Hematologic and immunophenotypic factors associated with development of Rhodococcus equi pneumonia of foals at equine breeding farms with endemic infection

Rhodococcus equi causes severe pyogranulomatous pneumonia in foals and in immunocompromised people. In mice, both CD4+ and CD8+ T lymphocytes contribute to host defense against R. equi, but CD4+ T lymphocytes are required for pulmonary clearance of the bacteria. In this prospective study of 208 foals at two equine breeding farms with endemic R. equi infections, we collected peripheral blood samples at 2 and 4 weeks of age and at the time of diagnosis of R. equi pneumonia. Samples were analyzed for concentrations of total and differential leukocytes, EqCD4+ and EqCD8+ T lymphocytes, and B lymphocytes. Thirty (14.4%) foals developed R. equi pneumonia. At the 2nd week of life, affected foals had significantly lower concentrations of white blood cells (WBC) and segmented neutrophils, significantly lower proportions of EqCD4+ T lymphocytes, and significantly higher proportions of EqCD8+ T lymphocytes. The EqCD4:EqCD8 ratio was significantly lower for affected foals. At the 4th week of life, affected foals had significantly lower concentrations of segmented neutrophils and EqCD4+ T lymphocytes than did unaffected foals. The ratio of EqCD4:EqCD8 was significantly lower for affected foals. Two- and 4-week-old foals with ratios of EqCD4:EqCD8<3 were significantly more likely to develop R. equi pneumonia. There was a significant farm effect which diluted our statistical power to detect differences; however; after adjusting for the farm effect, 2-week-old foals with ratios of EqCD4:EqCD8<3 remained significantly more likely to develop R. equi pneumonia. There were no significant differences in immunophenotypic variables between affected foals (at the time of diagnosis) and age-matched control foals. These data suggest that there are hematologic and immunophenotypic differences between affected and unaffected foals during the first 2-4 weeks of life, prior to onset of clinical signs of R. equi pneumonia. These differences may represent important immunologic mechanisms associated with increased susceptibility of individual foals to infection with R. equi. Because there was considerable overlap between values for affected and unaffected foals, we cannot yet recommend immunophenotyping of foals at endemically-infected farms as a clinically useful screening tool to identify foals at increased risk of developing R. equi pneumonia.     

Immunologic Profiles of Peripheral Blood Leukocytes and Serum Immunoglobulin G Concentrations in Perinatal Mares and Neonatal Foals (Heavy Draft Horse)

The purpose of this study was to examine sequential changes in the immunologic parameters of perinatal mares and neonatal foals of the heavy draft horse. Blood samples were collected from clinically healthy pregnant mares and their newborn foals every week from 1 month before the expected foaling date, and 1 hour, 1 day (24-48 hours), and 1, 2, 3, and 4 weeks after foaling. Peripheral blood samples were used to examine total leukocyte counts (n = 20), differential leukocyte counts (n = 20), lymphocyte subpopulations (n = 13), lymphocyte responses to mitogens (n = 10), neutrophil phagocytic function (n = 12), and serum immunoglobulin G (IgG) concentrations (n = 10). In perinatal mares, remarkable changes observed included increased neutrophils, decreased lymphocytes, decreased CD4⁺ T lymphocytes, and decreased lymphocyte responses to mitogens at delivery. These changes were speculated to be the result of physical stress associated with delivery. In neonatal foals, increase in the phagocytic function of neutrophils, and increase in serum IgG concentration after suckling colostrum and increase of lymphocytes accompanied by physiologic growth were observed. Compared to dams, foals showed lower phagocytic function of neutrophils before suckling and fewer lymphocytes and lower lymphocyte responses to mitogens within 1 day after birth. This study revealed immunologic dynamics in perinatal mares and neonatal foals. Immunologic functions are suppressed in foaling mares and are immature in neonatal foals, especially before colostral intake. We expect these data will be useful for further studies in the field of clinical immunology, and preventive medicine.     

