Transfusion With 2 L of Hyperimmune Plasma is Superior to Transfusion of 1 L or Less for Protecting Foals Against Subclinical Pneumonia Attributed to Rhodococcus equi

Investigation of the relationship between pulmonary lesions based on lung ultrasound and respiratory clinical signs in foals with suspected pulmonary rhodococcosis

Rhodococcus equi is a widely recognized bacterium responsible for pneumonia in preweaned foals. On endemic farms, foals with a subclinical course of the disease usually outnumber those presenting clinical signs. The disease is typically chronic and mainly manifests as fever and dyspnoea. Currently, field diagnosis is often based on lung ultrasound (LUS); however, both diagnostic and therapeutic approaches vary among practitioners and considerably change over time. This longitudinal, prospective study was designed to describe the appearance and progression of rhodococcal pulmonary lesions during the first months of life based on LUS and to evaluate the relationship between the presence and severity of rhodococcal pulmonary lesions and the occurrence of respiratory clinical signs in foals from farms with endemic R. equi infections. Nearly 26% of foals demonstrated respiratory signs highly suggestive of pulmonary rhodococcosis, and approximately 70% of the foals had abnormalities detected on LUS without concurrent clinical signs. The appearance and development of LUS abnormalities were age-related. An abscess diameter exceeding 15 mm in LUS and other pleural lesions were significantly linked with the occurrence of clinical signs suggestive of pulmonary rhodococcosis (P < 0.001) and may be considered predictive factors of rhodococcal pneumonia in foals.

Rhodococcus equi-What is New This Decade?

Foals become infected shortly after birth; most develop subclinical pneumonia and 20% to 30% develop clinical pneumonia that requires treatment. It is now well established that the combination of screening programs based on thoracic ultrasonography and treatment of subclinical foals with antimicrobials has led to the development of resistant Rhodococcus equi strains. Thus, targeted treatment programs are needed. Administration of R equi-specific hyperimmune plasma shortly after birth is beneficial as foals develop less severe pneumonia but does not seem to prevent infection. This article provides a summary of clinically relevant research published during this past decade.

Transfusion of hyperimmune plasma for protecting foals against Rhodococcus equi pneumonia

The bacterium Rhodococcus equi causes pneumonia in foals that is prevalent at breeding farms worldwide. In the absence of an effective vaccine, transfusion of commercial plasma from donor horses hyperimmunised against R. equi is used by many farms to reduce the incidence of pneumonia among foals at farms where the disease is endemic. The effectiveness of hyperimmune plasma for controlling R. equi pneumonia in foals has varied considerably among reports. The purposes of this narrative review are: 1) to review early studies that provided a foundational basis for the practice of transfusion of hyperimmune plasma that is widespread in the US and in many other countries; 2) to summarise current knowledge of hyperimmune plasma for preventing R. equi pneumonia; 3) to provide an interpretive summary of probable explanations for the variable results among studies evaluating the effectiveness of transfusion of hyperimmune plasma for reducing the incidence of R. equi pneumonia; 4) to review mechanisms by which hyperimmune plasma might mediate protection; and 5) to consider risks of transfusing foals with hyperimmune plasma. Although the weight of evidence supports the practice of transfusing foals with hyperimmune plasma to prevent R. equi pneumonia, many important gaps in our knowledge of this topic remain including the volume/dose of hyperimmune plasma to be transfused, the timing(s) of transfusion, and the mechanism(s) by which hyperimmune plasma mediates protection. Transfusing foals with hyperimmune plasma is expensive, labour-intensive, and carries risks for foals; therefore, alternative approaches for passive and active immunisation to prevent R. equi pneumonia are greatly needed. This article is protected by copyright. All rights reserved.

Rhodococcus equi Foal Pneumonia: Update on Epidemiology, Immunity, Treatment, and Prevention

Pneumonia in foals caused by the bacterium Rhodococcus equi has a worldwide distribution and is a common cause of disease and death for foals. The purpose of this narrative review is to summarise recent developments pertaining to the epidemiology, immune responses, treatment, and prevention of rhodococcal pneumonia of foals. Screening tests have been used to implement earlier detection and treatment of foals with presumed subclinical R. equi pneumonia to reduce mortality and severity of disease. Unfortunately, this practice has been linked to the emergence of antimicrobial resistant R. equi in North America. Correlates of protective immunity for R. equi infections of foals remain elusive, but recent evidence indicates that innate immune responses are important both for mediating killing and orchestrating adaptive immune responses. A macrolide antimicrobial in combination with rifampin remains the recommended treatment for foals with R. equi pneumonia. Great need exists to identify which antimicrobial combination is most effective for treating foals with R. equi pneumonia and to limit emergence of antimicrobial-resistant strains. In the absence of an effective vaccine against R. equi, passive immunisation remains the only commercially-available method for effectively reducing the incidence of R. equi pneumonia. Because passive immunisation is expensive, labour-intensive, and carries risks for foals, great need exists to develop alternative approaches for passive and active immunisation.

