Summary
Heaves or broken wind is a respiratory disease of horses resulting in signs of
chronic coughing, decreased exercise tolerance, difficulty breathing and
abnormal lung sounds. These signs occur as the result of narrowing of the small
airways of the lungs caused by: inflammation and thickening of their tissues;
constriction of the smooth muscles that surround them; and accumulation of
mucous and exudates within their lumens. The end result is trapping of air in
the lungs (emphysema). Technically heaves is called chronic obstructive
pulmonary disease (COPD). However, recurrent airway obstruction may be a better
name because most cases go into remission when their environment is changed.
Difficulty in breathing recurs when susceptible horses are again exposed to
mouldy feeds or dusty conditions. Animals with clinical signs adopt a
characteristic breathing strategy with very high peak flows at the start of
exhalation which decreases rapidly as exhalation continues.
Causes of Heaves
Although heaves has been recognized as a disease of horses for centuries, its
precise cause remains uncertain. Most published evidence suggests that the
inflammation of the small airways (bronchiolitis) is the result of an allergic
response that occurs following exposure of the lower airways to dust and moulds,
particularly those that come from poorly cured hay.
Heaves occurs most frequently in the northern hemisphere (Europe and North
America) among horses kept in stables. It also occurs in pastured horses in
certain regions of Great Britain and the southern United States where it is
thought to result from exposure to various pollens. Signs do not become obvious
until a large number of airways are affected and therefore, many more horses are
probably affected than is recognized. The disease is rare in warm dry climates
and in New Zealand and Australia where horses spend little time in stables.
Common risk factors for the occurrence of clinical signs are exposure to poorly
cured, mouldy or dusty feeds, confinement to a stable environment, inadequate
stable ventilation, straw bedding and being 6 years of age or older. The
incidence of the disease may be highest in ponies as they are frequently kept in
less than ideal conditions and fed poor quality hay.
Other possible causes of heaves include specific toxins (3-methylindole)
absorbed from the intestinal tract, exposure to cold air, genetic predisposition
and viral respiratory infections. To date there is no solid evidence that any of
these factors play a role in the occurrence of heaves but researchers are still
exploring these theories.
Development of Heaves
Bronchiolitis (inflammation of small airways) is considered to be the most
important abnormality of affected animals but spasm of the smooth muscles
surrounding the bronchioles (bronchospasm) also plays a role. The airways of all
horses become exposed to dusts, spores of moulds and a number of temperature
resistant bacteria that are aerodynamically small enough (0.5-0.3 µm) to be
inhaled deep into the respiratory system. However, the airways of horses with
heaves are considered to be hypersensitive to these substances.
With the narrowing of the small airways of the lungs affected horses experience
increasing difficulty in both pulling air into the lungs and pushing it back out
again. As the condition advances, severely affected animals are seen to contract
the muscles of the abdominal wall during the last phase of exhalation causing
the floor of the abdominal wall to lift up at the very end of exhalation. This
abdominal lift requires extra effort of some of the muscles of the abdomen.
These muscles may enlarge visibly causing what is known as a "heave
line".
Although there remains some dispute about it, the type of hypersensitivity
occurring in these animals appears to be delayed rather than acute. Animals in
remission and exposed to mouldy feeds take about 90 minutes to start developing
signs of respiratory disease. The process begins with inflammation of the
airways followed by thickening of their linings, accumulation of mucous and
inflammatory debris, loss of the effectiveness of the mechanisms that normally
remove mucous from the airways and spasm of the smooth muscles that are wrapped
around the small airways. The end result is narrowing of thousands of tiny
airways throughout the lungs and loss of the ability to ventilate the lungs
fully and efficiently. Affected lung tissue looses its normal elasticity and
this combined with narrowing of the airways prevents normal ventilation and
oxygenation of the lungs. This process results in poor oxygenation of the blood
and subsequent loss of exercise tolerance and poor performance.
An other consequence of this condition may include so called bleeders or
"exercise induced pulmonary haemorrhage". This is thought to occur as
the result of laboured breathing with marked pressure differences developing
across the delicate tissues of the lung and their tiny blood vessels. Bleeding
may result when these pressure differences become so large that blood vessels
start to rupture and blood escapes into the airways occurs.
With narrowing of the small airways it becomes increasingly difficult for
affected horses to push the normal volume of air out of the lungs at the end of
exhalation. As a result, the lungs tend to remain over-inflated at the end of
exhalation resulting in a condition known as functional or reversible emphysema.
