Neglected Parasitic Infections in the United States

Neglected Parasitic Infections in the United States
Parasitic infections are a global public health problem and often affect the poorest people in low-income countries. However, US residents may also acquire parasitic infections in the United States, in their country of origin, or during international travel. The Centers for Disease Control and Prevention (CDC) has prioritized 5 parasitic infections for public health action in the United States: Chagas disease, cysticercosis, toxocariasis, toxoplasmosis, and trichomoniasis. Referred to as the "neglected parasitic infections (NPIs) in the United States," these infections share common characteristics in that they affect large numbers of people and are often underrecognized, and preventive or curative interventions are often lacking.

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NPIs: Key Gaps in Prevention and Control
Additional work is needed in 3 major areas to advance prevention and control of NPIs in the United States: (1) better estimates of disease burden and a better understanding of how to reduce the risk of acquiring NPIs, (2) improved diagnostic tests and medications, and (3) expansion of programs that provide proven interventions.^[1]

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Chagas Disease: A Vector-Borne Parasitic Infection
Chagas disease, caused by the protozoan parasite Trypanosoma cruzi, can lead to severe cardiac and gastrointestinal disease. Most people acquire infection through contact with triatomine bugs carrying T cruzi in areas of Latin America where Chagas disease is common. Infection can also be transmitted congenitally and by transfusion and transplantation. In Latin America, an estimated 8 million people have Chagas disease, and approximately 12,000 deaths are attributed to Chagas disease annually.^[2] Chagas disease has 2 phases: acute and chronic. The acute phase lasts 6-8 weeks and is often asymptomatic. In the absence of treatment, people enter the chronic phase of infection. The development of clinical disease occurs after years to decades in approximately 20%-30% of chronically infected people. Disease manifestations include arrhythmias, heart failure, stroke, gastrointestinal megasyndromes, and sudden death.^[3,4]

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Chagas Disease: Key Gaps in Prevention and Control
An estimated 300,000 people with Chagas disease live in the United States; most are unaware of their infection.^[5] The parasite, vector bugs, and infected mammals have been found throughout the southern half of the United States. Human infections acquired in the United States have been reported.^[6] Although only a single case of congenital transmission has been documented, probably as many as 300 babies with Chagas disease are born every year in the United States.^[5,7] Better diagnostic tests and medications for treatment are needed to improve patient care, and research is needed to define transmission risks and develop strategies to prevent new infections.

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Neurocysticercosis: A Preventable Cause of Seizures
Neurocysticercosis is caused by infection with larvae of the parasite Taenia solium. People become infected after ingesting eggs excreted in the feces of human carriers of adult tapeworms. The carriers acquire infection by ingesting T solium cysts in undercooked pork.^[8] Often asymptomatic, the most common presentation is seizures, followed by headache, focal deficits, and hydrocephalus.^[8,9] Diagnosis often requires neuroimaging and confirmatory serology, unless a scolex is visualized on imaging. Initial management focuses on symptom control, which may include antiepileptic medication, corticosteroids, or surgery to relieve intracranial pressure. Anthelminthic therapy or surgical resection of cysts may be indicated. Hand hygiene and treating tapeworm carriers are the cornerstones of prevention efforts.

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Neurocysticercosis: Key Gaps in Prevention and Control
We estimate that there are at least 1000 incident hospitalizations for neurocysticercosis annually, around 2% of people who are hospitalized die, and around 2% of emergency department visits for seizures are related to the disease. Although laboratory diagnosis of neurocysticercosis has improved, tests that differentiate active from inactive disease are needed. We also need a better test to detect carriers of adult tapeworms, so that they can be identified and treated to prevent transmission of neurocysticercosis. Randomized trials are needed to optimize management of subarachnoid disease, determine the best corticosteroid as well as steroid-sparing regimens, and guide surgical therapy for intraventricular disease.^[10]

