Haemonchosis - Sheep

Haemonchosis

Epidemiology of 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

Pathogenesis of Haemonchosis

• 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

Forms of Haemonchosis

• 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

Diagnosis of Haemonchosis

• Clinical signs (anaemia)
• Season
• Faecal egg count (may be very high)
• Low blood haemoglobin, PCV and red blood cell count
• Elevated blood pepsinogen
• Post-mortem examination:
  • >2000 adult worms
  • Numerous haemorrhagic lesions on gastric mucosa
  • Brown fluid in abomasum
  • Pale oedematous carcass
  • Expansion of red marrow from epiphysis into medullary cavity of long bones

Control of Haemonchosis

Temperate Areas

• Dose ewes around lambing

Tropical and Sub-Tropical Areas

• Dose all stock every two to four weeks during the rainy season

Vaccine

• Hidden antigen vaccine currently under development