Riboflavin (vitamin B2) deficiency causes 'curled toe paralysis' in young chicks, which is a common foot abnormality in which the toes curl inward (medially). Riboflavin is an essential, water-soluble vitamin that is required for proper functioning of the nervous system. When chicks do not have enough riboflavin in their bodies, which can result from a riboflavin deficiency in the breeding chickens, and/or by not receiving adequate sources during early growth. Without having enough riboflavin in the body, it causes peripheral nerve damage that leads to degeneration of the sciatic nerve (the nerve which is found along the back of the chick's leg to the foot). This causes the inward curling of the chick's toes, giving the feet a fist-like appearance.
Progression of clinical signs
Chicks that are fed a riboflavin-deficient diet will begin to show signs at about 8 to 14 days following hatch. They will slowly develop progressive symmetrical paresis starting with initial signs of reduced growth rate, weakness and sometimes diarrhea. Affected chicks soon become reluctant to move due to weakness, followed by inward curling of their toes. Without use of the legs, the muscles in the legs will start to atrophy and may eventually extend outward out from underneath the body of the chick.
During advanced stages of this condition, chicks are seen more frequently resting on their hocks, trying to walk as little as possible. It is at this later stage that chicks are at high risk of death from starvation, due to inability to reach food or water sources, or from getting trampled on by other chicks. The condition is still treatable however, as the chick's peripheral nerves are able to rapidly regenerate once riboflavin levels are restored.
Riboflavin requirements for chickens fluctuate depending on genetics, stage of growth, environmental conditions, level of activity, health status, and other dietary components and synthesis.
Riboflavin requirements are highest for newly hatched chicks and for chickens used for breeding. The NRC (1994) recommends that poultry species require between riboflavin at 1.8 - 4 mg/kg (0.45 - 1.8 mg/lb) of diet. However, more recently conducted research studies have found that the NRC's recommendation is not sufficient for modern breeds of chickens, breeders, or growing chicks. All chickens should receive a diet with a minimum riboflavin content of 4.4 mg/kg (2.0 mg/lb)
Dietary Sources of Riboflavin
Riboflavin is one of the vitamins most likely to be deficient in poultry feeds. Only a few feed ingredients that are used in poultry feeds that contain enough riboflavin to contribute to the requirements for growth and reproduction. Toxicity concerns
There are no reports of riboflavin toxicity studies in poultry. According to the NRC, most data from studies conducted on rats have found that dietary levels between 10 and 20 times the requirement (possibly 100 times) can be tolerated safely.