by Phyllis Hanlon, Contributing Writer

Schistosomiasis, a disease caused by infection with freshwater parasitic worms belonging to the genus Schistosoma, is one of the most prevalent parasitic infections in the world. The Global Network reports that approximately 240 million people in 78 countries, with the greatest impact in Africa, are infected with this parasite. Where schistosomiasis exists, it has been found to be a major cause of pulmonary arterial hypertension (PAH), according to a review out of the University of Kent in the United Kingdom.

Ghazwan Butrous, MB, ChB, PhD, examined the problem and offered details regarding the life cycle of the parasite and the impact schistosomiasis has on the pulmonary system.

Dr. Butrous explained that a human excretes the Schistosoma eggs into fresh water via urine or feces and when they hatch, enter freshwater snails as intermediate hosts. These organisms then multiply in the snail before they leech into the water in search of another host. When the worms enter a human or other mammal through the skin, they travel through the organs in the body, including the heart, lungs, liver, bladder and others, and produce eggs. The female parasite can produce thousands of eggs daily and the host reacts to this infestation by developing inflammation around the parasite egg.

According to Dr. Butrous, studies in his lab of mice infected with schistosomiasis showed that pulmonary vessels in 46 percent of lungs examined had remodeling around the granuloma (a structure of inflammatory cells), and “…that pulmonary vascular remodeling in PAH associated with schistosomiasis is caused by the local presence of granulomas.”

Due to the scarcity of epidemiologic studies in areas of high schistosomiasis prevalence, the exact number of PAH cases due to schistosomiasis is not known, but Dr. Butrous offered some insight into the matter. He wrote, “It is suggested from our experimental observations that the changes in the pulmonary vasculature after Schistosoma infection are far more common, but may not always be associated with significant increase in pulmonary vascular resistance or full clinical manifestations of PAH.” Additionally, Dr. Butrous found that the number of eggs and where they were deposited in the lung affects how severe the remodeling is and how it impacts the development of PAH.

Detecting schistosomiasis can be challenging. Dr. Butrous indicated that early symptoms (acute condition) might include fever at night, cough, shortness of breath, muscle pain, headache, stomach tenderness, increased white blood cell count and swelling of the lymph nodes. At this stage, the condition can be resolved; however, chronic cases may present with symptoms similar to other forms of PAH. Undetected, the parasite can repeatedly deposit eggs, causing severe changes to the pulmonary vessels and eventually lead to pulmonary hypertension and right heart failure. While he noted that there are no tests currently to help diagnose PAH due to schistosomiasis, Dr. Butrous emphasized the importance of performing right heart catheterization to evaluate mean pulmonary arterial pressure, right ventricular function and pulmonary artery wedge pressure. He added that an echocardiogram could reveal an enlarged heart or other negative changes in right-sided vessels.

Treatment options, supported only by observational evidence, for schistosomiasis-induced PAH follow current trends, according to Dr. Butrous, i.e., phosphodiesterase-5 inhibitors (e.g., sildenafil or tadalafil) or endothelin receptor antagonists (e.g., ambrisentan, bosentan, macitentan). He added that active cases might be treated with praziquantel, an anthelmintic drug used to expel parasitic worms from the body. Although anti-inflammatory medications might help, their effectiveness on the pulmonary system is unclear. Dr. Butrous suggested that additional studies would help to better identify prognosis and appropriate treatment options.

This paper originally appeared in Advances in PHAdvances in Pulmonary Hypertension: Official Journal of the Pulmonary Hypertension Association is a quarterly publication directed by an editorial board of renowned experts with the oversight of the PHA’s Scientific Leadership Council. Its mission is to help physicians in their clinical decision-making by informing them of important trends affecting their practice and providing an analysis of the impact of new findings in peer-reviewed publications. Each article is reviewed and approved by members of the Editorial Advisory Board. The upcoming issue will be printing in the next few weeks and will focus on applying diagnosis and treatment guidelines into clinical practice. To sign up to receive notifications when the latest issue is available and to view past issues, visit Advances in PH is distributed to 40,000 physicians, clinicians and researchers around the world each quarter.