PATHOLOGICAL EFFECT OF THE PARASITE UPON THE HOSTn 7.6.1 INFECTION IN HUMANS
In humans (intermediate hosts) E. multilocularisis found exclusively in the liver and is referred
to as alveolar echinococcosis, or alveolar hydatid disease.
The metacestode progressively invades surrounding tissue by forming a protrusion
from the germinal layer, which protects itself from the host response by synthesis of an
outer laminated layer. A hepatic lesion forms which macroscopically appears as a dispersed
mass of tissue consisting of small cysts and vesicles. The lesions caused by the parasite’s
development form focal zones of calcification and necrosis. Microscopic examination
shows evidence of proliferation of host fibrous tissue, reflecting the host tissue’s
response to the metacestode.
The clinical signs of the disease are hepatic carcinoma and cirrhosis. Immunodiagnosis
is based on the presence of a specific antibody known as the Em-2 antibody, detected by
the ELISA technique. The Em-2 antibody is directed against purified E. multilocularis
carbohydrate antigen. Parasitic lesions can be detected by computed tomography and show
up as indistinct and ill-delineated solid masses.
n 7.6.2 ANTIBODIES
During the course of the infection all of the different classes of immunoglobulins are detected.
In particular there is an increased serum level of IgE as well as an increase in the level
of IgE bound to circulating basophils. The antibodies may restrict the growth of the
metacestode and are probably involved in immunopathological mechanisms, resulting in
a certain amount of granulomatous tissue. There are amyloid and immune complex
deposits in the infected liver.
n BOX 7.4
The disease caused by E. multilocularisshows up in different forms which are referred to
as disease states:nPatients susceptible to the disease develop intensive proliferating intra-hepatic E. multi-
locularisalveolar cysts. Circulating antibodies known as anti-Em-2 (echinoccosis mem-
brane) antibodies can be detected in the body fluids. There is an increase in the number
of lymphocytes, particularly cytotoxic T lymphocytes (CD8+T cells). When removed from
the patient these respond to E. multilocularisantigen in vitro. In addition, high levels of
IL-5 responsible for eosinophil production are found in the serum of such patients.
nPersons who are susceptible to the infection but resistant to the disease develop calcified
lesions that do not contain living parasites. The debris from the laminated layer stimu-
lates and maintains the production of antibodies against antigen (Em-2). There is a high
proliferative response of lymphoyctes to Em-2 antigen resulting in an increased number
of T helper cells (CD4+T cells).
nPersons who are immunologically resistant to the infection. These are healthy indi-
viduals with serological activity to Em-2 antigen but no signs of disease. There is no
post-oncospheral development and these individuals appear to be immunologically
resistant to infection. But if their serum antibodies are tested, they can respond specifically
to the parasite.
nPersons who are constitutionally resistant to the infection. They have a high rate of
exposure to the infection but show no immunological reaction.