Performance of five serological assays for diagnosis of Rhodococcus equi pneumonia in foals

Diagnosis of bacteria from the CMNR group in farm animals

The CMNR group comprises bacteria of the genera Corynebacterium, Mycobacterium, Nocardia, and Rhodococcus and share cell wall and DNA content characteristics. Many pathogenic CMNR bacteria cause diseases such as mastitis, lymphadenitis, and pneumonia in farmed animals, which cause economic losses for breeders and represent a threat to public health. Traditional diagnosis in CMNR involves isolating target bacteria on general or selective media and conducting metabolic analyses with the assistance of laboratory biochemical identification systems. Advanced mass spectrometry may also support diagnosing these bacteria in the clinic's daily routine despite some challenges, such as the need for isolated bacteria. In difficult identification among some CMNR members, molecular methods using polymerase chain reaction (PCR) emerge as reliable options for correct specification that is sometimes achieved directly from clinical samples such as tracheobronchial aspirates and feces. On the other hand, immunological diagnostics such as the skin test or Enzyme-Linked Immunosorbent Assay (ELISA) for Mycobacterium tuberculosis yield promising results in subclinical infections with no bacterial growth involved. In this review, we present the methods most commonly used to diagnose pathogenic CMNR bacteria and discuss their advantages and limitations, as well as challenges and perspectives on adopting new technologies in diagnostics.

Current and Future Advances in the Detection and Surveillance of Biosecurity-Relevant Equine Bacterial Diseases Using Loop-Mediated Isothermal Amplification (LAMP)

Horses play an important role throughout the world, whether for work, culture, or leisure, providing an ever-growing significant contribution to the economy. The increase in importation and movement of horses, both nationally and internationally, has inevitably allowed for the global equine industry to grow. Subsequently, however, the potential for transmission of fatal equine bacterial diseases has also escalated, and devasting outbreaks continue to occur. To prevent such events, disease surveillance and diagnosis must be heightened throughout the industry. Current common, or "gold-standard" techniques, have shown to be inadequate at times, thus requiring newer technology to impede outbreaks. Loop-mediated isothermal amplification (LAMP) has proven to be a reliable, rapid, and accessible tool in both diagnostics and surveillance. This review will discuss equine bacterial diseases of biosecurity relevance and their current diagnostic approaches, as well as their respective LAMP assay developments. Additionally, we will provide insight regarding newer technology and advancements associated with this technique and their potential use for the outlined diseases.

Fecal concentration of Rhodococcus equi determined by quantitative polymerase chain reaction of rectal swab samples to differentiate foals with pneumonia from healthy foals

BACKGROUND

Diagnostic accuracy of real-time, quantitative PCR (qPCR) assays to quantify virulent Rhodococcus equi using rectal swab samples has not been systematically evaluated.

OBJECTIVE

To evaluate the accuracy of qPCR of rectal swab samples to differentiate foals with pneumonia from healthy foals of similar age from the same environment.

ANIMALS

One hundred privately owned foals born in 2021 from 2 farms in New York.

METHODS

An incident case-control study design was used. Rectal swabs were collected from all foals diagnosed with R. equi pneumonia at 2 horse-breeding farms (n = 47). Eligible pneumonia cases (n = 39) were matched by age to up to 2 healthy (n = 53) control foals; rectal swabs were collected from control foals on the day of diagnosis of the index case. DNA was extracted from fecal swabs and the concentration of virulent R. equi (ie, copy numbers of the virulence-associated protein A gene [vapA] per 100 ng fecal DNA) was estimated by qPCR.

RESULTS

The area under the ROC curve for qPCR of fecal swabs was 83.7% (95% CI, 74.9-92.6). At a threshold of 14 883 copies of vapA per 100 ng fecal DNA, specificity of the assay was 83.0% (95% CI, 71.7-92.4) and sensitivity was 79.5% (95% CI, 66.7-92.3).

