Efficacy of liposomal gentamicin against Rhodococcus equi in a mouse infection model and colocalization with R. equi in equine alveolar macrophages

Rhodococcus equi, a facultative intracellular pathogen and an important cause of pneumonia in foals, is highly susceptible to killing by gentamicin in vitro. However, gentamicin is not effective in vivo, due to its poor cellular penetration. Encapsulation of drugs in liposomes enhances cellular uptake. The objectives of this study were to compare liposomal gentamicin and free gentamicin with respect to their uptake by equine macrophages and intracellular colocalization with R. equi and to compare the efficacies of liposomal gentamicin, free gentamicin and clarithromycin with rifampin for the reduction of R. equi CFU in a mouse model of infection. After ex vivo exposure, a significantly higher mean (±SD) percentage of equine alveolar macrophages contained liposomal gentamicin (91.9±7.6%) as opposed to free gentamicin (16.8±12.5%). Intracellular colocalization of drug and R. equi, as assessed by confocal microscopy, occurred in a significantly higher proportion of cells exposed to liposomal gentamicin (81.2±17.8%) compared to those exposed to free gentamicin (10.4±8.7%). The number of R. equi CFU in the spleen was significantly lower in mice treated with liposomal gentamicin compared to that of mice treated with free gentamicin or to untreated control mice. Treatment with liposomal gentamicin also resulted in a significantly greater reduction in the number of R. equi CFU in the liver compared to treatment with clarithromycin in combination with rifampin. These results support further investigation of liposomal gentamicin as a new treatment for infections caused by R. equi.

VapA-specific IgG and IgG subclasses responses after natural infection and experimental challenge of foals with Rhodococcus equi

Rhodococcus equi is a common cause of pneumonia in young foals worldwide and has considerable economic effects on the global equine industry. Despite ongoing efforts, no vaccine is currently available to prevent rhodococaal pneumonia. This is due, in part, to an incomplete understanding of the protective immune response to this bacterium. While antibodies to VapA, a lipoprotein produced by virulent R. equi, are useful in differentiating antibody production in response to pathogenic versus non-pathogenic strains, the significance of the humoral response of foals to this lipoprotein remains poorly defined. The objectives of this study were to evaluate changes in VapA-specific IgG and IgG subclasses after exposure and infection of neonatal foals. Experimental foals included those challenged with R. equi at 1 (n=18), 2 (n=4) and 3 (n=6) weeks of age. Confirmed naturally infected (n=7) and not infected (n=3) foals were also included. All foals were bled 24h after birth and weekly thereafter for a period of 8 weeks. Antibody changes over time were evaluated. Following birth, VapA-specific IgGs significantly (p<0.05) decreased over time in all foals as a result of normal decay of passively transferred antibodies. Both VapA-specific IgGa and IgG(T) significantly increased (p<0.05) after experimental challenge, however, the rise in IgG(T) occurred earlier. Only a significant (p<0.05) increase in VapA-specific IgG(T) over time was seen after natural infection. Whether VapA-specific IgG(T) can be used to differentiate rhodococcal from other pneumonias requires further investigation under field conditions.

Antimicrobial use in critically ill horses

OBJECTIVE

To discuss controversies surrounding antimicrobial use in critically ill horses.

DATA SOURCES

PubMed searches from 1970-present for terms including, but not limited to: "horse," "foal," "antimicrobial," "prophylaxis," "infection," "surgery," "sepsis," and "antimicrobial resistance."

HUMAN DATA SYNTHESIS

Increasing bacterial antimicrobial resistance has changed first-line antimicrobial choices and prompted shortening of the duration of prophylactic and therapeutic treatment. The need to decrease bacterial resistance development to critically important antimicrobials has been highlighted.

