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Diagnosis of neurological EHV

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01 November 2011, at 12:00am

MARION McCULLAGH presents the second in her series of reports on the September congress of the British Equine Veterinary Association, covering the clinical pathology session

ANDY Durham of the Liphook Equine Hospital described “How to diagnose: Neurological EHV” in the clinical pathology session at the BEVA congress, chaired by Keith Chandler. Equine herpesvirus-1 (EHV-1) is the most clinically important of the equine herpes viruses and it is probably responsible for all the cases of equine herpesvirus myeloencephalopathy (EHM). All horses will encounter EHV-1: respiratory disease or subclinical infections are the most usual outcome, while abortion, neonatal death and EHM are seen less frequently. Around 90% of the horse population are latent carriers of EHV-1 and various types of stress can cause virus shedding. EMH is likely to occur in older horses (over three to five years); horses are more at risk than ponies; females more than males; and horses that have been vaccinated previously are 10 times more likely to get the disease. The D752 strains of EHV-1 give a severe viraemia which has high magnitude and duration which allows the infection to get into the endothelial cells of the spinal cord. These strains are involved in over 75% of cases of EHM. Repeat attacks of EHM are not reported. Ataxia and weakness, the main neurological signs of EHM, may be preceded by respiratory signs, pyrexia or abortion. Morbidity ranges from 10 to 90%, mortality from 5 to 30%. In the last four years there have been 35 cases in 15 outbreaks in the UK. The condition develops over one or two days and then stabilises. Horses that are recumbent for more than 24 hours are unlikely to recover. Horses need good nursing and may need to be supported in slings. Cauda equina signs such as urinary retention, faecal incontinence and tail flaccidity may be seen with cranial and cerebral signs, such as altered mentation and head pressing seen less frequently. Differentials include cervical myopathy, grass staggers and sacral fracture, with sacral fracture most likely to be seen in young horses. Virus isolation is slow but sure as it takes from three to seven days for the laboratory to grow the virus. Gauze swabs, either nasal or nasopharyngeal, need to be transported in virus transport media, or virus can be recovered from buffy coat samples taken from heparinised blood. In-contact horses should be sampled. Polymerase chain reaction (PCR) gives a result more quickly and it is more sensitive and specific than virus isolation. Samples are used in the same way as for virus isolation, or a dry swab can be sent in a sterile tube, but it is always worth discussing the method of sampling with the laboratory so that the best protocol is chosen. Serology should give an increase in titre of at least four-fold in samples taken 10 to 20 days apart, though this information is retrospective. Cerebrospinal fluid changes are inconsistent and not specific. There may be yellow colouring, increased protein concentration and possibly an increase in monocytes. Post mortem examination of brain and spinal cord shows vasculitis and thrombosis of small blood vessels. Immunohistochemistry, in situ hybridisation and PCR can help to confirm the presence of virus. The Horserace Betting Levy Board Code of Practice www.hblb.org.gov.uk needs to be read and a copy given to whoever is in charge of the horse. 

Strangles

Richard Newton of the Animal Health Trust gave his advice on “How to diagnose strangles”. He said, “If it looks like strangles, treat it as strangles.” Those are the easier cases, the problem lies in sorting out an atypical endemic problem so that efficient control methods can be used and freedom from disease  established reliably. A carrier horse can remain healthy in an outbreak, due to its underlying immunity. The disease is caused by Streptococcus equi which shares ancestry with the very closely related S. zooepidemicus. These are Gram-positive, Lancefield group C, beta-haemolytic bacteria and they look alike and behave similarly in culture. Research into their genomes has allowed identification by PCR but the well-tried methods of serology, bacterial culture and biochemical behaviour are still in regular use. Swabs can be taken from nostrils or nasopharynx, washes from nostrils or guttural pouches or pus can be aspirated from abscesses. Samples are grown on selective culture media and the growth patterns such as fermentation of sugars allows confirmation of the identity of S. equi. If these techniques give a negative answer, the bacteria may have died in transit or the wrong sites may have been sampled. In experimental studies, S. equi cleared from the nasopharynx within 24 hours of inoculation and could be found by needle aspiration of the local lymph nodes before the glands burst. The original PCR assay developed in the 1990s was useful in detecting persistently-infected horses but it became apparent that it was insufficiently sensitive and so there was a risk of this test giving false negative results. Now we have a quantitative, real-time PCR assay which distinguishes between S. equi and S. zooepidemicus and gives the answer within a few hours. However, it reacts to dead bacteria as well as live ones so a positive PCR needs to be confirmed by culture. Some samples can give negative PCR results but be positive on culture. The ELISA serum antibody blood test has been refined so that it  reacts to surface proteins which are specific to S. equi. These proteins give a strong response during the acute phase of the disease though early testing may give false negatives and  recovered noninfectious horses will test positive as the test is demonstrating the horse’s immune response rather than the presence of the bacteria itself. It fails to distinguish the vaccine
strain from true infection. It is a valuable tool when it is used to assess the herd situation either following an outbreak or before introducing horses to new premises. All these methods involve sending samples to a suitable laboratory which means that there is a time lag between sampling and results. The Animal Health Trust has worked with commercial partners to develop pointof-
care methods that can be done “beside the horse”. The PCR test uses a disposable test card which is put into a machine and this  gives a result in less than 30 minutes. The serum antibody blood test uses a lateral flow device which gives a result in about 10 minutes.