical lymphodenopathy
for which treatment is largely supportive with enforced bed rest
and analgesics to control the pain2. The name infectious
mononucleosis refers to the appearance of infected white blood
cells, as they appear to have a grossly distorted single nucleus,
together with an increase in the number of monocytes1.
However, diagnosis of the disease cannot be made on cell appearance
alone as other disease states can produce cells of similar
appearance such as malignant lymphoma3. Diagnosis of IM
has therefore relied upon the demonstration of IgM antibodies
directed against the causative agent, the Epstein-Barr
virus4.
Epstein-Barr
Virus
The Epstein-Barr virus (EBV)
was first described in 1964. It is a member of the herpes virus
group having the ability to immortalise human B lymphocytes. The
virus is ubiquitous, through genital and oropharyngeal secretions,
and as such, in most populations, over 95% of individuals show some
level of EBV antibody5.
The only known target cells
of EBV are B lymphocytes and epithelial cells. During the lytic
phase of the disease, the virus enters epithelial cells via the
calls CD21 surface cell receptor. Once inside the cell it inserts
itself into the nuclear genome. The virus utilises the cell genome
for synthesis of its own proteins and for replication into virons.
Replication causes a proliferation of B lymphocytes. Latency is
characterised by an increase in the number of circulating B
lymphocytes which correlates with the severity of the disease. EBV
Nucleic Antigen (EBNA) produced during latency allows T lymphocyte
cells to seek out and destroy the virus. Cytokines are produced by
the T cells in response to increased production of B lymphocytes
and it is thought that the cytokines contribute to the symptomology
of the disease5. The T cell response is so efficient in
eliminating the disease that serious complications only arise in
those with depleted immunity, and the majority of patients
seroconvert with few symptoms. A small number of infected B
lymphocytes circulate following primary infection, leading to a
permanent carrier state. Unlike other herpes viruses, EBV does not
lead to re-infection except in the
immuno-compromised6.
Epidemiology
Infectious mononucleosis can
occur at any age, but peak incidence occurs between 15 and 19 years
of age with 345-671 cases per 100,000 per year2.
Although young children may become infected with EBV, the majority
seroconvert with few or no symptoms, whereas infection of
adolescents or young adults results in the disease state in 30-75%
of cases6. In adulthood, the disease can produce severe
symptoms, although by this age the majority of the population would
have already been in contact with the virus with little or no
consequence, and so development of the disease in this age group is
less common2. EBV infection results in permanent carrier
state, but differs from other herpes viruses in that during the
latent phase of the disease there is no resurgence of viral
activity. However, transplant patients and the immuno-compromised
may show some viral reactivity following primary
infection6.
In childhood, EBV infection
is associated with low socio-economic status, poor hygiene and
crowding. For this reason, IM is less common in developing
countries as the majority of the population would have already
acquired immunity to the EBV virus during childhood. In affluent
populations, improvements in living conditions over the last few
decades has led to shift in the peak age of EBV infection, with
many more individuals not coming into contact with the virus until
adolescence, which has led to an increased incidence of infectious
mononucleosis7.
Transmission of EBV is
primarily though oropharyngeal secretions from carriers and
infected individuals and as such IM is commonly referred to as the
‘kissing disease’. The rate of shedding of EBV varies from
individual to individual, although it follows that the
immuno-suppressed shed EBV at a far greater rate. Of previously
infected individuals, 20% intermittently shed virus in saliva, and
can be recovered from infected individuals in 15-20% of attempts.
The requirement for salivary contact explains how the disease is
easily transmitted to partners but the incidence in the home
schools, hospitals, college dormitories or military barracks is
relatively low6.
Diagnosis
Although IM can develop
severe symptoms such as splenic rupture, and haemolytic anaemia,
many IM patients exhibit vague symptoms such as a fever and sore
throat that are common to many other disease states such as
that caused by cytomegalovirus, lymphoproliferative disorders or
even a common cold. In addition to the symptoms, many of the immune
responses observed in IM are also found in other disease states. As
EBV establishes life long infection in the host, identification of
IM cannot be confirmed with the presence of EBV alone. This
presents a diagnostic challenge to the physician. Diagnosis of IM
therefore relies on a combination of haematological, serological
and symptomatic assessment of the patient in order to decide on the
best course of treatment for the illness6.
Haematological indicators of
IM are the rise in white blood cell count to approximately
10-15,000 cells per mm2 in the first 2 to 3 weeks of the
disease. This leads to lymphocytosis in approximately 70% of cases.
Both B and T lymphocytes contribute to 10-30% of the characteristic
increase in atypical lymphocytes but in older patients this
increase is not as marked. Compared with normal lymphocytes, these
atypical cells are generally larger with a distorted shaped
nucleus. However, the lymphocytes are not exclusive to IM and may
be associated with other disease states such as that caused by
cytomegalovirus, viral hepatitis, measles, rubella, and drug
reactions. In addition to lymphocytosis, more than 50% of patients
develop mild thromobocytopenia6.
The most effective serology
test for IM is the demonstration of IM heterophile antibody.
Heterophile antibodies have the capacity to react with antigens
that are unrelated to the one producing the antibody response. IM
heterophile antibody will react with sheep, bovine and horse
erythrocytes but does not react with EBV specific antigens. Other
heterophile antibodies such as Forssman and serum sickness
antibodies are produced by a variety of other diseases, and these
must be differentiated from IM heterophile antibody for accurate
diagnosis8.
