Effect of age and mitogen on the frequency of interleukin-4 and interferon gamma secreting cells in foals and adult horses as assessed by an equine-specific ELISPOT assay

Equine blood cultures: Can we do better?

Blood culture is considered the gold standard test for documenting bacteraemia in patients with suspected bacterial sepsis in veterinary and human medicine. However, blood culture often fails to yield bacterial growth even though the clinical picture is strongly suggestive of bacterial sepsis, or contaminating organisms can overgrow the true pathogen, making accurate diagnosis and appropriate management of this life-threatening condition very challenging. Methodology for collecting blood cultures in equine medicine, and even in human hospitals, is not standardised, and many variables can affect the yield and type of microorganisms cultured. Microbiological culture techniques used in the laboratory and specific sample collection techniques, including volume of blood collected, aseptic technique utilised, and the site, timing and frequency of sample collection, all have substantial impact on the accuracy of blood culture results. In addition, patient-specific factors such as husbandry factors, the anatomical site of the primary infection, and changing microflora in different geographic locations, also can impact blood cultures. Thus, blood cultures obtained in practice may not always accurately define the presence or absence of, or specific organisms causing, bacteraemia in horses and foals with suspected sepsis. Erroneous blood culture results can lead to inappropriate antimicrobial use, which can result in poor outcomes for individual patients and contribute to the development of antimicrobial resistance in the patient's microflora and the environmental microcosm. This review summarises current indications and methodology, and specific factors that may be optimised, for equine blood culture, with particular focus on available literature from neonatal foals with suspected bacterial sepsis. To standardise and optimise blood culture techniques in horses and foals, future research in this area should be aimed at determining the optimal volume of blood that should be collected for culture, and the ideal site, timing, and frequency of sample collection.

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.

Vaccination of foals with a modified live, equid herpesvirus-1 gM deletion mutant (RacHΔgM) confers partial protection against infection

Equid herpesvirus-1 (EHV-1) causes respiratory and neurological disease and late gestation abortion in pregnant mares. Current vaccines contain either inactivated or live EHV-1, but fail to provide complete clinical or virological protection, namely prevention of nasopharyngeal shedding and cell-associated viraemia. Thus, the development of novel products, such as modified live virus (MLV) vaccines which stimulate virus-specific, humoral and cell mediated immune responses more effectively remains a priority. Two groups of weaned foals (n = 6 each group) were used in a longitudinal, prospective, experimental study to evaluate immune responses elicited by two vaccinations with a glycoprotein M (gM) deletion mutant of EHV-1 (RacHdeltagM). Following two concurrent intranasal and intramuscular inoculations six weeks apart, vaccinated (8.4 ± 0.2 months old) and control foals (6.2 ± 0.4 months) were challenge infected intranasally with EHV-1 Ab4/8 four weeks after the second vaccination and clinical signs and virological replication measured. Vaccination caused no adverse events, but did stimulate significantly higher complement fixing and virus neutralizing antibodies in serum compared with control foals at either equivalent or pre-vaccination time points. Virus-specific nasopharyngeal antibody levels and cytotoxic T lymphocyte responses were not significantly different between the groups. Following challenge infection, these immune responses were associated with a reduction in clinical signs and virological replication in the vaccinated foals, including a reduction in duration and magnitude of pyrexia, nasopharyngeal shedding and cell-associated viraemia. We conclude that the RacHΔgM MLV primed EHV-1-specific humoral immune responses in weaned foals. However, complete virological protection by vaccination against EHV-1 requires further research.

Modulation of TNF-α, IL-1Ra and IFN-γ in equine whole blood culture by glucocorticoids

Glucocorticoids are important drugs in the treatment of many inflammatory, autoimmune and allergic diseases in humans and animals. We investigated the effects of hydrocortisone and dexamethasone on TNF-α, IL-1Ra and INF-γ release in stimulated whole blood cell culture from healthy horses. Whole blood cell cultures proved to be useful for the characterization of the anti-inflammatory properties of new drugs. Diluted equine whole blood was exposed to lipopolysaccharide (LPS) and PCPwL (a cocktail consisting of phythemagglutinin E, concanavalin A, pokeweed mitogen and lipopolysaccharide) in the presence or absence of hydrocortisone and dexamethasone (10^-12 - 10^-5 M). TNF-α and IL-1Ra (LPS) as well as IFN-γ (PCPwL) levels were measured in the supernatants using specific enzyme-linked immunosorbent assay (ELISA). The LPS-induced TNF-α and IL-1Ra as well as the PCPwL-induced IFN-γ levels were more potently suppressed by dexamethasone than by hydrocortisone in a concentration-dependent manner. Dexamethasone inhibited TNF-α, IL-1Ra and IFN-γ with the half maximal inhibition concentration (IC₅₀) values of 0.09 μM, 0.453 μM and 0.001 μM, respectively, whereas hydrocortisone inhibited these cytokines with lower IC₅₀ values of 1.45 μM, 2.96 μM and 0.09 μM, respectively. Our results suggest that the equine whole blood test system is useful and reliable to evaluate drug effects and immunological alterations and offers several advantages including simple and cheap performance in physiological and pathological conditions.