The type of anticoagulant used for plasma collection affects in vitro Rhodococcus equi assays

Objective

The efficacy of Rhodococcus equi-specific hyperimmune plasma (HIP) is usually evaluated in vitro. Anticoagulants (AC) used for plasma collection can negatively impact bacterial replication but their effect on R. equi growth has not been evaluated. The aim of this study was to establish the effect that AC routinely used in veterinary medicine (ACD, K2EDTA, Li Heparin, and Na Citrate) have on in vitro R. equi growth. To assess this, in vitro assays commonly used to test HIP efficacy (direct effect on microorganism and macrophage infection), were performed using each AC and non-treated bacteria.

Results

There was no direct effect of ACD, Li Heparin or Na Citrate on R. equi growth. These AC significantly (p < 0.05) delayed growth for 12 h following opsonization. The number of R. equi colonies after macrophage infection was significantly (p < 0.05) lower 72 h post-opsonization with Na Citrate. K2EDTA inhibited the formation of R. equi colonies by 12 h in all the assays. In conclusion, AC should be taken into consideration when interpreting in vitro results as their negative effect on bacterial growth may be mistakenly interpreted as HIP efficacy. ACD and Li Heparin appear more appropriate for the selected assays.

Randomized, controlled trial comparing Rhodococcus equi and poly-N-acetyl glucosamine hyperimmune plasma to prevent R equi pneumonia in foals

Background

Hyperimmune plasma raised against β‐1→6‐poly‐N‐acetyl glucosamine (PNAG HIP) mediates more opsonophagocytic killing of Rhodococcus equi (R equi) than does R equi hyperimmune plasma (RE HIP) in vitro. The relative efficacy of PNAG HIP and RE HIP to protect foals against R equi pneumonia, however, has not been evaluated.

Hypothesis

Transfusion with PNAG HIP will be superior to RE HIP in foals for protection against R equi pneumonia in a randomized, controlled, blinded clinical trial.

Animals

Four hundred sixty Quarter Horse and Thoroughbred foals at 5 large breeding farms in the United States.

Methods

A randomized, controlled, blinded clinical trial was conducted in which foals were transfused within 24 hours after birth with 2 L of either RE HIP or PNAG HIP. Study foals were monitored through weaning for clinical signs of pneumonia by farm veterinarians. The primary outcome was the proportion of foals that developed pneumonia after receiving each type of plasma.

Results

The proportion of foals that developed pneumonia was the same between foals transfused with RE HIP (14%; 32/228) and PNAG HIP (14%; 30/215).

Conclusions and Clinical Importance

Results indicate that PNAG HIP was not superior to a commercially available, United States Department of Agriculture‐licensed RE HIP product for protecting foals against R equi pneumonia under field conditions.

Antibody activities in hyperimmune plasma against the Rhodococcus equi virulence -associated protein A or poly-N-acetyl glucosamine are associated with protection of foals against rhodococcal pneumonia

The efficacy of transfusion with hyperimmune plasma (HIP) for preventing pneumonia caused by Rhodococcus equi remains ill-defined. Quarter Horse foals at 2 large breeding farms were randomly assigned to be transfused with 2 L of HIP from adult donors hyperimmunized either with R. equi (RE HIP) or a conjugate vaccine eliciting antibody to the surface polysaccharide β-1→6-poly-N-acetyl glucosamine (PNAG HIP) within 24 hours of birth. Antibody activities against PNAG and the rhodococcal virulence-associated protein A (VapA), and to deposition of complement component 1q (C՛1q) onto PNAG were determined by ELISA, and then associated with either clinical pneumonia at Farm A (n = 119) or subclinical pneumonia at Farm B (n = 114). Data were analyzed using multivariable logistic regression. Among RE HIP-transfused foals, the odds of pneumonia were approximately 6-fold higher (P = 0.0005) among foals with VapA antibody activity ≤ the population median. Among PNAG HIP-transfused foals, the odds of pneumonia were approximately 3-fold (P = 0.0347) and 11-fold (P = 0.0034) higher for foals with antibody activities ≤ the population median for PNAG or C՛1q deposition, respectively. Results indicated that levels of activity of antibodies against R. equi antigens are correlates of protection against both subclinical and clinical R. equi pneumonia in field settings. Among PNAG HIP-transfused foals, activity of antibodies with C՛1q deposition (an indicator of functional antibodies) were a stronger predictor of protection than was PNAG antibody activity alone. Collectively, these findings suggest that the amount and activity of antibodies in HIP (i.e., plasma volume and/or antibody activity) is positively associated with protection against R. equi pneumonia in foals.

Current Trends in Understanding and Managing Equine Rhodococcosis

The aim of this review was to summarize studies on equine rhodococcosis over the last decade. For many years Rhodococcus equi has remained one of the major health challenges in the equine breeding industry worldwide. Recently, many novel approaches and ideas have been described and some of them were initially implemented into the clinical practice. This study reviews a variety of new information about neonatal susceptibility, clinical appearance, considered and applied diagnostic procedures and treatment alternatives, factors limiting accurate prognosis, ideas regarding environmental management and prophylaxis considerations. Although multiple research were conducted, the main problems such as high morbidity and mortality, a lack of reliable prevention strategies and treatment limitations are still unresolved and require further scientific effort.