TAPEWORMS (CESTODES)

JOANN L. COLVILLE DVM , DAVID L. BERRYHILL PHD , in Handbook of Zoonoses, 2007

ETIOLOGY: PARASITIC

Adult tapeworms are found in the intestinal tracts of their definitive, or final, hosts. Each adult tapeworm consists of a head (scolex), which attaches the tapeworm to the intestinal wall, neck, and various numbers of segments, developing from the neck region. As new segments are formed at the neck, older segments are pushed back. Tapeworms are hermaphroditic; each segment has two sets of male and female reproductive organs, which will fill the segment with fertile eggs as the segment is pushed back from the neck. When the segment is full of eggs, it detaches itself from the adult tapeworm and is passed in the feces (Figure 40).

Each genus and species of tapeworm has at least one intermediate host, which ingests the tapeworm eggs. After the eggs hatch, the immature tapeworms migrate out of the intestine of the intermediate host and travel to various tissues in the body, depending on the genus of tapeworm. The immature tapeworm enters tissue in the intermediate host and is enclosed in a cyst, in which young tapeworms develop to an infective stage. Definitive hosts are infected by eating the cystic tissues of intermediate hosts infected with immature tapeworms.

There are three tapeworms of zoonotic importance in North America: Dipylidium caninum, Diphyllobothrium latum, and Echinococcus spp.

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ANIMALIA

Lynn Margulis University of Massachusetts at Amherst , Michael J Chapman Marine Biological Laboratory Woods Hole, Massachusetts, USA , in Kingdoms and Domains (Fourth Edition), 2009

(Flatworms)

Greek platys, flat; helmis, worm

GENERA

Bothrioplana

Convoluta

Dipylidium

Dugesia

Echinococcus

Fasciola

Hymenolepis

Opisthorchis

Planaria

Procotyla

Schistosoma

Taenia

Platyhelminthes are free-living and symbiotrophic flatworms. The soft body of the flatworm is bilaterally symmetrical. Structures for capturing and consuming prey are localized in the anterior end except in many free-living flatworms, in which the mouth is anterior. Flatworm organs are composed of tissues and are organized into systems. Flatworms are the simplest metazoans to possess an embryonic intermediate tissue layer, the mesoderm. The platyhelminth worm, like the cnidarian, lacks an anus. The flatworm middle tissue layer, a loose mesoderm called parenchyma, never splits into a cavity (coelom) in which internal organs are suspended. Flatworms and other animals without a coelom are called acoelomates. Flatworms, having three tissue layers, are triploblastic, have spiral cleavage in their eggs, and yet are among the least complex of bilaterally symmetrical true metazoans.

Flatworm classification is constantly being reviewed and revised. The symbiotrophic forms, which undergo a transformation of the epidermis to a cuticle-like neodermis during development, are generally placed among two or three traditional classes within the group Neodermata. Two established classes contain the Trematoda, or flukes, and the single to multi-host Cestoda, the tapeworms. Some workers divide the Trematoda into the Digenea, or multi-host trematodes, which are internal necrotrophs, and Monogenea, which typically have a single host and are largely external necrotrophs. The free-living flatworms make up a number clusters or clades (using cladistic methodologies), the relationships of each remain unresolved. The most distinctly primitive group, the Acoela (Acoelomorpha), may not be a platyhelminth group at all.

There are about 20,000 species of flatworm altogether. Some species are richly colored. Others harbor symbiotic algae called zoochlorellae producing a green color. Most necrotrophic forms and those free-living forms that inhabit caves and underground water are colorless. Tapeworms are the largest platyhelminths; some reach a length of more than 30   m. The smallest are less than 1   mm in length.

Flatworms are masters of adaptation, exploiting an enormous variety of habitats. Some live in bat guano, others in the mantle fold of various Mollusca (Phylum A-26) where as symbionts they feed on the particles not consumed by the mollusc host. Members of many animal phyla, certainly an enormous number of vertebrates, play host to ubiquitous flatworm symbiotrophs. In sediments low in molecular oxygen, a few flatworms utilize energy by oxidizing hydrogen sulfide. A survey of the phylum reveals that flatworms tolerate an immense temperature range from minus 50 degrees to plus 47 degrees.

