A. pacificus tapeworms are the most widely distributed endoparasitic helminth of seals, and infections occur in temperate areas of the North and South Pacific regions and in some southern temperate zones of the Atlantic and Indian oceans (1) (Technical Appendix Table 1). In contrast, human infections have been reported almost exclusively from the Pacific coast of South America, mainly from Peru and, in a relatively few records, from Chile and Ecuador (Figure 1).

The ability of the A. pacificus tapeworm to expand its distribution area globally is demonstrated by infections of humans in Spain, which have recently been confirmed by molecular data (6,31). The source of human infection in Europe remains to be clarified, but commercial import of marine fish stored on ice from areas to which the parasite is endemic, such as Chile or Ecuador, may be a plausible explanation. Spain is the third largest importer of fish and seafood in the world and imports fresh or chilled fish (i.e., those that may harbor infective plerocercoids of diphyllobothriid tapeworms) (2). The import of fish products from South America is critical in the spread of the parasite; countries to which A. pacificus tapeworm is endemic (i.e., Ecuador, Chile, and Peru) represent major exporters (6). Travel-associated cases or migration of humans may also result in distribution of diphyllobothriid cestodes to area outside endemic zones.

The life cycle of A. pacificus tapeworms is not completely known, and no data on the first intermediate hosts are available. Because marine mammals serve as definitive hosts, the life cycle is undoubtedly completed in the sea, unlike the freshwater cycle of most other human-infecting diphyllobothriids (2). Thus, we may assume that the cycle includes marine copepods as the first intermediate hosts, marine fish as the second intermediate hosts, and fish-eating mammals, including humans, as the definitive hosts (2) (Figure 2).

Humans become infected with A. pacificus tapeworms when they eat raw or insufficiently cooked marine fish or food items made from these fish. In coastal regions of Peru, dishes made with raw fish, such as cebiche, tiradito, and chinguirito, are popular and represent the main source of human infections (2,32). Several marine fish inhabiting waters off the Peruvian coast have been reported as potential intermediate hosts of A. pacificus, but their actual spectrum has never been critically reviewed.

The plerocercoids of A. pacificus are encysted in membraneous cysts in the viscera, on the peritoneum or in the stomach wall; some have also been found outside of the intestinal wall and in the gonads (32) (Figure 3). However, they have never been found in musculature. The cysts are thin-walled, oval, pearly white, and measure 2–4 mm in diameter (33,34). Excysted plerocercoids are relatively large (total length of 4–22 mm), and their anterior end (future scolex) possesses distinct bothria measuring 0.5–1.4 mm in length (Figure 3); the surface of plerocercoids is wrinkled and covered with microtriches ≈4 µm long. The species identification of these plerocercoids as A. pacificus was confirmed by sequencing of the cox1 gene (1).

Baer (33) first reported plerocercoids of A. pacificus from 2 species of marine fish caught on the coast of Peru: the Eastern Pacific bonito Sarda chiliensis and Atlantic Spanish mackerel Scomberomorus maculatus. However, the first morphological description of plerocercoids supposedly belonging to A. pacificus was made by Tantalean (33), who found plerocercoids in the peritoneum and gonads of the lorna drum, Sciaena deliciosa, and the Peruvian banded croaker, Paralonchurus peruanus. Plerocercoids of other diphyllobothriid cestodes may also use marine fish as intermediate hosts (33). To date, plerocercoids allegedly from A. pacificus tapeworms were found in 21 fish species of 12 phylogenetically unrelated and ecologically distant families of different orders, including 1 shark species (Technical Appendix Table 2). However, only 8 fish species were confirmed as suitable second intermediate hosts of A. pacificus by experimental infections of dogs or genotyping (Technical Appendix Table 2). Other fish species may serve as second intermediate hosts, as can be assumed from anamnestic data of humans infected with A. pacificus tapeworms (Technical Appendix Tables 2, 3), but their actual role in transmission must be confirmed by finding A. pacificus plerocercoids. Documented prevalence of fish infection with A. pacificus plerocercoids has seldom exceeded 20% (Technical Appendix Table 2). We dissected 79 fish of 5 species collected off the coast of Lima, Peru and found 66 plerocercoids in the body cavity of 2 species with intensity of 2–3 per fish (Technical Appendix Table 2, Figure 3).

Diphyllobothriosis is notoriously known as a potential cause of vitamin B12 avitaminosis and megaloblastic anemia (35). However, this effect of the parasite on its human host is rare, and most cases in which these conditions were reported as human infections with D. latum tapeworms occurred in Finland after World War II (2). Clinical symptoms of diphyllobothriosis are usually mild; the most common clinical signs are abdominal discomfort or pain and diarrhea (2).

