‘Rattles’ is the name given to a lung infection (bacterial pneumonia) in foals caused by the bacteria Rhodococcus equi (R. equi). The disease is called ‘Rattles’ because of the rattling sound a sick foal can make while trying to breath with pus in the lungs.

This bacteria is tough and can survive in soil.  The foal becomes infected when it breathes in the bacteria in dusty environments.

The foal is usually infected in the first few days of life.  The foal does not show symptoms for several weeks to months after infection, therefore most foals are between 2-5 months of age when they become sick.  This often coincides with summertime and the heat can significantly worsen the illness.

R. equi causes abscesses (‘balls’ of pus, sometimes larger than a tennis ball) to form in the lungs and they are almost impossible to detect without tests like x-ray and ultrasound.  The first obvious symptoms are usually a cough and maybe discharge from the nose.

Many foals do not show any symptoms at all, but some foals can fail to gain weight or lose weight, spike a fever, go off their food, have trouble breathing and even die.

Not all foals will be infected, in fact many will not.  It is not yet known exactly why some foals get sick and others do not but it is related to how the immune system is able to fight the disease.

Rattles is one on the most researched diseases in horses because of the significant effect it can have on foals being reared in intensive situations like the thoroughbred breeding farms.

Breeding farms are particularly at risk because of the high stocking rate of horses.  R. equi is predominantly spread by the horse ingesting the bacteria from the soil.  The bacteria then survives the long passage from the mouth all the way through the horse’s intestines, where it can multiply, and is then passed out in the manure.

The infected manure spreads into the soil and other horses then pick up the bacteria and spread it further.  The more bacteria that is in the soil, the more is picked up in dust and breathed in by the foal.

The bacteria rarely causes disease in the intestines or the abdomen, it mostly causes disease of the lungs.  Other bacteria can cause infections in the lungs, but Rhodococcus equi is the most likely suspect, especially in older foals on a farm with a history of Rattles.

Adult horses usually do not get R. equi infections because their immune system has developed the ability to fight the infection.  Most foals do not develop this stronger immune system until they are older than 3 months of age.

Once a property is contaminated, preventing the disease becomes a challenge.


1. Minimizing contamination and exposure:  Regular manure pick-ups and low stocking densities can decrease how much bacteria is contaminating the environment.

Keeping the foal in a ‘dust-free’ environment also helps.  Research has found that foals confined to stables for the first 2-4 weeks of life are much less likely to develop infections.

A ‘dust-free’ environment can be very challenging on large breeding farms, especially when the foals are born toward the end of a dry winter and spring when there is not much grass around.

2. Boosting the foal’s immune system:  A plasma product has been produced that is rich in antibodies that help protect the foal against R. equi. The plasma is administered by a veterinarian, a bit like a blood transfusion, preferably when the foal is 24 hours old, and then 21 days later.  There is some controversy as to how well the plasma works and whether it is worth the expense.

3. Early detection:  Early detection means the foal can be treated before the disease gets serious, and which means shorter treatment times.  Ultrasound is considered the cheapest and most effective screening test for the early detection of abscesses forming in the foal’s chest.  Ideally the foals are examined every 2 weeks from 2 weeks of age.

Ultrasound can detect small abscesses on the surface of the lung.


Blood tests: The pus in the lungs causes a distinctive increase in the number of white cells in a foal’s blood. It also causes a large increase in an inflammatory protein called fibrinogen. White cell counts and fibrinogen levels are relatively cheap and useful for demonstrating the presence of infection, however there are other diseases that can also cause them to become elevated.

Lung washes: A tube is passed into the foal’s lungs and a sample of pus is collected and the type of bacteria is identified. This is a very useful test, but as you can imagine, it is a bit uncomfortable for the foal for a few minutes and if they are very sick it is not good to stress them further.

Radiographs (x-rays): X-rays are good at detecting abscesses in the lungs however they are more expensive and require the foal to stand very still.


Some recent large studies on farms in South America have shown that for small abscesses (< 2cm), antibiotic treatment may not be necessary. Foals that were not given antibiotics healed as quickly as foals that were administered antibiotics.

For foals with large abscesses that are showing symptoms like coughing and weight loss, treatment involves daily administration of two different types of antibiotics as a paste into the foal’s mouth, for a period of between 1 to 6 months (until the abscesses have disappeared on ultrasound, then continue for several weeks afterward).

The antibiotics are usually a combination of rifampin and one of erythromycin, clarithromycin or azithromycin.

Erythomycin, clarithromycin and azithromycin can cause diarrhea in an adult horse if accidently ingested – sometimes severe and life threatening. Care must be taken to wipe away any antibiotics from the outside of the foal’s mouth so the mare does not accidentally ingest the antibiotic, either directly from the foal’s mouth or when it washes off in the drinking water.


Occasionally we see Rhodococcus equi causing different problems in foals, such as:

1. Abscesses in other parts of the body, including the intestine, liver, joints, bones and brain.

2. Excessive and abnormal stimulation of the immune system causing inflammation of the joints, muscles, internal lymph nodes and eyes.

It is rare to see R. equi infections in an adult horse.  When it does occur it usually means that there is a problem with that horses’ immune system.

One adult mare I saw recently was a rare ancient breed of pony (with a very small gene pool in Australia, which often leads to immune deficiencies).  This mare had small but deep abscesses covering her hind legs and belly.  The infection had not responded to penicillin antibiotics like most skin infections would.  This mare had a 3 month old foal at foot that had recently developed a cough.  Both mare and foal were diagnosed with Rhodococcus equi infections.

Humans with immune deficiencies can also be susceptible R. equi infections.


1. If your foal develops a cough that doesn’t go away after 1-2 days.

2. If your foal has a cough and a snotty (mucous and/or pus) nasal discharge.

3. If your foal is not interested in nursing or eating or has laboured breathing.

4. If you are able to take the foal’s temperature, and it is greater than 39°C.