Worms and Human Disease

pleural cavity in 6–10 days and enter the
lung capsule in 15–20 days. The first eggs
are passed 60–70 days after infection.
Although Paragonimus is hermaphrodite,
two worms are necessary for fertilization to
take place (Fig. 19 and Plate 4). This is true
for the usual diploid form of P. westermani,
but there is also a triploid form, in which
parthenogenesis can occur; the adults of the
latter are larger and more pathogenic and
the eggs differ in size and shape (see Blair
et al., 1999). The adult worms can live for
20 years but usually die after about 6 years.
CLINICAL MANIFESTATIONS
There are no recognizable symptoms
accompanying the migratory phases.
The first signs are usually fever with a
dry cough, sometimes accompanied by
bloodstained sputum containing eggs after
rupture of the cysts. Chest pain can be
severe and, if the infection is heavy, there
is sometimes increasing dyspnoea and
bronchitis. Many patients with haemoptysis
are diagnosed as suffering from
pulmonary tuberculosis.
Cerebral paragonimiasis may result in
epileptic seizures, as well as headaches,
visual disturbances and symptoms of
meningitis.
PATHOGENESIS
Pulmonary. As flukes grow in the lungs
they cause an inflammatory reaction and
The Trematodes 35
become surrounded by a cyst, possibly
caused by softening followed by fibrosis
(Plate 4). However, as the cyst wall is usually
lined with columnar epithelium, it is
probable that, in most cases, it represents
the expanded wall of a bronchus. The capsule
is collagenous and oedematous, with
plasma cells, eosinophils, neutrophils,
macrophages and fibroblasts. The cyst measures
about 1 cm in diameter after 90 days
and is filled with a thick brownish fluid
containing eggs. Bronchopneumonia may
result when a cyst bursts or retains an
opening into a bronchus. Eggs are often
present in the lung tissues and cause
pseudotubercles similar to those caused by
schistosome eggs, with infiltration by
eosinophils and lymphocytes, followed by
giant cells and fibroblasts.
Extrapulmonary. Adults are sometimes
found in many parts of the body, particularly
in the organs of the abdominal cavity
or subcutaneous tissues. In these sites the
worms are rarely fertilized and presumably
have developed from larvae that lost their
way. This occurs more frequently with
species of Paragonimus which are less
adapted to humans than P. westermani.
Most serious complications follow the
presence of flukes in the brain (Fig. 20).
This may be common in areas of high
endemicity (e.g. over 5000 cases per
annum in South Korea). Adults found in
Fig. 20. Radiograph of head of a girl with soap-bubble type of calcification surrounding cysts containing
adult Paragonimus in occipital lobes.