CONCLUSIONS AND CLINICAL IMPORTANCE

Although fecal concentrations of virulent R. equi are significantly higher in pneumonic foals than healthy foals of similar age in the same environment, qPCR of rectal swabs as reported here lacks adequate diagnostic accuracy for clinical use.

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.

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.

Serological epidemiological surveillance for vapN-harboring Rhodococcus equi infection in goats

Rhodococcus equi causes suppurative pneumonia in foals aged 1-3 months; moreover, it has emerged as a pathogenic cause of zoonotic diseases. After the initial report of the ruminant-pathogenic factor VapN encoded by the novel virulence plasmid pVAPN, several reports have described ruminant infections caused by vapN-harboring R. equi. Herein, we conducted a serological epidemiological surveillance in goats at a breeding farm (Farm A) and characterized the vapN-harboring R. equi isolates from this farm. First, we established a simple screening enzyme-linked immunosorbent assay (ELISA) using recombinant glutathione S-transferase-tagged VapN as an immobilized antigen. This method revealed that the VapN antibody titers were elevated in 12 of 42 goats. Subsequently, we attempted to isolate R. equi from the goat feces and soil of Farm A. choE⁺/vapN⁺R. equi was isolated from the feces of Goat No. 27 and a soil sample near the shed. The pulsed-field gel electrophoresis (PFGE) patterns of five vapN-harboring R. equi strains isolated from Farm A in 2013 and 2019 were investigated and found to be the same except for the strain (OKI2019F1). However, no difference was observed in VapN expression and growth in macrophages among these vapN-harboring R. equi isolates. Our results revealed that some goats had histories of vapN-harboring R. equi infections, and two genomic types of vapN-harboring R. equi were found in isolates from Farm A. Ruminant-specific (pVAPN-carrying) R. equi might be an unrecognized pathogen in Japan and further studies are required to determine its prevalence and distribution.

Seroprevalence of Rhodococcus equi in horses in Israel

Rhodococcus equi is a common cause of pneumonia in foals and has extensive clinical, economic and possibly zoonotic consequences. This bacterium survives well in the environment and may be considered as normal flora of adult horses. Certain strains of this bacterium are extremely virulent in foals, and early identification and intervention is crucial for prognosis. Rhodococcus equi is endemic in many parts of the world and occasionally isolated in Israel. This study was designed to evaluate R. equi seroprevalence in adult horses in Israel to indirectly indicate the potential level of exposure of susceptible foals. Sera were collected from 144 horses during spring 2011 and from 293 horses during fall 2014, and the presence of antibodies against virulent R. equi was detected by enzyme-linked immunosorbent assay. Equine seroprevalence of R. equi was found to be 7.6% in 2011 and 5.1% in 2014. Only one farm had seropositive horses in 2011, whereas several farms had seropositive horses in 2014. No significant risk factors for seropositivity were found. Rhodococcus equi appears to be endemic in Israel. This is the first survey of R. equi in Israel that provides information on the epidemiology of this important bacterium.

Validation and evaluation of VapA-specific IgG and IgG subclass enzyme-linked immunosorbent assays (ELISAs) to identify foals with Rhodococcus equi pneumonia

REASONS FOR PERFORMING STUDY

Rhodococcus equi (Rhodococcus hoagii/Prescottella equi) is a common cause of foal pneumonia, but its diagnosis remains a challenge for equine veterinarians. While the VapA-specific (virulence-associated protein A) immunoglobulin G (IgG) enzyme-linked immunosorbent assay (ELISA) has low sensitivity and specificity for detecting pneumonic foals, little is known about VapA-specific IgG subclasses.

OBJECTIVES

To evaluate the performance of VapA-specific ELISA for IgG and its subclasses IgGa, IgGb and IgG(T) in the early diagnosis of pneumonia caused by R. equi.

STUDY DESIGN

Assay validation followed by assessment of diagnostic performance using archived samples from animals of known status.

