General pathology of Helminthic and protozoal diseases

GENERAL PATHOLOGY OF HELMINTHIC AND PROTOZOAL INFECTIONS  

Parasitic helminths or worms, are important causes of disease in all species of animals. Their life cycle is complex: Most alternate between sexual reproduction in the final host and asexual multiplication in an intermediate host or vector except Strongyloides, the larvae which can become infective inside the intestine

Two important consequences of this lack of multiplication by the adult worms

  1. Disease is usually caused by inflammatory responses against their eggs or larvae rather than the adults (e.g.Schistisomiasis).
  2. Disease is in proportion to the number of parasites that affect the individual ( e.g.10 hook worms can cause little damage, whereas 1000 hookworms cause severe anaemia by consuming 100 ml of blood per day)

Phylum 1.Platyhelminthes: Trematodes and cestodes

Phylum 2.Nemathelminthes: Nematodes

Phylum 3.Acanthocephala: Thorny –headed worms

Effects of helminthic parasites on the host

1.Mechanically interfere with function

  • Obstruct blood vessels or lymphatics
    • Right ventricle and pulmonary artery : Dirofilaria immitis in dogs
    • Carotid arteries : Elaeophora schneideri ( sheep)
    • Lymphatics: Dracunculus insignis(dogs)
    • Mesenteric arteries-Strongylus vulgaris(horses)
    • Aorta : Spirocerca lupi (dogs), S.vulgaris (horses)
    • Vena cava- Schistosoma bovis, S.haematobium
  • Obstruct ducts or tracts : Bile duct (Liver flukes), oesophagus (Spirocerca lupi), intestinal lumen (Ascarids), respiratory tracty (Filaroides osleri, Metastrongylus apri) and urinary tract (Dioctophyma renale)
  • Attach to or use functional tissue – Stomach mucosa (Trichostrongylus axei- sheep and cattle), Draschia megastoma (Horse), small intestine (Hook worms) , caecum and large intestine (Strongyles-Horses, caecal worms-dogs)
  • Act as foreign body with resultant tissue reactions displacing normal structures : Schistosome ova, dead larvae (T.canis, D.immitis)

2. Invade and displace cells and tissues, producing necrosis, loss of function and hypersensitivity reactions

  • Skin- Hook worm larvae, Habronema larvae, Onchocerca larvae
  • Liver – Giant liver flukes, kidney worm larvae, ascarid larvae, Echinococcus and cysticercus cysts
  • Intestinal wall- Nodular worm (Oesophagostomus sp.), larvae of Strongyles
  • Brain and spinal cord- Coenurus, echinococcus, filarial or other helminth larvae
  • Lung- Lung worm, ascarid larvae, hook worms
  • Musculature-Trichinae, cysticerci

3. Suck blood and thereby cause anaemia

  • · Hook worms (dogs, cattle)and stomach worms(cattle, sheep)

4. Use food needed by the host

  • · Tape worms and ascarids

5. Induce or predispose to neoplasia

  • · Oesophagus-Spirocerca lupi, Urinary bladder(Schistisoma haematobium in humans), liver (Cysticercus fasciolaris)

6. Introduce bacterial or other infection into tissues of the host

  • · Lungs (lung worms, ascarid larvae), intestinal wall (hook worms, nodule worms, salmon flukes-dogs), perirenal tissues (Stephanurus dentatus -pigs), caecum ( histomonad of turkeys)

7. Eat up tissues of the host

  • · Ascarids and stomach worms

8. Secrete toxic products (haemolysins, histolysins, anticoagulants)

  • Hook worms, nodule worms, stomach worms, Strongyles
  • The subkingdom Protozoa (Kingdom : Protista) is composed of various single celled eukaryotes
  • The vast majority of eukaryotes are free living and not pathogenic to either animals or humans
  • The important pathogenic protozoa are
    • Leishmania, Trypanosoma, Giardia,Tritrichomonas,Trichomonas, Histomonas, Entamoeba, Klossiella, Hepatozoon, Eimeria, Caryospora, Isospora, Cryptosporidium, Besnoitia, Neospora, Toxoplasma, Sarcocystis, Plasmodium, Babesia, Theileria, Cytauxzoon, Encephalitozoon, Balantidium, Pneumocystis

The subphylum Mastigophora includes those protozoa whose trophozoites have one or more flagella.

These may occur as extracellular parasites e.g. Trichomonas,Trypanosoma sp.and Giardia or as intracellular parasites e.g. Leishmania and Histomonas. The life cycles includes

  • Direct transmission with no extra-host stage
  • Direct transmission with resistant cyst stage outside their mammalian hosts(Giardia)
  • Indirect transmission with parasitic stage in some insect, which serve as an intermediate host (Trypanosmoa and Leishmania)
  • In Histomonas, the trophozoites enter and live in eggs of the round worm Heterakis gallinae

The subphylum Sarcodina includes those protozoa which utilize pseudophodia for locomotion.e.g. Entamoeba. These are extracellular parasites of intestine and their life cycle is direct

The phylum Apicomplexa which contains most of the important pathogenic protozoan parasites. They lack means of locomotion . Their life cycle may be

  • Direct with an encysted extra-host form e.g. Eimeria, Isospora, Klossiella and Cryptosporidium
  • Indirect with an intermediate host e.g. Sarcocystis, Toxoplasma, Babesia, plasmodium, Besnoitia and Neopora . Babesia, plasmodium and Theilria are all intracellular parasites

The phylum Microspora contains Encephalitozoon cuniculi which is transmitted through spores in the urine and may also be transmitted in utero

The phylum Ciliophora contains dikaryotic protozoa ( i.e. having two nuclei) with cilia or other cilary structures for locomotion e.g.Balantidium coli which is transmiited through cysts.

 
Last modified: Thursday, 22 March 2012, 6:09 AM