Common variable immunodeficiency in a horse

A 12-year-old Quarter Horse mare that was nonresponsive to medical treatment was evaluated for chronic respiratory disease and hepatobiliary disease. Serum immunoglobulin concentrations were measured by use of radial immunodiffusion that revealed trace to nondetectable concentrations of IgG, IgG(T), IgM, and IgA. Use of serum protein electrophoresis confirmed agammaglobulinemia by the absence of the expected peak in the gamma region. In addition, vaccination with tetanus toxoid did not result in specific immunoglobulin production. Flow cytometric analysis of blood lymphocyte subpopulations revealed the absence of B cells in blood. Immunohistochemical analysis of tissue sections revealed the absence of B lymphocytes in bone marrow and spleen, with occasional B cells in the peripheral lymph nodes. Blood lymphocyte proliferation assays revealed weak responses to pokeweed mitogen and no response to stimulation with lipopolysaccharide. Considering the age and sex of the horse, results of the immunologic tests suggested a diagnosis of common variable immunodeficiency.     

Age-related quantitative alterations in lymphocyte subsets and immunoglobulin isotypes in healthy horses

OBJECTIVE: To characterize age-associated changes in lymphocyte population subsets and immunoglobulin isotypes. ANIMALS: 30 healthy young light-breed horses (5 to 12 years old) and 30 healthy aged light-breed horses (> 20 years old). PROCEDURE: Lymphocyte subset populations were identified, using monoclonal antibodies to cell surface markers CD5, CD4, CD8, and IgG. Subset populations were quantitated by use of flow cytometric analysis of antibody-stained cells. Serum immunoglobulin concentration was determined using single radial immunodiffusion. RESULTS: Absolute cell counts of total lymphocytes, T cells, CD4+ and CD8+ T cells, and B cells were decreased in aged horses, compared with young horses. There was a significant decrease in the percentage of CD8+ cells and an increase in the CD4+-to-CD8+ cell ratio in the aged population, compared with young horses. However, serum concentration of IgG, IgG(T), IgM, or IgA did not differ with age. CONCLUSIONS AND CLINICAL RELEVANCE: In horses, total lymphocyte count and lymphocyte subset cell counts decrease with age. Age-matched control values are necessary for optimal evaluation of hematologic variables in aged horses. The decrease in lymphocyte subset cell counts in healthy aged horses mimics that seen in other species and may contribute to an age-associated decrease in immunocompetency.     

Rhodococcus equi foal pneumonia: protective effects of immune plasma in experimentally infected foals

The immunoprophylactic capacity of specific immune plasma was evaluated in pony foals infected experimentally with Rhodococcus equi. Immune plasma, produced by repeated parenteral administration of viable R. equi to adult horses, was harvested and frozen. Group I (six control foals) and Group II (six principal foals) received lactated Ringers solution and immune plasma respectively at three and five days of age. R. equi were aerosolised into a caudal lung lobe of all foals at seven days of age. Clinical signs, haematological alterations, immune responses, thoracic radiographs and technetium99m pulmonary perfusion scans were monitored. All foals were destroyed and complete post mortem examinations performed. All foals developed pneumonia as evidenced by clinical, radiographic and perfusion alterations, but the survival rate of principal foals was significantly (P less than 0.01) greater than that of control foals. Five control foals developed terminal disease, whereas all principal foals recovered. There was no significant (P greater than 0.05) difference in temperature response, or peripheral blood leucocyte, neutrophil or fibrinogen concentrations between groups. ELISA values for R. equi antibody were significantly (P less than 0.001) greater in principal foals following treatment, but there was no significant (P greater than 0.05) difference in IgG or IgM concentrations between groups. Results of the haemolysis inhibition assay indicated that equi factor neutralising antibodies were transferred by immune plasma to the principal foals. Post mortem examinations of five control foals destroyed at approximately three weeks post infection because of terminal disease, revealed severe pyogranulomatous pneumonia. One control and all principal foals were either free of lesions or had resolving lesions and/or minimal scar formation at three months post infection.(ABSTRACT TRUNCATED AT 250 WORDS)     