VETERINARY DATA SYNTHESIS

Veterinary medicine has followed a similar trend but often without a high-level evidence. Common dilemmas include diseases in which the theoretically most effective drug is a reserved antimicrobial, the inability to differentiate infectious from noninfectious disease, the duration and necessity of prophylactic antimicrobials and use of antimicrobials in primary gastrointestinal disease. These problems are illustrated using examples of purulent infections, neonatal sepsis, colic surgery, and treatment of colitis. Although enrofloxacin, cephalosporins, and doxycycline, in contrast to gentamicin, reach therapeutic concentrations within the lungs of healthy horses, the first two should not be used as first line treatment due to their reserved status. Due to the high risk of bacterial sepsis, antimicrobial treatment remains indispensable in compromised neonatal foals but shortening the length of antimicrobial treatment might be prudent. One prospective randomized study demonstrated no difference between 3 and 5 days of perioperative antimicrobial treatment in colic surgery but shorter durations were not evaluated. High-level evidence to recommend antimicrobial treatment of adult horses with undifferentiated diarrhea does not exist.

CONCLUSIONS

Few evidence-based recommendations can be made. Commonly used antimicrobial combinations remain the mainstay for treating purulent infections. Antimicrobial treatment for compromised foals should not extend beyond recovery. Continuation of prophylactic antimicrobials >3 days is likely unnecessary after colic surgery; shorter durations might be equally effective. Antimicrobial prophylaxis in adult horses with diarrhea is unlikely to be beneficial.

Large intra-abdominal abscess treatment by marsupialization in 2 alpacas

OBJECTIVES

To report the clinical signs, diagnostic findings, surgical treatment, postoperative complications, and long-term outcome in 2 adult alpacas with large intra-abdominal abscesses treated by marsupialization of the abscess to the ventral body wall.

STUDY DESIGN

Clinical report.

ANIMALS

Alpacas (n = 2).

METHODS

Two alpacas each presented with decreased appetite, lethargy, poor body condition, and marked abdominal distension. A complete physical examination, abdominal radiography, and ultrasonography confirmed the presence of large intra-abdominal abscesses. Streptococcus equi ssp. zooepidemicus was cultured from the abscess fluid of alpaca 1, and a Gram-negative bacillus from alpaca 2.

RESULTS

Both alpacas had exploratory celiotomy with marsupialization of the abdominal abscess to the ventral body wall. Postoperatively, alpacas were administered systemic antibiotics and non-steroidal anti-inflammatory drugs, and the abscesses were lavaged for several days. Alpaca 2 required a second surgical procedure. The abscesses ultimately resolved in both alpacas, with body wall hernia formation as the only major complication. Both alpacas had good long-term outcomes.

CONCLUSION

Marsupialization to the ventral abdominal wall with concurrent antibiotic treatment should be considered as a treatment option for alpacas with large intra-abdominal abscesses in which complete surgical resection of the abscess is not possible. An abdominal wall hernia can result from this procedure, and clients should be informed of this potential complication before surgery.

Effects of age and macrophage lineage on intracellular survival and cytokine induction after infection with Rhodococcus equi

Rhodococcus equi, a facultative intracellular pathogen of macrophages, causes life-threatening pneumonia in foals and in people with underlying immune deficiencies. As a basis for this study, we hypothesized that macrophage lineage and age would affect intracellular survival of R. equi and cytokine induction after infection. Monocyte-derived and bronchoalveolar macrophages from 10 adult horses and from 10 foals (sampled at 1-3 days, 2 weeks, 1 month, 3 months, and 5 months of age) were infected ex vivo with virulent R. equi. Intracellular R. equi were quantified and mRNA expression of IL-1β, IL-4, IL-6, IL-8, IL-10, IL-12 p40, IL-18, IFN-γ, and TNF-α was measured. Intracellular replication of R. equi was significantly (P<0.001) greater in bronchoalveolar than in monocyte-derived macrophages, regardless of age. Regardless of the macrophage lineage, replication of R. equi was significantly (P=0.002) higher in 3-month-old foals than in 3-day old foals, 2-week-old foals, 1-month-old foals, and adult horses. Expression of IL-4 mRNA was significantly higher in monocyte-derived macrophages whereas expression of IL-6, IL-18, and TNF-α was significantly higher in bronchoalveolar macrophages. Induction of IL-1β, IL-10, IL-12 p40, and IL-8 mRNA in bronchoalveolar macrophages of 1-3-day old foals was significantly higher than in older foals or adult horses. Preferential intracellular survival of R. equi in bronchoalveolar macrophages of juvenile horses may play a role in the pulmonary tropism of the pathogen and in the window of age susceptibility to infection.