Influential factors inducing suboptimal humoral response to vector-based influenza immunisation in Thoroughbred foals

CONTEXT

Numerous equine influenza (EI) epizooties are reported worldwide. EI vaccination is the most efficient methods of prevention. However, not all horses develop protective immunity after immunisation, increasing the risk of infection and transmission.

OBJECTIVES

This field study aimed to understand the poor response to primary EI vaccination.

STUDY DESIGN

The EI antibody response was measured in 174 Thoroughbred foals set in 3 stud farms (SF#1 to SF#3) over a 2years period. All foals were immunised with a commercial recombinant canarypox-based EI vaccine. Sera were tested by single radial haemolysis against the A/equine/Jouars/4/06 EIV strain (H3N8) at the time of the first vaccination (V1), 2weeks and 3months after the second immunisation (V2), 2days and 3months after the third immunisation (V3).

RESULTS

The frequency of poor-responders (no detectable antibody titres) was surprisingly elevated after V2 (56.8%), increased to 81.7% at V2+3months and reached 98.6% at V3. The frequency of poor-responder was still 19.2%, 3months after V3. Two independent influential factors were identified. The short (V2+2weeks) and mid-term (V2+3months, V3+3months) antibody levels were positively correlated to the age at V1 (p-value=0.003, 0.031 and 0.0038, respectively). Presence of maternally-derived antibodies (MDA) at V1 was negatively correlated with antibody levels after V3 only (p-value=0.0056). Given that SF#1 antibody response was below clinical protective levels at all-time points studied, the annual boost immunisation (V4) was brought forward by 7.0±1.1months. V1 was delayed by 7weeks the following year, which significantly increased short- and mid-term antibody titres (p-value=9.9e-07 and 2.31e-07, respectively).

CONCLUSION

The age and MDA at first immunisation with the canarypox-based IE vaccine play an independent role in the establishment of antibody levels. This study also highlights the benefit provided by serological surveillance to evaluate herd immunity and to implement corrective management/vaccination measures.

Regulatory T cells in early life: comparative study of CD4+CD25high T cells from foals and adult horses

The immune system of mammals is subject to continuous development during the postnatal phase of life. Studies following the longitudinal development of the immune system in healthy children are limited both by ethical considerations and sample volumes. Horses represent a particular valuable large animal model for T regulatory (Treg) cells and allergy research. We have recently characterised Treg cells from horses, demonstrated their regulatory capability and showed both their expansion and induction in vitro. Insect bite hypersensitivity (IBH) is a common allergy in horses resembling atopic dermatitis and studies have shown that first exposure to allergens in adult life results in an increased incidence of IBH. The aim of the present study was to characterize circulating CD4⁺CD25^highFoxP3⁺cells in foals, evaluate their suppressive capability and their in vitro induction compared to adult horses. 19 foals (age range, 1–5 months), their adult mothers and six one-year-old horses (yearlings) were included in the study. The proportion of FoxP3⁺ cells within the circulating CD4⁺CD25^high population was significantly higher in foals (47%) compared to their mothers (18%) and to yearlings (26%). Treg cells from foals also displayed a higher suppressive capability. Furthermore, CD4⁺CD25^high cells in foals could be induced in vitro from CD4⁺CD25⁻ cells in a significantly higher proportion compared to mares. These cells also displayed a significantly enhanced suppressive capability. In summary these findings support the notion that exposure of horses to allergens during maturation of the immune system assists the establishment of induced (i)Treg driven tolerance.

The effectiveness of anti-R. equi hyperimmune plasma against R. equi challenge in thoroughbred Arabian foals of mares vaccinated with R. equi vaccine

This study aimed to determine the effectiveness of a pregnant mare immunization of a Rhodococcus equi (R. equi) vaccine candidate containing a water-based nanoparticle mineral oil adjuvanted (Montanide IMS 3012) inactive bacterin and virulence-associated protein A (VapA), as well as the administration of anti-R. equi hyperimmune (HI) plasma against R. equi challenge in the mares' foals. The efficacy of passive immunizations (colostral passive immunity by mare vaccination and artificial passive immunity by HI plasma administration) was evaluated based on clinical signs, complete blood count, blood gas analysis, serological response (ELISA), interleukin-4 (IL-4) and interferon gamma (IFN-γ), total cell count of the bronchoalveolar lavage fluids (BALF) samples, reisolation rate of R. equi from BALF samples (CFU/mL), lung samples (CFU/gr), and lesion scores of the organs and tissue according to pathological findings after necropsy in the foals. The vaccination of pregnant mares and HI plasma administration in the foals reduced the severity of R. equi pneumonia and lesion scores of the organs and tissue by 3.54-fold compared to the control foals. This study thus indicates that immunization of pregnant mares with R. equi vaccine candidate and administration of HI plasma in mares' foals effectively protect foals against R. equi challenge.