In some cases, the disease progresses to the point that permanent,
non-reversible damage in at least some portions of the lung tissue. There is no
quick and reliable method of determining if this has occurred, although
persistent failure to respond to treatment and management of the disease and the
examination of lung biopsies may point in this direction. In many cases, much of
the loss of lung function that occurs in cases of heaves is reversible with the
aid of careful management of the animal and its environment.
What Heaves Looks Like
Horses do not usually show signs of heaves until they are 6 years of age or
older. The earliest clinical signs are coughing at exercise, when eating or when
exposed to dusty environments. The respiratory rate begins to increase from the
normal of 8-12/min to 24-36/min and this is accompanied by an increase in the
depth of respiration. As breathing becomes more difficult, a distinct lift of
the floor of the abdomen will be observed at the end of exhalation and the
nostrils will begin to flare during inspiration. By placing your ear close to
the nostrils you may detect a wheezing sound as the horse breaths. Intermittent
nasal discharge composed of mucus and pus (mucopus), evident in both nostrils,
is a common sign. The quantities of this material may be quite large when the
horse puts its head down to eat after having had it tied above the ground for 2
or 3 hours. Following exercise the heart rate will be faster than expected and
after vigorous exercise, some blood may be evident in the nasal discharge. With
the aid of a stethoscope, wheezing and crackling sounds may be heard over the
lung fields and fluid, fluttering sounds may be heard in the trachea. The
abnormal lung sounds are easiest to detect if the horse is made to take deep
breaths with the aid of a rebreathing bag or following exercise. These abnormal
sounds are caused by the accumulation of mucopus in the respiratory tract and
the narrowing of airways in the lungs. In the most advanced stages of the
disease, horses lose weight and appear in severe respiratory distress with
marked flaring of the nostrils and abdominal breathing. There may also be an
obvious accentuation of the spaces between the ribs associated with extreme
efforts to breath.
The course of the disease depends largely on the degree of effort that is put
into improving the conditions under which the horse is kept. Complete or near
complete recovery from the signs has been reported in horses turned out to
pasture or by moved into a well ventilated stall, fed cubed or pelleted roughage
with dampened grain, and kept on bedding that is virtually dust and mould free,
such as shredded paper or high quality wood shavings.
In summary, the important indication of heaves in the horse are:
1. Horses older than 6 years of age
2. Prolonged or repeated stabling or exposure to other dusty environments
3. Having a chronic cough and reduced exercise tolerance
4. Difficulty breathing in an otherwise healthy animal
5. Wheezing and crackling lung sounds heard with a stethoscope
6. Laboured breathing (abdominal lift, flaring nostrils) in horses at rest
7. Worsening of clinical signs when the horse is exposed to poor quality feeds
or environments
8. Improvement or remission of clinical signs when the horse is kept at pasture
or in a near dust free environment and fed a cubed ration for 7-21 days
Special Examination of the Horse With
Heaves
The results of a complete blood count and serum biochemistry tests are usually
normal in horses with heaves. The principle abnormality detected by blood tests
is a decrease in oxygen content of arterial blood from the normal of about 77
mm/hg to about 61 mm/hg.
Lung function tests have been used in the study of heaves but the equipment
required to perform these tests is highly specialized and only available in a
few laboratories. The results of pulmonary function tests have shown that horses
with heaves have distinct changes in air flow rates, decreased elasticity of the
lung tissues and increased resistance to the flow of air in the airways.
The examination of fluid samples obtained from the trachea (tracheal or
trans-tracheal aspirates) is also useful in making the diagnosis of heaves. With
the aid of a flexible endoscope passed through the nasal cavity and down the
trachea, large quantities of mucopus may be observed in the lower part of the
trachea. Samples of this fluid contain large numbers of white blood cells,
largely as a result of an increase in the number of neutrophils.
Techniques have also been developed to permit sampling of the region of the
small airways. The technique, bronchoalveolar lavage (BAL), is performed by
passing an endoscope or a specialized BAL tube down the trachea and pushing it
until it lodges in a small airway. Collection fluid is injected down the tube,
flooding a small portion of the lung and then withdrawn and submitted to the
laboratory. The cells suspended in the fluid are then concentrated and examined
under a microscope.
In some cases, lung biopsies may be taken in an effort to determine if permanent
changes have occurred in the lung tissue. Radiographs taken of the chest when
clinical signs are severe may show an increase in lung density associated with
the infiltration and thickening of the airways by inflammatory cells. If there
is a history of bleeding with exercise then radiographs may show a marked
increase in the density in the upper part of the lung field. In specialized
research facilities, nuclear scintigraphy or imaging has been used to further
characterize these lesions.
