The effect of immunosuppression on resistance to Rhodococcus equi in mice.

Rhodococcus equi, a natural pathogen of horses, produces lesions in mice following experimental infection. The effect of various immunosuppressing agents on the sequential development of these lesions has been assessed by measuring the growth of R. equi following intravenous or intranasal challenge and by histological examination. Cyclophosphamide treatment of mice, challenged intranasally, resulted in the development of lesions not unlike that seen in experimental and natural infection in foals. Cortisone acetate also impaired bacterial clearance from the lungs and affected the accumulation of mononuclear cells at infective foci. Most of the agents chosen to impair macrophage function failed to affect the resistance of mice to R. equi. Carbon, carrageenan and silica failed to alter significantly the growth kinetics of R. equi. Dextran sulphate depressed the rate of pulmonary clearance of organisms and affected the ability of animals to eliminate R. equi following rechallenge. Overall, these results support other evidence that cell mediated immunity is involved in host resistance to R. equi and that activated macrophages play a role in acquired immunity to this organism.

Early events associated with experimental infection of the murine lung with Rhodococcus equi.

Pneumonia due to Rhodococcus equi was induced in the murine lung by deposition of a known dose of organisms. From serial estimations of bacterial numbers in the lungs of inoculated mice, analysis of the cellular composition of bronchoalveolar lavage fluid and morphological examination of the lungs, events in the host-parasite interaction were followed until day 7. Early bacterial clearance from the lung was dose-dependent but was not sustained. A proportion of the inoculated R. equi was susceptible to the early nonspecific phagocytic cell response, and the contribution of neutrophils to bacterial clearance appeared largely limited to the first 24 hours. A substantial fraction of the organisms survived in the alveoli, probably within macrophages. The contribution phagocytes make to resistance against R. equi is similar to that which prevails in infection with Listeria monocytogenes.

Experimental infection of mice with Rhodococcus equi: differences in virulence between strains.

The growth kinetics in outbred mice of clinical and environmental isolates of Rhodococcus equi were followed by serial bacterial enumeration of organ homogenates. Clinical isolates multiplied until Day 4 before being progressively cleared, but could still be recovered from the liver at 3-4 weeks post-infection. Intravenous inoculation of clinical strains was associated with histopathological responses very similar to those elicited by intravenous infection with various facultative intracellular parasites. Whereas lesions in mice and foals at 7-9 days following respiratory infection are those of severe bronchopneumonia with massive consolidation, a week later the patterns of host response have diverged as the murine lesions resolve. The type strain, NCTC 1621 and 4-6 environmental isolates were eliminated without prior multiplication and these strains caused negligible lesions. The two environmental strains which behaved as the clinical strains were recovered from a stud with an R. equi problem. No association of colonial morphology of R. equi with virulence was apparent.

The immunological response of foals to Rhodococcus equi: a review.

Normal horses of all ages regularly show evidence of having responded immunologically to R. equi, thus adding serological support to epidemiological evidence that this organism is a normal intestinal inhabitant. More animals from "diseased" farms show a stronger antibody response when compared with foals from "healthy" farms. Various serological tests have been used to detect evidence of infection and to relate antibody level to severity of disease. Anti-R. equi IgG antibody levels, as measured by ELISA, are raised significantly during natural infection. Clinical severity of pneumonia can be correlated with lower specific antibody responses. Following experimental infection, immunological responses can be detected by complement fixation, indirect immunofluorescence, ELISA, lymphocyte blastogenesis and skin testing. Very little work has been carried out to evaluate vaccines against R. equi infection and results have not been encouraging. Success in treatment has been reported following passive immunisation. Administration of immune leucocyte extracts has had no effect on morbidity or mortality rates. The widespread distribution of this organism, together with the relative infrequency of disease caused by it, suggest that R. equi may initiate infection only in such circumstances as a very high infectious challenge, immunological immaturity or deficiency in the host and genetic predisposition.

A note on hydrolytic enzymes of Corynebacterium equi.

One hundred and five strains of Corynebacterium equi from various sources were examined for the production of 11 extracellular enzymes. Only lipase and phosphatase activity were detected in all strains. A small number of strains produced deoxyribonuclease but these strains were not confined to any one source. It seems likely that C. equi does not rely on a powerful array of extracellular enzymes to induce pathology.

Antibiotic susceptibility testing: caeci caecos ducentes?

Antibiotic susceptibility testing is frequently conducted by diagnostic laboratories and some clinicians rely on the information which such testing provides. In this review of the procedure the rationale for interpretation of the agar diffusion test is outlined and the factors which affect determination of zone size and minimum inhibitory concentration (MIC) discussed. The relationship between zone size and MIC is considered, as well as the clinical relevance of MIC breakpoints. Against this background we assess the value of disc diffusion tests for veterinary practice.

Capsular serotypes of Rhodococcus equi.

One hundred strains of Rhodococcus equi from various animal species and sources in Australia were examined for capsular serotype. Eighty-four of the strains fell into the existing 7 serotypes, and just under half of the strains belonged to serotype 1. Isolates from the intestines and faeces of horses, cattle, pigs and other species, and from soil, were found to belong to the same serotypes as those recovered from the lungs of foals with R. equi pneumonia. There was no clear relationships between capsular serotype and source of origin of the isolates.

Experimental Corynebacterium equi infection in mice.