Immunity to Rhodococcus equi: antigen-specific recall responses in the lungs of adult horses

Rhodococcal pneumonia is an important disease of young horses that is not seen in immunocompetent adults. Since all foals are normally exposed to Rhodococcus equi in their environment, we hypothesized that most develop protective immune responses. Furthermore, these antigen-specific responses were hypothesized to operate throughout adult life to prevent rhodococcal pneumonia. A better understanding of the mechanisms of immune clearance in adult horses would help define the requirements for an effective vaccine in foals. Adult horses were challenged with virulent R. equi by intrabronchial inoculation into the right lung, and pulmonary immune responses were followed for 2 weeks by bronchoalveolar lavage. Local responses in the inoculated right lung were compared to the uninfected left lung and peripheral blood. Challenged horses rapidly cleared R. equi infection without significant clinical signs. Clearance of bacteria was associated with increased mononuclear cells in bronchoalveolar lavage fluid (primarily lymphocytes) and inversion of the normal macrophage:lymphocyte ratio. There was no significant increase in neutrophils at 7 days post-challenge. Flow cytometric analysis of bronchoalveolar lavage fluid demonstrated that clearance correlated with significant increases in pulmonary T-lymphocytes, both CD4+ and CD8+. Prior to challenge, most adult horses demonstrated low proliferative responses when pulmonary lymphocytes were stimulated with soluble R. equi ex vivo. However, clearance was associated with marked increases in lymphoproliferative responses to soluble R. equi antigen and recombinant VapA, a virulence associated protein of R. equi and candidate immunogen. These results are compatible with previous work in mice which showed that both CD4+ and CD8+ T-cells play a role in immune clearance of R. equi. Recognition of VapA in association with clearance lends further support to its testing as an immunogen. Importantly, the cellular responses to R. equi challenge were relatively compartmentalized. Responses were more marked and the sensitivity to antigen dose was increased at the site of challenge. The blood, including peripheral blood mononuclear cells, was an insensitive indicator of local pulmonary responses.     

Humoral immune response of foals to experimental infection with Rhodococcus equi

Humoral immune response to Rhodococcus equi in experimentally infected foals was studied with the enzyme-linked immunosorbent assay (ELISA) method. Class-specific antibodies were measured by ELISA in the sera of foals after intratracheal or oral inoculation with R. equi ATCC 6939 or T 48 and in the lung washings of a foal after intratracheal inoculation or of normal horses. After intratracheal or oral inoculation with R. equi, serum antibodies were first detected in immunoglobulin G (IgG) followed by IgM and IgA classes, but significant levels of IgM and IgA developed only in the foal infected intratracheally with R. equi T 48. Only the foal infected intratracheally with T 48 developed pneumonia. Anti-R. equi IgG and IgA antibodies appeared in lung washings of the intratracheally infected foal. There were differences in the antibody response to R. equi among the intratracheally infected foals, the orally infected foal and the naturally infected foal. These results suggest that the humoral immune response to R. equi may be affected by the type of R. equi strain and the route and extent of R. equi exposure.     

Survey of arabian foals in poland for immune system disorders

During the foaling seasons of 1983 and 1984, 228 (76%) of 300 Arabian foalsborn in Poland were analyzed for immune system disorders by performing leukocyte differential counts and by quantitating serum concentrations of IgM and IgG. IgM concentrations and absolute lymphocyte counts were within normal limits for all foals tested. Twelve foals (5.3%) demonstrated failure or partial failure of colostral IgG transfer (foal serum IgG < 400 mg/dl). All 12 foals survived. No cases of combined immunodeficiency, selective IgM deficiency or agammaglobulinemia were detected among the 228 foals tested.     