Some free-living flatworms are marine inhabitants; most are freshwater forms, and several dwell in moist soil. Soil flatworms are mainly tropical whereas aquatic forms are more abundant in temperate than in tropical waters. Free-living and non-neodermatid necrotrophic forms have several undulipodia per cell. By simultaneously sweeping ventral cilia through secreted mucus and generating muscular waves, free-living flatworms glide over surface films on water, plants and soil. On the ventral surface of free-living flatworms, duoglands (adhesive and releaser cells) secrete either adhesive that attaches the worm to its substrate or a substance that detaches it. Most free-living aquatic species are benthic, a few swim with undulations or loop along substrate like caterpillars, and some live among the plankton.

Free-living flatworms are detritus feeders, carnivores, and scavengers. They eat insects or crustaceans (A-21), tunicates (A-35), bivalve molluscs (A-26), other worms, bacteria, mastigotes (Phylum Pr-28), ciliates (Phylum Pr-8), and diatoms (Phylum Pr-18). Most free-living flat worms are marine; some inhabit the digestive tract of sipunculans (A-23) and echinoderms (A-34); a few are terrestrial in damp habitats or are freshwater species. Digestive systems of free-living flatworms range from a straight or branched gut to absence of a gut; food moves from the pharynx of acoel (lacking a gut) free-living flatworms into loose digestive cells. Some jab food by using a proboscis separate from the mouth. Others "vacuum out" soft parts of their prey by using a tubular, muscular pharynx, which may project through the mouth on the ventral side. Digestive enzymes secreted into the gut begin digestion; intestinal cells continue digestion by engulfing food in food vacuoles.

Necrotrophic flatworms undergo a phenomenon in which the epidermis is replaced by a new skin, the neodermis, during maturation. Thus, all cilia are lost and movement within the host is carried out by detaching and reattaching the sucker or variations of the sucker.

All flukes (digenean and monogenean trematodes) are internal or external necrotrophs, usually of vertebrates. The digenean trematodes have a life cycle that includes several types of larvae and sometimes an intermediate host or hosts. Trematode larvae include miracidium, sporocyst, redia, cercaria, and metacercaria. Schistosomiasis (bilharziasis), caused by several species of the blood fluke Schistosoma, is currently the second most prevalent infectious disease worldwide (malaria is first). Cercariae, which are distinctive swimming larvae with a tail and sucker, are carried by snails that spread schistosomiasis. Snails release cercariae; the cercariae swim, attach to and penetrate human skin between the fingers and toes, and then mature into adult worms and migrate to take up residence in the liver and other organs of the human host. The disease results from the human host's immune response to schistosome eggs deposited in host tissues by activation of lymphocytes and other immune cells; urinary tract and bowel blockage also can result. Monogenean trematodes typically have only one larval type, the onchomiracidium, which is released alive. Larvae move about the host or locate other hosts and attach. The larva matures, a neodermis replaces the ciliated epidermis, and the cycle repeats itself. Trematodes have one or two suckers; some trematodes feed through their oral suckers.

Tapeworms (cestodes) are exclusively internal necrotrophs that usually attach inside the gut of vertebrates by means of a specialized structure, the scolex. The scolex may contain exclusively suckers or a combination of suckers and other structures enabling a firm grasp of the host tissue. Tapeworms lack a gut. Microvilli (minute tissue projections) absorb nutrients (amino acids and sugars) from the hosts exploited by tapeworms. The typical tapeworm body plan consists of the scolex followed by repeated segments, each with reproductive organs; these sexually reproducing segments bud from the tapeworm's neck. Many tapeworms, however, lack segments. Like flukes, most tapeworms have intricate life cycles with several distinctive larval types. Others, typically without segments, have simpler life cycles, and may represent progenetic forms of more typical segmented tapeworms.

Figure A. Dorsal view of gliding Procotyla fluviatilis, a live freshwater turbellarian flatworm from Great Falls, Virginia. Its protrusible pharynx connects to a branched intestine visible through its translucent body. Bar=1   cm. [Photograph courtesy of R. Kenk; drawing by L. Meszoly; information from R. Kenk.]

The large surface area of free-living and symbiotrophic flatworms relative to their volume has physiological implications. Oxygen, carbon dioxide, and ammonia exchange across the body surface. Like cnidarians and ctenophores, flatworms are blood-, lung-, and heartless. In symbiotrophic flatworms, gases and nutrients diffuse into tissues of the flatworm from the host digestive system or from water. Free-living flatworms ingest food. Platyhelminth worms that have a gut discharge solid waste through their mouths because they lack an anus. Protonephridia are the organs of excretion and osmoregulation in flatworms, except in free-living flatworms that lack a gut. Protonephridia are extremely primitive excretory structures that are composed of ciliated cells, called "flame cells", that collect dissolved wastes. Protonephridia regulate water and ions by wafting liquid through ducts that exit to the outside through pores.

