Owners and veterinarians are often concerned about morbidity and mortality of geriatric horses with gastrointestinal disease. The association between age and survival for equine surgical colic cases has been recently studied using statistical models (Proudman et al. 2002, 2005a,b; Mair and Smith 2005). Proudman et al. (2002) reported that survival did not decrease with increasing age for surgical colic cases overall nor for horses with small intestinal lesions (Proudman et al. 2005a). Similarly, Mair and Smith (2005) found no association between age and short-term survival. In another study, there was, however, a positive association between age and the risk of post operative death in horses with large intestinal disease undergoing surgery (Proudman et al. 2005b). These studies included very young horses (i.e. age ≤1 year) (Mair and Smith 2005; Proudman et al. 2005b) and did not specifically evaluate the geriatric horse compared to the mature nongeriatric horse nor did they evaluate specific reasons for nonsurvival in these cases. Only horses that were recovered from general anaesthesia were included in the analysis (Proudman et al. 2002, 2005a,b), which can cause a selection bias whereby horses in a particular age class with only a good prognosis may be recovered leading to artificial inflation of the survival rate of the particular age class of horse.
Recently, it was reported that geriatric horses (age ≥16 years) overall and with gastrointestinal disease that were admitted on an emergency basis, had a lower survival compared to mature horses of other age categories (Southwood et al. 2008). Possible reasons for a lower survival for geriatric horses include: 1) geriatric horses being more critically ill at admission compared to mature horses; 2) different or more serious causes of colic in geriatric horses; 3) alterations in healing, inflammatory and immune response associated with or independent of pituitary pars intermedia dysfunction (PPID) leading to more post operative complications; or 4) euthanasia without treatment because of unwillingness of owners to invest in geriatric horses or a perceived poor prognosis for survival.
The higher morbidity and mortality could be explained by geriatric horses being more critically ill and having more serious disease. In Southwood et al. (2010) it was shown that geriatric horses were not more critically ill than mature horses. However, there was some evidence that geriatric horses may have moreserious disease compared to mature horses. The more common lesions that cause colic in geriatric horses are often different to those that cause colic in nongeriatric horses. This difference could have an impact on survival. The most common lesions identified in geriatric horses presented to a referral hospital included PPID, strangulating small intestinal lesions particularly strangulating lipoma of the small intestine, laminitis, large colon impaction and gastric ulcers (Carson-Dunkerley and Hanson 1996; Freeman and Schaeffer 2001; Brosnahan and Paradis 2003). Horses with a strangulating lipoma had a lower survival (80%) compared to horses with epiploic foramen entrapment and miscellaneous strangulating lesions (90–95%) (Freeman and Schaeffer 2001) and it could be inferred from this that geriatric horses also had a lower survival than nongeriatric horses.
Aging and PPID can alter healing, immune function and the inflammatory response (Fermaglich and Horohov 2002; McCue 2002), which could result in a higher post operative complication rate and subsequent mortality. Aging in horses was associated with alterations in markers of inflammation with a shift toward a proinflammatory state (McFarlane and Holbrook 2008). Geriatric horses (age ≥16 years) had increased expression of interleukin (IL)-6, IL-8 and interferon-γ as well as an increase in tumour necrosis factor-α (TNF-α) release from peripheral blood mononuclear cells after lipopolysaccharide (endotoxin) stimulation (McFarlane and Holbrook 2008). Circulating TNF-α is cytotoxic (MacKay et al. 1991; MacKay 1992) and high levels have been associated with mortality (Morris et al. 1991; Barton and Collatos 1999; Kyaw et al. 2008). The immune impairment associated with ageing can lead to an increase in morbidity and mortality as a result of infectious disease (Pawelec 2006). Geriatric horses (age >16 years) that were positive for Salmonella spp. on faecal culture had a 17-fold increase in mortality compared to other horses (H. Aceto, unpublished data). Therefore, there are sufficient reasons that geriatric horses may not do as well as mature horses following colic surgery.
Carson-Dunkerley and Hanson (1996) reported an overall short-term survival rate of 67% for geriatric horses aged ≥17 years; with 76% of horses recovering from general anaesthesia and 94% of horses managed medically surviving to discharge. These lesions and survival rates were not compared to a mature nongeriatric population. More recently, a study comparing survival rates for geriatric vs. nongeriatric horses following exploratory celiotomy was published (Krista and Kuebelbeck 2009). There were several limitations with the latter study including: 1) inclusion of horses age from one day to 19 years in the nongeriatric age category 2) use of cases from the late 1990s 3) lack of information on admission data to determine if the geriatric and nongeriatric population of horses were of similar critical illness 4) no clear distinction between survival of horses subjected to euthanasia during surgery vs. horses recovered, and 5) very low numbers of geriatric horses. Further, the authors reported a 27% survival rate for geriatric horses with small intestinal strangulating lesions. We believe that the survival rate for geriatric horses with small intestinal strangulating lesions in our hospital is higher than that reported (Krista and Kuebelbeck 2009). Our null hypothesis was that the short-term survival for geriatric horses showing signs of colic would not be different than that for mature nongeriatric horses. Therefore, to address the hypothesis, the objective of the present study was to compare short-term survival of geriatric and mature nongeriatric horses with signs of colic.