Platyhelminths (flatworms)

🪱 Platyhelminthes (Flatworms)

  • Bilateral symmetry

  • Body cavity lacking (acoelomata)

  • Three layers

  • Hermaphrodites (some exceptions)

  • No respiratory and circulatory system

  • Simple excretory system

  • Centralized nervous system

The Platyhelminthes class includes:

  • Monogenea: Direct life cycle

  • Trematoda

  • Cestoda

  • Turbellaria: Non-parasitic

☘ Trematodes

Characteristics

  • Endoparasites, occasionally ectoparasites (Aspidogastrea, Transversotrematidae)

  • Parasites of vertebrates (excluding Aspidogastrea)

  • More than 2700 genera and 8000 species

  • Oral and ventral sucker

  • Parasitizing all organ systems

  • Complex indirect life cycle with 1-3 intermediate hosts

Taxonomy

  • Aspidogastrea: None of veterinary or medical importance

  • Digenea

    • Diplostomida

      • Brachylaimoidea

      • Diplostomoidea

      • Schistosomatoidea

    • Plagiorchiida

    • Other superfamilies: 19 (including Echinostomatoidea, Opistorchioidea, and Gorgoderoidea)

Digenean Trematodes

  • All digenea are parasitic (endoparasites of vertebrates)

  • Dorso-ventrally flattened body with simple anatomy and without segmentation

  • No coelom, but animals are filled with mesodermal parenchyma

  • No blood vessels, simple ladder nervous system

  • Possess two suckers (oral and ventral acetabulum) for attachment, oral sucker contains the mouth

Excretion System

  • Protonephridial system + paranephridial system

Reproductive System

  • Most trematodes are hermaphrodites (cross as well as self-fertilization occurs)

  • Gonochorists (family Schistosomatidae) exhibit sexual dimorphism

Digestive System

  • The gut of trematodes is blind ending but can be quite extensive and highly branched

Surface of the Fluke Body

  • Tegument

Trematode Life Cycle Examples

  1. Fasciola hepatica (Fasciolidae)

  2. Opisthorchis viverrini (Opistchorchiidae)

  3. Schistosoma (Schistosomatidae)

Trematode Families of Veterinary Importance

Family

Examples

Paramphistomidae

Paramphistomum cervi, Calicophoron daubneyi

Fasciolidae

Fasciola hepatica, Fascioloides magna, Fasciolopsis buski

Dicrocoeliidae

Dicrocoelium dendriticum, D. hospes

Opisthorchiidae

Opisthorchis viverrini, O. felineus

Paragonimidae

Paragonimus westermani

Schistosomatidae

Schistosoma bovis, S. mattheei, S. japonicum, S. mansoni

🐑 Family Fasciolidae

Fasciola hepatica (Common Liver Fluke)

  • Adults: 2-4 cm, feed on blood

  • Localization: Liver (bile ducts)

  • Definitive host: Cattle, sheep, goat, buffalo

  • Worldwide losses in livestock production: $3.2 billion US/year

  • Infection of humans: More than 2.6 million are infected

  • Intermediate host (IMH): Freshwater snails family Lymnaeidae (Galba truncatula)

Fasciola gigantica

  • Cattle, buffaloes, sheep, humans

  • Subtropical and tropical areas of Asia and Africa

Fascioloides magna

  • Deer and domestic ruminants in North America and Central Europe

Intermediate Host of Fasciola hepatica : Galba truncatula

  • Small gasteropod mollusc, dark shell

  • Survival to freezing/drying: Overwintering in the mud.

  • Optimal conditions: Slightly acid pH, still/slow running water

  • Optimal temperature range: 15-22°C. Stop under 5°C.

Epidemiology of Fasciola hepatica (and Galba spp.)

  1. Adequate habitat for the intermediate host (humid areas along rivers/running water irrigation facilities, ponds, small water amounts)

  2. Temperature (optimal development rate of both parasite and host: 15°C)

  3. Humidity (earth saturated by rains offers the best environmental setting: egg maturation, miracidia survival, cercarial dissemination)

Increase of environmental amount of infective metacercariae = CLINICAL FORMS INCREASE

Fasciola hepatica in the UK

  • Summer infection of I.H.