METHODS

Serum samples from exposed (n = 125) and nonexposed adult horses (n = 10) and from experimentally challenged and naturally infected foals were used for ELISA validation. Post mortem and tissue culture records of the last 24 years from the Institute for Experimental Pathology at the University of Iceland in Keldur, Iceland laboratory were evaluated to confirm the absence of R. equi cases in Iceland. The diagnostic performance of VapA-specific IgG and its subclasses was evaluated using banked serum samples from pneumonic (n = 21) and healthy foals (n = 80). To evaluate each IgG assay, a cut-off value was selected based on receiver operating characteristic curve analysis and used to calculate sensitivity and specificity. The intra- and interassay coefficients of variation were calculated for each ELISA.

RESULTS

Using sera from Iceland, where R. equi infection has not been reported, the VapA-specific IgG ELISA differentiated exposed from nonexposed horses. When used to identify infected foals, VapA-specific IgG, IgGa and IgGb had no diagnostic value. In contrast, IgG(T) had high sensitivity and specificity.

CONCLUSIONS

Horses from Iceland are not exposed to VapA(+) R. equi and can serve as negative controls. VapA-specific IgG subclasses, with the exception of IgG(T), are poor predictors of disease. Further investigation on the use of IgG(T) as a diagnostic tool in field conditions is needed.

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.

Development and evaluation of three mortality prediction indices for cold-stunned Kemp's ridley sea turtles (Lepidochelys kempii)

Kemp's ridley sea turtle is an endangered species found in the Gulf of Mexico and along the east coast of the USA. Cold-stunned Kemp's ridley turtles are often found stranded on beaches of Massachusetts and New York in November and December each year. When found alive, turtles are transported to rehabilitation centres for evaluation and treatment. Blood gas and chemistry analytes of major clinical relevance in sea turtles were selected to develop mortality prediction indices (MPI)s. Testing the diagnostic performance of various combinations of blood gas and chemistry analytes by receiver operating characteristics (ROC) analysis resulted in the development of three mortality prediction indices. The sensitivity and specificity of the best performing MPI (based on three blood analytes: pH, pO2, and potassium) was 88 and 80%, respectively. Using ROC analysis, the area under the curve = 0.896 (95% confidence interval = 0.83-0.94). The use of validated MPIs based on four blood analytes (pH, pCO2, pO2, and potassium) could be useful for better diagnosis, treatment, and prognosis of cold-stunned sea turtles when admitted to rehabilitation facilities.

Development of ELISA test for determination of the level of antibodies against Rhodococcus equi in equine serum and colostrum

Rhodococcus equi infection occurs worldwide and is one of the major causes of losing foals in the first six months of life. The application of serological tests in the diagnostics of rhodococcosis is limited, however they play a crucial role in immunological studies. The objective of this study was to develop and standardize ELISA test for the determination of the level of antibodies against Rhodococcus equi in equine serum and colostrum.Bacterial cell lysate was used as antigen. The test was standardized on 175 sera obtained from adult horses kept on rhodococcosis-free and endemic farms. Positive and negative control sera were used. The test detected IgG antibodies mainly against VapA protein, which was confirmed by Western blot analysis. The test was easy to perform, did not require inactivation of sera and had low well-to-well variation. The shelf life of antigen-coated ELISA plates was 21 days.The test allowed to reveal significant increase of R. equi-specific antibodies in both serum and colostrum in response to the vaccination (p<0.001). Therefore it can be applied to the evaluation of efficacy of immunization. Moreover, no statistically significant difference in the baseline antibody level in adult horses from rhodococcosis-free and endemic farm was revealed (α=0.05).

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.

