Susceptibility of ponies to infection with Streptococcus pneumoniae (capsular type 3)

Naturally-occurring serotype 3 Streptococcus pneumoniae strains that lack functional pneumolysin and autolysin have attenuated virulence but induce localized protective immune responses

Streptococcus pneumoniae is an important cause of fatal pneumonia in humans. These bacteria express virulence factors, such as the toxins pneumolysin and autolysin, that drive host inflammatory responses. In this study we confirm loss of pneumolysin and autolysin function in a group of clonal pneumococci that have a chromosomal deletion resulting in a pneumolysin-autolysin fusion gene Δ(lytA'-ply')593. The Δ(lytA'-ply')593 pneumococci strains naturally occur in horses and infection is associated with mild clinical signs. Here we use immortalized and primary macrophage in vitro models, which include pattern recognition receptor knock-out cells, and a murine acute pneumonia model to show that a Δ(lytA'-ply')593 strain induces cytokine production by cultured macrophages, however, unlike the serotype-matched ply+lytA+ strain, it induces less tumour necrosis factor α (TNFα) and no interleukin-1β production. The TNFα induced by the Δ(lytA'-ply')593 strain requires MyD88 but, in contrast to the ply+lytA+ strain, is not reduced in cells lacking TLR2, 4 or 9. In comparison to the ply+lytA+ strain in a mouse model of acute pneumonia, infection with the Δ(lytA'-ply')593 strain resulted in less severe lung pathology, comparable levels of interleukin-1α, but minimal release of other pro-inflammatory cytokines, including interferon-γ, interleukin-6 and TNFα. These results suggest a mechanism by which a naturally occurring Δ(lytA'-ply')593 mutant strain of S. pneumoniae that resides in a non-human host has reduced inflammatory and invasive capacity compared to a human S. pneumoniae strain. These data probably explain the relatively mild clinical disease in response to S. pneumoniae infection seen in horses in comparison to humans.

Epidemiology and pathogenicity of zoonotic streptococci

Zoonotic infections caused by Streptococcus spp. have been neglected in spite of the fact that frequency and severity of outbreaks increased dramatically in recent years. This may be due to non-identification since respective species are often not considered in human medical diagnostic procedures. On the other hand, an expanding human population concomitant with an increasing demand for food and the increased number of companion animals favour conditions for host species adaptation of animal streptococci. This review aims to give an overview on streptococcal zoonoses with focus on epidemiology and pathogenicity of four major zoonotic species, Streptococcus canis, Streptococcus equi sub. zooepidemicus, Streptococcus iniae and Streptococcus suis.

Evidence for transferrin allele as a host-level risk factor in naturally occurring equine respiratory disease: a preliminary study

REASONS FOR PERFORMING STUDY

Recurring respiratory infections can contribute to prolonged burdens of disease, especially in younger horses and better knowledge of factors and effective interventions, such as vaccines, should improve therapeutic and preventive strategies.

OBJECTIVES

To identify factors and infections associated with naturally occurring respiratory disease in recently weaned Welsh Mountain ponies maintained at pasture and to determine whether ponies vaccinated with an experimental inactivated bacterial vaccine had lower burdens of disease and infection compared to nonvaccinated controls. Transferrin allele, which may influence the ability of pathogenic bacteria to acquire iron, was examined specifically as a host-level risk factor.

METHODS

Twice weekly clinical evaluations and weekly microbiological samplings over a 10 week period were conducted in 29 ponies, of which 12 received an experimental bacterial vaccine, 12 received placebo and 5 were untreated. A multilevel modelling approach suitable for analysing longitudinal data containing repeated observations was used to identify factors associated with disease and to evaluate any effect from vaccination.

RESULTS

Analyses demonstrated significant variation in clinical disease between ponies that possessed different alleles of iron binding transferrin protein but no significant effect from vaccination. Of the 29 ponies monitored, 14 possessing homozygote or non-F2 heterozygote transferrin D alleles demonstrated significantly less clinical disease (P < 0.001), whereas 14 possessing the F2 transferrin allele demonstrated significantly more clinical disease (P < 0.001). These effects remained apparent even after the significant effects of tracheal bacterial isolates, clinical score the previous week and repeated observations from the same ponies were accounted for.

CONCLUSIONS AND POTENTIAL RELEVANCE

Results provide evidence for a potential genetic basis for variation in susceptibility to clinical equine respiratory disease of bacterial origin, although more work is required to corroborate this conclusion.

