Rhodococcus equi in foals

Rhodococcus equi is a disease which affects foals from 3 weeks to 6 months old, infected by the rhodococcus equi bacteria. Incubation time is between 3 to 6 days. Only foals are affected by the illness, but adults can be carriers if they live in a contaminated environment.
Symptoms are essentially respiratory, but can also be digestive (diarrhoea) or locomotor and affect joints and bones (polyarthritis, osteomyelitis, suppurative myositis…)
Morbidity rate ( % of animals infected) is around 17 % including all breeds.
Mortality ( % of deaths) is high and can reach over 80 % in the absence of treatment for those most affected.

The pathogen for foal pneumonia is the bacteria Rhodococcus equi. This bacteria has the following characteristics :
•    Is extremely widespread in the general environment ;
•    Presents a wide variety of bacterial strains which are not all pathogenic (virulent) ;
•    Is present in the digestive tract of adult horses and can multiply in droppings if temperature and humidity levels are favorable ;
•    Is present in airborne dust, especially during dry weather, and can directly infect the foal’s mucous linings (e.g when the foal follows his dam and breathes in the dust she raises) ;
•    Persists for long periods in the environment, in the soil (especially when it is dry), especially in sparse pasture, areas where the horses stand for long periods (around feeders and drinking troughs) and in bare earth paddocks;
•    Is destroyed by phenolic disinfectants (based on phenol or carbolic acid).

This bacteria is mainly transmitted through breathing it in : foals inhale the bacteria in fecal matter and contaminated dust, and are thus infected.
The high number of foals affected every year is due to several factors : high concentration of foals, gatherings, more frequent transportation, the number of healthy adult carriers of the bacteria, as well as contaminated premises and soils.
The risk of a foal developing the disease when exposed is the result of three determining factors :
•    The virulence of the bacteria ;
•    The environment ;
•    The foal’s general condition

Several clinical forms of the disease have been reported :
•    An acute repiratory form : pneumonia or bronchpneumonia, often with abscessation is the most common
•    The intestinal form, more insidious in its development, often causing abdominal abscessation ;
•    A musculoskeletal form : affects the locomotor system, especially the joints with arthritis.
Genrally clinical signs include the following :
•    Hyperthermia (high temperature) ;
•    Coughing, which can lead to respiratory distress ;
•    Possible lack of appetite ;
•    General lethargy.

The development of the illness can be insidious, with subtle clinical signs, and then appear brutally with signs of acute respiratory disease, difficulty in breathing and sudden death.
For foals infected by rhodococcus equi there is often a fatal outcome for those severely affected.
The most sensitive foals are aged betwen 1 and 3 months. This is when the immunity from the mother’s colostrum begins to decrease, and before the foal’s own immune system has had time to become effective
It appears however that foals are often contaminated very early on after birth, as soon as the first week, due to a lack of antibodies in the dam’s colostrum.
Foals born late in the season (June or July), when it is hotter, are more sensitive due to a favorable environment for the development of the bacteria (heat and humidity), but also to its dissemination in dry conditions, and to a higher concentration of mares and foals.
Foals aged under 2 months old often develop the respiratory form (pneumonia), intestinal forms are mainly recorded in older foals (between 3 to 6 months old).

Clinical diagnosis of rhodococcus equi is based on the clinical signs described above (pneumonia with a high temperature, lethargy, coughing and nasal discharge), but as these symptoms are non-specific, infection by Rhodococcus equi can only be confirmed with complementary laboratory analyses.
In fact, even though bronchopneumonia is a major cause of death in foals, the main agent responsible for pneumonia is a streptococcus bacteria (S. equi zooepidemicus), which is pathogen as soon as it enters damaged mucous lining, and for which the treatment is different.
 

Imagery techniques can be of use in determining the extent of damage to the lungs, or even in some cases to help a therapeutical decision and to monitor the efficiency of the treatment. Thus, an X-ray, or even better a pulmonary ultrasound scan, are very important diagnosis and monitoring tools which should be implemented before deciding on the treatment to follow.
 Indeed, ultrasound scanning allows to establish a map of the number and the size of any abscesses, and to define a damage score, which , in correlation to the clinical signs will help determine the therapeutical options, which may include surveillance with no treatment. Regular ultrasound screening enables to keep a watch, or even to anticipate, the sudden development of the disease in stud farms and is a part of useful preventative measures in high risk environments.
The presence of abscesses alone does not mean that the infection is rhodococcus equi, or that the outcome is inevitably fatal : abscesses are signs of bronchopneumonia, but do not indicate the nature of the germ. The size and number of abscessess, combined with the foal’s general condition, reflect the seriousness of the infection and the need to act medically.

Laboratory diagnosis of rhodococcus equi is established through bacteriological analysis and identification of the bacteria from a respiratory tract sample (transtracheal wash sample, or nasal swab) or a sample from the joints of affected foal. Serology or PCR tests carried out on these same samples, or blood or fecal samples can also be useful.
Blood samples taken for fibrinogenesis and blood counts alone will only confirm an infection and inflammatory condition, without givong the cause.
Like all bacteria, R. equi is not virulent, even in foals (the expression of a virulence plasmid is necessary),  detection of the germ should not therefore automatically trigger treatment, but should lead to increased suveillance, and vigilance regarding the appearance of distinctive clinical signs.
To confirm diagnosis on a dead foal, a bacteriological and/or PCR test should be carried out to identify Rhodococcus equi.

