Runting-Stunting Syndrome in Broilers

By F. Dustan Clark and Frank T. Jones, University of Arkansas, Division of Agriculture and published in Avian Advice, Spring 2008.
calendar icon 27 August 2008
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The microbial agents causing a number of intestinal diseases in young broilers have not yet been identified and such conditions are often called “viral enteritis” (Anonymous, 2008). However, agents causing similar signs in young birds have been reported around the world and have been called runting stunting syndrome (RSS), malabsorption syndrome, brittle bone disease, infectious proventriculitis, helicopter disease and pale bird syndrome (Rebel et al., 2006)

More on the Authors
Dr. Dustan Clark
Extension Poultry Health Veterinarian
Dr. Frank Jones
Extension Section Leader

Runting-stunting syndrome (RSS) was first reported in the 1940’s, became well known to the commercial industry in the 1970’s and has since been reported around the world (Rebel et al. 2006). RSS continues to cause economic hardship in the broiler industry through decreased body weights, elevated feed conversions, reduced uniformity, reduced livability, plant downgrades and secondary diseases (Anonymous, 2008; Zavala and Barbosa, 2006).

Recognizing Runting Stunting Syndrome

While symptoms of RSS can vary dramatically, birds are generally affected by RSS early in life with symptoms and mortality peaking at about 11 days. After placement RSS affected birds may huddle around feeders and waterers, or may persistently peck at the walls. Feed consumption is often depressed. A sizable proportion of the flock may be involved and while affected birds that are not culled may not die, they never recover. Often flock mortality is unaffected, but flock uniformity which normally runs about 70% decreases to about 35%. As feathers appear on affected birds, they are smaller than normal and may be curled especially at the wing tips (helicopter disease) (Zavala, 2006). The legs and beak of affected birds may appear pale in color (pale bird syndrome) and some birds may have rickets or broken legs (brittle bone disease) (Rebel et al., 2006).

When diseased birds are necropsied, the livers are generally small, but gall bladders are enlarged. Intestines are thin and translucent with large amounts of fluids along with poorly digested feed present in the lumen (Zavala, 2006). Intestines of affected birds may appear enlarged whereas the stomachs (proventriculi) may appear inflamed (Shapiro et al., 1998, Guy, 1998). The normal intestinal growth of the jejunum (the portion of the intestine where much of the digestion and nutrient absorption takes place) is interrupted by RSS (Esmail, 1988; Rebel et al, 2006). Pancreases from diseased birds degenerate and digestive enzymes are reduced. Droppings from affected birds are unusually loose, vents are soiled and litter may become damp, enhancing the possibility of secondary infections (Zavala, 2006; Zavala and Sellers, 2005).

What causes Runting Stunting Syndrome?

Researchers have not reproduced all the field symptoms of RSS experimentally and believe that several viruses, bacteria and other pathogens may be involved. Reovirus was originally thought to be the cause of RSS, but adenovirus, enterovirus, rotavirus, parvovirus and others may also be involved. Bacteria often isolated from RSS birds (E. coli, Proteus micabilis, Enterococcus faecium, Staphylococcus cohnii, Clostridium perfringes, Bacteroides fragilis and Bacillus licheniformis) are commonly found in the intestinal tract and may cause secondary infections, aggravating the initial lesions (Rebel et al., 2006). Brooding at cool temperatures tends to worsen RSS symptoms, as does short down-time between flocks. Certain strains of birds appear to be more susceptible to the effects of RSS than others and male birds are more severely affected than females (Zavala and Barbosa, 2006). However, it is interesting to note that researchers have found that resistant broiler strains have stronger immunological responses than susceptible strains. This difference is particularly pronounced when gut immunity is compared (Rebel et al., 2006). Some researchers have suggested that the poor growth and retarded feathering (which are consistently observed in RSS cases) are due to a common underlying infection, while virtually all other symptoms result from other infections or management factors.

Controlling Runting Stunting Syndrome

RSS often appears suddenly and disappears equally suddenly, making it difficult to determine effective control measures. However, it is important to remember that RSS is a disease of young birds with symptoms and mortality peaking at about 11 days so control efforts should be focused early in the life of the flock. Control efforts should focus in three primary areas: Biosecurity, good poultry house management and vaccination.

When RSS is reported in an area, it is important for the industry in the area to tighten Biosecurity procedures to reduce the possibility of exposure and to slow the spread of the disease. It is particularly important to emphasize procedures that control farm visitors, properly manage disposal of mortality and limit vermin infestations (rodents, wild birds and insects).

The objective of proper poultry house management is to provide an environment for the birds that is virtually stress free. In RSS situations, poultry house management is doubly important. Good management starts before the birds arrive. A minimum of 12 days of downtime should be allowed between flocks. Since litter has been shown to transmit the disease, it should be removed if birds have broken with RSS. If it is not possible to remove the litter, heat the litter to 100°F for 100 hours or compost the litter in the poultry house to lessen the possibility of passing the disease to the next flock via litter. The brood chamber should be cleaned and disinfected as thoroughly as possible prior to chick placement. Since low brooding temperatures have been shown to worsen the effects of RSS, DO NOT reduce brooding temperatures to save fuel. Check on birds often and maintain a house environment that is as stress free as possible. Remove dead birds quickly and cull severely if RSS breaks. The application of vinegar or other acidifiers via water may reduce spread of the disease. Supplemental vitamins and minerals in both breeder and broiler feeds has also been shown to improve immunity in chicks and their ability to deal with RSS.

Certain strains of reovirus (e.g. 1733 and 2408) were originally implicated as the cause of RSS and vaccines have been developed for such strains. While vaccination of broilers for RSS may be effective about 50% of the time, a consistent vaccination program for breeders often provides long term benefits (Shane, 2008, van der Heide, 2000). RSS vaccination programs for breeders generally provide protection for adult birds, reducing the possibility of spread to young birds. In addition, immunity in breeder hens is passed to chicks, helping to protect them from the disease.


Runting stunting syndrome (RSS) has caused economic losses in the poultry industry for over three decades. While the reovirus was originally thought to cause RSS, further research has shown that other viruses and bacteria are likely involved. Control of RSS involves Biosecurity, good poultry house management and vaccination.


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Zalvala, G. and H. Sellers. 2005. Runting-stunting syndrome. The Informed Poultry Professional Issue 85:1-4. http://www. Accessed 3/31/08.

August 2008
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