Rhodococcus (Corynebacterium) equi: bactericidal capacity of neutrophils from neonatal and adult horses

The capacity of hematogenous polymorphonuclear neutrophilic leukocytes (PMNL) to kill Rhodococcus equi was compared in horses of various ages. A radioisotope bactericidal assay was used to determine the capacity of PMNL to kill R equi. Assays were conducted on PMNL from horses in 3 groups: group I, 13 foals with a mean age of 3.3 days; group II, 10 group-I foals at a mean age of 35.7 days; and group III, adult dams of group-I foals. Bacteria were obtained from the lungs of a foal with R equi pneumonia and opsonized with fresh adult equine serum that contained R equi specific antibody. The mean peak percentage of R equi killed by PMNL was 78.9 for group I, 90.1 for group II, and 87.9 for group III. There was no significant difference (P greater than 0.05) among groups; however, 15% of foals in group I (2 foals) had a mean peak percentage of 30.5 killed, which was significantly (P less than 0.05) lower than the percentage for other foals in group I. The results of our investigation indicated that the capacity of PMNL to kill opsonized R equi is similar in neonatal, young, and adult horses. However, some neonatal foals have a substantially lower capacity to kill R equi, which may be an important factor in the pathogenesis of R equi infections.     

Foal IgG and opsonizing anti-Rhodococcus equi antibodies after immunization of pregnant mares with a protective VapA candidate vaccine

The aim of this study was to evaluate serum IgG antibody levels and opsonizing activity in foals from pregnant mares immunized with either proteins from an R. equi strain containing virulence-associated protein A (VapA), an immunodominant surface-expressed lipoprotein encoded by a virulence plasmid crucial for virulence in foals, or a whole killed virulent R. equi preparation. Forty-eight pregnant mares were distributed into three groups, i.e. 24 immunized with R. equi VapA protein antigen associated with a water-based nanoparticle adjuvant (Montanide IMS 3012), 8 immunized with whole killed R. equi, and 16 non-immunized as control. Serum IgG and opsonizing capacity were evaluated during pregnancy in mares, and up to day 45 post-delivery in foals in which R. equi infections were recorded in the first 6 months of life. Pregnant mares immunized with virulent R. equi proteins developed higher serum IgG and opsonic activity which were transferred to the foals than either in the whole R. equi immunized or the control group. Four foals developed pneumonia in the control group while none in immunized groups. Results support further evaluation of VapA protein antigen associated with a water-based nanoparticle adjuvant as a candidate vaccine for immunization of pregnant mares resulting in passive antibody-mediated protection of foals.     

Transient hypogammaglobulinaemia of the young in a Thoroughbred yearling

A 12-month-old Thoroughbred colt was presented for chronic recurrent respiratory disease characterised by persistent nasal discharge, intermittent pyrexia and coughing of several months' duration. These episodes were responsive to empirical antimicrobial therapy but returned once therapy was discontinued. History, physical examination and initial diagnostics were suggestive of bacterial pneumonia and possible underlying immunodeficiency. Serum immunoglobulin analysis and peripheral blood lymphocyte phenotyping revealed low serum IgG and IgM concentrations. In this 12-month-old colt, low serum IgG concentration was the most diagnostic parameter and supported a presumptive diagnosis of transient hypogammaglobulinaemia of the young. The colt responded favourably to repeated treatment with antimicrobials for bacterial pneumonia, and serial immunologic testing over the following months showed gradual increase in serum IgG and IgM concentrations, supporting the diagnosis of delayed development of humoral immunity in this patient.     

