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meninges from osteomyelitis of the skull, or through direct extension via anatomic anomalies of the skull (Giannoni et al. 1997; Gallagher et al. 1998; Jones et al. 2002; Reid 2004). Periorbital cellulitis, as was present in this horse, and ophthalmic vein thrombosis have also been documented as a result of sinusitis and as leading to meningitis (Giannoni et al. 1997; Reid 2004). Escherichia coli, the organism isolated in this case, is a
common pathogen in septic foals that develop meningitis from haematogenous dissemination, but is less prevalent in the mature horse (Furr 2008). In human patients, high levels of E. coli bacteraemia (>103 colony-forming units/ml), are required for the organism to cross the blood brain barrier and colonise (Kim 2002). The collection of blood for culture may have facilitated identification of the route of infection into the CNS. The 3 separate foci of infection (CNS, lower palpebrae, right naris) suggested an immune deficiency as a potential factor in the development of meningitis and prompted screening of the immune status of this horse via serum immunoglobin concentrations. Three cases have been reported of horses presenting with bacterial meningitis and subsequent recurrent infections that were ultimately diagnosed with common variable immunodeficiency (CVID; Pellegrini-Masini et al. 2005). Horses with CVID are commonly deficient in one or more immunoglobulins and may display persistent lymphopenias, neither of which were present in this case. In addition, horses affected by CVID frequently have a history of recurrent infections, fevers, and poor responses to vaccines, while this case did not (Flaminio et al. 2002, 2009; Pellegrini-Masini et al. 2005). Immune deficiencies including asplenia, agammaglobulinaemia, CVID and human immunodeficiency virus are predisposing factors for human patients in the development of bacterial meningitis (Honda and Warren 2009). Treatment of bacterial meningitis in the mature equid is
rarely successful, and usually culminates in euthanasia. However in this case, targeted antimicrobial therapy, combined with adjunct corticosteroid and nonsteroidal anti-inflammatory drugs produced total resolution of clinical signs of meningitis, apart from the cranial nerve deficits that were gradually improving at discharge. The residual unilateral loss of vision may have been due to the initial suspected traumatic incident, as trauma is a common cause of acute, permanent blindness in the horse (Martin et al. 1986), or may have been due to irreversible optic nerve or CNSdamagefrom the meningitis. In man, third-generation cephalosporins are the mostwidely utilised antimicrobial for bacterial meningitis (Tunkel et al. 2004).However, ceftiofur hasbeendescribed in veterinary literature as being less effective at penetrating the blood brain barrier due to a high level of protein binding (Mitchell et al. 2007). Inflammation of the blood brain barrier may have allowed better penetration of both the ceftiofur and gentamicin initially utilised in this case. Third-generation cephalosporins with improved blood brain barrier penetration include cefotaxim, cefepime, ceftazidime and ceftriaxone (Furr 2008). Chloramphenicol achieves therapeutic levels in the CSF, and could also have been considered (Furr 2008). Enrofloxacin, which also hasgoodpenetration into the CSF,was used for the latter portion of treatment. Initial antimicrobial selections were made based on availability, cost and the presumed inflammation of the blood brain barrier. The use of dexamethasone as an adjunct to antimicrobial therapy is controversial, but is widely utilised in both human
and veterinary medicine (Tunkel et al. 2004). Purported benefits include reduction in cerebral oedema and inflammation within the CSF (Quagliarello and Scheld 1997). A study performed on 143 children with bacterial meningitis demonstrated that patients who received a dose of dexamethasone prior to initiation of antimicrobial therapy had fewer days with fevers, although the outcome following disease was not significantly different between the dexamethasone and control groups (Wald et al. 1995). However, a prospective, double-blinded study in 301 adults with acute bacterial meningitis revealed that patients administered dexamethasone every 6 h for the first 4 days of antimicrobial therapy had a significantly better survival rate than those patients who received the placebo treatment (DeGans and Van De Beek 2002). While it remains remotely plausible that the immunosuppressive effects of dexamethasone lead to the development of meningitis in this case, it is unlikely given that the horse received only 2 low doses prior to onset of neurological signs. While exceedingly rare, there are reports of horses that
have returned to full function, with little or no residual neurological deficits, following bacterial meningitis. In a report of 2 horses with presumed bacterial meningoencephalitis, one horse recovered completely while the other developed mild aggression towards pasture mates but was otherwise healthy (Newton 1998). In another report of 5 cases of mature bacterial meningitis, one horse recovered fully, and a second had only residual facial nerve paralysis. The 3 remaining animals were subjected to euthanasia following a rapid disease course (Mitchell et al. 2006). Three cases of successful medical and surgical treatment of intracranial abscesses have also been reported (Allen et al. 1987; Cornelisse et al. 2001). In the case reported here, 2 months following discharge from the hospital, the horse was considered healthy with the exception of mild right sided facial paralysis and blindness in the left eye. The lack of a menace response, dazzle reflex, or PLR in conjunction with relatively normal ERG findings indicated that the lesion causing the blindness was most likely to have been in the optic nerve between the globe and optic chiasm. It is possible, albeit unlikely, that the horse may regain vision over time, as a previous report of 4 horses that developed post traumatic optic nerve atrophy did not regain vision (Martin et al. 1986). The case described here involves an acute onset of
bacterial meningitis following suspected periocular trauma that resolved with prolonged antimicrobial therapy and supportive care. Although trauma initially seemed a likely route of infection in this case, the identification of 3 identical bacterial species in 3 remote sites is highly suggestive of haematogenous dissemination of bacteria, from a nasal abscess, or from local extension of periocular infection. Recovery from bacterial meningitis in the mature horse is an infrequent occurrence, but can be achieved if treated rapidly and aggressively following the onset of clinical signs.
Authors’ declaration of interests No conflicts of interest have been declared.
Manufacturers’ addresses
1Vetone, Meridian, Idaho, USA. 2Vedco, St. Joseph, Missouri, USA. 3Pfizer, New York, New York, USA.
© 2013 EVJ Ltd
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