Ureia como marcador de diluição em amostras de lavado traqueobrônquico de potros com rodococose

No estudo das infecções e dos mecanismos inflamatórios do trato respiratório posterior, a diluição desconhecida de amostras de lavados traqueobrônquicos leva a um grave problema na interpretação das concentrações de várias substâncias. Foi investigada a amplitude da diluição em uma situação clínica verdadeira, com o objetivo de determinar a validez do uso da dosagem da ureia em lavados traqueobrônquicos para correção da diluição. As amostras do estudo consistiram em lavados traqueobrônquicos obtidos de sete potros com infecção por Rhodococcus equi. Foi realizada a contagem celular total e diferencial e comprado com o quadro clínico e a recuperação bacteriana de todas as amostras. Os fatores de diluição dos lavados variaram entre 14,3 e 130 (média 59,7). O uso da ureia como marcador de diluição melhorou a exatidão na determinação da concentração total de células nos lavados. Estes resultados sugerem que a prática de mensurar substâncias em lavado traqueobrônquico sem correção da diluição possa induzir a falsas conclusões.

Current understanding of the equine immune response to Rhodococcus equi. An immunological review of R. equi pneumonia

Rhodococcus equi is recognised to cause chronic purulent bronchopneumonia in foals of less than 6 months of age. Virulent strains of the bacteria possess a large 80-90 kb plasmid encoding several virulence-associated proteins, including virulence-associated protein A (VapA), which is associated with disease. R. equi pneumonia can represent significant costs and wastage to the equine breeding industry, especially on stud farms where the disease is endemic. This article reviews knowledge of the equine immune response, both in the immune adult and susceptible neonate, with respect to this pathogen. Humoral immune responses are addressed, with a discussion on the use of hyperimmune and normal adult equine plasma as prophylactic tools. The role that innate immune mechanisms play in the susceptibility of some foals to R. equi infection is also highlighted. Likewise, cell-mediated immune components are reviewed, with particular attention directed towards research undertaken to develop an effective vaccine for foals. It is possible that the implementation of a single immunoprophylaxis strategy to prevent R. equi infection on farms will yield disappointing results. Combined prophylactic protocols that address husbandry practices, environmental and aerosol contamination levels, enhancement of innate immunity, good quality hyperimmune plasma for the neonate, and vaccinal efficacy in the developing foal may be required.

Proline-glutamic acid-proline-lysine repetition peptide as an antigen for the serological diagnosis of strangles

The reactivity of the proline-glutamic acid-proline-lysine (PEPK) repetition peptide antigen in 3176 serum samples was investigated to evaluate its utility as an antigen for the serological diagnosis of strangles. The reactivity of the sera of horses infected with Streptococcus equi subspecies equi was high when the peptide had several PEPK repetitions. However, as the number of PEPK repetitions increased, the reactivity of the antigen with the sera of horses infected with Streptococcus equi subspecies zooepidemicus also increased. In horses infected experimentally with S equi, the reactivity of the PEPK antigen with five repetitions increased one week after inoculation and continued to increase during the following four weeks. The optical density (OD) values of test sera from horses infected experimentally with S equi and sera from horses that had recovered from strangles were high. The od values of sera from horses that had recovered from an experimental infection with S zooepidemicus and of sera from healthy horses were comparatively low.

Clinical evaluation of a peptide-ELISA based upon N-terminal B-cell epitope of the VapA protein for diagnosis of Rhodococcus equi pneumonia in foals

A total of 227 field samples from naturally exposed foals aged between 3 weeks and 6 months were used in an evaluation of a peptide-based enzyme-linked immunosorbent assay (ELISA) for diagnosis of Rhodococcus equi infection. A biotinylated peptide derived from the virulence-associated protein A (VapA) of R. equi, a horse pathogen, was synthesized and designated as PN11-14. The peptide corresponds to the N-terminal B-cell epitope TSLNLQKDEPNGRASDTAGQ of the VapA protein. Based upon a serum immunoglobulin (Ig)G titre of 512 as a positive cut-off value for the R. equi infection, the ELISA provided the overall sensitivity of 47.62%, specificity of 69.67% and an accuracy of 59.47% with a positive predictive value of 57.47% for true R. equi pneumonia. The assay was improved by detecting VapA-specific IgGb antibodies against N-terminal B-cell epitope of the VapA protein rather than IgG antibodies. The VapA-IgGb ELISA showed the overall sensitivity of 70.47%, specificity of 72.13% and accuracy of 71.36% with a positive predictive value of 68.52%. Diagnosis of R. equi disease in 6-week-old foals showed that the VapA-IgGb ELISA provided an increasing trend (P=0.0572) in sensitivity of 82.4% in comparison with the VapA-IgG ELISA which showed the sensitivity of 58.8%. However, differences in specificity of both tests were statistically insignificant (P=0.357) as analysed by the McNemar test. These results indicated that detection of VapA-specific IgGb antibodies may be a better predictor of R. equi disease in foals.

