Page 1093 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

1068 CHAPTER 10

VetBooks.ir can be difficult and requires either isolation of the of acyclovir) is superior to acyclovir because of sub-
Ante-mortem confirmation of the diagnosis
stantially greater bioavailability and the increased
virus from nasopharyngeal swabs (viral shedding is
highest in the first 5 days after infection) or blood likelihood of reaching sustained therapeutic plasma
concentrations. The efficacy of valacyclovir in horses
samples (viraemia classically occurs 4–10 days experimentally infected with a neuropathic strain
post-infection). In reality, it is advisable to collect of EHV-1 has been recently reported, but evidence
anticoagulant blood samples and nasal swabs from of usefulness in clinical cases of EHM is currently
suspected cases. Demonstration of a four-fold rise lacking
in virus neutralising or complement fixing antibody At present there are no proven methods for pre-
titres between acute and convalescent samples taken venting EHM. Vaccines that are currently used to
7–10 days apart may also support an ante-mortem prevent EHV-1 respiratory disease and abortion do
diagnosis of EHM. Complement fixation antibody not offer protection against myeloencephalopathy.
titres are preferable, because many horses have high Routine vaccination may reduce the likelihood of
levels of virus neutralising antibody, which is long exposure to the virus by reducing the incidence of
lived. Many cases have high levels of complement other EHV-1 diseases. The value of primary vac-
fixation antibodies (>1:160) at the onset of clini- cination or booster vaccination of in-contact horses
cal signs. Identification of seroconversion of other during an outbreak is hotly debated and the potential
horses on the farm with respiratory or neurological therapeutic or preventive benefits in the context of
disease is suggestive. an outbreak are yet to be established.
A definitive post-mortem diagnosis is based on Management practices are important in helping to
histology and immunohistochemistry, or PCR to prevent the introduction and subsequent dissemina-
detect viral antigen in CNS tissues. tion of EHV-1 infection (e.g. isolating new animals
for 3 weeks and maintaining distinct herd groups
Management according to age, gender and occupation). Pregnant
The treatment of EHM is mainly supportive and mares should not have access to the general popu-
there is little evidence-based specific treatment avail- lation and stress should be minimised. The ability
able. The outcome is broadly related to the sever- of EHV-1 to remain latent for prolonged periods
ity and extent of the CNS lesions in each case. The of time and then re-emerge complicates infection-
main features of treatment include evacuation of the control measures.
bladder and rectum and providing sling support for
paretic horses. Maintenance of adequate nutrition Prognosis
and hydration is important. Flunixin meglumine is The prognosis is reasonable provided affected horses
helpful in reducing pyrexia and may have a beneficial remain standing. Full recovery is possible, but weeks
effect on the CNS vasculitis. Treatment strategies to months may be required. The prognosis is poor
to minimise thromboembolic lesions in the CNS if there is clinical deterioration or no improvement
include heparin, DMSO, aspirin and pentoxyphyline. over the first week of treatment.
At this time, there is little evidence to support
the use of corticosteroids and antiherpetic agents LISTERIOSIS
in EHM. The theoretical benefit of corticosteroids
(dexamethasone, 0.05–0.1 mg/kg i/v q24 h) may be Listeriosis is a rare bacterial infection in horses,
to inhibit the inflammatory response in the infected caused by Listeria monocytogenes. It has been associated
vascular endothelial cells, and its use in EHM cases with septicaemia, abortion, keratoconjunctivitis, diar-
is frequently recommended. The aim of antiherpetic rhoea and also meningoencephalitis. The organism is
therapy is to reduce the cell-associated viraemia and ubiquitous in the environment, and sites of infection
minimise the degree of CNS endothelial cell infec- can include wounds, umbilical remnants, oral (via
tion. Of the antiherpetic drugs available, pharma- contaminated or poorly prepared haylage/silage) or
cokinetic data suggest that valacyclovir (a prodrug nasal mucosa and conjunctiva. Immunocompromise