Equine Veterinary Journal ISSN 0425-1644 DOI: 10.1111/evj.12307
Detection of bacteraemia and host response in healthy neonatal foals E. S. HACKETT*, D. P. LUNN†, R. A. FERRIS, D. W. HOROHOV‡, M. R. LAPPIN and P. M. MCCUE Department of Clinical Sciences, Colorado State University, Fort Collins, USA † College of Veterinary Medicine, North Carolina State University, Raleigh, USA ‡ Department of Veterinary Science, Maxwell H. Gluck Equine Research Center, University of Kentucky, Lexington, USA. *Correspondence email:
[email protected]; Received: 03.03.14; Accepted: 01.06.14
Summary Reasons for performing the study: Neonatal sepsis is a common problem in foals and is a primary cause of death in the post natal period. Transient bacteraemia and subsequent host responses have not been described in the equine neonate. Objectives: The primary objective of this study was to determine if transient bacteraemia occurs in foals within the first 72 h of life. Additional objectives included description of bacterial organisms associated with transient bacteraemia and concurrent cytokine gene expression in healthy foals. Study design: Prospective observational study in healthy foals. Methods: Blood was aseptically collected for bacterial culture from observed spontaneously born foals at birth and 1, 2, 3, 4, 8, 12, 24, 48 and 72 h following birth. Samples taken at birth, 4, 12, 24, 48 and 72 h were analysed for interferon gamma (IFNγ), interleukin (IL)-1, IL-2, IL-6, IL-8, IL-10, IL-18 and monocyte chemotactic protein 1 (MCP1) cytokine gene expression quantified by RT-PCR. Results: Bacteria were cultured from 9 of 70 samples submitted for blood culture. The positive samples were from 4 of the 7 foals, all of which remained healthy throughout and subsequent to the study. All positive blood cultures were from blood samples obtained at 12 h of age or earlier and IL-10 elevation coincided with positive blood cultures in healthy foals. Cytokine gene expression fluctuated with age. Conclusions: Positive blood cultures suggest transient bacteraemia may occur in healthy foals early in the post natal period. Age corrected normal values may be necessary to interpret cytokine concentration in diseased populations. Keywords: horse; sepsis; transfer of immunity; passive transfer
Introduction Sepsis is a leading cause of morbidity and mortality in the equine neonate [1]. Reports identify mortality rates in foals with bacterial sepsis of 25–45% [2–4]. Factors that predispose foals to infection include maternal placentitis and dystocia and failure of transfer of passive immunity. Post natal routes of infection include entry via the respiratory tract, umbilicus and gastrointestinal tract. Of these, the gastrointestinal route of infection is implicated most frequently [5] as the foal does not discriminate between maternal immunoglobulin G (IgG) and other molecules absorbed in the gastrointestinal tract. Whether this permissiveness extends to molecules as large as bacteria in foals is unknown but in healthy humans, bacterial translocation from the gastrointestinal tract is well documented [6]. Thus, it is likely that healthy foals undergo transient bacteraemia in the post natal period. Further, post natal transient bacteraemia may result in sepsis if foals are unable to clear the bacteria with host defences or if an uncontrolled nonspecific inflammatory response is triggered. Inflammatory mediators of systemic inflammation, including cytokines interferon gamma (IFNγ), interleukin (IL) 1, IL-6, IL-18, IL-2 and IL-10, and chemokines IL-8 and monocyte chemotactic protein 1 (MCP1), are upregulated in human neonatal sepsis [7–11]. Though cytokines are known to be predictive of early sepsis in other neonatal species [12], there is limited knowledge of the foal host response to events and disease in early life. Interferon gamma gene expression in healthy foal peripheral blood mononuclear cells was low when evaluated within 72 h of birth and did not reach adult levels until 3 months of age [13]. Gene expression of various cytokines were reported from foals within the first month of life, although without specified ages and health status, host responses were difficult to interpret [14]. In disease, IL-1 and IL-8 expression was correlated to clinicopathological variables in septic foals [15]. In a separate report, IL-8 expression was lower in septic vs. nonseptic foals, though not associated with survival [16]. Increased IL-10 gene expression has been associated with death or euthanasia of foals with neonatal disease in one report [17], although others have had conflicting results [16]. Interleukin 8 gene expression was greater in diseased foals 20 g/l in 5 foals, 12.8 g/l in one foal and 2.5 g/l in one foal. In the foal with serum IgG of 2.5 g/l, 500 ml of frozen-thawed colostrum was administered by nasogastric tube. The source of the colostrum was a related mare on the same farm from which colostrum was frozen during the previous foaling season. The foal treated with colostrum had a serum IgG of >20 g/l at 24 h. All foals continued to nurse and maintained good
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TABLE 1: Blood culture results in 7 foals evaluated for transient bacteraemia post birth. Timing of blood sampling was recorded relative to birth time Sample time (h)
At birth
1
2
3
4
8
12
24
48
72
Foal 1 Foal 2 Foal 3 Foal 4 Foal 5 Foal 6 Foal 7
B B -
-
P
B BM -
B -
B -
B SS -
-
-
-
Positive culture result indicated by letter with B = Bacillus spp., M = Micrococcus spp., P = Propionibacterium spp. and SS = Streptococcus spp. and Staphylococcus spp.
health without clinical signs of disease or need for medical intervention. All mares bonded appropriately with their respective foals and did not require medical care. Bacteria were cultured from 9 of 70 samples submitted for blood culture (12.9%, Table 1). Isolates included Bacillus spp. (7), Micrococcus spp. (1), Streptococcus spp. (1), Staphylococcus spp. (1) and Propionibacterium spp. (1). The positive samples were from 4 of 7 foals that remained healthy throughout and subsequent to the study. All positive blood cultures were from blood samples obtained at 12 h of age or earlier, in foals with IgG concentration >20 g/l at 12 h. Blood samples in the 2 foals with the earliest positive blood cultures were obtained at 9 min (Foal 1) and 3 min (Foal 2) after birth. Relative quantities of gene expression calibrated to the first sampling period, as soon as possible following birth, were determined (Table 2) and represented graphically over the study period (Figs 1a–h). An early rise in cytokine gene expression was observed in the post natal period. There was a main effect of sample time in IL-1 (P = 0.02), IL-6 (P = 0.01) and IL-8 (P
