Page 66 - Journal of Special Operations Medicine - Fall 2016

at least 6 weeks after exposure. Follow-up occurred at 6 Figure 2 Schistosomiasis life cycle, available at
weeks, and serologic examinations revealed one patient http://www.cdc.gov/parasites/schistosomiasis/biology.html.
who was negative and one patient who had an indeter-
minate result. Testing for the patient with the indeter-
minate result was repeated at 12 weeks, which revealed
a second indeterminate result. A specimen was sent to
the Centers for Disease Control and Prevention (CDC)
at 16 weeks, which again yielded an indeterminate re-
sult. The patient was then prescribed a 3-day course of
20mg/kg praziquantel twice daily.

Basic Science

Life Cycle
Schistosomal life begins as eggs excreted into freshwater
via the urine or feces of an infected person. The eggs
6
hatch into miracidiae that find and infect snails. In this
host, the miracidiae reproduce and develop. They leave
6
the snail as cercariae looking for a human host. These
6
cercariae penetrate human skin, transform into schisto- fever, dry cough, wheeze, abdominal pain, diarrhea,
somules, and migrate to blood vessels, which takes 10 nausea, vomiting, and rash. 2,5,6,13 In the chronic form,
to 72 hours. From these vessels, they travel through patients develop hematuria and/or hematospermia with
6–9
the lungs to the liver where they mature. Finally, they S. haematobium and hematochezia with S. mansoni. 1,6,13
travel to their definitive locale, where they pair with a
mate. Schistosoma mansoni reside in the mesenteric Exposure
6
vessels, whereas S. haematobium reside in the genitouri-
nary vessels. At these locations, the pairs produce eggs For schistosomes to survive at a location, there must be
6
that penetrate the viscera and are excreted via urine and a triad of freshwater, human excrement, and host snails.
feces. The process from skin penetration to ova produc- When humans come into contact with cercaria-infested
7
tion typically requires 4 to 9 weeks and causes evolving freshwater, they are at risk of infection. However, the at-
symptoms along the way (Figure 2). 7 tack rate for Schistosoma after a freshwater exposure is
not well defined in the scientific literature. Areas where a
Clinical Presentation high percentage of snails are infected with Schistosoma
Historically, patients present with Katayama syndrome, do not always correlate with human infection rates.
14
the constellation of urticarial rash, fever, nonproduc- Further, there is no definitive way to ascertain the cercar-
tive cough, and elevated eosinophils after freshwater ial challenge to a person when he or she is exposed. In a
14
exposure, but this is often not the case in travelers. laboratory study, Cooper et al. noted that after 1 hour of
9
5,6
Many present with minimal or no symptoms. Reports exposure, 88% of cercariae interacted with human skin,
indicate that 34% to 59% of travelers with schistoso- but that does not necessarily correlate with infection. The
miasis are asymptomatic. 2,5,10 In fact, up to 41% of as- cercariae can die in the skin or fail to successfully mature
ymptomatic at-risk patients presenting to a travel clinic into adults. While a case study of 18 travelers in Mali
are ultimately diagnosed with schistosomiasis. 2,10 This is reported an attack rate of 92% to 100% for S. haemato-
troublesome, as schistosomiasis develops into a chronic bium, this is the exception, not the rule. In retrospective
15
infection that can lead to transverse myelitis, renal fail- studies, schistosomal infection in travelers ranges from
ure, bladder cancer, chronic colitis, portal hypertension, 18% to 28% for frequent freshwater contact and 13% to
or other complications secondary to erroneous metasta- 19% for occasional contact. 5,10 There is no difference in
sis of ova. 5,6,9,11 infection rates based on urban versus rural living, deep
10
versus shallow water exposure, or area of water access
16
When travelers do present with acute symptoms, the such as beaches, vegetation, or boats. Swimming has
5
earliest manifestation is known as cercarial dermatitis, been noted to be a more significant risk factor than wad-
or “swimmer’s itch.” 5,12 Patients complain of skin irri- ing, but wading remains a risk factor. Most important
5
tation and develop 1–3cm erythematous lesions at the for deployers is the fact that a single exposure can lead to
sites of dermal penetration. The lesions appear within an infection with the parasites. It is equally important for
a few hours to 1 week after freshwater contact. 7,12 The medics to maintain a high index of suspicion in personnel
most common symptoms in acute schistosomiasis are traveling to the tropics.

48 Journal of Special Operations Medicine Volume 15, Edition 3/Fall 2016