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Malabsorption Syndrome

Broiler Runting Syndrome, Infectious Proventriculitis, Helicopter Syndrome, Runting-stunting Syndrome, Pale Bird Syndrome, Brittle Bone Syndrome, Femoral Head Necrosi

Malabsorption syndrome (MAS), also known as runting-stunting syndrome (RSS), is a syndrome in which a number of chickens in a flock appear considerably small and dwarf-like. MAS was first described in the 1970s and affects chickens worldwide. MAS is most commonly seen in "meat-type" chicken breeds or broilers. The specific cause of MAS is not well understood, several different significant viruses have been identified in chickens with MAS, such as chicken astrovirus, avian nephritis virus, chicken parvovirus, reoviruses, chicken calicivirus, chicken megrivirus, and sicinivirus 1.

Clinical Signs

Chicks with MAS usually begin to show evidence of stunted growth at about 6-7 days of age. At this time, they may also look disoriented and pale. At about 10-12 days of age, the bodies of affected chicks are noticeably small relative to the length of the primary feathers of the wing and beak. The chick's beak and legs are much paler than the other birds. Some chicks may show "helicopter" feathers or curled wing tips, and possibly other feather abnormalities.

Common clinical signs of chickens with MAS may include:
  • Stunted growth: Where birds have much smaller bodies than other chicks.
  • Pale skin pigmentation: Chick's beak and legs appear as a paler color than normal birds.
  • Poor feathering: Chicks may have smaller than normal feathering and usually curled wing tips.
  • Leg weakness: Some affected chicks may have rickets, with weak or broken legs.
Chicks with MAS often develop secondary infections. Studies have shown that male chicks are more severely affected than females.


MAS is transmitted by direct physical contact with infected chicken, turkeys and ducks. Outbreaks have also occurred in flocks that were vaccinated by REV-contaminated vaccines.

Clinical Signs

Stunted growth
Undigested feed in feces
Poor feed conversion ratios
Slow feathering
Broken, twisted, or curled wing tips
Eating feces
Leg weakness
Reluctance to move
Huddling around feeders and drinkers
Incessantly pecking at the walls


  • History
  • Clinical signs
  • Physical exam
  • PCR
  • Postmortem lesions


Supportive care: Provide supplemental vitamins and minerals



Scientific References

Age Range

Occurs most frequently in chicks 3 to 6 weeks in age.

Risk Factors

  • Brooding at cool temperatures and short downtime between flocks - have been associated with worsening of MAS signs
  • Male chickens are more severely affected than females.