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.

Equine ELISPOT assay to study secretion of IFNγ and IL-4 from peripheral blood mononuclear cells

Human and mouse immune system cells are the most frequently used specimens in ELISPOT assays. In an effect to expand the application of ELISPOT assay to other species, we developed matched antibody pairs for ready-to-use kits designed for studying the frequency of equine IFNγ- and IL-4-secreting peripheral blood mononuclear cells (PBMCs). Equine PBMCs were stimulated with either concanavalin A (Con A) or calcium ionomycin mixed with phorbol 12-myristate 13-acetate (CaI + PMA). We found that Con A, in general, had a more profound stimulating effect than CaI + PMA on IL-4 secretion, whereas both stimulatory and inhibitory effects were observed on IFNγ secretion. Our data demonstrate a large dynamic range in IFNγ and IL-4 secretion among different donors, which may reflect animal health and serve as a valuable diagnostic marker.

Equine neonates have attenuated humoral and cell-mediated immune responses to a killed adjuvanted vaccine compared to adult horses

The objectives of this study were to compare relative vaccine-specific serum immunoglobulin concentrations, vaccine-specific lymphoproliferative responses, and cytokine profiles of proliferating lymphocytes between 3-day-old foals, 3-month-old foals, and adult horses after vaccination with a killed adjuvanted vaccine. Horses were vaccinated intramuscularly twice at 3-week intervals with a vaccine containing antigens from bovine viral respiratory pathogens to avoid interference from maternal antibody. Both groups of foals and adult horses responded to the vaccine with a significant increase in vaccine-specific IgGa and IgG(T) concentrations. In contrast, only adult horses and 3-month-old foals mounted significant vaccine-specific total IgG, IgGb, and IgM responses. Vaccine-specific concentrations of IgM and IgG(T) were significantly different between all groups, with the highest concentrations occurring in adult horses, followed by 3-month-old foals and, finally, 3-day-old foals. Only the adult horses mounted significant vaccine-specific lymphoproliferative responses. Baseline gamma interferon (IFN-γ) and interleukin-4 (IL-4) concentrations were significantly lower in 3-day-old foals than in adult horses. Vaccination resulted in a significant decrease in IFN-γ concentrations in adult horses and a significant decrease in IL-4 concentrations in 3-day-old foals. After vaccination, the ratio of IFN-γ/IL-4 in both groups of foals was significantly higher than that in adult horses. The results of this study indicate that the humoral and lymphoproliferative immune responses to this killed adjuvanted vaccine are modest in newborn foals. Although immune responses improve with age, 3-month-old foals do not respond with the same magnitude as adult horses.

Effects of two commercially available immunostimulants on leukocyte function of foals following ex vivo exposure to Rhodococcus equi

The objective of this study was to determine the effect of immunostimulants on neutrophil, macrophage, and lymphocyte function following ex vivo exposure to Rhodococcus equi. Eighteen foals were randomly assigned to one of 3 treatment groups. Treatment consisted of inactivated Propionibacterium acnes (PA), inactivated parapoxvirus ovis (PPVO), or saline (control) administered on days 0 (7 days of age), 2, and 8. Bronchoalveolar lavage (BAL) fluid and blood were collected on days 0 (baseline), 12, 24 and 36. Intracellular replication of R. equi in macrophages, cytokine induction by R. equi-infected macrophages, phagocytic and oxidative burst activity of neutrophils, lymphoproliferative responses, and cytokine induction of proliferating lymphocytes were measured. Neutrophils from foals treated with PPVO had significantly greater ability to phagocytize R. equi and undergo oxidative burst on day 12 and day 24 compared to baseline values. On day 24, foals treated with PPVO had significantly greater phagocytosis and oxidative burst than foals treated with PA. Treatment with PA resulted in significantly less intracellular proliferation of R. equi within monocyte-derived macrophages on day 12 compared to control foals. The ability of R. equi to replicate in BAL macrophages decreased significantly (P=0.005) with time with lower replication in BAL macrophages of older foals compared to younger foals, regardless of treatment. On day 12, TNF-α induction in monocyte-derived macrophages and IL-12 p40 induction in BAL macrophages infected with R. equi was significantly higher in foals treated with PPVO than in controls. Lymphoproliferative responses and IFN-γ induction were not significantly different between groups.

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.