Rhodococcus equi risk factors

Rhodococcus equi (R. equi) is a bacteria which is particularly common in the soil and droppings (telleric and ubiquitous). Its main target is the foal aged less than 6 months old. The most common clinical form is respiratory, characterised by suppurative bronchopneumonia. Digestive forms (enterocolits) and bone and joint infections (arthritis, osteomyelitis) have also been observed.

Transmission is mainly through inhaling dust from the environment contaminated by virulent R.equi. Clinical signs often develop insidiously with serious internal damage before the first symptoms appear.

Early diagnosis is difficult and antibiotic treatment is specific, costly and lengthy. In addition, the prophylaxis means at present available (hyperimmune plasma and autovaccines) are not entirely effective when there is a high environmental risk.

A study carried out with veterinarians enabled to determine a prevalence (number of cases in a given equine population) in Basse Normandie which is of 1,2 %. It seems that the disease is epizootically present in some studs (i.e they have cases of rhodococcus equi every year), whereas others remain free from the disease.

1) Respiratory form of rhodococcus equi : suppurative bronchopneumonia detected on autopsy

2) Digestive form of rhodococcus equi : abnormally enlarged and abscessed lymph nodes in the caecum detected on autopsy

3) Bone and joint form of rhodococcus equi : spinal osteomyelitis (bone abscesses) detected on autopsy

It is now recognised that knowledge of the risk factors with regard to breeding is paramount in establishing effective preventative measures.

The goal of this case-control study carried out in 2005 was to establish risk factors of rhodococcus equi in relation to the structure and practices of breeding farms in  Basse Normandie. Vetrinarians from the region enabled us to recruit the studs enzootically affected by pneumonia caused by R. equi.

This first step enabled us to establish that there was a considerable difference between the size of the affected studs (number of foalings in 2004) and the size of Basse Normandie studs (see figure showing the results). The larger size studs had significantly higher risk of being affected by pneumonia caused by R. equi.

So that we were able to determine the other potential risk factors, we structured the study so that we could eliminate the size factor (by choosing the case-control studs and those affected by the disease, according to a balanced ratio of size categories)

Finally, the study was conducted over 93 voluntary studs from Basse Normandie : 64 case control studs, and 29 affected studs. An investigator visited each of them and completed a study questionnaire. This questionnaire collected a lot of information regarding the general characteristics of the breeding practices, the pastures, paddocks, buildings dedicated to the horses’ accomodation, and the buildings dedicated to reproductive activities. Information regarding breeding management and sanitary measures implemented was also recorded.
The date was then analysed statistically (univariate and multivariate analyses using logistic regression) so as to determine the variables which were significantly associated to the status of the stud (affected or case-control)

Size of the stud farm, density of animals and brood mares stationed temporarily

The annual number of births is therefore a good indicator of the degree of risk for a stud to be affected by pneumonia due to  R. equi.

So as to eliminate the size factor and look into the other risk factor, our study was built in such a way as to have the same ratio of enzootic studs and case-study studs in each size category (annual number of foalings)
This approach can explain the fact that risk factors linked to size, such as Permanent Grass covered Surface (PGCS) dedicated to horses, and the resident horses do not appear in our study. On the other hand the parameters linked to the density of horses appeared significant :

• A density of over one horse/hectare was significant with 3,5 times more risk of becoming an enzootic stud;
• In the affected studs a density of over 2 mare and foal pairs per hectare was frequently the case ;
• Maximal brood mare capacity was frequently considered to be reached or even exceeded in the affected studs;
• A number of temporarily residing mares of over 20 also appears to be a higher risk factor of being an affected stud.

This confirms the significant influence of the number of animals, whether concentrated or not, on the risk of rhodococcus equi on a particular stud. This influence may reflect a higher number of possibilities of infection due to, on the one hand, to more frequent opportunities of environment contamination by a virulent strain of  R. equi in the droppings of of mares and foals, and on the other hand, more frequent opportunity of contact between foals.

Foals affected by R. equi  pneumonia are mainly infected through breathing in dust particles contaminated with virulent  R. equi. The bacteria can then multiply in the digestive tract of infected foals and huge quantities of  R. equi are then excreted particularly in the first 8 weeks of life. Moreover, a large population of foals can lead to a higher R. equi contamination of the environment. The mares, to a lesser degree can also be a source of virulent  R. equi.

A contagion phenomenon between foals due to airborne bacteria has also been reported with infected foals, and clinically healthy foals who have high concentration of virulent  R. equi in their breath, and who can therefore become a significant source of bacteria for sensitive foals.

