A Systematic Overview of Zoonotic Helminth Infections in North America

Blaine A. Mathison, BS, M(ASCP); Bobbi S. Pritt, MD, MS, DTM&H


Lab Med. 2018;49(4):e61-e93. 

In This Article

Abstract and Introduction


Zoonotic helminths are frequent human parasites that generally complete their natural life cycle in the absence of humans. This review encompasses 30 common or clinically relevant zoonotic helminth infections that are endemic to North America. For each organism or disease, the following information is briefly reviewed: taxonomy, biology, and life cycle, epidemiology, route of transmission for the human host, clinical manifestations, pathologic features, treatment, and laboratory diagnosis. Illustrations are provided for select parasites.


Most parasitic helminth (worm) infections of humans originate among animals, whether by coevolution between humans and parasites after centuries of exposure or by new and emerging relationships due to increased human activity into areas and habitats that allow for exposures to novel etiologies. Zoonotic infections, in the broad sense, are those that cycle between humans and nonhuman animals.

In this review, we apply the term zoonotic to those parasites that usually complete their natural cycle in the absence of a human host. There is no discrimination as to whether humans can serve as adequate definitive hosts; often, they can. The geographic scope of this review is North America; Hawaii is included as part of United States in the political sense, although it should be noted that Hawaii is not considered part of North America in terms of biogeography.

In the medical and clinical realms, helminths are members of 1 of 4 broadly unrelated groups of animals: nematodes (roundworms), cestodes (tapeworms), trematodes (flukes), and acanthocephalans (thorny-headed worms; see also the Glossary). Members of these groups are all multicellular animals. Most parasitic species have multihost life cycles, involving a vertebrate definitive host and 1 or more invertebrate or vertebrate intermediate hosts. Nuances defining and separating the various groups, as well as the zoonotic helminths in each group, will be discussed systematically in detail later in this article and are outlined in Table 1.

Helminth infections can involve nearly all sites in the human body and can be associated with a broad range of clinical manifestations.[1,2] Although this is also true for zoonotic helminths, it is important to note that the site(s) of infection and clinical manifestations may differ from what is observed in the natural animal host. Some zoonotic helminth infections cause minimal to no symptoms in humans (eg, Dipylidium caninum, Hymenolepis nana), whereas others, particularly those that migrate throughout the body, can cause serious, life-threatening disease (eg, Baylisascaris procyonis, Trichinella spp, Toxocara spp).[1,2]

Means of diagnosing helminth infections in the clinical laboratory can vary greatly depending on the species and specimen type. Most helminths that colonize the human intestinal tract are identified by the gross morphology of adult worms or proglottids in stool specimens, or the identification of eggs and larvae in ova and parasite (O&P) examinations of stool. Filarial nematodes that use humans as the definitive host are usually identified by the finding of microfilariae in blood or skin snips. Most helminth infections can be identified at some point in their development by the histological examination of tissue specimens. Serologic and molecular (nucleic-acid amplification tests [NAATs]) assays are also available for select etiologies. Still, morphologic analysis remains the criterion standard for the identification of most parasitic infections.

Diagnosis of zoonotic helminths presents several inherent challenges. First, many species are not common and therefore may not be familiar to most bench technologists or microbiologists. Secondly, if the parasites do not mature to adults or reproduce in the human host, traditional diagnostic methodologies, such as O&P examinations of stool or examination of blood films, may not be appropriate. Many zoonotic helminthiases are best diagnosed by the examination of histopathologic preparations of biopsy specimens and require the expertise of a pathologist or experienced parasitologist.

This review encompasses the 30 most common and/or clinically relevant zoonotic helminth infections that are endemic to North America (Table 1). It also provides a brief description of the taxonomy, biology and life cycle, epidemiology, route of transmission for the human host, clinical manifestations, pathologic features, treatment, and laboratory diagnosis for these organisms. Illustrations are provided for select parasites (Figures 1–4). The reader is directed to several comprehensive publications to assist in the identification of helminth and other parasitic infections.[1–8] The DPDx website, managed by the United States Centers for Disease Control and Prevention (CDC),[2] contains illustrations of life cycles and an extensive image library showing the morphologic features of many parasites in a variety of clinical specimens. Also, the DPDx Team at the CDC offers a free telediagnostic/online diagnostic service (https://www.cdc.gov/dpdx/contact.html) for rapid diagnosis of parasitic infections.[9] The website for the CDC's National Center for Emerging and Zoonotic Infectious Diseases (https://www.cdc.gov/ncezid/) is a useful source for up-to-date information regarding zoonotic infections in the United States.

Figure 1.

Gross and microscopic images of nematodes. A, Ascaris lumbricoides (inset shows a close-up of the ascarid "lips"). B, Pseudoterranova sp, (note the mucron [arrow, inset]). C, Dirofilaria tenuis. D, Gongylonema pulchrum (note the presence of cuticular bosses, or rounded swellings [arrows] [eggs, inset]). E, Thelazia gulosa (note the cup-shaped buccal cavity [arrow]). F, Angiostrongylus cantonensis, L4 larva in cerebrospinal fluid (CSF). Images B, C, and E courtesy of the Centers for Disease Control (CDC)–DPDx website; Image D courtesy of D. Jane Hata, PhD; Image F from Pritt BS.8 Reproduced with permission.

Figure 2.

Gross cestodes, trematodes, and acanthocephalans. A, Dibothriocephalus spp, proglottids (inset, close-up of centrally-located uteri). B, Dipylidium caninum, proglottids (scale = 1 mm). C, Fasciola hepatica. D, Paragonimus kellicotti. E, Macracanthorhynchus sp. F, Moniliformis moniliformis. Image C courtesy of the Centers for Disease Control and Prevention (CDC)–DPDx. Image E courtesy of Marc Couturier, PhD, D(ABMM).

Figure 3.

Helminth eggs. A, Ascaris lumbricoides (fertile, mamillated). B, A. lumbricoides (fertile, decorticated). C, A. lumbricoides (infertile, mamillated). D, A. lumbricoides (infertile, decorticated). E, Dibothriocephalus sp F, Dipylidium caninum. G, Hymenolepis nana. H, Hymenolepis diminuta. I, Fasciola hepatica. J, Paragonimus species. K, Macracanthorhynchus ingens. L, Moniliformis moniliformis. All images captured at ×400 magnification. All images courtesy of the Centers for Disease Control and Prevention (CDC)–DPDx.

Figure 4.

Helminths in histological preparations of tissue specimens. A, Capillaria hepatica in liver (hematoxylin/eosin [H&E] staining, original magnification [OM] ×400; inset at ×1000). B, Trichinella spiralis in muscle—note the presence of stichocytes (arrow) and bacillary bands (arrowheads) (H&E, OM ×400). C, Baylisascaris procyonis in brain (H&E, OM ×400). D, Dirofilaria species in subcutaneous nodule (OM ×200). E, Gnathostoma species in colon (OM ×100). F, Spirometra sp in soft tissue (H&E, OM ×40); inset shows a close-up (OM ×1000) of the calcareous corpuscles. G, Taenia sp coenurus from the chest wall (H&E, OM ×40). H, Echinococcus granulosus (H&E, OM ×100). All images except E are from Pritt BS.8 Reproduced with permission.