The simplest flatworm nervous system consists of light-sensitive pigment-cup eyespots (either single or in groups) connected to a cluster of nerve cells (brain) in the head and ventral, longitudinal nerve cords. The nervous system of flatworms ranges in complexity from this simple system to the more primitive nerve net of acoel free-living flat worms resembling that of cnidarians and ctenophores. Free-living flatworms detect chemicals, food, objects, and currents with sensory pits or tentacles on the sides of the head. When flatworms wander away from a scent source, they turn from side to side more frequently and so eventually home in on the source.

Triclad free-living flatworms and cestodes have prodigious powers of regeneration and reproduce sexually or asexually. Slices of Dugesia, a triclad, regenerate to form entire worms. Planarians (freshwater species of triclad free-living flatworms, an order of free-living flatworms characterized by a gut having three branches) and taeniid cestodes reproduce asexually as well as sexually. Almost all flatworm species are simultaneous hermaphrodites. Each individual flatworm bears ovaries and testes. Self-fertilization is rare in free-living flat worms and common in cestodes. In mating pairs of hermaphrodites a copulatory bursa receives sperm or, in some free-living flatworms, a hypodermic-like penis injects sperm through the body wall into the body of the mate. Some flatworm sperm have no tails; others have a [9(2)+1] arrangement or a [9(2)+0] arrangement (see Figure I-3). Ribbons of fertilized free-living flatworm eggs are laid in cocoons. Freshwater flat worms "glue" eggs to stones. Eggs of most free-living flat worms hatch into miniature adults. A few marine free-living flatworms develop ciliated larvae known as Muller's larvae. Necrotrophic flatworms frequently have complex reproductive cycles, with a succession of larval stages. Schistosoma, a fluke, is dioecious. Bothrioplana, a free-living flat worm, exhibits parthenogenesis—females asexually produce females.

The origin of this phylum is uncertain because flatworms fossilize poorly. Prevailing theory maintains that the bilaterally symmetrical, triploblastic, acoelomate pattern of early free-living flat worms was ancestral to the coelomates. Arguments that the Platyhelminthes represent secondarily reduced coelomates persist, however, and some data from molecular studies suggests they are spiralian protostomes; their coelom, anus, and multicellular excretory organs having been lost. Free-living flatworms logically preceded parasitic forms, but the relationship between platyhelminth worms and other metazoan phyla remains unclear.

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Parasites

Colin G. Scanes , Samia R. Toukhsati , in Animals and Human Society, 2018

16.4 Cestodes (Tapeworms)

16.4.1 Overview

Tapeworms are parasites that inhabit the intestines. They consist of a head (scolex) imbedded into the mucosa, a neck, and a body consisting of a series of segments or proglottids ( Fig. 16.10). These contain male and female gonads.

Figure 16.10. Cestodes.

(A) Adult beef tapeworm (Ta. saginata). (B) Scolex (head) of pork tapeworm (Ta. solium) showing hooks and suckers for attachment to the intestine. (C) Mature proglottids from beef tapeworm (Ta. saginata) after injecting with lactophenol cotton blue. (D) Cestode eggs (diameter 30–34 μm) from Taenia sp.

 Source: Part A–D: Courtesy CDC, 2017. Parasites. Available from: https://www.cdc.gov/parasites/ including Flukes http://www.cdc.gov/parasites/fasciola/; Parasites—Angiostrongyliasas (also known as angiostrongyliasis infection) http://www.cdc.gov/parasites/angiostrongylus/; Parasites—Ascariasis http://www.cdc.gov/parasites/ascariasis/; Parasites—Dracunculiasas (also known as guinea worm disease) http://www.cdc.gov/parasites/guineaworm/; Parasites—Hookworm http://www.cdc.gov/parasites/hookworm/; Parasites: Neglected parasitic infections https://www.cdc.gov/parasites/npi/; Parasites—Schistosomiasis http://www.cdc.gov/parasites/schistosomiasis/epi.html; Taeniasis http://www.cdc.gov/parasites/taeniasis/biology.html; Parasites—Toxocariasis (also known as roundworm infection) (http://www.cdc.gov/parasites/toxocariasis/gen_info/faqs.html); Parasites—Trichuriasis (also known as whipworm infection) http://www.cdc.gov/parasites/whipworm/biology.html.