Age-related changes in lymphocyte subsets of quarter horse foals

OBJECTIVE: To characterize changes in lymphocyte subsets over time in foals from birth to 18 weeks of age, accounting for differences among individuals, and to determine the effect of overnight storage of blood samples on foal lymphocyte subset concentrations. ANIMALS: 8 healthy Quarter Horse foals from birth to 18 weeks of age. PROCEDURE: Blood samples were collected longitudinally from birth to 18 weeks of age and a CBC performed on each sample. The samples were stained for lymphocyte markers, either immediately or after overnight storage and analyzed by flow cytometry. RESULTS: Total leukocytes, total lymphocytes, and the absolute concentrations of all lymphocyte subsets increased significantly with age. The proportions of B29A+, CD21+, and-equine major histocompatability complex class-II molecule+ lymphocytes increased significantly with age. The proportion of equine (Eq) CD5+, EqCD8+, and EqWC4+ lymphocytes decreased significantly with age. Significant differences among foals were found with respect to initial concentrations with respect to initial concentrations, but not with respect to the rate of increase of the various subsets tested. Significant differences were not found in subset values when comparing blood samples stained on the day of collection or after overnight storage at room temperature (approx 21 C) or under refrigeration. CONCLUSIONS AND CLINICAL RELEVANCE: These results are consistent with an increase in subset numbers and proportions over time, but with individual differences among foals. The observation of individual differences in subsets among foals suggests that there may be individual differences in susceptibility to infectious disease during the perinatal period. The absence of an effect of overnight storage makes field studies of lymphocyte subset concentrations more feasible.     

Immunoglobulin G2 deficiency with transient hypogammaglobulinemia and chronic respiratory disease in a 6-month-old Holstein heifer.

A 6-month-old Holstein heifer that was nonresponsive to medical treatment was evaluated for chronic respiratory disease. Complete blood count and serum chemistry revealed neutrophilic leukocytosis and low globulin levels. Assays for bovine leukemia virus, bovine virus diarrhea, and bovine leukocyte adhesion deficiency were negative. Serum globulin subclass assays revealed transient low concentrations of immunoglobulin (Ig) G1 and IgA, persistent low IgG2, and subnormal IgM. Vaccination with 2 doses of multiple, inactived viruses induced seroconversion for most viruses. Flow cytometric analysis of blood lymphocyte subpopulation demonstrated an increase in CD5+ B-cells. Blood lymphocyte proliferation and neutrophil function tests were normal. Results of immunologic assays indicated IgG2 deficiency with transient hypogammaglobulinemia.     

Colostral and serum IgG, IgA, and IgM concentrations in Standardbred mares and their foals at parturition

Immunoglobulin G, IgM, and IgA concentrations were measured in serum collected from 36 Standardbred mares within 12 hours of foaling, in colostrum collected within 6 hours of foaling, and in serum collected from foals 24 to 48 hours after birth. In serum collected from mares after parturition, mean concentrations of IgG, IgM, and IgA were 2,463.9 +/- 1,337.3 mg/dl, 136.4 +/- 218 mg/dl, and 305.2 +/- 237.5 mg/dl, respectively. In serum from foals, mean concentrations of IgG, IgM, and IgA were 1,953.3 +/- 1,635 mg/dl, 33.8 +/- 30.4 mg/dl, and 58.4 +/- 42.2 mg/dl, respectively. In colostrum, mean concentrations of IgG, IgM, and IgA were 8,911.9 +/- 6,282.2 mg/dl, 957 +/- 1088.1 mg/dl, and 122.9 +/- 77.3 mg/dl, respectively. The IgG concentrations in foal serum were poorly correlated with IgG concentrations in colostrum (r = 0.462, P less than 0.01). Correlations of IgM or IgA concentrations in serum from foals with IgM or IgA concentrations in colostrum and correlations of IgG concentrations in serum from mares with those in colostrum were not significant (P less than 0.01). Of 36 foals, 1 (2.8%) had a serum IgG concentration less than 400 mg/dl. Of 36 foals monitored for 4 months, 6 developed infectious respiratory tract disease requiring antimicrobial therapy at ages varying from 55 to 113 days; these infections were probably not related to failure or partial failure of passive transfer of antibody.     