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Toxocariasis: A Zoonotic Parasitic Disease
Toxocariasis is a parasitic zoonotic infection caused by the roundworms Toxocara canis or Toxocara cati. Infected dogs and cats shed Toxocara eggs in their feces; humans become infected when they ingest the eggs, usually by touching contaminated hands to their mouth.^[11] The majority of cases of toxocariasis occur in children; they are at high risk for infection owing to their play habits and poor hygiene. Toxocariasis can manifest clinically as ocular toxocariasis, when the eye is affected, or as visceral toxocariasis, when such organs as the lung or liver are affected.^[12] Symptoms can include vision loss, cough, wheezing, and abdominal pain and can range from mild to life-threatening. Diagnosis of toxocariasis is made by clinical presentation and testing for antibody to the Toxocara parasite; anthelminthic drugs are used for treatment, although some disease manifestations, such as vision loss, may be irreversible.

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Toxocariasis: Key Gaps in Prevention and Control
Although the serologic prevalence of Toxocara was found to be 14% in the US population in a national survey, it remains unknown how many people have clinical disease.^[13] The development of a diagnostic test that can differentiate between current disease and past infection is needed, and prevention strategies, such as deworming of dogs and cats and covering sandboxes to reduce exposure to animal feces, should be promoted.^[14]

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Toxoplasmosis: Infects 1 Million in US Annually
Toxoplasmosis is caused by the protozoan parasite Toxoplasma gondii. Although T gondii can infect most warm-blooded animals, cats are the only host that shed the organism in their feces.^[15,16] The people at most risk for serious complications from toxoplasmosis are pregnant women (who can transmit the parasite to their child, resulting in ocular and neurologic disease) and immunocompromised persons, who can develop encephalitis or disseminated disease.^[15-17] People are usually infected by (1) eating raw or undercooked meat containing the parasite; (2) ingesting food, soil, or water contaminated by cat feces; (3) congenital transmission from a newly infected pregnant woman to her child; and (4) organ transplant or blood transfusion.^[15-17] T gondii infects approximately 1 million persons each year in the United States, of which an estimated 4800 persons develop symptomatic eye disease.^[18,19]

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Toxoplasmosis: Key Gaps in Prevention and Control
Continued epidemiologic and laboratory investigation is needed to determine the prevalence of and risk factors for T gondii infection.^[20] Better diagnostic tests, including polymerase chain reaction technology, and better tests to help determine the time of infection in pregnancy are needed to improve clinical management. Improved ways of preventing T gondii contamination of meat, especially field-raised meat, and development of a cost-effective T gondii vaccine for cats to prevent shedding of the organism in feces could reduce the incidence of toxoplasmosis.

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Trichomoniasis: A Common Sexually Transmitted Parasitic Infection
Trichomoniasis is caused by the protozoan parasite Trichomonas vaginalis. This is the most prevalent nonviral sexually transmitted infection and one of the most common parasites in the United States, affecting about 3% of US women and an estimated 3.7 million people nationwide.^[21] It can cause urogenital symptoms (eg, itching, burning, and discharge) in both women and men, although most people do not know that they are infected. Trichomoniasis increases the risk of getting or spreading other sexually transmitted infections, including HIV. In pregnant women, trichomoniasis is linked with preterm delivery of a low-birthweight baby. Untreated, infections may persist for months or years. Because treatment is simple -- a single 2-g dose of a nitroimidazole antibiotic -- testing is important for people at high risk.^[22] Although the traditional diagnostic method is wet-mount microscopy, new highly sensitive molecular tests can detect parasite nucleic acid in urine.

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Trichomoniasis: Key Gaps in Prevention and Control
Even though T vaginalis infection is more common than gonorrhea or chlamydia, it has received little attention from the general public.^[23,24] Several newly available diagnostic assays may improve the ability to identify these infections in persons with or without symptoms. Further investigation is needed to establish the true public health impact and costs of this infection, and to determine how associated adverse events and health disparities can be prevented. Emergence of antimicrobial resistance is concerning, given the lack of currently available treatment alternatives.^[25]