    • Metacercariae on pasture from August to October

    • Miracidia comes from egg laid in spring or overwintered

    • Development of cercariae takes place in summer

    • PERIOD OF MAJOR RISK FOR HEAVY INFECTIONS

  • Winter infection of I.H.

    • Metacercariae on pasture from May to June

    • Cercariae come from molluscs infected in previous autumn and that overwintered within molluscs.

    • LESS IMPORTANT THAN THE SUMMER INFECTION

Fasciola hepatica in France

Three periods of risk year-round:

  1. Release on pasture, March-April (overwintered metacercariae)

  2. End of June, beginning of July (metacercariae coming from molluscs that restarted development in the beginning of spring). Risk especially for lambs born in winter.

  3. From September on, MAIN RISK PERIOD (metacercariae coming from molluscs born the same year). Affecting all the animals.

Fasciola hepatica: Pathogenicity

  • PARASITIC MIGRATION WITHIN LIVER PARENCHYMA (young flukes): Hemorrhage, damaged liver functions

  • FLUKES IN BILIARY DUCTS (adult flukes): Blood-feeding (5ml/blood/parasite/day), mucosal damage/erosion due to spines on dorsal surface

Clinical Signs of Fasciolosis

Several clinical forms:

  • Acute fasciolosis (type I, type II)

  • Subacute fasciolosis

  • Chronic fasciolosis

  • Sudden deaths

  • Anemia

  • Pale mucosae

  • Hypoalbuminaemia

  • Oedema (bottle jaw), ascites

  • Enlarged and painful livers

  • Poor body condition

  • Decreased milk production

  • Reduction of weight gains

Ovine Fasciolosis

CLINICAL PICTURE DEPENDS ON BURDEN OF INFECTION

  • Acute:

    • Many metacercariae (>2000) / short period of time: diffuse liver hemorrhages (flukes migrations)

    • Signs: (autumn/beginning of winter): sudden death of many animals 2 weeks pi. Surviving animals anemic and weak.

  • Sub-acute:

    • Less metacercarie (500-1500) / longer period of time: some parasites in biliary ducts - others still migrating in hepatic tissue

    • Clinical signs (autumn/winter): anemia, jaundice, death 8-10 weeks pi if not treated; 1-2 weeks of clinical signs before death. Hypoalbuminemia: oedema, ascites.

  • Chronic:

    • Few metacercarie (200-500) / diffuse period of time, several hepatic damages (hepatic fibrosis, cicatrization of migratory lesions, parasites in the biliary ducts, hyperplastic angiocolitis from physical damage)

    • Signs (late winter/spring): anemia, weight loss, edema, ascites

Bovine Fasciolosis

  • Chronic disease is most commonly described in cattle

  • Acute and subacute disease: young animals (RARELY)

  • Calcification of biliary ducts (typical appearance of liver)/enlargement of gallbladder

  • Possible co-infection with Ostertagia combined clinical syndrome

  • CATTLE IMMUNITY removes infection after less than one year, while in sheep infection can be life-lasting

Diagnosis of F. hepatica

  • Post mortem = gross pathology

  • Coprology: Faecal egg counts

  • Immunodiagnosis

  • Biochemical analysis

  • Molecular (PCR, LAMP)

Post Mortem Diagnosis

  • Typical pathological changes in livers:

    • Fibrosis

    • Calcification of bile ducts

    • Hyperplasia of bile ducts

Coprology

  • Sedimentation technique or flotation with heavy-density solutions (>1.45)

  • Eggs detectable from 9-12 weeks post infection

  • Intermittent excretion of eggs false negative results

  • Only 40-60% sensitivity

  • Eggs: yellowish-brown in color, 120-150 m

  • Differ. diagnosis: Paramphistomum cervi

Immunodiagnosis

  • ELISA detection of specific antibodies in serum or milk (individual serum samples vs. bulk tank milk samples)