Tapeworms that infect people include the following (WHO, 2015):

•

Genus Taenia causing Taeniasis and cysticercosis

•

Taenia solium (pork tapeworm)

•

Taenia saginata (beef tapeworm)

•

Taenia asiatica (Asian tapeworm)

•

Taenia multiceps

•

Genus Hymenolepis causing hymenolepiasis

•

Rat tapeworm (Hymenolepis diminuta)

•

Dwarf tapeworm (Hymenolepis nana)

•

Other tapeworms

•

Dipylidium caninum

•

Diphyllobothrium sp. (fish tapeworm)

•

Echinococcus granulosus (dog tapeworm)

•

Spirometra

16.4.2 Taenia

Ta. solium (pork tapeworm) results in 2.8 million DALY (WHO, 2016). Larval Ta. solium infection in the brain (neurocysticercosis) is a major cause of epilepsy in developing countries with it estimated that Ta. solium is the cause of 30% of epilepsy cases in many endemic areas where people and roaming pigs live in close proximity (Del Brutto et al., 1992; Harhay et al., 2010; Ndimubanzi et al., 2010). Ingestion of Ta. solium eggs can lead to neurocysticercosis. The CDC estimates that there are 1000 new hospitalizations in the USA per year due to neurocysticercosis; this being a major cause of infectious seizures (CDC, 2017). The life cycle of Ta. solium and Ta. saginata are shown in Fig. 16.11 (WHO, 2016).

Figure 16.11. Life cycle of beef and pork tapeworms.

 Source: Courtesy CDC, 2017. Parasites. Available from: https://www.cdc.gov/parasites/ including Flukes http://www.cdc.gov/parasites/fasciola/; Parasites—Angiostrongyliasas (also known as angiostrongyliasis infection) http://www.cdc.gov/parasites/angiostrongylus/; Parasites—Ascariasis http://www.cdc.gov/parasites/ascariasis/; Parasites—Dracunculiasas (also known as guinea worm disease) http://www.cdc.gov/parasites/guineaworm/; Parasites—Hookworm http://www.cdc.gov/parasites/hookworm/; Parasites: Neglected parasitic infections https://www.cdc.gov/parasites/npi/; Parasites—Schistosomiasis http://www.cdc.gov/parasites/schistosomiasis/epi.html; Taeniasis http://www.cdc.gov/parasites/taeniasis/biology.html; Parasites—Toxocariasis (also known as roundworm infection) (http://www.cdc.gov/parasites/toxocariasis/gen_info/faqs.html); Parasites—Trichuriasis (also known as whipworm infection) http://www.cdc.gov/parasites/whipworm/biology.html.

16.4.3 Diphyllobothrium sp.

There is a growing incidence of the infection diphyllobothriosis with fish tapeworms including Diphyllobothrium latum, Diphyllobothrium dendriticum, Diphyllobothrium nihonkaiense, and Adenocephalus pacificus (syn. Diphyllobothrium pacificum) (Chen et al., 2014; Kuchta et al., 2013, 2015). The increase in infections is due to consumption of raw or insufficiently cooked fish (Kuchta et al., 2013). Diphyllobothriosis infection is usually asymptomatic, but can be associated with abdominal pain, diarrhea, nausea, and weight loss.

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Food Hazards: Physical, Chemical, and Biological

Pradeep Kumar Singh , ... Ram Lakhan Singh , in Food Safety and Human Health, 2019

Tapeworms (Cestodes)

Tapeworms live in human intestines, where they feed on the partially digested food there. The three common types of tapeworms are Taenia solium, found in pork; Taenia saginata, found in beef; and Diphyllobothrium latum, found in fish. Tapeworm eggs are for the most part ingested through food, water, or soil contaminated with human or animal host excrement. After ingestion, they develop into larvae, which can move out of the intestines and form cysts in different tissues, for example, lungs and liver. The infection of tapeworms in the intestine usually causes no symptoms. However, some people experience upper abdominal discomfort, diarrhea, loss of appetite, and sometimes anemia. Sickness is by and large perceived when the infected person passes segments of proglottids in the stool.