Interaction of Rhodococcus equi with phagocytic cells from R. equi-exposed and non-exposed foals

The interaction of Rhodococcus equi with alveolar macrophages from adult horses, foals experimentally exposed to R. equi (sensitized foals) and non-exposed foals was studied using in vitro bactericidal assays, cytochemical staining and transmission electron microscopy. It was demonstrated that R. equi is a facultative intracellular parasite, able to survive and multiply within the alveolar macrophages of the host by interfering with phagosome-lysosome fusion. Opsonization of R. equi with antibody against capsular components was associated with increased phagosome-lysosome fusion and significantly enhanced (P less than 0.05) killing of the organism by alveolar macrophages from non-exposed foals. Macrophages from non-exposed foals were able to ingest the non-opsonized organism, but unable to kill greater than 65% of the infective dose by 6 h post-exposure. Alveolar macrophages from sensitized foals behaved as adult macrophages, able to kill greater than 95% of the infective dose by 6 h. Lymphocyte factors, derived by in vitro incubation of sensitized peripheral blood lymphocytes with R. equi surface antigens, enhanced macrophage bactericidal activity. Macrophages from non-exposed foals incubated in the presence of the lymphocyte factors had a 50% increase in killing of R. equi, while sensitized macrophages incubated with lymphocyte factors had a greater than 100% increase in killing capacity.     

Influence of age and plasma treatment on neutrophil phagocytosis and CD18 expression in foals

The aim of this study was to evaluate the influence of age and plasma treatment on neutrophil phagocytosis, CD18 expression and serum opsonic capacity in foals in field settings. Microbial infections constitute a large threat in young foals and neutrophil functions are crucial for the defense. Blood samples were obtained from 13 foals at seven time points between the ages of 2 and 56 days and once from 16 adult horses. Six of the foals were treated with adult plasma at the age of 1 week. Neutrophil phagocytosis of yeast after various opsonizations and the expression of complement adhesion receptor CD18 were analysed by flow cytometry. Autologous serum opsonization resulted in 52+/-6.1% phagocytic neutrophils in 2-day-old foals (n = 12), a significantly lower rate than in adult horses (mean 84+/-3.1%; n = 16). In foals, yeast ingestion per neutrophil was also lower than in adults. Opsonic capacity increased with age (p < 0.05), reaching adult levels at 3-4 weeks. An increase in serum opsonic capacity followed plasma treatment (p < 0.05). The phagocytic capacity of foal neutrophils at the time-points studied was equal to or higher than that in the adults, when pooled adult horse serum or anti-yeast IgG was used as opsonin. In foals, serum IgG concentration was negatively correlated to serum opsonic capacity. CD18 receptor expression was higher in neutrophils from foals (<21 days old) than in those from adult horses (p < 0.05). The results indicate that foals are transiently deficient in serum opsonic capacity, which negatively affects their capacity for neutrophil phagocytosis. These changes in serum opsonins, unrelated to IgG, may be important factors in susceptibility to infections in foals.     

Absorption of bovine colostral immunoglobulins G and M in newborn foals

The uptake of colostral IgG and IgM, their serum half-lives, and the rates of endogenous synthesis of IgG and IgM were evaluated in 6 newborn foals fed bovine colostrum (principals) and 6 foals allowed to suckle their dams (controls). The principal foals were fed 400 ml of bovine colostrum (IgG, 10,000 mg/dl and IgM, 200 mg/dl) at 2-hour intervals, from 2 to 20 hours after foaling (total dose, 4 L). Serum IgG and IgM concentrations were determined by single radial immunodiffusion from birth to 98 days of age. At foaling, principal foals had no detectable serum equine IgG, but 1 control foal had serum equine IgG of 185 mg/dl. After ingestion of colostrum, there was no significant difference in the maximal serum bovine IgG concentration (range, 1,350 to 3,300 mg/dl) in the principal foals, and maximal serum equine IgG concentration in the control foals (range, 500 to 6,000 mg/dl). The calculated biological bovine and equine IgG half-life in the principal and control groups was 9.4 and 26 days, respectively. Endogenous IgG synthesis was first detected in 1 principal foal at 3 days of age, but was detected first between 28 and 42 days in the other principal foals. Starting on day 56 there was no significant difference in serum equine IgG concentration between groups. At foaling, foals in both groups had low equine IgM concentrations. In the control foals, there was marked individual variation in the increases in equine IgM concentration (range, 5 to 73 mg/dl) after ingestion of colostrum.(ABSTRACT TRUNCATED AT 250 WORDS)     