  • Detection of F. hepatica antigen in serum or faeces (coproantigen ELISA)

  • Western Blot

  • Advantage: high sensitivity, specificity, early detection (since 2-3 weeks post-infection)

Disadvantage of ELISA

  • Persistency of antibodies after treatment

  • Cut off

  • ELISA Individual serum samples Individual milk samples Bulk tank milk samples screening of fasciolosis on herd level, detectable from 20 % positive individuals in herd

  • Spatial distribution of F. hepatica in dairy cattle in Sweden

  • Spatial distribution of F. hepatica in beef cattle in Sweden

Coproantigen Detection

  • Detection of coproantigen in animal faeces

  • Bio-X Diagnostics kit

  • Detectable from 4-7 weeks PI

PCR

  • A = primer Kralova-Hromadova (amplified region 112 bp, gene ITS-2)

  • B = primer Cucher (ampl. reg. 405 bp, cox-1)

    1. F. hepatica adult

    2. F. magna adult

    3. Snail infected with F. hepatica

    4. Snail infected with F. magna

    5. Snail infected with H. cylindracea

  • PCR detection of F. hepatica larval stages in snails

  • PCR detection of F. hepatica eggs in faeces

  • PCR for distinguishing of F. hepatica, Paramphistomum cervi, Fascioloides magna eggs

Control of Fasciolosis

  • Anthelmintics (flukicides)

  • Alternative approaches

    • When?

    • Which drug?

    • Withdrawal period?

    • Withholding time?

  • Survey of flukicides

  • Active compounds that can be used for different stages of liver fluke infection in sheep and cattle, respectively *Resistance to flukicides

Fascioloides magna (Giant Liver Fluke)

  • DH: cervids, cattle, sheep, goats

  • IMH: Galba truncatula and other lymnaeid snails

  • Adults 4-10 cm, in liver parenchyma (pseudocysts)

🐌 Dicrocoelium dendriticum (Lancet Liver Fluke)

  • DH: ruminants, occasionally in other mammals

  • Adults 7-15 mm, in bile ducts and gallbladder

    1. IMH: terrestrial snail (Helicella, Zebrina)

    2. IMH: ants (Formica)

  • Adult fluke (7-15 mm)

  • Sporadic in humans

  • Egg (40 m)

🐮 Paramphistomum cervi and Calicophoron daubneyi (Rumen Flukes)

  • Adults 6-12 mm, attached to rumen mucosa, eventually in the duodenum

  • DH: ruminants (mainly cattle, red deer)

  • IMH: aquatic snails (Planorbidae, Lymnaeidae)

  • P. cervi pathogenicity vs. C. daubneyi pathogenicity

  • C. daubneyi in France, Spain, UK

🩸 Schistosomes

  • S. bovis, S. mattheei, S. curasoni Africa

  • S. spindale, S. nasale, S. japonicum Asia

🪢 Class Cestoda (Tapeworms)

  • Segmented body: scolex and proglottids (strobila)

  • Hermaphrodites only (each proglottid contains both male and female reproductive organs)

  • No GIT tract

  • 2-host life cycles

Cestodes in Ruminants

  • Anoplocephalidae: Anoplocephala, Moniezia

  • Taeniidae: Taenia spp., Echinococcus spp. (ruminants as intermediate hosts)

Anoplocephala

  • Species: A. magna (80 cm), A. perfoliata (8 cm), Paranoplocephala mamillana (0.6-5 cm)

  • Parasites of horses

  • Habitat: small or large intestine

  • IMH: Oribatid mites (cysticercoid)

  • Scolex: without rostellum and hooks, lappets (A. perfoliata)

  • Mature proglottids: much wider than longer

  • Symptoms: catharral or ulcerative enteritis, colic, and diarrhea in heavy infections

  • Therapy: praziquantel, pyrantel

Moniezia

  • Species: M. expansa (2 m), M. benedeni

  • Parasites of ruminants

  • IMH: oribatid mites

  • Mature proglottids: much wider than longer

  • Eggs are triangular with piriform apparatus (56-67 m)