10/4/2022:  In 2021, the Equine Help Desk was called to consult on a foal with a chronic Rhodococcus infection. The foal had not recovered sufficiently following lengthy treatment with multiple types of antibiotics, and deteriorated as soon as treatment was discontinued. It eventually became apparent that the Rhodococcus bacteria had caused inflammation not only in the lungs but also elsewhere. Resistance to antibiotics also played a role. In these types of cases, good diagnostics provide essential information for treatment and prognosis.

After six weeks of antibiotics prescribed by the owner’s veterinarian, the foal was in poor condition when admitted to a secondary clinic. The animal showed clear clinical signs of pneumonia, severe emaciation, no appetite and uveitis in both eyes. A mucous sample taken from the airways (trans-tracheal wash) and an X-ray showing lung inflammation with abscesses and an almost complete lack of air in pulmonary tissue, resulted in the diagnosis of Rhodococcus pneumonia. The foal was in such poor condition that euthanasia was the only option, after which the animal was submitted to GD for necropsy. The question was why this foal reacted so much worse to the prescribed treatment than we are used to seeing in cases like this.

Rhodococcus equi and resistance

Problems with Rhodococcus infections in foals occur with varying frequency in the Netherlands. They are more commonly found in countries where it is warmer and more dusty, such as Australia. R. equi is present in the soil worldwide. Inhalation of dust particles with R. equi is the main source of infection, alongside the ingestion of soil and faeces when grazing. Foals in particular can develop severe infections. As the summers become warmer and drier in the Netherlands, the number of cases may increase in the future. In countries with a higher prevalence, antibiotics are sometimes more rapidly deployed against these bacteria, also preventively. However, this does increase the risk of resistant strains developing.

Diagnostics

Good clinical examination, combined with imaging and laboratory diagnostics are important to get clarity. A PCR test gives rapid and sensitive insight into the question of whether a pathogenic Rhodococcus is involved, while bacteriological testing using an antibiogram can provide information on the most effective treatment. Bacteriological diagnostics are time-consuming and less sensitive, but do give essential information on the best antibiotics to use.

EPD and resistance result in low chances of survival
The literature shows reasonably good chances of survival for foals in general, as long as the Rhodococcus infection is limited to the lungs and is treated promptly and accurately. The chances of survival are significantly lower once there are infections in other organs or parts of the body (extrapulmonary disease, EPD). Resistant Rhodococcus bacteria are more commonly found in foals undergoing extended treatment, as was the case for this foal. Good diagnostics and a timely conclusion on possible resistance are extremely relevant for the prognosis.

Monitoring

Rhodococcus equi can infect humans, but does not appear to be a primary zoonosis worldwide. The bacteria only occasionally result in severe issues in immuno-compromised people. There is no central registration for the occurrence of Rhodococcus infections in the Netherlands. It may prove interesting to monitor the trends for this disease.

This was certainly an eye-opener

 Kim de Graaf: equine veterinarian at Veterinair Centrum Someren

“Rhodococcus is generally a farm problem, as the bacteria can survive long in the ground. I was only alerted to the foal towards the end of the process. Based on the clinical presentation, the animal had been treated by the owner’s veterinarian for Rhodococcus, but deteriorated once the antibiotics were discontinued. The foal had very abnormal inflammation values, considerable eye infections and pulmonary abscesses. What stood out to me were the SAA values in the blood, an acute phase protein. We often use this test to detect an infection, but the SAA value was repeatedly zero in this case. On the other hand, the white blood cell count was the highest I have ever seen. In this case, we could therefore clearly not trust the SAA on its own. All other signs pointed to Rhodococcus and the foal had been treated with the correct combination of antibiotics, for a sufficiently long period of time.

In my experience, Rhodococcus is generally very treatable, and it was therefore striking that the foal had deteriorated so much. We therefore submitted a transtracheal wash to GD, and a PCR test did indeed detect pathogenic Rhodococcus. As the foal continued to deteriorate, euthanasia became the only option. GD conducted a pathological examination, also to determine whether any other factors were present. There was disastrous damage to the lungs, as well as extrapulmonary inflammations. The foal had multiple large abscesses in its stomach. An antibiogram was the next diagnostic step, which showed resistance, an eye-opener. As equine veterinarians, we were taught that Rhodococcus is always sensitive to the combination of antibiotics that was used.

Together with Linda van den Wollenberg, I presented the case to the Professional Network of Equine Veterinarians. It then became apparent that colleagues regularly see foals with Rhodococcus. They all agreed that they never test for sensitivity to antibiotics. We really need to make antibiograms more regularly, also to gain insight into the degree of resistance in the Netherlands. Our clinic has agreed to do this for a number of foals with Rhodococcus this year. We also discussed the tendency of many equine veterinarians to use SAA testing as an indicator of when to discontinue the use of antibiotics. These tests can certainly play a role as an initial indication of an acute infection. Once the score is low however, this does not necessarily mean that there is no longer any infection. We have now agreed with the professional network that we are on the alert for cases of SAA values that are different from those expected. This will give us a better understanding of the situations in which such values increase or decrease.”
 

Good diagnostics are absolutely essential

Linda van den Wollenberg: equine veterinarian at GD

“Pathological examination of this foal showed that the Rhodococcus bacteria had caused inflammation at many more locations in the body other than the lungs. The animal also had a large abscess in its stomach, eye inflammations and inflammation in the joints. Rhodococcus was suspected right from the start, and the animal had been treated with targeted antibiotics. The veterinarian in the referring clinic, Kim de Graaf, was keen to know why the foal had not reacted to this treatment with a combination of antibiotics. In her practice, earlier cases had generally reacted well to this treatment.

At GD, we detected the bacteria using a PCR test, followed by bacteriological testing. In this case, an antibiogram was also made, specifically for the combination of the antibiotics prescribed in such cases. This foal had been treated with a combination of azithromycin and rifampicin. The antibiogram showed that the isolated bacteria were reasonably resistant to this combination. So the case of this foal once again highlights that it is sensible to have an antibiogram made before using such antibiotics. This is also desirable from a legislation point of view. When foals do not respond well to treatment, I sincerely advise veterinarians to keep a very open mind. If the animal does not recover as expected, it is essential to use effective diagnostics for further examination.

The Professional Network of Equine Veterinarians meets twice a year. GD then provides information on new diagnostic developments. Equine veterinarians inform us of solutions that have proven useful in practice or that they have questions about. This case of Rhodococcus was jointly presented to the network by the treating veterinarian Kim de Graaf and myself. After all, having a sparring partner can often result in very interesting findings. And you are more likely to work out a problem in detail or solve it. We can all learn from such a case, and I too am happy to have gained more knowledge.”

An article on monitoring equine animal health, which featured a number of cases including this Rhodococcus case, was published in April 2022 in the Dutch veterinary medicine journal Tijdschrift voor Diergeneeskunde.