When normal mice are challenged intravenously with Corynebacterium equi, the organisms are generally cleared from the liver, spleen and lung after an initial increase in numbers in these tissues. Viable organisms can still be recovered from the liver and spleen 5 weeks after challenge. The animals show no sign of infection even when tissue counts exceed 1 X 10(7). In-vivo growth rates of C. equi are similar when the challenge inoculum is 6 h and 21 h old. In-vitro growth temperature does not affect the pattern of in-vivo growth and clearance. There was no evidence of systemic involvement when 6 X 10(7) and 7 X 10(7) organisms were administered orally and subcutaneously, respectively. Following intranasal challenge with 1.5 X 10(6) organisms mice can clear the lungs within 4 weeks, but when challenged with 10 times this number of organisms, the clearance rate is slowed markedly and pneumonia results. Administration of carrageenan before bacterial challenge reduces clearance rates from the tissues but does not affect mortality rates. Cyclophosphamide treatment with C. equi infection converts a non-lethal challenge into a fatal infection and impairs clearance from the tissues.

Corynebacterium equi in the gastrointestinal tract of ruminants.

Corynebacterium equi has been recovered from the gastrointestinal tract of sheep and cattle. It can be found in all parts of the gut, and around 80% of animals have the organism in one or more intestinal sites. C. equi could be detected in the faeces of sheep which were kept caged and free from recontamination by the organism.

The isolation of antibiotic resistant coliforms from meat and sewage.

Beef and pig carcases, meat products, frozen chickens, and sewage were examined in 3 separate surveys for antibiotic resistant coliforms. Escherichia coli was isolated from 18 of 50 beef carcases; the numbers were low and resistance was found only to tetracycline. E. coli was isolated from 45 to 50 pig carcases; the numbers were high and showed a range of patterns of multiple antibiotic resistance. In meat products, the proportion of E. coli in contaminating organisms was low, and most resistance found was to tetracycline and streptomycin. E. coli was isolated from 66 of 75 chickens and these gave 23 patterns of antibiotic resistance, often multiple. Sewage from hospital or domestic origin and abattoir effluent yielded approximately 10(6) coliforms/ml, most of which were resistant to one or more antibiotics; few of those from hospital or domestic origin however, were classified as E. coli of faecal origin. Twenty-four patterns of resistance were found in coliforms from domestic sewage, 19 from hospital sewage and 11 from abattoir effluent. Transfer of resistance, often multiple, was achieved from 55% of 447 resistant strains to an E. coli K12 recipient. Much more information is required on the prevalence of R-factors in bacteria associated with food producing animals and their products.

Corynebacterium equi: in vitro susceptibility to twenty-six antimicrobial agents.

The minimal concentrations of 26 antimicrobial agents required to inhibit growth of 100 isolates of Corynebacterium equi in vitro have been determined. The most active agents were penicillin G, doxycycline, erythromycin, lincomycin, and the aminoglycosides.

Epidemiology of Corynebacterium equi in horses.

Using a selective medium, the epidemiology of Corynebacterium equi in horses was studied. Twelve horse studs in different geographical locations were investigated. In addition, a group of horses was monitored over a period of time to determine whether faecal carriage of C equi is constant or intermittent. It was found that C equi could be recovered from every property tested and from all horse breeds. The organism was found in mares, stallions, yearlings and two year olds, and foals aged from 10 days to several months. It appears that C equi is widespread and that its occurrence is not related to any particular geographical location nor to a limited number of properties within a given area. Furthermore, it seems likely that C equi is part of the normal equine faecal flora. The results challenge the notion that C equi is primarily soil associated and do not support the concept that there are infected and non-infected areas.

Corynebacterium equi in cattle and pigs.

The cervical lymph nodes of pigs, the retropharyngeal and submandibular lymph nodes of cattle and faecal samples from both animal species were examined for the presence of Corynebacterium equi. The organism was recovered from 19 (35 per cent) of 54 porcine cervical lymph nodes and from 0 of 54 bovine retropharyngeal and submandibular lymph nodes. Fifteen (50 per cent) of 30 bovine faecal and 11 (35 per cent) of 31 porcine faecal samples yielded C. equi.

Corynebacterium equi in cattle and pigs.

Summary The cervical lymph nodes of pigs, the retropharyngeal and submandibular lymph nodes of cattle and faecal samples from both animal species were examined for the presence of Corynebacterium equi. The organism was recovered from 19 (35 per cent) of 54 porcine cervical lymph nodes and from 0 of 54 bovine retropharyngeal and submandibular lymph nodes. Fifteen (50 per cent) of 30 bovine faecal and 11 (35 per cent) of 31 porcine faecal samples yielded C. equi.

Selective medium for Corynebacterium equi isolation.

The development of a selective medium for the isolation of Corynebacterium equi is described. The medium has been used to examine fecal samples from 127 horses of which 90 have been found to carry the organism.

The capsules of Corynebacterium equi and Streptococcus equi.

The capsules of Corynebacterium equi and Streptococcus equi were examined by electron microscopy after staining with ruthenium red. They were compared with the capsule of Klebsiella pneumoniae which had previously been examined using the same procedure (Springer & Roth, 1973). The capsule of C. equi had a laminated appearance. When S. equi was grown on solid medium, its capsule appeared as radially arranged projections capped by a thick electron dense layer. When grown in liquid medium, S. equi produced a capsule which showed as short thick projections with no layer external to them.