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Parasitology

ELLIS C. GREINER , DOUGLAS R. MADER , in Reptile Medicine and Surgery (Second Edition), 2006

TAPEWORMS

Tapeworms are flatworms comprising a scolex (holdfast organ) and a chain of repetitive sections (proglottids). Each proglottid increases in maturity as they move farther from the scolex with budding of new sections. The adults reside in the small intestine of their DH, and they all have indirect life cycles (Figure 21-26). Tapeworms parasitize all groups of reptiles except crocodilians.

Tapeworms can outcompete the host for basic nutrients, but if the host is on a good plane of nutrition, tapeworms are not considered to be pathogens. Reptiles may serve as an IH as the larvae tapeworms, not the adult, are found in the reptiles. These larvae are often subcutaneous, causing bumps or ridges in the skin, and make a reptile less attractive for display purposes.

Many of the adult tapeworms of reptiles are in the families Anoplocephalidae, Diphyllobothriidae, and Proteocephalidae. Anoplocephalidae typically use a mite or an insect as the IH. Diphyllobothriidae life cycle uses two IHs, typically an aquatic crustacean as the first and a vertebrate as the second (Table 21-3).

Proteocephalidae cycles have the eggs eaten by copepods in which the procercoid larva develops, and these are infective to the DH. But they first develop into plerocercoids, and these develop in the solid organs, such as the liver. The parasite then wanders through the host, and if it reaches the lumen of the intestine, it attaches and matures. Adult tapeworms may be diagnosed with fecal flotation if the eggs are released from the proglottids or with visualization of free proglottids in the feces and then a search for eggs in normal saline solution. All of the tapeworm eggs (except species of the Diphyllobothriidae) from reptile feces should contain a fully formed oncosphere with six hooks (Figures 21-27 to 21-33).

Larval tapeworms of Spirometra (spargana) may be found in the viscera (see Figure 72-11) or subcutaneously in snakes and lizards, and the same is true for the tetrathyridia of Mesocestoides. The latter should contain an inverted scolex with four suckers, but this is not always obvious in the experience of the senior author.

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Cestoda (Tapeworms)

Seppo Saari DVM , ... Sven Nikander DVM, PhD , in Canine Parasites and Parasitic Diseases, 2019

Abstract

Tapeworms (Cestoda) are flat, tape-like worms with the size, depending on the species, from only couple of millimeters up to several meters. Dogs usually act as their definitive hosts, but infections in them are generally nonpathogenic. Dogs may, however, be a source of a human infection, since some of the species are zoonotic. As a thumb rule, the tapeworm cannot be directly transmitted from one dog to another, the development and transmission must happen via one or several intermediate hosts. Dogs roaming free or allowed to eat prey or other uncooked meat from an intermediate host, are at risk to acquire an infection. A flea that contains tapeworm larval stages is another common source of canine infection. The head of tapeworms, scolex, contains structures, such as grooves, suckers or hooks, which enable the worm to attach to the gut wall. The major part of the tapeworm is called strobila and it consists of segments, proglottids. They each contain both male and female reproductive organs. Tapeworms do not have a digestive tract. Instead, they absorb their nutrients from the gut contents of the host directly through their outside surface, tegument. New proglottids are constantly formed in the neck of the worm and along the length of the tapeworm they mature. The most distal segments are filled with embryonated eggs. Segments detach and are passed in the feces. Cestodes important in veterinary medicine belong to the orders Cyclophyllidea or Pseudophyllidea. Cyclophyllidea tapeworms lack a uterus opening for egg release and eggs are distributed within the segments. Thus, eggs are not detected with sufficient sensitivity with the common methods in fecal analysis. Pseudophyllidea cestodes have a uterus opening for secreting the eggs directly into the fecal mass. These eggs are easily found in routine fecal analysis. Treating tapeworm infections is usually done with praziquantel or epsiprantel. Prevention includes an effective flea control and avoiding raw meat.

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Biology and Diseases of Ruminants (Sheep, Goats, and Cattle)

Wendy J. Underwood DVM, MS, DACVIM , ... Adam Schoell DVM, DACLAM , in Laboratory Animal Medicine (Third Edition), 2015

ii Abdominal Or Visceral Cysticercosis and Echinococcosis (Hydatid Cyst Disease)

Tapeworm eggs from the primary host contain an oncosphere, which hatches and penetrates the intestinal wall when ingested by the intermediate host. The second-stage larva develops in the intermediate host and is called a metacestode which is a space occupying cystic structure. Metacestode forms are tapeworm specific and include the cysticercoid (microscopic and in small intermediate hosts such as insects of mites); cysticercus (small blister to ping-pong ball sized structures); coenurus (usually intracranial in the host) and hydatid cysts (usually intraabdominal in the host). The coenurus and hydatid cysts can become quite large, contain multiple larvae, and can locally bud and spread. When the metacestode form is ingested by the primary host, the larval brood capsules containing protoscolices evaginate to form the tapeworm head called the scolex.

