Haemonchosis

Haemonchus has a high biotic potential, egg to L3 development occurs very fast in hot humid conditions

Tropical and Subtropical Areas

Disease outbreaks depend on local rainfall
Parasite survives prolonged dry periods by arrested development
Development resumed just prior to rainy season e.g. Australia, Brazil, Middle East and Nigeria
Arrested development less common in areas with more frequent rainfall e.g. East Africa
Several cycles of infection annually

Temperate Areas (less favourable for Haemonchus L3 survival)

Ewe is primary source of infection (relatively few L3 overwinter on pasture)
Eggs hatch and develop to L3
Most L3 ingested by ewes and lambs in late summer become arrested (EL4, abomasal wall) → resume development following spring → acute haemonchosis (epidemiology equivalent to type 2 ostertagiosis)
A few L3 ingested by lambs do not become arrested → acute haemonchosis (if sufficient L3 ingested; equivalent to Type 1 ostertagiosis)
This is most likely to happen in hot thundery summer periods
Normally, only one cycle of infection annually

Moderate infection (2000 worms) leads to
- Increased red blood cell turnover (from 4 months to 3 weeks)
- Loss of haemoglobin and iron (normally recycled)
- Depleted iron reserves leading to anaemia and death

Hyperacute (0-7 days)
- Sudden death in apparently healthy sheep;
- Severe anaemia (tropics mainly)
Acute (1-6 weeks)
- Oedema, loss of condition, lethargy, death
- Anaemia, hypoalbuminaemia
Chronic (2+ months)
- Progressive weight loss or reduced weight gain
- Similar appearance to poor nutrition

Haemonchosis - Sheep