Examination of the Lung Tissue From Horses
With Heaves
Heaves is not usually a fatal disease so there are few published studies upon
the examination of lung tissue from affected animals. The information that is
available confirms that the major lesion is bronchiolitis (inflammation of
airways with a diameter of 2 mm or less) and emphysema (trapping of air in the
lung). There are plugs of mucus in the bronchioles, a build up of fibrous tissue
around the airways (peribronchial fibrosis) and infiltration of the tissues
surrounding the bronchioles with inflammatory cells (white blood cells). In the
large airways there is evidence of loss of the ciliated epithelial cells which
are important in moving secretions up the trachea and out of the lungs. With the
exception of the peribronchiolar fibrosis and the destruction of some of the
fine membranes or alveoli that occurs due to severe emphysema, most of the
lesions observed in the lungs of horses with heaves are reversible in nature.
Distinguishing Heaves From Other
Diseases
Heaves can be confused with other diseases including severe bacterial pneumonia,
pleurisy, parasitic pneumonia (lung worm infections), certain neoplasms or
cancers of the thoracic cavity and diaphragmatic hernia. Distinguishing these
diseases from heaves is accomplished through careful physical examination of
affected animals. In general, horses suffering respiratory distress due to
diseases other than heaves look very ill, depressed and/or painful. Many animals
with these conditions suffer a sudden loss of appetite and have distinct
abnormalities in their complete blood counts. The diagnosis these diseases may
be aided by radiographic or ultrasonographic examination of the thorax.
Control and Management of Heaves
It is well recognized that the best treatment for heaves is to remove the animal
from the environment that appears to be causing the problem and by reducing
exposure to dusts and moulds. Unless every reasonable effort is made to meet
these objectives, none of the other recommended treatments is likely to be
effective. In experimental studies, horses suffering from acute signs of heaves,
subsequent to exposure to mouldy feeds, experience substantial remission of
clinical signs one to three weeks after being moved to a dust free environment
and fed cubed roughage.
Where practical, the best approach to management is to keep susceptible horses
in the open air. Many owners are reluctant to keep horses outside during cold
weather but there is ample evidence to show that horses, in good bodily
condition, given adequate nutrition, a good windbreak and overhead shelter do
very well in our cold, dry climate. Attempting to keep horses with heaves
indoors requires well ventilated stalls bedded with shredded paper, peat or high
quality shavings. Some of these cases can be managed by feeding high quality hay
that has been thoroughly soaked prior to feeding; others require a cubed or
pelleted ration. Feeding horses on the ground may assist in draining
inflammatory exudates collected in the trachea.
Treatment of Heaves
Many drugs have been used in the treatment of heaves. Unfortunately, although
most veterinary practitioners and horse owners believe that at least some
treatments are beneficial, there is very little published scientific evidence to
support this view or to allow us to compare the effects between treatments. Most
modern treatments for heaves centre around the use of drugs to decrease the
amount of inflammation and the accumulation of inflammatory exudate in the
respiratory tract, to dilate the airways, and to increase the rate of clearance
of mucus and inflammatory debris from the respiratory tract. Drugs used to
decrease inflammation include corticosteroids such as dexamethasone or
prednisolone, administered over a period of one to three weeks. A whole variety
of bronchodilators have also been used, including atropine and clenbuterol.
Traditionally, most of these treatments have been given by injection or by
mouth. More recently there has been a move towards the administration of
corticosteroids and bronchodilators as inhalants and special devices are now on
the market that permit the use of products that have been developed for use in
the treatment of asthma in humans.
Prevention of Heaves
In those parts of the world where horses are kept out of doors and grazed most
of the year, heaves rarely occurs. On the other hand, heaves is common in those
regions where horses are kept in stalls, bedded on straw and fed hay. Hay baled
at 15-20% moisture and carefully stored will contain little dust and few spores.
Horses fed this sort of roughage, all of their lives, should have few problems
with heaves. On the other hand, hay baled with a moisture content of 20% or
greater may become contaminated with the types of moulds that have been shown to
result in the acute onset of clinical signs of heaves. The higher the moisture
content of the hay when bailed, the greater the risk and degree of
contamination.
The period of exposure to moulds required to initiate the first clinical signs
of heaves is not known. However, it is clear the safest and most logical
approach to the prevention of this disease would be to limit exposure to those
feeds and conditions that are known to cause acute signs in susceptible animals.
Further, these management procedures should be initiated at birth rather than
after signs of the disease have developed.