Abdominal or visceral cysticercosis is an occasional finding in ruminants at slaughter. The 'bladder worms' typically affect the liver or peritoneal cavity and are caused by the larval form (metacestode) of Taenia hydatigena , the common tapeworm of the dog family. The larval intermediate of another tapeworm of canids, Echinococcus granulosus, also may form hydatid cysts, particularly in liver and lungs. Ruminants are intermediate hosts of both parasites, and are infected by feed or water contaminated with gravid segments or ova. Although larval migration may cause nonspecific signs such as anorexia, hyperthermia, and weight loss, affected animals are usually asymptomatic. Infestation is usually diagnosed at necropsy or slaughter, and may result in condemnation of carcasses. Minimizing exposure to canine feces-contaminated feeds and water effectively interrupts the life-cycle. Research animals may have been exposed prior to purchase.

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Alimentary System and the Peritoneum, Omentum, Mesentery, and Peritoneal Cavity1

Howard B. Gelberg , in Pathologic Basis of Veterinary Disease (Sixth Edition), 2017

Cestodes.

Tapeworms, although frequently found in the alimentary system, are generally of little clinical significance. They require two and sometimes three hosts, often including arthropods and other invertebrates, to complete their life cycles. Tapeworms attach to the gut wall by means of their anterior scolex, which may have hooks in addition to four suckers ( Fig. 7-133). Although they can cause some damage at the site of attachment, generally they compete with the host for nutrients. Lacking an alimentary system, they absorb nutrients through their surface. Tapeworms are flat, segmented, and hermaphroditic, reproducing by addition of segments or proglottids. Examples of tapeworms are Anoplocephala spp. in horses, Moniezia spp. in ruminants, and Diphyllobothrium and Dipylidium spp. in dogs and cats. Mesocestoides spp. can infect dogs and cats. In some cases this parasite can perforate through the intestine and proliferate in the peritoneal cavity (see Fig. 7-25).

Taenia and Echinococcus spp. are the most destructive of the cestodes. Although carnivores are the definitive hosts, the larval forms reside in the viscera and body cavities of the intermediate hosts, usually ruminants, pigs, horses, or rodents (see Fig. 8-59). Human beings can also become infected, and sometimes it takes 20 or 30 years for clinical disease to appear. The damage in the intermediate hosts may be quite severe.

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Beetles (Coleoptera)

William L. Krinsky , in Medical and Veterinary Entomology (Third Edition), 2019

Intermediate Hosts of Parasites

Tapeworms (cestodes), flukes (trematodes), roundworms (nematodes), and thorny-headed worms (acanthocephalans) of many species that infest domestic and wild animals use beetles as intermediate hosts. Animals become infested by ingesting parasitized beetles that contaminate feed or bedding (tenebrionids, carabids) or that are attracted to animal dung (scarabaeids) or by ingesting water in which infective beetles have disintegrated.

Two tapeworms that infest the small intestines of poultry are the broad-headed tapeworm (Raillietina cesticillus) and Choanotaenia infundibulum (Fig. 9.11). Both parasites cause enteritis and hemorrhaging in chickens, turkeys, pheasants, and guinea fowl. A few tenebrionids and scarabaeids and more than 35 species of carabid beetles, notably in the genera Amara and Pterostichus, are intermediate hosts for R. cesticillus (Cheng, 1973). Some tenebrionid and dermestid species, including the lesser mealworm beetle, are intermediate hosts for C. infundibulum. Proglottids or tapeworm eggs ingested by beetle larvae or adults develop into cystercerci (encysted larvae) that can then infest birds that eat the beetles. Chicks are most susceptible to serious infestations and often die from worm burdens.

Figure 9.11. Tapeworm, Choanotaenia infundibulum (Cestoda: Dilepididae), parasite in small intestine of chickens, turkeys, pheasants, and guinea fowl; uses tenebrionid and dermestid beetles as intermediate hosts.

 Photograph by Nancy C. Hinkle.

