Neglected Parasitic Infections in the United States: Needs and Opportunities

Parasitic infections are a major global health burden. The impact of debilitating diseases caused by parasites is greatest among those who struggle to meet their daily basic needs and access basic health care services in low-income countries. However, persons who have or are at risk for parasitic infections are present in every income and social strata, and residents of the United States and other developed nations are not unaffected. For some persons living in the United States, these parasitic infections are acquired in their own immediate environment; for example, exposure to feces from domestic dogs or cats puts children at risk for toxocariasis and toxoplasmosis. For others, chronic parasitic infections acquired years ago in other areas of the world can manifest with severe illness later in life, such as neurocysticercosis leading to adult– onset epilepsy or Chagas disease leading to severe cardiomyopathy requiring heart transplant. We know much less than we should about the health and economic burden and impact of parasitic diseases in developed countries, including the United States (Table 1). This issue of the American Journal of Tropical Medicine and Hygiene features brief reviews of five parasitic infections that remain a significant health problem in the United States: Chagas disease, cysticercosis, toxocariasis, toxoplasmosis, and trichomoniasis. These five diseases, which are among those that Centers for Disease Control and Prevention (CDC) refers to as neglected parasitic infections (NPIs) in the United States, have different epidemiologic profiles and modes of transmission and require tailored prevention and control strategies. However, in the United States these NPIs share key characteristics, including 1) the surprisingly large number of persons infected or at risk, especially those living in poverty; 2) the potential for underreporting and missed diagnoses, largely because of lack of clinician awareness and poor availability of optimal diagnostic tests; and 3) the dearth of interventions that can prevent or cure illness. As a result of these issues, these diseases have not received needed attention or resources in the United States. These reviews are part of an effort to raise awareness and promote public health actions that can be taken now with available knowledge and tools and highlight areas for further clinical and epidemiologic research and action. Assessments and reviews of these diseases often highlight the remarkable lack of information about who is at highest risk for infection, how many people in the United States are infected, and how to best intervene to prevent infection from progressing to disease. Estimates of disease burden are often based on serologic markers from cross-sectional population studies that indicate a person has been infected but not whether that person has active disease (e.g., toxoplasmosis and toxocariasis). In addition, the estimates may be projected from geographically focused studies that might not be representative of the true population burden (e.g., trichomoniasis, Chagas disease, and neurocysticercosis). The public health community also has few good estimates of the economic burden imposed by these NPIs. Limited estimates, such as one study that examined hospital discharge data on neurocysticercosis for 2009 in California estimated statewide hospital charges and costs at $17 million and $5 million, respectively, suggesting that the nationwide economic burden is considerable. The morbidity caused by NPIs extends across the lifespan, including blindness or severe developmental deficits in children (toxocariasis, toxoplasmosis), infertility and poor birth outcomes (trichomoniasis) or epilepsy (neurocysticercosis) in young adults, and cardiomyopathy sometimes requiring transplant in middle age adults (Chagas disease). Symptoms can develop in persons with Chagas disease, toxoplasmosis, and cysticercosis years after an initial mildly symptomatic or subclinical infection. Furthermore, a few intriguing studies have shown associations between some NPIs and complex chronic diseases with enormous health impact, such as studies linking antibody evidence of Toxoplasma infection and mental illness or Toxocara infection and asthma. Appropriate and timely medical management of a person with an NPI is critical, but the public health actions needed to identify and prevent infection and disease fall to the under-resourced and overburdened public health sector. For example, a person with neurocysticercosis needs careful medical management to control neurologic symptoms but can also benefit from public health follow up because testing of household members for taeniasis (intestinal Taenia solium infection) might identify and treat the typically asymptomatic human source of the person’s infection. Women bear most of the burden of symptomatic trichomoniasis, and they can be successfully treated with a short course of metronidazole in clinics for women’s health or sexually transmitted infections. However, without finding and treating their male partners, reinfection is likely. Among U.S. residents, Chagas disease risk is much higher among persons who previously lived in rural and impoverished areas in Mexico, Central America, or South America, where the vector of Chagas disease, the triatomine or kissing bug, is commonly found near or in dwellings. Chagas disease patients are increasingly being identified during routine blood donor screening; their health care providers often turn to the public health sector for assistance with diagnosis and treatment decisions. Furthermore, identification of a person with Chagas disease can trigger additional recommendations such as testing others in the family (for example, children of an infected mother). Targeted screening programs in some immigrant communities might identify other infected persons and *Address correspondence to Monica E. Parise, Parasitic Diseases Branch, Division of Parasitic Diseases and Malaria, Center for Global Health, Centers for Disease Control and Prevention, 1600 Clifton Road NE, Mailstop A06, Atlanta, GA 30333. E-mail: [email protected]