Treatment of rhodococcus equi is based on :

• Diagnosis following clinical examination, imagery with ultrasound pulmonary scan and laboratory results ;
• Choice of foals to be treated with regard to the clinical context, the damage score from the ultrasound scan and the environment : foals presenting subtle or no symptoms, with few negative ultrasound images usually heal with no medical treatment;
• Specific course of antibiotics subject to regulations, since the molecules used are normally reserved for human medical use ;
•  Regular therapeutical and clinical screening of the symptoms of the disease, particularly using pulmonary ultrasound screening scores ;
• Choice of when to stop the treatment based on clinical criteria, the stage of the foal’s immunity and the environment (climate, concentration of animals, healthy carrier animals) ;
•  Hygiene measures on the premises, particularly regarding management of the droppings and dust, also that of carrier animals, including brood mares.

Once the diagnosis is established, the adequate treatment recommended for foals infected by rhodococcus equi consists in compulsary administration of two antibiotic families concurrently, usually given orally : macrolides and rifampicin.
When the diagnosis is not precise, and while waiting for laboratory results, penicillin and gentamicin can be administered, this is sometimes useful and complys with the regulations regarding the use of antibiotics :

• Decree dated 22nd July 2015 relative to good practices for the usage of medicines containing one or several antibiotic substances in veterinary medicine ;
• Decree 2016-317 dated 16th March 2016 relative to the prescription and delivery of medicines used in veterinary medicine containing one or several critically important antibiotic substances;
• Decree dated 18th March 2016 determining the list of critically important antibiotic substances as provided for by article L5144-1-1 of the Public Health Law.

Treatment is pescribed by a veterinarian  by means of a written prescription, following clinical examination and justified diagnosis of the disease, especially since the antibiotics used are subject to restrictions defined by law.
The length of the treatment should be adapted to the therapeutical result, reconsidered on a regular basis by the vet, and should not exceed 3 weeks without obvious clinical response.
Administering antibiotics on a preventative basis on a healthy foal is completely INEFFECTIVE and FORBIDDEN.
In the same way, self-medicating treatment without prior veterinary examination and prescription is forbidden and subject to legal charges.
Medical treatment should be accompanied by hygiene measures (clening the stables, dampening the ground, reducing the amount of dust) so as to limit thhe spread of the contamination.
A cure with no after-effects is not always possible.
There is no vaccine against rhodococcus equi (foal and/or dam) and it is so far not conceivable due to the complexity of the disease.
Using autovaccines, although does not protect specifically against rhodococcus equi, does stimulate general immunity and can help in the fight against the disease.

The risk of contamination by Rhodococcus equi is higher in breeding studs where the disease has already been detected in previous years, in dry dusty fields and in dirty draughty stables.
In these cases it is advisable to take samples of dust to look for the presence of the bacteria :

• Take 3 samples on 4 different sites (stable, circulation areas, fencing, paddocks) ;
• The laboratory will provide the necessary equipment to take the samples together with a guide on how to carry out the sampling.

The risk of the having an outbreak of the disease increases if :

• There is a large number of bacteria present : >10 000 germs per gram ;
• The strain isolated is pathogenic compared to a non-pathogenic strain (presence of the virulence plasmid) ;
• If the strain isolated is resistant to antibiotics, compared to a sensitive strain.

Mares

Contamination of sensitive animals should be restricted by :

• Favoring early births and avoiding concentrations of foals from 1 to 3 months old when the weather is very hot;
• Isolating and protecting mares from the others when they are about to give birth ;
• Seperating mares with foals from the others and establishing small groups of mares, identifying carrier mares to optimise surveillance in the following years

Using autovaccines on the mares on high risk premises or with a prior history of Rhodococcus equi can be implemented with the treating veterinarian.

Foals from 1 to 3 months old

Early diagnosis and close clinical surveillance, including of subclinical forms, thanks to ultrasound scans, is the best prevention.
In addition to a clinical examination and imagery which will confirm pneumonia, it is only the specific laboratory tests to identify the germ which enable to diagnose rhodococcus equi.

Healthy foals

•  Vigilance and keeping a close watch on the foal’s general state of health and behaviour (suckling, wakefulness and reactivity) ;
• Taking the temperature on a daily basis and keeping an eye on breathing frequency.

For a suspected case (foal with a teperature of >38,5°C,  or rapid breathing even if there is no coughing)

• Seperate from the other mares due to foal, and mares with foal (e.g loose box at the other end of the yard) ;
• Apply hygiene and preventative measures against contamination : do not go from a supected case to a healthy foal, or to a mare due to foal without washing and disinfecting your hands, wear overalls dedicated to looking after affected foals…. ;
• Call in the vet for a clinical examination if in doubt or in the case of an abnormal temperature > 38,5°C, and ask the vet to carry out the necessary examinations for an etiological diagnosis of pneumonia..

Quarantine => avoid introducing into the stud a mare whose foal is either suspect or ill. Ir they h    ave to enter the premises, make sure they are isolated immediately.

Prevention regarding Rhodococcus equi is based for the main part on extreme care in applying the measures to the environment.

Stables and loose boxes

• They should be kept clean, well ventilated and with particular attention paid to the foal’s well-being ;
• Droppings should be removed every day ;
• Clean thoroughly and disinfect every two weeks using a disinfectant (some disinfectants have an in vitro proven action on a reference strain of Rhodococcus equi);
• It is advisable to  disinfect all surfaces after cleaning : material, vehicles, stables, premises, animal food storage and preparation material. Pay particular attention to common equipment and premises (insemination and scan restraint stock), as well as loose boxes (disinfect between horses).

Paddocks

In order to prevent the bacteria from multiplying on the premises, and risking contaminating due to dust one should :

• Avoid concentration of too many mares with foals in paddocks;
• Water the areas where there is a lot of dust and where the horses move around a lot ;
• Poo-pick the paddocks and shelters ;
• Keep the grassy paddocks for mares with foals ;
• Regularly move the drinking troughs and feeders to avoid trampling.