Dust

It was commonmy thought that there was a link between the concentration of  R. equi  in the soil and the prevalence of the disease on a breeding farm. This idea is today contested. On the other hand the association between the concentration of virulent airborne R. equi  and the number of cases of pneumonia due to R. equi on a stud, has recently been demonstrated.

In fact, it appears from our study that several parameters which reflect increased exposure of foals to the airborne bacteria : paddock soils on sand or earth, lack of grass cover in the paddocks, no rest period with no animals for the paddocks, dusty environment,  bare earth surface around the restraint stock for ultrasounds and/or insemination, poor ventilation for the mare and foal loose boxes.

A study carried out in Australia (Muscatello, Appl. Environ. Microbiol., 2006) brought to light sites which were at high risk, i.e sites with with higher concentrations of airborne virulent  R. equi . These sites were usually dry, bare and dusty, especially the pathways used to move horses from one field to another, the paddocks and the waiting areas. In another study carried out in Ireland (Muscatello, Equine Vet. J., 2006), where climate conditions are different from those found in Australia, particularly regarding higher humidity, it appeared that airborne virulent R. equi was more of a challenge in the buildings.

In our study, risk factors relating to dust probably reflect an increase in concentration of airborne virulent R. equi  with an increased challenge over the whole stud (dusty environment) :

• Inside : poor ventilation of the stables ;
• Outside : presence of paddocks on sand or earth surfaces ;
• In between area : waiting area for ultrasounds and/or inseminations on a bare earth surface.

Late foalings

It appears to be a dominating parameter, probably linked to the fact of superimposing the hot dry season to the period of maximum sensitivity of the foal to infections when born later in the year.

Indeed, foals are most sensitive to infection between the age of 1 to 3 months old, period when the immunity from the dam drops, and when the foal’s acquired immunity is not yet developed enough. (see figure)

Thus, foals born later in the season, reach the critical sensitivity age during the hot months, when the infectious challenge is greater. It is therefore not surprising that late foalings represent a major risk factor in our study.

Foreword

Large breeding farms, or those with a high concentration of horses are by essence at risk to Rhodococcus equi infections. Ideally, the first thing to do would be to modify these parameters : reduce the number of horses, increase the PGCS. If thses changes are not economically possible, the farm will have a higher risk of being affected by cases of  R. equi  pneumonia, and will need to implement other preventative measures to limit the number of infected foals.

Corrective measures for the risk factors established by the study

• Reduce the numbr of mares temporarily present : < 20 ;
• Favor early foalings : prepare the mares (living conditions, food, artificial lighting)
• Reduce exposure of the foals to dust :
  • Identify the risk areas, in each stud, and every year, so as to intervene in the correct areas. The location of areas to be considered can change every year depending on:
    • The region : variation of the climate and /or breeding practices where the animals are either confined, or on the contrary farmed extensively;
    • The year and the time of year : period of heat and drought;
    • The stud farm: breeding methods, nature of the ground inside the buildings and outside, quality of the ventilation in the buildings;
  • Modify the environment:
    • Modifying the nature of the ground (paddocks, waiting areas) ;
    • Improve ventilation in the buildings ;
    • Move the foals’ drinking and eating places around  ;
    • Reduce the time the foals spend in dusty areas (such as areas used while waiting for ultrasound scans and/or insemination);
    • Implement rotation on the grazing areas;
    • Reduce the size of the groups of mares with foals;
    • Watering the ground ;
    • Reseeding of the grazing areas.

• Regular poo-picking (inside and outside) ;
• Regularly clean and disinfect the foals’ living areas (e.g by using phenolic derivatives) ;
• Use well-reasoned liming for the pastures  (Ferry, 2010) ;
• Use good composting practices for the manure (Hebert, 2010) ;
• Check the passive immunity transfer : assess the quality of the mare’s colostrum (colotest), make sure the foal takes the colostrum, check the transfer of antibodies from the mare to the foal (serum immunoglobin blood test) build up a colostrum bank;
• Medical prophylaxis : hyperimmune plasma, auto vaccines (strains of R.equi from the premises).

Moreover, one should not forget that regularly observing the foals is a key element for detecting symptoms early on, implementing a treatment, and thus with a better prognosis..

This page is from a brochure and study carried out by the Anatomopathological and epidemiological Observatory Unit for major and newly emerging Equine Disease  (Dr J. Tapprest, Dr C. Laugier, A. Saison, C. Sevin, D. Courtois, F. Duquesne, N. Foucher) from AFSSA (ANSES) Dozulé. Collaboration : Dr M. Henry-Amar (GRECAN – Caen).

The brochure is available for free download at the Haras Nationaux library, and can be accessed by clicking HERE.