Ascites In Meat-Type Chickens Caused By Right Heart Failure

Agdex#:	451/662
Publication Date:	November 2000
Order#:	00-101
Last Reviewed:	November 2000
History:	Replaces Factsheet Ascites in Meat-Type Chickens Caused by Right Heart Failure, Order No. 89-094
Written by:	R. J. Julian - Professor of Pathobiology/Ontario Veterinary College, University of Guelph

Table of Contents

1. Introduction
2. Clinical Signs
3. Postmortem Examination and Diagnosis
4. Why Do Meat-Type Chickens Get Ascites?
5. Control

Introduction

Definition:

Ascites is an accumulation of non-inflammatory fluid in one or more of the abdominal spaces. There may be clots of yellow material in the fluid.

Ascites caused by right heart failure (RHF) has been an important cause of illness and death in broiler chickens raised at high altitude for many years. The number of cases in the U.K., Canada, the United States, and other areas where broilers are grown at low altitude is also notable and coincides with a continuing genetic and nutritional improvement in feed efficiency and rate of growth. The incidence of ascites is above 1% in some broiler flocks and many roaster flocks, and it is occasionally 15%-20% in roaster flocks.

Increased pulmonary (lung) arterial pressure can be produced in mammals and birds by lack of oxygen. The heart must circulate blood more rapidly to provide the same amount of oxygen to the body. In both chickens and mammals, lack of oxygen in the body causes a marked increase in the number of blood cells, and this makes the blood more viscid and more difficult to pump through the lung. The right side of the heart enlarges in response to the increased work and this eventually leads to RHF if the heart has to continue working harder than normal.

At low altitude it is not lack of oxygen in the air that causes the increased work for the heart, and dust or fumes in the pens are not likely causes unless they cause pneumonia. Carbon monoxide interferes with oxygen uptake and carbon monoxide levels (from the brooders) have been above normal in some barns where RHF has been a problem. The most important cause of increased work for the heart at low altitude appears to be the high oxygen requirement of rapid growth in the modern broiler combined with restricted space for blood flow through the small blood vessels of the lung.

Clinical Signs

Occasionally young broilers will develop ascites, particularly if there is too much salt in the feed or water or if lung diseases like aspergillosis are present. But if the lung is normal, deaths from ascites are greatest after 5 weeks. Clinically affected broilers have a pale head and a shrunken comb and in white chickens the feathers lose their bright white sheen. The abdominal skin may be red and vessels in the skin congested. Since growth stops as RHF develops, affected broilers are smaller than their pen mates are. Their abdomen is distended with fluid and the pressure results in an increased respiratory rate and reduced exercise tolerance.

Postmortem Examination And Diagnosis

At postmortem there is a large or small quantity of clear yellow fluid and clots of fibrin in the abdomen. The liver may be swollen and congested, or firm and irregular with edema (fluid), and have fibrin adherent to the surface. It may be nodular or shrunken; it may be white with edema under the capsule or have large or small blebs of edema in the sacs around the liver. There is a mild to marked increase in fluid in the sac around the heart and occasionally there is inflammation of the outer surface of the heart. There is enlargement of the right side of the heart and thickening of the muscle of the right side. The right atrium and vena cava are very dilated. Frequently there is thinning of the left heart. The lungs are extremely congested and edematous.

Not all broilers that die from RHF have ascites. Death may occur before clinical signs are observed and affected broilers frequently die on their back. At necropsy there may be a swollen liver, venous congestion, a dilated right atrium and vena cava and thickening of the right heart, as well as marked lung congestion and edema (death is likely from respiratory failure). The intestine may or may not be empty but the heart changes will differentiate RHF from flip-over.

Why Do Meat-Type Chickens Get Ascites?

The lungs of birds are rigid and molded into the thoracic cavity. They cannot expand like mammalian lungs. The capillaries can expand only a little to allow for increased blood flow. The lungs of chickens grow less rapidly than the rest of the body, and lung capacity does not keep up with the very rapid growth of muscle in fast growing broiler chickens.

Ascites caused by RHF has been produced by rickets that make breathing difficult, and increases the number of cells in the blood. Cold is a common cause of outbreaks of ascites in small farm flocks because it increases the blood flow through the lung.

Ascites at low altitude may be caused by lack of sufficient space for blood flow in the lung. Predisposing factors that increase the amount of oxygen required, reduce oxygen–carrying capacity of the blood, increase blood volume, or interfere with blood flow through the lung, may result in flock outbreaks of ascites. A diagrammatic summary of possible causes of increased pulmonary arterial pressure is shown in Figure 1.

Control

Slowing growth to reduce the oxygen required after 30 to 35 days of age can prevent ascites caused by primary pulmonary hypertension. Restricting feed, feeding a mash diet, or using a less dense diet accomplishes this. Long dark periods are also effective, but should be started at day 3 and require covered fan outlets. Limiting the number of hours of light slows growth and reduces the incidence of flip-over, lameness, leg deformity as well as ascites. A lighting program of 3 hours light to 9 hours dark may be effective. Lengthen each light period by 1 hour a week starting at day 18. Control environmental temperature, humidity and air movement to prevent excessive body heat loss.

Prevent ascites caused by other factors by avoiding the cause. Avoid moldy litter. Sodium level in feed should not be above 2000 parts/10⁶. Routinely analyze feed for sodium directly. Do not estimate from the chloride level, since high levels of sodium from sources other than NaCl may be present. If sodium is present in the water supply, reduce sodium in the feed by 2 to 3 times the level present in the water. (For 500 parts/10⁶ of sodium in the water reduce feed sodium 1000-1500 parts/10⁶). Do not use water containing over 1000 parts/10⁶ of sodium for young broilers.

Altitudes above 1,800 to 2,000 m are unsatisfactory for meat-type chickens and growth must be slowed to prevent mortality. More care to prevent chilling is also necessary at high altitude.

Figure 1

Diagrammatic summary of possible causes of increased pulmonary arterial pressure in broiler chickens resulting in increased right ventricular workload. Text Equivalent of Figure 1.

 

Adapted from: Julian, R.J. The effect of increased sodium in the drinking water on right ventricular hypertrophy, right ventricular failure and ascites in broiler chickens. Avian Pathology 16: 61-71, 1987.