Curled-toe paralysis is caused by riboflavin (vitamin B2) deficiency in young chicks. It is one of the most characteristic signs associated with the vitamin deficiency, resulting in the curling of the chicks toes. The condition nervous system related due to peripheral nerve damage, related to degeneration of the sciatic nerves (the nerves along the back of the chick's leg to the foot). The damage can be reversed if treated quickly, however in longstanding cases where treatment is delayed the condition will become permanent. Although curled-toe paralysis is seen commonly in newly hatched chicks, studies have shown that only about 10% of riboflavin-deficient chicks develop clinical signs. Riboflavin deficiency in chicks is most often the result of breeding from riboflavin-deficient parents.
What is Riboflavin?
Riboflavin, also known as vitamin B2, is an essential, water-soluble B vitamin. It helps the body transform proteins, fats, and carbohydrates into energy. Riboflavin also helps maintain healthy skin and eyes, and is necessary for building and maintaining body tissues.
Clinical Presentation
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 (despite a good appetite), weakness and sometimes diarrhea. Affected chicks soon become reluctant to move, followed by intermittent flexing and inward curling of 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.
Nutritional Riboflavin Requirements
Nutritional 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), however the recommended riboflavin levels based on research conducted by DSM Nutrition is as follows:
| Age/Life Stage | mg/kg |
|---|
| Newly Hatched Chicks (0 - 10 wks) | 6 to 7 |
| Young & Growing (10 - 20 wks) | 5 to 6 |
| Laying hens (Actively laying eggs) | 5 to 7 |
| Breeders (20 wks & older)* | 10 to 12 |
| Broiler/'Meat' Breed Chicks (0-18 wks) | 6 to 8 |
| Broiler/'Meat' Breeds* (19 wks & older) | 12 to 16 |
Dietary Food 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. Milled rice and wheat does not contain much riboflavin, since most of the vitamin is in the germ and bran which are removed during this process. Fermentation significantly increases the proportions of riboflavin present in the free form. Good food sources for riboflavin include
green leafy vegetables and forages, particularly alfalfa, which leaves having the highest concentrations of the vitamin.
Vitamin Safety 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.
