A non-invasive method for obtaining samples for bacterial culture from foals with suspected pneumonia has been described by workers in Japan. Dr Hashikura and colleagues developed a technique of naso-tracheal intubation using a single silicon tube passed through the nose into the trachea. They assessed the reliability of the technique for the investigation of Rhodococcus equi infections in foals. They found that the results compared favourably with alternative techniques.
Rhodococcus equi affects foals up to six months of age. It causes an intractable suppurative pneumonia with abscessation of the abdominal lymph glands and associated diarrhoea.
The differentiation of Rh. equi from other causes of pneumonia can be difficult. Diagnostic tests that may be useful include haematology (complete blood count, and fibrinogen) , radiography and serology. Culture of bacteria from the lower airways is indispensable in differentiating Rh. equi from other causes of pneumonia, and in determining the antibiotic sensitivity of the organism concerned.
The trans-tracheal aspirate has become a well used technique. It involves passing a tube into the trachea through a needle placed through the skin at the lower third of the neck. It has the advantage that it avoids contamination of the sample by bacteria from the upper airways. But it is invasive and may cause complications, such as mediastinal emphysema, local infection, tracheal haemorrhage and damage to the cartilage rings of the trachea. One way of avoiding these complications is to sample the tracheal exudate by passing a tube through an endoscope. But this requires more expensive equipment and is more difficult to perform in foals.
The naso-tracheal method described by the authors avoided the complications of the trans-tracheal method , without requiring expensive equipment such as endoscopes. They developed a technique which uses simple aspiration through a single silicon tube passed through the nose in to the trachea.
One hundred and seventeen foals, ranging in age from about 3 weeks to 3 months, were selected for the study. They had signs of increased temperature, cough, nasal discharge, increased respiratory rate and abnormal breathing sounds. A transtracheal aspirate (TTA) was collected from 11 foals; a nasotracheal aspirate (NTA) was collected from 96 foals. Both TTA and NTA were collected from a further 10 foals. The foals were sedated for both procedures.
The proportion of foals in which Rh equi was isolated was similar for both methods.
Rh equi can occur as virulent and avirulent forms. The avirulent form may be found in the upper airways of foals but it does not cause disease. The virulent and avirulent forms can be differentiated using Polymerase Chain Reaction tests. All Rh equi isolated in the study were virulent.
Dr Hashikura listed the advantages of naso-tracheal aspiration:
lower risk of complications compared with tracheal puncture
it can be performed easily, safely and effectively by one clinician in the field situation.
it is available for repeated use in the same foal
tests show it produces similar results to trans-tracheal aspiration
the use of a large diameter silicon catheter allows for collection of more exudate than with transtracheal aspiration
it requires no expensive equipment
But: it may limit the accuracy of bacterial culture because of contamination from the upper airway.
The foals which were infected with Rh equi were slightly younger than those affected with other cases of respiratory disease. According to the authors, this supports the suspicion that foals are infected with virulent Rh equi within one month of birth, and signs show at 30-45 days. Previously, the same authors have suggested that, on farms with previous cases of Rh equi infection, foals should be screened for the disease at 30-45 days. Their screening protocol included physical and serological examination, ELISA tests and tracheal aspiration. They now suggest that nasotracheal aspiration might be very useful in field practice instead of the more invasive transtracheal aspirate method.
"We suggest that naso-tracheal aspiration can be used as an alternative to trans-tracheal aspiration , especially for the diagnosis of pneumonia in foals " said Dr Hashikura.
Dr Takai added "The method is very popular in Hidaka, Hokkaido, Japan and about 60% of washings by naso-tracheal aspiration from suspected foals with 3 weeks to 3 months of age are Rh equi-positive. We think that the most important pulmonary pathogen in foals less than 3 month is Rh equi, and this method is very useful for definitive diagnosis."
For more details see: S Hashikura, T Higuchi, S Taharaguchi, Y Orita, Y Nanao, S Takai. Evaluation of nasotracheal aspiration as a diagnostic tool for Rhodococcus equi pneumonia in foals.Equine Vet J (2000) 32 (6) 560 - 564