Study of serum amyloid A concentrations as a means of achieving early diagnosis of Rhodococcus equi pneumonia

REASONS FOR PERFORMING STUDY

Prognosis of Rhodococcus equi pneumonia can be challenging because the course of the disease is often insidious and overt clinical signs are subtle. Early diagnosis is considered desirable because it may offer the chance of more successful implementation of treatment and, thereby, improved outcome. Serological tests have previously failed to be accurate for early detection or diagnosis. Measurement of serum amyloid A (SAA) prior to and at the time of clinical signs was therefore chosen in order to assess its potential clinical use.

OBJECTIVE

To determine whether SAA concentrations differentiate foals affected with R. equi pneumonia from unaffected foals, either prior to the onset of disease or at the time of onset of clinical signs.

HYPOTHESIS

SAA concentrations are significantly higher among foals that develop R. equi pneumonia than in foals from the same environment that remain clinically unaffected.

METHODS

Serum samples were obtained from 212 foals 7-14 days and 196 foals 21-28 days post partum, and from affected foals and age-matched controls at the time of onset of signs of pneumonia. SAA concentration was determined for each sample.

RESULTS

There were no significant differences between SAA concentrations of foals with R. equi and clinically unaffected foals during the 2 periods of examination or at the time of onset of clinical signs of R. equi pneumonia.

CONCLUSIONS

Concentrations of SAA are variable among foals with R. equi pneumonia and cannot be used reliably either as an ancillary diagnostic tool or to screen for early detection of disease during the first month post partum.

POTENTIAL RELEVANCE

Bimonthly monitoring concentration of SAA is not useful as a screening test for early detection of R. equi pneumonia and does not facilitate diagnosis of this disease when used according to the protocol of this study.

Recognition of a B-cell Epitope of the VapA Protein of Rhodococcus equi in Newborn and Experimentally Infected Foals

The aim of this study was to evaluate the usefulness of the previously identified B-cell epitope TSLNLQKDEPNGRASDTAGQ of the VapA protein of Rhodococcus equi and its association with R. equi pneumonia. A modified peptide designated PN11-14 corresponding to the epitope was recognized by all sera from experimentally infected foals with virulent R. equi ATCC103+ containing the virulence plasmid but not by its plasmid-cured derivative ATCC103- strain. Marked levels of VapA-specific immunoglobulin (Ig)G were detected in all sera from the ATCC103+ infected foals at 2 weeks after the infection. One control animal had high titres as determined by the peptide enzyme-linked immunosorbent assay (ELISA), indicating the ELISA may not absolutely differentiate between foals with R. equi pneumonia and healthy exposed foals in farms where the prevalence of disease is high. However, numbers of animals used were small. Further evaluation of the peptide ELISA with field samples is necessary to determine whether the assay is diagnostically useful. This study showed that levels of passive transfer of maternal IgG antibodies to the epitope in newborn foals could be measured. Interestingly, the maternally derived antibodies were found to significantly (P<0.05 by Student's t-test) decline 2 weeks after birth. Seroconversion against naturally occurring VapA expressing R. equi could be detected in some foals at 4 weeks of age. Antibodies to the epitope peaked and were significantly (P<0.05) greater in foals aged between 6 and 8 weeks. These results indicated that the peptide ELISA could be used to monitor anti-VapA antibodies in foals, particularly those at the age of 4-6 weeks. It is possible that the ELISA may be of some use as a diagnostic test on farms where R. equi is non-endemic. Further studies using large number of field samples are needed to verify this assumption.