Association between respiratory disease and bacterial and viral infections in British racehorses

Respiratory disease is important in horses, particularly in young Thoroughbred racehorses, and inflammation that is detected in the trachea and bronchi (termed inflammatory airway disease [IAD]) is more significant in this population in terms of impact and frequency than other presentations of respiratory disease. IAD, which is characterized by neutrophilic inflammation, mild clinical signs, and accumulation of mucus in the trachea, may be multifactorial, possibly involving infections and environmental and immunological factors, and its etiology remains unclear. This 3-year longitudinal study of young Thoroughbred racehorses was undertaken to characterize the associations of IAD and nasal discharge with viral and bacterial infections. IAD was statistically associated with tracheal infection with Streptococcus pneumoniae (capsule type 3), Streptococcus zooepidemicus, Actinobacillus spp., and Mycoplasma equirhinis and equine herpesvirus 1 and 4 infections, after adjustment for variation between training yards, seasons, and age groups. The association with S. pneumoniae and S. zooepidemicus was independent of prior viral infection and, critically, was dependent on the numbers of organisms isolated. S. pneumoniae was significant only in horses that were 2 years old or younger. The prevalence and incidence of IAD, S. zooepidemicus, and S. pneumoniae decreased in parallel with age, consistent with increased disease resistance, perhaps by the acquisition of immunity. The study provided evidence for S. zooepidemicus and S. pneumoniae playing an important etiological role in the pathogenesis of IAD in young horses.

A case control study of factors and infections associated with clinically apparent respiratory disease in UK Thoroughbred racehorses

A matched case control study was used to determine infections and other factors associated with clinically apparent respiratory disease in young racehorses in training in the UK. A total of 170 cases, defined as horses with sudden onset coughing, nasal discharge or pyrexia, were identified and matched to 632 non-affected controls by trainer and time period. Factors examined included age, sex, time since entry into the training yard, time since last race and different infections including tracheal and nasopharyngeal (NP) bacteria and viruses. Multivariable conditional logistic regression (CLR) modelling was used to evaluate the risk of being a case for variables after adjustment for other factors. Three analyses were conducted using clinical cases as outcomes, which were compared with: (i) controls without evidence of subclinical inflammatory airway disease (IAD) (ii) controls with evidence of subclinical IAD and (iii) all controls irrespective of IAD status. A fourth analysis was conducted comparing the two groups of controls, i.e. those with and without IAD. Younger horses and those that had entered training more recently were at increased risk of suffering episodes of clinically apparent respiratory disease. Among the infections, increasing numbers of Pasteurella/Actinobacillus spp. in tracheal washes were associated with increasing risk of clinical disease. Tracheal infection with Streptococcus zooepidemicus was associated with both clinical respiratory disease and subclinical IAD when compared with controls with no evidence of IAD. This explained the lack of association between clinical cases and S. zooepidemicus when all controls were used. Tracheal isolation of Mycoplasma felis was also associated with clinical disease after controlling for other factors. An inverse association was identified between risk of clinically apparent disease and isolation from tracheal washes of the transient, non-pathogenic bacteria Staphylococcus and Acinetobacter spp. There was no significant association identified between clinical disease and infection with equine herpesviruses-1 and -4, rhinoviruses-1 and -2 or adenovirus. Equine influenza was significantly associated with clinical respiratory disease but it was a very rare infection in this well-vaccinated population, only occurring in three cases.

A case-control study of respiratory disease in Thoroughbred racehorses in Sydney, Australia

In order to investigate the role of infectious agents in the aetiology of lower respiratory tract disease in Thoroughbred racehorses, a matched case-control study was conducted. Cases were identified by the presence of coughing, and were compared to a control population matched on time of sample collection and location within the same training establishment. Tracheal wash samples were collected from 100 cases and 148 controls. Case horses were more likely than controls to have endoscopic and cytological evidence of airway inflammation. There was no significant association between serological evidence of infection by commonly implicated respiratory viruses and coughing. Similarly, mycoplasma were rarely isolated and were not associated with disease. In contrast, there was a strong association between isolation of greater than a total of 10(3) colony-forming units/ml of tracheal wash and coughing. Individual bacterial species associated with disease included Streptococcus zooepidemicus, Streptococcus pneumoniae, Streptococcus suis, Streptococcus sanguis, Pasteurella spp and Bordetella bronchiseptica. This study provides evidence of the role of bacterial infection in the aetiology of lower respiratory tract inflammation in racehorses. However, in 58% of cases, few or no bacteria were isolated. Hence, at the time of identification of disease, there was no evidence of viral, bacterial or mycoplasmal infection in the majority of coughing horses. The aetiology of the signs observed in these horses requires further investigation.

Attitudes of Australian veterinarians about the cause and treatment of lower-respiratory-tract disease in racehorses

A questionnaire was administered to members of the Australian Equine Veterinary Association to investigate their attitudes and behaviours regarding the cause, diagnosis and treatment of lower-respiratory-tract disease in racehorses. The most-important perceived risk factors related to the level of exposure and resistance to infectious agents, whereas factors associated with racing and climatic factors were lower ranked. By far the most-commonly implicated primary cause of disease was respiratory viruses. However, specific diagnostic tests (such as viral serology or virus isolation) were rarely performed. By far the most-common diagnostic procedure was auscultation (without a rebreathing bag). The relative importance of diagnostic procedures was influenced by the proportion of total workload comprising horse work. Many respondents believed that bacteria become secondarily involved in disease; accordingly, antibiotic therapy was the most-commonly reported therapy. However, bacteriology of lower-airway-fluid samples was not commonly performed. The most-commonly used antibiotics were the potentiated sulphonamides (either alone or in combination with mucolytics or bronchodilators), followed by procaine penicillin and penicillin combined with gentamicin. Mainly, treatment would appear to be based on empirical beliefs rather than exhaustive investigation of individual cases.