Endogenous endophthalmitis caused by Enterococcus faecalis in a cat

A three‐year‐old male neutered domestic shorthair cat was presented for loss of vision associated with hyperthermia, lethargy and anorexia. Ophthalmic examination revealed a bilateral panuveitis. Cytological examination of aqueous and vitreous humours was performed and revealed a suppurative inflammation associated with numerous cocci. Enterococcus faecalis was identified by bacterial culture from aqueous and vitreous humour. No primary infection site was identified. Active uveitis resolved after systemic antibiotic therapy, but the vision loss was permanent. To the authors’ knowledge, this is the first reported case of endogenous bacterial endophthalmitis secondary to E. faecalis infection in a cat.

Rhodococcus equi: the many facets of a pathogenic actinomycete

Rhodococcus equi is a soil-dwelling pathogenic actinomycete that causes pulmonary and extrapulmonary pyogranulomatous infections in a variety of animal species and people. Young foals are particularly susceptible and develop a life-threatening pneumonic disease that is endemic at many horse-breeding farms worldwide. R. equi is a facultative intracellular parasite of macrophages that replicates within a modified phagocytic vacuole. Its pathogenicity depends on a virulence plasmid that promotes intracellular survival by preventing phagosome-lysosome fusion. Species-specific tropism of R. equi for horses, pigs and cattle appears to be determined by host-adapted virulence plasmid types. Molecular epidemiological studies of these plasmids suggest that human R. equi infection is zoonotic. Analysis of the recently determined R. equi genome sequence has identified additional virulence determinants on the bacterial chromosome. This review summarizes our current understanding of the clinical aspects, biology, pathogenesis and immunity of this fascinating microbe with plasmid-governed infectivity.

Streptolysin-O/antibiotics adjunct therapy modulates site-specific expression of extracellular matrix and inflammatory genes in lungs of Rhodococcus equi infected foals

The addition of streptolysin-O (SLO) to the standard antibiotics regimen was shown to be superior to antibiotics alone after experimental infection of foals with Rhodoccocus equi (R. equi). The objective of this study is to investigate this response by determining the site-specific expression of extracellular matrix (ECM) and inflammatory response genes in biopsy samples taken from three distinct lung regions of the infected foals. Twenty-four foals were challenged by intrabronchial instillation of R. equi and assigned to four treatment groups: SLO/antibiotics adjunct therapy, antibiotics-only therapy (7.5 mg/kg clarithromycin and 5 mg/kg rifampin), SLO-only, and saline-only treatments. Treatments were administered twice daily for 16 days unless symptoms progressed to the point where the foals needed to be euthanized. Gene expressions were determined using custom-designed equine real-time qPCR arrays containing forty-eight genes from ECM remodeling and inflammation pathways. A non-parametric Wilcoxon signed-rank test for independent samples was applied to two pairs of time-matched comparison groups, SLO/antibiotics vs. antibiotics-only and SLO-only vs. saline-only, to document the significant differences in gene expressions within these groups. Several genes, MMP9, MMP2, TIMP2, COL1A1, COL12A1, ITGAL, ITGB1, FN1, CCL2, CCL3, CXCL9, TNFα, SMAD7, CD40, IL10, TGFB1, and TLR2, were significantly regulated compared to the unchallenged/untreated control foals. The results of this study demonstrate that enhancement of clinical responses by SLO is consistent with the changes in expression of critical genes in ECM remodeling and inflammatory response pathways.

A review of foal diarrhoea from birth to weaning

Diarrhoea is among the most common clinical complaints in foals. Aetiologies, diagnostic testing and recommended interventions for specific causes of enterocolitis are summarised. Many mild to moderately affected foals can be managed in an ambulatory setting, while others will benefit from more intensive care at a referral centre.

Ophthalmic lesions in neonatal foals evaluated for nonophthalmic disease at referral hospitals

OBJECTIVE

To determine types and frequency of ophthalmic lesions detected in neonatal foals evaluated for nonophthalmic disease at 3 veterinary referral hospitals and to investigate associations between systemic and ophthalmic diseases in these foals.