Phenotypic comparison of ileal intraepithelial lymphocyte populations of suckling and weaned calves

Ileal intraepithelial lymphocyte (IEL) suspensions from suckling calves (1-3 weeks old) and weaned calves (3-6 months old) were phenotyped to determine whether there were differences in the lymphocyte populations consistent with postnatal maturation of the mucosal immune system. Flow cytometric comparisons of IEL from the two age groups revealed the presence of significantly larger proportions of CD4+ T lymphocytes and CD8+ T cells in the weaned animals. In contrast, there was a significantly larger proportion of B-B2+ IEL in the suckling calves. Freshly isolated IEL from both groups of calves expressed mRNA for TNF-alpha and IFN-gamma, but not IL-4 or IL-10. The B-B2+ IEL population was more closely examined by flow cytometry. These cells co-expressed IgM and CD21. However, they did not express IgA, IgG1, nor any of several additional leukocyte differentiation molecules. Immunohistochemical data confirmed the presence of IgM+ lymphocytes, and the paucity of IgA+ and IgG1+ lymphocytes in suckling calf ileum. However, substantial numbers of IgA+ and IgG1+ cells were observed in weaned calf ileum. Together, the data are consistent with ongoing postnatal maturation of the gut mucosal immune system.     

Evaluation of equine breeding farm management and preventative health practices as risk factors for development of Rhodococcus equi pneumonia in foals

OBJECTIVE: To determine whether foal management practices, environmental management, and preventative health practices are risk factors for development of Rhodococcus equi pneumonia in foals. DESIGN: Prospective matched case-control study. ANIMALS: 2,764 foals on 64 equine breeding farms with 9,991 horses. PROCEDURE: During 1997, participating veterinarians completed paired data collection forms for comparison; 1 for an affected farm (containing > or = 1 foal with pneumonia caused by R equi) and 1 for a control farm. Information collected pertained to stabling facilities, environmental management, foal husbandry, and preventative equine health practices. RESULTS: Matched farm data compared by use of conditional logistic regression indicated that personnel on affected farms were more likely to attend foal births, test foals for adequacy of passive immunity, administer plasma or other treatments to foals to supplement serum immunoglobulin concentrations, administer hyperimmune plasma prophylactically to foals, vaccinate mares and foals against Streptococcus equi infection, and use multiple anthelmintics in deworming programs. Affected farms were also more likely to have foals that developed other respiratory tract disorders and were approximately 4 times as likely to have dirt floors in stalls used for housing foals as were control farms. CONCLUSIONS AND CLINICAL RELEVANCE: Rhodococcus equi pneumonia does not appear to be associated with poor farm management or a lack of attention to preventative health practices. Housing foals in stalls with dirt floors may increase the risk for development of R equi pneumonia.     

Unusual Patterns of Serum Antibodies to Streptococcus Equi in Two Horses with Purpura Hemorrhagica

An enzyme-linked immunosorbent assay (ELISA) was developed for use in horses to determine serum titers of antibodies of the immunoglobulin classes IgA, IgG, and IgM to Streptococcus equi M-like protein and culture supernatant protein antigens. Serum antibodies were determined in 28 adult horses, including 9 horses with recent S. equi infections, 17 horses without known exposure to S. equi, but without a history of respiratory disease in the preceding 4 months, and 2 horses with clinical purpura hemorrhagica. Serum IgA titers to culture supernatant protein antigen were highest in recently infected horses (P less than 0.001). Serial determinations of antibody titers in the horses with purpura showed that IgG antibodies to both S. equi M-like protein and culture supernatant protein antigens were undetectable initially, but later rose coincidental with clinical recovery from the disease. Possible mechanisms for these findings are discussed.