Retrospective study of the relationships between age, inflammation and the isolation of bacteria from the lower respiratory tract of thoroughbred horses

A total of 1235 tracheal aspirates taken from 724 thoroughbreds in race training, aged from two to 10 years, were examined cytologically and bacteriologically. An inflammation scoring system on a scale of 0 to 9 was devised to allow the severity of lower airway disease to be assessed from the cytological results. The inflammation scores were closely related to the isolation of bacteria (P<0.001), and the most common bacterial isolates were Streptococcus zooepidemicus, Streptococcus pneumoniae and Pasteurella/Actinobacillus-like species. Lower airway disease was less common in older horses (P = 0.031), and the groups at highest risk were the two- and four-year-olds. Lower airway inflammation was more common in the four-year-olds at National Hunt yards than in the four-year-olds at flat racing yards (P = 0.040, odds ratio = 3.80).

Molecular characterization of equine isolates of Streptococcus pneumoniae: natural disruption of genes encoding the virulence factors pneumolysin and autolysin

Although often considered a strict human pathogen, Streptococcus pneumoniae has been reported to infect and cause pneumonia in horses, although the pathology appears restricted compared to that of human infections. Here we report on the molecular characterization of a group of S. pneumoniae isolates obtained from horses in England and Ireland. Despite being obtained from geographically distinct locations, the isolates were found to represent a tight clonal group, virtually identical to each other but genetically distinguishable from more than 120 divergent isolates of human S. pneumoniae. A comprehensive analysis of known pneumococcal virulence determinants was undertaken in an attempt to understand the pathogenicity of equine pneumococci. Surprisingly, equine isolates appear to lack activities associated with both the hemolytic cytotoxin pneumolysin, often considered a major virulence factor of pneumococci, and the major autolysin gene lytA, also considered an important virulence factor. In support of phenotypic data, molecular studies demonstrated a deletion of parts of the coding sequences of both lytA and ply genes in equine pneumococci. The implications of these findings for the evolution and pathogenicity of equine S. pneumoniae are discussed.

Respiratory disease in thoroughbred horses in training: the relationships between disease and viruses, bacteria and environment

A longitudinal study of respiratory disease in racehorses was carried out to assess its relative associations with different infectious agents and to examine any role that the environmental conditions might play. The relationships between coughing, nasal discharge, pyrexia and lower respiratory tract disease were also examined to provide information for improving clinical diagnosis, particularly of disease of the lower respiratory tract. Lower airway disease was closely associated with infection with Streptococcus zooepidemicus. It was also found that equine herpesvirus seroconversions and S pneumoniae infections were independently associated with the development of nasal discharge. Coughing was a specific, but insensitive measure of lower respiratory tract disease (specificity 84 per cent, sensitivity 38 per cent). However, horses that coughed were very likely to have had lower airway disease for more than one month. Horses housed on straw in loose boxes were twice as likely to suffer from lower airway disease as those kept on shredded paper in American barns. The study was not large enough to assess the significance of rarer infections but it did improve the definition of the problem of respiratory disease in racehorses and revealed some of the trends in the associations between viruses, bacteria and the environment in respiratory disease.

Equine pulmonary disease: a case control study of 300 referred cases. Part 1: Examination techniques, diagnostic criteria and diagnoses

Three-hundred adult horses, referred from 1990 to 1993 inclusively, for pulmonary examination were assessed using standardised history taking and clinical, intrapleural pressure, arterial blood gases and pH, bronchoscopic and tracheal and broncho-alveolar lavage fluid (BALF) cytological examinations. Two-hundred and thirty-five cases were referred with overt signs of pulmonary disease and the remaining 65 cases were referred for pulmonary examination because of reduced exercise (usually racing) performance or prolonged dyspnoea after racing. No pulmonary disease was detected in 30 cases. The 270 horses with pulmonary disease included 148 cases (54.8%) of chronic obstructive pulmonary disease (COPD), 45 (16.7%) of infectious or post infectious pulmonary disease, 7 (2.6%) of Streptococcus zooepidemicus pulmonary infection, 7 (2.6%) of lungworm infection, 16 (5.9%) of primary exercise induced pulmonary haemorrhage (EIPH), 9 (3.3%) of chronic idiopathic hypoxaemia, 20 (7.4%) of miscellaneous identified pulmonary disorders and 18 (6.7%) of undifferentiated pulmonary disorders. Two cases of primary summer associated obstructive pulmonary disease (SPAOPD) were observed, but 9.5% of COPD affected horses additionally suffered from SPAOPD.

Pathology of equine pneumonia associated with transport and isolation of Streptococcus equi subsp. zooepidemicus

Seven horses that died of pneumonia associated with transport yielded Streptococcus equi subsp. zooepidemicus (S.z.) from their pulmonary lesions. These lesions were divisible roughly into two types, serous haemorrhagic pneumonia and multiple foci of coagulative necrosis, which were considered to reflect a temporal difference in the process of lesion formation. Immunohistologically, S.z. antigen was detected in both types of lesion. Acute necrotic lacunar tonsillitis was considered to play an important role in the onset of the pneumonia.