DESIGN

Prospective cross-sectional study.

ANIMALS

70 foals < 30 days old.

PROCEDURES

Complete ophthalmic examinations were performed. Signalment, clinical signs, mentation during ophthalmic examination, results of clinicopathologic tests, and diagnosis of systemic disease were recorded. Descriptive data analysis including a χ(2) test for associations was performed.

RESULTS

Most foals (39/70 [55.7%]) with systemic disease had ≥ 1 ophthalmic lesion detected. Of the 39 foals with ophthalmic disease, 24 (61.5%) had potentially vision-threatening lesions. Clinically important abnormalities included conjunctival hyperemia or episcleral injection (30/70 [42.9%]), uveitis (18/70 [25.7%]), ulcerative keratitis (13/70 [18.6%]), nonulcerative keratitis (10/70 [14.3%]), entropion (8/70 [11.4%]), retinal hemorrhage (8/70 [11.4%]), and cataract (6/70 [8.6%]). Foals with sepsis were significantly more likely to have uveitis than were those without sepsis. Foals with sepsis and uveitis were also significantly less likely to survive to discharge than were foals that had sepsis without uveitis. Acquired ophthalmic disease (detected in 37/70 [52.9%] foals) was significantly more common than congenital ophthalmic disease (detected in 9/70 [12.9%]).

CONCLUSIONS AND CLINICAL RELEVANCE

Ophthalmic lesions were detected in 55.7% of neonatal foals with systemic disease. Acquired ophthalmic disease was more commonly detected than congenital ophthalmic disease. Foals with sepsis were more likely to have uveitis than were foals without sepsis. A complete ophthalmic examination is indicated in neonatal foals evaluated for systemic disease.

Diagnosis, treatment, control, and prevention of infections caused by Rhodococcus equi in foals

Rhodococcus equi, a gram-positive facultative intracellular pathogen, is one of the most common causes of pneumonia in foals. Although R. equi can be cultured from the environment of virtually all horse farms, the clinical disease in foals is endemic at some farms, sporadic at others, and unrecognized at many. On farms where the disease is endemic, costs associated with morbidity and mortality attributable to R. equi may be very high. The purpose of this consensus statement is to provide recommendations regarding the diagnosis, treatment, control, and prevention of infections caused by R. equi in foals.

Rhodococcus equi pneumonia in the foal--part 1: pathogenesis and epidemiology

Rhodococcus equi pneumonia is a worldwide infectious disease of major concern to the equine breeding industry. The disease typically manifests in foals as pyogranulomatous bronchopneumonia, resulting in significant morbidity and mortality. Inhalation of aerosolised virulent R. equi from the environment and intracellular replication within alveolar macrophages are essential components of the pathogenesis of R. equi pneumonia in the foal. Recently documented evidence of airborne transmission between foals indicates the potential for an alternative contagious route of disease transmission. In the first of this two-part review, the complexity of the host, pathogen and environmental interactions that underpin R. equi pneumonia will be discussed through an exploration of current understanding of the epidemiology and pathogenesis of R. equi pneumonia in the foal.

Pharmacokinetics of an orally administered methylcellulose formulation of gallium maltolate in neonatal foals

Gallium is a trivalent semi-metal with anti-microbial effects because of its incorporation into crucial iron-dependent reproductive enzyme systems. Gallium maltolate (GaM) provides significant gallium bioavailability to people and mice following oral administration and to neonatal foals following intragastric administration. To study the prophylactic and therapeutic effects of GaM against Rhodococcus equi pneumonia in foals, we developed a methylcellulose formulation of GaM (GaM-MCF) for oral administration to neonatal foals. Normal neonatal foals were studied. Six foals received 20 mg/kg and another six foals received 40 mg/kg of GaM-MCF orally. Serial serum samples were collected and serum gallium concentrations were determined using inductively coupled plasma mass spectroscopy. Gallium was rapidly absorbed (T(max) of 4 h), and a mean C(max) of 0.90 or 1.8 microg/mL was achieved in foals receiving 20 or 40 mg/kg respectively. Marked variability existed in C(max) among foals: only half of the foals receiving 20 mg/kg attained serum concentrations of >0.7 microg/mL, a level suggested to be therapeutic against R. equi by previous studies. Mean elimination half-life was 32.8 or 32.4 h for foals receiving 20 or 40 mg/kg respectively. The results of this study suggest that at least 30 mg/kg orally every 24 h should be considered in future pharmacodynamic and efficacy studies.

