Parasitic Diseases – Effects of Helminthic Parasites on the Host

Overview: Effects of Helminthic Parasites on the Host

Helminths (worms) can harm the host through several non-exclusive mechanisms.
Damage may be direct (mechanical, nutritional, chemical) or indirect (immunological, infectious, neoplastic).
Severity depends on: worm burden, developmental stage, anatomical location, duration of infection, and host factors (age, nutrition, immunity).

1. Mechanical Interference with Normal Physiology

1.1 Obstruction of Blood or Lymph Channels

Right ventricle / pulmonary artery – $Dirofilaria\;immitis$ (dog heartworm) ➔ right-sided heart failure, pulmonary hypertension.
Carotid arteries – $Elaeophora\;schneideri$ (sheep) ➔ cerebro-ischemic signs, dermatitis (“sorehead”).
Lymphatic channels – $Dracunculus\;insignis$ (dogs, raccoons) ➔ lymphangitis, edema.
Mesenteric arteries – $Strongylus\;vulgaris$ (horse) ➔ verminous arteritis, colic from intestinal ischemia.
Aorta – $Spirocerca\;lupi$ (dog) & $S.\;vulgaris$ (horse) ➔ aneurysms, thrombo-embolism.
Vena cava – $Schistosoma\;bovis$ , $S.\;haematobium$ ➔ pulmonary emboli, hematuria (in humans).

1.2 Obstruction of Ducts or Hollow Viscera

Bile duct – liver flukes (e.g., $Fasciola$ spp.), ascarids, fringed tapeworms ➔ cholestasis, jaundice.
Esophagus – $Spirocerca\;lupi$ (dogs) ➔ dysphagia, aneurysm formation.
Intestinal lumen – large ascarid or tapeworm burdens ➔ impaction, volvulus.
Respiratory tract – $Filaroides\;osleri$ (dogs), $Metastrongylus\;apri$ (pigs) ➔ bronchial granulomas, pneumonia.
Urinary tract – $Dioctophyma\;renale$ (giant kidney worm) ➔ hydronephrosis, renal failure.

1.3 Attachment to / Utilization of Functional Tissue

Stomach mucosa – $Trichostrongylus\;axei$ (ruminants), $Draschia\;megastoma$ (horse) ➔ chronic gastritis, ulceration.
Small intestine – hookworms (e.g., $Ancylostoma$ , $Uncinaria$ ) attach, suck blood, trigger villus atrophy.
Cecum / large intestine – equine strongyles, caecal worms of turkeys & dogs ➔ typhlitis, colitis.

1.4 Foreign-Body Reactions & Space Occupation

Eggs or dead larvae act as irritants; granulomatous inflammation may displace or compress vital tissue.
• Examples: schistosome ova; dead larvae of $Toxocara\;canis$ , $Dirofilaria\;immitis$ .

2. Invasion, Migration & Cellular Destruction

Parasites physically burrow through tissues, causing necrosis, hemorrhage, fibrosis, and Type I/IV hypersensitivity.

Target Tissue	Representative Helminths	Consequences
Skin	Hookworm larvae (cutaneous larva migrans), $Habronema$ spp. (summer sores), $Rhabditis$ , $Onchocerca$ , $Elaeophora$ , $Stephanofilaria\;stilesi$	Dermatitis, pruritus, non-healing ulcers
Liver	Giant liver flukes, kidney-worm larvae, cysticerci, hydatid/coenurus cysts, ascarid larvae, $Capillaria\;hepatica$	Parenchymal necrosis, fibrosis, hepatic insufficiency
Intestinal wall	Nodular worms ( $Oesophagostomum$ ), strongyle larvae	Granulomas, peritonitis
CNS	Coenurus, hydatid, filarial & other larvae	Cerebral pressure, ataxia, seizures
Lung	Lungworms, migrating ascarids, hookworms	Bronchitis, pneumonia, “verminous” granulomas
Muscle	$Trichinella$ (encysted larvae), cysticerci	Myositis, pain, weakness

3. Hematophagy & Resultant Anemia

Hookworms (dogs – $Ancylostoma\;caninum$ ; cattle – $Bunostomum$ ) ingest host blood daily (≈ $0.1–0.3\;mL$ per adult). Severe infections ➔ microcytic, hypochromic anemia.
Stomach worms (ruminants – $Haemonchus\;contortus$ , $Ostertagia$ ) cause d\”bottle jaw\” edema from hypoproteinemia.

Pathophysiology:
$\text{Blood\;Loss} \; (mL) = N_{worms} \times \text{Blood\;ingested\;per\;worm\;per\;day}$
Chronic hemorrhage ➔ ↓ PCV, ↓ Hb, compensatory erythropoiesis.

4. Nutrient Theft & Competition

Tapeworms (e.g., $Taenia$ , $Dipylidium$ ) absorb pre-digested nutrients across their tegument.
Ascarids (especially in young animals) physically occupy lumen and compete for lipids & vitamins.
Consequence: growth retardation, ill-thrift, “pot-belly.”

5. Parasite-Induced or Parasite-Associated Neoplasia

Organ	Helminth	Noted Lesion
Esophagus (dog)	$Spirocerca\;lupi$	Osteosarcoma, fibrosarcoma in periesophageal granulomas
Liver (rat)	$Cysticercus\;fasciolaris$ (larval $Taenia\;taeniaeformis$ )	Hepatic fibrosarcoma
Urinary bladder (human)	$Schistosoma\;haematobium$	Squamous cell carcinoma

Mechanisms: chronic inflammation, oxidative stress, mitogenic excretory/secretory (E/S) products.

6. Secondary Bacterial/Viral/Protozoal Infections

Parasite migration disrupts barriers, introducing or predisposing to pathogens.
• Lungs – lungworms, ascarid larval tracks ➔ Pasteurella, Mycoplasma co-infection.
• Intestinal wall – hookworms & nodule worms allow Salmonella; salmon flukes (dogs) vector Neorickettsia helminthoeca (salmon poisoning).
• Cecum (turkeys) – $Heterakis\;gallinarum$ carries Histomonas meleagridis (blackhead).
• Perirenal tissues – $Stephanurus\;dentatus$ predisposes to pyelonephritis.

7. Tissue Consumption & Chemical Toxicosis

7.1 Direct Tissue Consumption

Ascarids & stomach worms browse on mucosal epithelium ➔ villus blunting, gastritis.

7.2 Secretion of Toxic/Modulatory Products

Helminths release hemolysins, proteases (histolysins), anticoagulants to facilitate feeding & migration.
Main culprits: hookworms, nodular worms, stomach worms, strongyles.
Systemic effects: anemia, edema, impaired clotting, allergic reactions.

$\text{Clotting\;Time}<em>{infected} > \text{Clotting\;Time}</em>{normal}$ (due to parasite anticoagulants)

8. Immunological & Hypersensitivity Consequences

Dead larvae/eggs = potent antigens ➔ granulomatous inflammation, “milk spots” on liver (pig ascarids).
Type I (IgE-mediated) reactions behind pruritic papules in cutaneous larva migrans.
Type III immune‐complex disease possible in heavy schistosome infections (glomerulonephritis).

9. Integrative Clinical Takeaways

Always correlate clinical signs (anemia, colic, dermatitis) with possible helminth etiologies.
Consider polyparasitism; multiple mechanisms may act simultaneously.
Chronic infections can lead to irreversible organ change (fibrosis, neoplasia).
Control strategies: anthelmintics, pasture management, vector control, sanitation.