Blackhead is a protozoan disease that mainly affects turkeys but can also affect chickens, pheasants and game birds. While chickens are relatively resistant, significant disease has been seen in breeding chickens and free-range layers.

Scientific name

Histomonas meleagridis


Caused by the protozoan organism Histomonas meleagridis, which has a very complex life cycle in birds and outside the host. It multiplies and damages the caecal wall in the bird, later migrating to the liver and causing liver necrosis.

Infection can occur from direct contact with infected birds or through ingestion of infected caecal worm eggs (Heterakis gallinae) or infected earthworms.

Other names

  • Histomoniasis


The blackhead organism can infect the egg of the caecal worm, where it can then pass into the environment. This organism combination can survive in the soil for up to 3 years, enabling transfer from one flock to the next. Earthworms can also play an important role in the spread of this disease by eating the caecal worm larvae. Once properties are infected, they can be difficult to clean up.

Life cycle

The life cycle of the protozoan organism (Histomonas meleagridis) is closely linked to the way in which it spreads. The organism infects the ovaries of caecal (Heterakis) worms which are picked up by poultry from the soil. The caecal worms produce infected worm eggs within the bird which are shed into the environment with the droppings, starting the whole process again.

Affected animals

  • turkeys
  • chickens
  • pheasants
  • game birds

Clinical signs

Signs of blackhead can include:

  • drowsiness and weakness in birds
  • ruffled feathers
  • lowered head, drooping wings and tail
  • depression
  • poor growth or weight loss
  • soiled vent feathers
  • mustard-yellow diarrhoea
  • darkened comb that may appear blue-black in colour.

How it is spread

Most birds are infected by eating caecal worm eggs which transport the blackhead organism safely through the gut. Droppings containing contaminated caecal worm eggs can rapidly spread the disease among poults.

Birds transmit the disease by eating caecal worm eggs or earthworms contaminated by Histomonas meleagridis. Rain brings earthworms to the surface and, if eaten, they can cause a blackhead outbreak in birds.

Caecal worm eggs can also be transferred mechanically on workers' shoes or boots and by other animals.

Risk period

In Queensland, blackhead is more common in summer and affects mainly chickens that are 3-4 weeks old, though outbreaks do occur in older birds.

Turkeys are most susceptible up to the age of about 18–20 weeks of age, with most deaths from blackhead occurring in birds 3-18 weeks of age. Losses in turkey production systems can be high. Chickens tend to be more resistant to blackhead and, as a precaution, should not be run with turkeys.


There are no effective treatments for blackhead. Successful control of blackhead depends on removing worm eggs from the cycle and providing areas that are not heavily contaminated. Providing sunny, well drained rearing facilities, particularly for turkey ranges, can reduce the survival of caecal worm (Heterakis) eggs.

Management factors include:

  • keeping turkeys and chickens apart to ensure drainage does not occur from chickens to turkeys
  • controlling caecal worm through suitable treatments currently available
  • brooding poults away from ranged turkeys and chickens to prevent ground contamination
  • keeping turkeys away from earthworms
  • ensuring that poult feeders and waterers are not contaminated with droppings, and are moved to clean areas regularly.