Disseminated Rhodococcus equi infection in dromedary camels (Camelus dromedarius)

Rhodococcus (R). equi, a recognized pathogen in horses, is emerging as a human opportunistic pathogen, especially in immunocompromized people. It affects also New World camelids, but there are no reports of R. equi infection in Old World camelids yet. Four cases of disseminated R. equi infection in adult breeding dromedaries occurred at one camel farm near Dubai within 16 months of each other. At necropsy the lungs were diffusely consolidated with large caseous areas. Histology revealed severe suppurative to necrotising pneumonia with multiple encapsulated abscesses. Immunohistochemistry enabled the detection of 15- to 17-kDa antigens (VapA) of R. equi in the lung sections. High numbers of R. equi were isolated from the lung lesions as well as from liver, spleen and mediastinal lymph nodes, indicative of septicaemia. The isolated strains were PCR-positive for the specific virulence plasmid (VapA-Gen) of R. equi, indicating virulent strains and containing an 85-kb type I plasmid. This is the first report of disseminated R. equi infection in Old World camelids. Since adult camels in general do not suffer from bacterial caused pneumonia (except tuberculosis), this is a new emerging disease for camels.

Characterization of the role of the pathogenicity island and vapG in the virulence of the intracellular actinomycete pathogen Rhodococcus equi

Rhodococcus equi, a facultative intracellular pathogen of macrophages, causes severe, life-threatening pneumonia in young foals and in people with underlying immune deficiencies. R. equi virulence is dependent on the presence of a large virulence plasmid that houses a pathogenicity island (PAI) encoding a novel family of surface-localized and secreted proteins of largely unknown function termed the virulence-associated proteins (VapACDEFGHI). To date, vapA and its positive regulators virR and orf8 are the only experimentally established virulence genes residing on the virulence plasmid. In this study, a PAI deletion mutant was constructed and, as anticipated, was attenuated for growth both in macrophages and in mice due to the absence of vapA expression. Expression of vapA in the PAI mutant from a constitutive promoter, thereby eliminating the requirement for the PAI-encoded vapA regulators, resulted in delayed bacterial clearance in vivo, yet full virulence was not restored, indicating that additional virulence genes are indeed located within the deleted pathogenicity island region. Based on previous reports demonstrating that the PAI-carried gene vapG is highly upregulated in macrophages and in the lungs of R. equi-infected foals, we hypothesized that vapG could be an important virulence factor. However, analysis of a marked vapG deletion mutant determined the gene to be dispensable for growth in macrophages and in vivo in mice.

Detection of Strain Variation in Isolates of Rhodococcus Equi from an Affected Foal Using Repetitive Sequence–Based Polymerase Chain Reaction

Rhodococcus equi is an important pathogen of foals aged 1–6 months. Evidence exists that foals are exposed to a wide diversity of R. equi strains in their environment. However, limited data are available regarding the extent to which genotypic variation exists among isolates infecting individual foals. Therefore, electrophoresis of repetitive sequence–based polymerase chain reaction (rep-PCR) amplicons in an automated microfluidics chip format was used to genotype 9 virulent R. equi isolates obtained from distinct anatomic locations in a single foal. Four of the isolates were obtained from different regions of the lung, and 5 were from abscessed intra-abdominal lymph nodes (LNs). Six distinct genotypes were identified among the 9 isolates. None of the pulmonary isolates was identical; however, a pulmonary isolate was found to be identical to an isolate recovered from a small intestinal LN, and another pulmonary isolate was identical to an isolate from a mesenteric LN. These results indicate that foals can be infected with multiple strains of virulent R. equi. Furthermore, identical strains can be found in multiple, remote anatomic locations in an infected foal, and this can occur for >1 strain in the same foal. The automated system used in the current study provided a rapid, reproducible, and discriminating method for typing R. equi isolates.