Parasite life cycles

Leishmania spp.

1. Sandfly injects promastigotes into host skin

2. Promastigotes enter macrophages transform into amastigotes

3. Amastigotes multiply by binary fission and infect other cells

4. Sandfly ingests amastigotes through blood

5. In sandfly midgut, amastigotes promastigotes multiply

6. Migrate to biting appendage and infect next host

Trypanosoma brucei

1. Tsetse fly injects metacyclic trypomastigotes into host

2. Transform into bloodstream trypomastigotes multiply by binary fission in body fluids

3. Tsetse fly ingests trypomastigotes through blood

4. Multiply in midgut migrate to salivary glands become metacyclic forms

5. Bite and infect next host

Tritrichomonas foetus

1. Direct transmission during copulation (cattle) or faecal-oral (cats)

2. Trophozoites colonise genital tract (cattle) or colon (cats)

3. Reproduce via binary fission

4. No cysts; cannot survive long outside host

Giardia intestinalis

1. Ingestion of cysts in contaminated water/food

2. Excyst in small intestine → release trophozoites

3. Trophozoites multiply by binary fission

4. Encyst in colon → cysts excreted in faeces

5. Cysts survive in environment; infective immediately

Cryptosporidium parvum

1. Ingest sporulated oocysts (immediately infective)

2. Sporozoites invade intestinal epithelium

3. Asexual reproduction (merogony) → sexual reproduction (gametogony)

4. Zygote forms thick-walled oocyst → excreted in faeces

5. Some oocysts rupture internally → autoinfection

Plasmodium spp.

1. Mosquito injects sporozoites

2. Sporozoites invade liver cells → schizogony → merozoites

3. Merozoites infect RBCs → trophozoite → schizont → merozoites

4. Some become gametocytes

5. Mosquito ingests gametocytes → sexual reproduction → sporozoites form in salivary glands

Eimeria spp.

1. Ingest sporulated oocyst

2. Sporozoites released → invade intestinal cells

3. Asexual stages (schizogony) → produce merozoites

4. Sexual stages (gametogony) → zygote forms oocyst

5. Oocyst shed in faeces → sporulates in environment (needs moisture + warmth)

Toxoplasma gondii

1. Cats shed unsporulated oocysts in faeces

2. Sporulate in environment → ingested by IH through food, water and soil

3. Sporozoites released in intestines and invade host cells, differentiating to tachyzoites

4. Tachyzoites differentiate to bradyzoites which form tissue cysts

5. Cats infected by eating tissue cysts or oocysts

6. Humans infected via undercooked meat, oocysts (soil), or transplacentally

Fasciola hepatica

1. Eggs passed in faeces → hatch in water → miracidium

2. Penetrates snail → becomes cercaria

3. Cercaria exits snail → encysts on vegetation as metacercaria

4. Ingested by host → excyst in SI → migrates through liver to bile ducts

5. Matures to adult → eggs passed in faeces

Schistosoma spp.

1. Eggs released into water via faeces/urine

2. Eggs in water hatch → miracidium penetrates snail

3. Develops into sporocyst → cercaria

4. Cercaria exits snail → penetrates host skin

5. Migrates via bloodstream → matures in mesenteric/bladder veins

Echinococcus granulosus

1. Eggs passed in dog faeces

2. Ingested by IH (sheep, humans) → oncosphere hatches → forms hydatid cyst

3. DH (dog) eats infected organs

4. Cysts develop into adults in SI and releases eggs

Echinococcus multilocularis

1. Eggs passed by fox/dog → ingested by rodent/human

2. Oncosphere migrates to liver → forms invasive alveolar hydatid

3. Predator eats infected rodent

4. Cysts develop into adults in SI and releases eggs

Dipylidium caninum

1. Gravid proglottids passed in faeces and release eggs

2. Flea larvae ingest egg packets

3. Develop into cysticercoid in flea

4. Host ingests adult infected flea → develops into adult tapeworm in intestine

Anoplocephala perfoliata

1. Eggs in faeces → eaten by oribatid mite

2. Cysticercoid develops in mite

3. Horse ingests mite while grazing → develops into adult in ileo-caecal junction

Taenia solium

1. Eggs passed in human faeces

2. Pigs ingest eggs

3. Oncospheres hatch, penetrate intestinal wall and circulate to muscles

4. Cysticerci in muscle

5. Humans ingest undercooked pork → develop into adult in SI

Taenia saginata

1. Eggs passed in human faeces

2. Cattle ingest eggs

3. Oncospheres hatch, penetrate intestinal wall and circulate to muscles

4. Cysticerci in muscle

5. Humans ingest undercooked beef → develop into adult in SI

Diphyllobothrium latum

1. Eggs passed in faeces hatch into coracidia in water

2. Coracidium eaten by crustacean and develop into procercoid

3. Fish eats crustacean → develops into plerocercoid which invades tissue

4. Human eats raw fish → develops into adult in intestine → eggs passed

Strongylus vulgaris

1. Eggs shed in faeces → hatch and develop into L3 on pasture

2. Horse ingests L3

3. Penetrates intestinal wall and migrates to cranial mesenteric artery

4. Larvae return to the intestine, mature to adults and shed eggs

Strongylus edentatus

1. Eggs shed in faeces → hatch and develop into L3 on pasture

2. Horse ingests L3

3. Migrates via the portal system to the liver, then through the peritoneal cavity

4. Larvae return to the intestine, mature to adults and shed eggs

Strongylus equinus

1. Eggs shed in faeces → hatch and develop into L3 on pasture

2. Horse ingests L3

3. Migrates through the liver and pancreas

4. Larvae return to the intestine, mature to adults and shed eggs

Cyathostomins

1. Eggs shed in faeces → hatch and develop into L3 on pasture

2. Horse ingests L3

3. Encyst in large intestinal mucosa (may undergo hypobiosis)

4. L4 larvae develop into adults and lay eggs

Ancylostoma caninum

1. Eggs shed in faeces (can be transmitted transplacentally/transmammary)

2. Infective larvae penetrate skin or is ingested

3. Migrates via lungs → coughed up and swallowed

4. Develop into adults in SI

5. Transmammary and transplacental transmission possible

Haemonchus contortus

1. Eggs passed and hatch into L3

2. L3 ingested through grazing

3. Develop in abomasum, feed on blood

Toxocara canis

1. Eggs ingested (shed through faeces) or larvae transmitted transplacentally/ transmammary

2. Larvae undergo liver-lung migration → swallowed

3. Adults in SI → eggs passed

4. Humans may ingest eggs → larva migrans

Parascaris equorum

1. Eggs passed → L2 inside egg is infective

2. Ingested → liver-lung migration

3. Adults in SI → eggs passed

Dirofilaria immitis

1. Mosquito ingests microfilariae from infected dog

2. Develops to L3 in mosquito

3. Injected into next host → migrates to pulmonary arteries → adults

4. Adults produce microfilariae

Trichuris vulpis

1. Eggs with bipolar plugs passed in faeces

2. Embryonate in environment (infective)

3. Ingested → larvae hatch in SI → move to caecum and mature

Filaroides osleri

1. L1 passed in sputum/faeces

2. Transmitted via saliva (e.g. dam to pup)

3. Larvae migrate to tracheal bifurcation → form nodules → adults lay L1

Ostertagia ostertagi

1. Eggs passed in faeces → L3 ingested

2. Enter gastric glands of abomasum → damage parietal cells

3. May undergo hypobiosis

4. Adults lay eggs → passed in faeces