Clinical signs related to human A. pacificus infection are poorly known and have been studied in more detail only 3 times, all in Peru: Lumbreras et al. (36) studied 32 cases, Medina Flores et al. (17) 21 cases, and Jiménez et al. (37) 20 patients. Additionally, 37 individual symptom reports have also been analyzed (Technical Appendix Table 3). From a total of 110 case-patients, 18 had no clinical signs, but most of the symptoms were mild or nonspecific, such as abdominal pain (n = 74), diarrhea (n = 37), weight loss (n = 17), nausea (n = 11), or vomiting (n = 5) (Technical Appendix Table 3). Megaloblastic anemia and vitamin B12 deficit were reported in 1 and 5 patients, respectively (36–38).

Typically, A. pacificus infections are registered after spontaneous elimination of tapeworms from the patient (Technical Appendix Table 3). Diphyllobothriosis caused by A. pacificus infection has sporadically reported in AIDS patients; García et al. (39) found only 4 (2%) of 217 AIDS patients infected with this tapeworm, but diarrhea may be a consequential complication and causes malabsorption and malnutrition among these patients.

Differential diagnosis of diphyllobothriid cestodes from human-infecting species of Taenia is easy and straightforward because they differ by the position of gonopores (median in diphyllobothriids versus lateral in taeniids). In contrast, identification of most diphyllobothriid cestodes from clinical material is usually impossible based only on their morphologic characteristics (2). The A. pacificus tapeworm represents one of the few exceptions because its proglottids possess papilla-like protuberances separated by semicircular pits between the genital atrium and the anterior margins of segments (1); these protuberances are absent in other species that cause diphyllobothriosis. In addition, A. pacificus eggs are somewhat smaller and more spherical than those of human-infecting species of Diphyllobothrium, and the worm’s genital atrium has an almost equatorial position, which distinguishes it from Diphyllobothrium, in which it has a more anterior position (1,6).

The only way to exactly determine the species of the causative agent and thus the origin of the infection is through sequencing and analysis of the parasites’ genes. To facilitate differential identification of morphologically indistinguishable human-infecting broad fish tapeworms (D. latum, D. dendriticum, D. nihonkaiense) and A. pacificus in clinical samples, a diagnostic method has been developed and optimized by Wicht et al. (40). The method is based on results of a multiplex PCR amplification of a selected gene (cox1) and does not involve sequencing; thus, this method represents a substantively less costly and easily interpretable approach to be used routinely, mainly by medical diagnostic laboratories.

Treatment of patients who have diphyllobothriosis is simple and highly effective by a single dose of niclosamide (2 g in adults) or praziquantel (2,36). Lumbreras et al. (36) sufficiently treated 32 case-patients by using a single dose of 10 mg/kg of praziquantel. However, a single administration of a 25–50 mg/kg dose is usually applied to ensure complete expulsion of diphyllobothriid tapeworms (2).

The imports of fishery products are subject to official certification. The national authorities must also guarantee that the relevant hygiene and public health requirements are met. The provisions are aimed at ensuring high standards and at preventing any contamination of the product during processing. Scholz et al. and Kuchta et al. compiled information for processing fish to avoid survival of plerocercoids of diphyllobothriid cestodes (2).

Human disease caused by infection with the Pacific broad tapeworm A. pacificus is endemic to the Pacific coast of South America, and most (>99%) clinical cases are reported from Peru. However, this tapeworm species occurs globally, and recent cases of human infection in Europe illustrate that more attention should be paid to this emergent fishborne zoonosis (6). The increasing popularity of eating raw or undercooked fish, import of fresh chilled or insufficiently frozen fish, and traveling and migration of humans represent risk factors that may contribute to a more global expansion of fishborne parasitoses caused by diphyllobothriid cestodes, including A. pacificus.

Samples of tapeworms found in humans should be processed adequately to enable molecular diagnosis and thus identification of the sources of human infection and the geographic origin of parasite infective stages (plerocercoids). Therefore, positive fecal samples or pieces of the strobila should be placed immediately to 96%–99% molecular-grade ethanol (i.e., not technical, denaturated ethanol). Samples should never be fixed with formalin unless part of the same sample is also fixed with ethanol. Fixed samples should be sent to a specialized parasitological laboratory, in which molecular and morphological identification can be performed. The laboratory of the Institute of Parasitology, Biology Centre of the Czech Academy of Sciences, České Budějovice, Czech Republic, is able to analyze and reliably identify clinical samples of diphyllobothriid cestodes free of charge. We highly recommend that representative samples be deposited in a parasite collection so that specialists can conduct further study if necessary.

For a better control of zoonotic disease caused by the Pacific broad tapeworm, gaps in our knowledge of its biology, epidemiology, and transmission should be filled. In particular, a limited knowledge of the fish intermediate hosts impedes a more effective control of fishery products and thus restriction of export of those fish that may harbor A. pacificus plerocercoids. Additionally, little is known about the factors that have contributed to the almost complete absence of human diphyllobothriosis outside South America, especially in the North Pacific, where A. pacificus tapeworms occurs frequently in fur seals but no human cases have been confirmed. The use of molecular markers for reliable identification of clinical samples should become an obligatory practice because it is necessary for a better understanding of the epidemiology of this zoonotic parasite.