...that tapeworms, which typically reside in canines, could cause lung disease?
The tapeworms, Echinococcus granulosus andE. multilocularis both induce pulmonary hydatid disease in humans when their eggs are accidentally ingested [1-4]. Once ingested, digestive enzymes disrupt the eggs allowing embryos to hatch out, penetrate the bowels, and enter the circulatory or lymphatic systems. These embryos migrate to various organs including the lungs where they form cysts. Pulmonary cysts, which can measure more than 10 cm in diameter, accommodate the larval cestode and progeny that form daughter cysts (Figure 1) [4-7]. Cysts also contain odorless, colorless, sterile fluid with antigenic elements. Over time or if the cyst ruptures, fluid can leak into the bronchi causing suffocation or anaphylactic shock; protoscolices, which can form new cysts, are also released perpetuating the disease state. Although the risk of infection is low in developed countries, hydatid disease is considered an occupational hazard as those most affected are people who work closely with sheep and herding dogs as well as populations in high endemic areas such as southern South America, the Middle East, Australia, and parts of China [1,3,8,9].
Although relatively uncommon today, hydatid disease has been observed in both humans and animals for centuries. From the time of ancient philosophers until the 19th century it was believed that parasites spontaneously arose from within the body, which was disproved by Francesco Redi (1626-1697) who showed parasites were of animal origin . Studies which furthered the field of parasitology include: Goeze's (1782) discovery of tapeworm scolices in pulmonary cysts, Felix Dujardin's (1801-1860) observation that tapeworms spend part of their lifecycle in another host, and Carl von Siebold (1852) who was the first to induce tapeworm infections in dogs using larval cestodes from sheep. Over the next 100 years, it was largely debated whether two separate species of tapeworms caused the altered appearance in cyst formation (unilocular vs. alveolar) or variants of the same tapeworm . In 1950, the debate was settled when it was discovered that E. granulosus and E.multilocularis both induce hydatid disease. Clinical research was advancing at the same time with Barrett's (1949) development of an operational technique, which enabled physicians to successfully remove an intact pulmonary cyst preventing cyst rupture and re-infection [2,3,10].
Our current understanding of parasitic tapeworms and how they form pulmonary cysts has allowed for better treatment of patients. Pulmonary hydatid disease can go unnoticed for more than 15 years; patients that eventually become symptomatic complain of cough, shortness of breath, and chest pains [1,6]. Diagnosis requires imaging, the most common being chest radiographs (Figure 2), and surgery, which is typically followed by chemotherapy, is needed to remove the cyst and prevent re-infection [1,2,5,6]. Preventive measures such as improved water sanitation and hygiene, as well as de-worming livestock and dogs, has helped decrease the incidence of pulmonary hydatid disease in endemic areas. So, although common companion animals in certain working conditions can cause the uncommon pulmonary hydatid disease, there are measures available to prevent and treat this disease.
Author: Alicia Waggoner
Chief Editor: Natalie Bauer, Ph.D. May. 2011
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