The presence of IM
heterophile antibodies was first demonstrated by Paul and Bunnell
in 1932 after they demonstrated the agglutination of IM heterophile
antibodies to sheep erthrocytes9. Following from this,
Davidsohn and Beer showed the need for differential absorption of
sera to remove other non-specific heterophile
antibodies10,11. Fletcher and Woolfolk showed that
antigens derived from bovine erythrocytes were more specific to the
IM heterophile antibody than erythrocytes produced from sheep or
horses12.
During the acute phase of
the illness, IM heterophile antibodies are produced in 80-90% of
cases. The IM heterophile antibodies are usually demonstrable one
week after onset of the illness, peaking at 2-4 weeks, and
declining to lower levels after 12 weeks2. One year
after the onset of the illness, IM heterophile antibodies have been
detected in patient's serum, although this is not
common4,8. If testing is carried out before sufficient
antibody is present then a false negative result can be obtained,
hence further testing is required at a later date.
In addition to this, 10-20%
of adults and 50% children fail to produce the IM heterophile
antibody. Diagnosis of IM based purely on the presence of IM
heterophile antibodies can therefore lead to false negative
results. EBV specific serology testing is therefore required in
patient's whose clinical symptoms suggest IM but no heterophile
antibody has been demonstrated2. However, these tests
are more costly than the demonstration of the IM heterophile
antibody.
EBV
Serology
Infection with EBV is
characterised by the development of specific antibodies to
antigenic components of the virus. These antigens appear at
different stages of infection and differ in lytic versus latent
infection. Antibodies to EBV antigens measured for clinical
purposes are those to viral capsid antigen (VCA), early antigens
(EA), and Epstein-Barr nuclear antigen (EBNA). EA are expressed
early in the lytic cycle, while VCA and membrane antigens are
structural proteins expressed late in the lytic cycle. EBNA is
expressed in cells that are latently infected. Antibodies to these
proteins are measured by enzyme immunoassays, indirect
immunofluorescence assays, and immunoblot
assays13,14.
VCA-IgM represents the most
useful EBV serology test in the diagnosis of acute IM. VCA-IgM is
usually measurable at symptom onset, peaks at 2-3 weeks, then
declines and becomes undetectable by 3-4 months; hence it is a good
indicator of primary infection. VCA-IgG rises shortly after symptom
onset, peaks at 2-3 months, then drops slightly but persists for
life. Antibodies to EBNA appear during convalescence and remain
present for life. Antibodies to EA appear transiently for up to 3
months during the acute phase of IM in 85% of patients. However, a
diagnosis of chronic EBV should not be based on the presence of
antibodies to EA since elevated anti-EA titres may also be found in
patient's with other diseases as well as healthy individuals with
past EBV infections13,14.
The most efficient and cost
effective method of diagnosing IM in the early stages of the
disease is therefore through demonstrating the presence of IM
heterophile antibodies. Clearview IM uses a
glycoprotein from bovine erythrocytes and is therefore highly
specific, requiring no pre-treatment of the specimen and producing
clear, unambiguous results.
References
- Davidsohn I. (1937) Serological Diagnosis of Infectious
Mononucleosis: JAMA. 108(4): 289-295.
- Bailey R.E. (1994) Diagnosis and treatment of infectious
mononucleosis. Am Fam Physician. 49(4), 879-88.
- Plumbley J.A., Fan H., Eagan P.A., Ehsan A., Schnitzer B. &
Gulley M.L. (2002) Lymphoid tissues from patients with infectious
mononucleosis lack monoclonal B and T cells. J Mol Diagn. 4(1):
37-43.
- Elgh F. & Linderholm M. (1996) Evaluation of 6 commercially
available kits using heterophile anigen for the rapid diagnosis of
Infectious Mononucleosis compared with Epstein-Barr virus specific
serology. Clin. Diag. Virol. 7, 17-21.
- Andersson J. (2000) An Overview of Epstein-Barr Virus from
Discovery to Future Directions for Treatment and Prevention:
Herpes. 7(3): 76-82.
- Moffat L.E. (2001) Infectious mononucleosis. Prim Care Update
Ob/Gyns. 8: 73-77
- Morris M.C. & Edmunds W.J. (2002) The Changing Epidemiology
of Infectious Mononucelosis. J Infect. 45, 107-132.
- Gray J.J., Caldwell J. & Sills M. (1991) The rapid
serological diagnosis of infectious mononucleosis. J Infect. 25,
39-46.
- Paul J.R. & Burnell W.W. (1932) The Presence of Heterophile
Antibodies in Infectious Mononucleosis. Am. J. Med. Sci. 183,
90-104.
- Davidsohn I. (1937) Seriological Diagnosis of Infectious
Mononecleosis. JAMA. 108, 289-295.
- Beer P. (1936) The Heterophile Antibodies in Infectious
Mononucleosis & After the Injection of Serum. J. Clin. Invest.
15, 591-599.
- Fletcher M.A., Woolfolk B.J. (1971) Immunochemical Studies of
Infectious Mononucleosis: 1 Isolation & Characterisation of
Heterophile Antigens from Haemoglobin-Free Stroma. J. Immunol. 107,
842-853.
- Field P. R. & Dwyer D. E. (1996) Difficulties with the
serological diagnosis of infectious mononucelosis: a review of the
RCPA Quality Assurance Programs. Pathology. 28, 270-276.
- NIH Conference (1993) Epstein-Barr virus infection: biology,
pathogenesis and management. Annals of Internal Medicine. 118,
45-58.