The beef tapeworm (Taenia saginata) can use dung beetles and carabids as intermediate hosts, although they are not essential for transmission. Beetles associated with infective dung or debris can ingest proglottids or eggs as in the case of poultry worms. Cattle and humans infested with the tapeworm may exhibit mild symptoms such as weight loss, abdominal pain, and increased appetite.

The dwarf tapeworms (Hymenolepis nana and H. diminuta) that usually infest rodents, especially rats and mice, can infest humans when the intermediate host beetles are accidentally ingested. Tenebrio molitor may act as an intermediate host for H. nana, although this worm is readily transmitted directly from one vertebrate host to another. Several species of tenebrionids (Tenebrio spp. and Tribolium spp.) are required intermediate hosts for H. diminuta. Larval and adult beetles infesting grain and cereals ingest worm eggs that develop into cysticercoid stages that infest rodents or humans, usually children, who ingest the beetles. Dwarf tapeworms produce minimal symptoms in rodents and people, although heavy infestations in children may cause abdominal pain, diarrhea, convulsions, and dizziness.

Beetles are known to be intermediate hosts for only a few trematodes. These are parasites of frogs that become infested by ingesting parasitized dytiscid beetles and pose no problem for other vertebrate animals.

Many nematodes infest livestock and wildlife, but only a few use beetles as intermediate hosts. Spirurid nematodes of various species infest livestock and, rarely, humans. Physocephalus sexalatus and Ascarops strongylina eggs develop in many species of scarabaeid dung beetles (Geotrupes spp., Onthophagus spp., and Scarabaeus spp.) that then may be ingested by pigs. Both wild and domestic swine can be infested with these stomach worms that cause digestive problems in heavily infested young animals. Gongylonema pulchrum is a parasite of the upper digestive tract of sheep, cattle, goats, and other ruminants as well as horses, dogs, and humans. The worms burrow in the mucosa and submucosa of the oral cavity and esophagus and may cause bleeding, irritation, numbness, and pain in the mouth and chest. Scarabaeid and tenebrionid beetles serve as intermediate hosts for the larvae. Scarabaeid dung beetles are also the intermediate hosts for Spirocerca lupi, the esophageal worm of dogs and wild canids. Physaloptera caucasica, another spirurid, often parasitizes monkeys in tropical Africa, where humans are also commonly infested. This nematode causes digestive distress by infesting the alimentary tract from the esophagus to the terminal ileum. Scarabaeid dung beetles are its intermediate hosts.

The acanthocephalans, aptly named for their thorny heads, include species found worldwide infesting swine, rodents, and carnivores such as dogs. Macracanthorhynchus hirudinaceus, which attaches to the small intestines of swine, causes enteritis and produces intestinal nodules that lower the value of these tissues when they are sold to make sausage casings. Eggs of this parasite are ingested by scarab beetle larvae of species belonging to various genera (Phyllophaga, Melolontha, Lachnosterna, Cetonia, Scarabaeus, and Xyloryctes), including May and June beetles, leaf chafers, dung beetles, and rhinoceros beetles. Infested beetle larvae, as well as the pupae and adults that develop from them, are infective to both pigs and humans. Humans and pigs often show no symptoms. However, in cases of heavy infestations, both human and porcine hosts may experience digestive problems, such as abdominal pain, loss of appetite, and diarrhea that can lead to emaciation.

Two other acanthocephalan worms that parasitize the small intestines of their hosts use scarab beetles or tenebrionids as intermediate hosts. They are Macracanthorhynchus ingens, which infests raccoons and occasionally dogs and humans, and Moniliformis moniliformis, a parasite of rodents and dogs.

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Parasites in farmed fish and fishery products

A. Levsen , ... B. Berland , in Improving Farmed Fish Quality and Safety, 2008

Cestoda – the tapeworms

Adult tapeworms occur in the intestine of various vertebrates including fish. Their bodies are typically long and flat, consisting of many segments called proglottids, where each is a gamete-producing factory. In contrast to trematodes, the body surface of tapeworms is covered with microvilli, which are tiny projections that formidably increase the surface area. As a digestive tract is completely absent, the microvilli facilitate absorption of nutrients from the host. At the anterior end, there is an attachment apparatus – the scolex – consisting of suckers and/or hooks. The life cycle of tapeworms typically includes the egg, one free-living larval stage, a procercoid and a plerocercoid in the first and second intermediate host, respectively, and the adult in the definitive host. Fish may serve as second intermediate host, definitive host, or in some species as both.

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