EQUINE HERPESVIRUS BEST MANAGEMENT PRACTICE

Transcription

1 Vet Times The website for the veterinary profession EQUINE HERPESVIRUS BEST MANAGEMENT PRACTICE Author : PHILIP IVENS Categories : Vets Date : February 24, 2014 PHILIP IVENS explains the various options available for diagnosis and treatment of clinical disease syndromes in horses caused by alpha herpesviruses EH1 and EH4 Summary Equine herpesviruses (EHVs) are some of the most widespread pathogens in the equine world. EHV1 and EHV4 cause diverse and important disease syndromes1. One of these equine herpes myeloencephalopathy (EHM) is potentially increasing in frequency and regarded as an emerging disease. The accurate diagnosis and treatment of EHV is a challenge and an evolving area. Practical biosecurity and herd health measures are critical to the prevention of EHV and the critical, but complicated role of vaccination is discussed. THERE are nine different equine herpesviruses (EHVs) EHV1 to EHV5 infect the domestic horse, while EHV6 to EHV9 are associated with infections in wild equids, including asses and zebra. The alpha herpesviruses EHV1 and EHV4 are respiratory pathogens, and are also responsible for abortion and neurological disease. EHV3 is a venereal pathogen causing coital exanthema and will not be discussed in this article. The gamma herpesviruses EHV2 and EHV5 may be clinically important in certain situations 1 / 14

2 causing ocular and respiratory disease. The management of these are beyond the scope of this article and we will focus on the commonest and most important clinical pathogens, EHV1 and EHV4 1. Different syndromes EHV1 is associated with three clinical disease syndromes: respiratory disease; abortion; and neurological disease equine herpes myeloencephalopathy (EHM). The virulence of individual isolates shows considerable variation. There are neuropathogenic and non-neuropathogenic strains; however, both can cause EHM and the differentiation is not very useful clinically 2. EHV4 is principally a cause of respiratory disease, although some highly virulent endotheliotropic, abortigenic strains exist. Diagnosis History and clinical signs are important, but usually insufficient on their own to confirm diagnosis. EHV infections often do not cause clinically apparent respiratory disease, and even then there is little to distinguish EHV respiratory disease from other viral and bacterial pathogens. A common misconception is monitoring for respiratory clinical signs provides a warning for impending abortion or EHM; most horses that abort or develop neurological disease after EHV infection do not show signs of respiratory disease first. When sampling, case selection is critical all samples for direct demonstration of virus should be collected from early clinical cases (ideally less than five days) whenever possible. This is best achieved by identifying in-contact cases with pyrexia that may have not yet shown any other clinical signs, although samples from clinical cases may also be taken especially if there appears to be only one horse affected. Proper selection of cases for sampling is particularly important in EHM cases where clinical signs appear towards the end of viraemia and viral shedding is waning or may have ceased. Samples 2 / 14

3 Nasopharyngeal swab A nasopharyngeal swab is a long, highly absorbent swab ( Figure 1 ) passed up the ventral meatus of the nasal passageways to the level of the medial canthus to ensure adequate sampling of the nasopharyngeal mucosa and placed in a viral transport medium (for example, a white top tube from the Animal Health Trust). The sample should be refrigerated and time minimised before it arrives at the laboratory. Quantitative polymerase chain reaction (qpcr): detects viral DNA, is sensitive, allows estimation of the amount of virus ( viral load ) in the sample 3-5 and gives quick results. Immunofluorescence (IF): detects viral antigen expressed on the surface of infected cells. It is quick, but not as sensitive as qpcr and does not give an estimate of the amount of virus. Virus isolation: traditionally regarded as the gold standard test, but is slow and insensitive compared to qpcr and IF. The virus is grown in cell cultures inoculated with the supernatant from the sample. This usually takes five to seven days of culture. Blood qpcr: to detect virus DNA in buffy coat cells (mainly lymphocytes). Serology: to detect antibody induced by infection. IgM: detectable four to five days after infection, peaking at 20 to 30 days and returning to baseline between 60 to 80 days 6. It is measured by complement fixation (CF) 7, 8. IgG: detectable eight to 10 days after infection, peaking at 30 to 40 days and persisting for many months (greater than nine months). It is measured by virus neutralisation (VN) or ELISA (this can differentiate between EHV1 and EHV4). Take a baseline sample and repeat the sample 10 to 14 days later 5. Significant increase in IgM antibody level is usually taken as threefold to fourfold increase. A high complement fixation titre (IgM) in a single serum sample from a non-vaccinated horse provides good preliminary evidence of infection and is a valuable initial diagnostic test in suspected EHM cases. Note, previous vaccination and maternal antibodies confound the interpretation of serological investigations. Haematology: non-specific and difficult to interpret. There is initial (first seven to 10 days) transient leukopaenia with lymphopaenia, which is replaced by a leukocytosis with lymphocytosis up to 21 days after infection. Tissue (for example, fetal, placental 9 or CNS) 3 / 14

4 Histopathology: characteristic eosinophilic inclusion bodies or vasculitis and often thrombosis of CNS blood vessels 10, 11. Immunohistochemistry (IHC): paraffin-embedded, formalin-fixed tissue that demonstrates the viral antigen. In situ hybridisation (ISH): paraffin-embedded, formalin-fixed tissue that demonstrates viral DNA. PCR: Fresh, frozen and fixed tissue samples. IF: Frozen sections from aborted fetal and placental tissues. Cerebrospinal fluid: EHM cases Cerebrospinal fluid (CSF) may be xanthochromic (yellow discolouration; Figure 2 ). Increased total protein without a concomitant increase in total white cell plus characteristic clinical signs are suggestive, but not diagnostic of, EHM. Antibodies to EHV1 in the CSF may be from leakage from the vasculitis, and may not be from local production, so this does not definitively confirm EHM, only EHV1 exposure. Treatment Respiratory disease Respiratory disease is generally mild and self-limiting, and does not require specific treatment. Rest, dust-free management and biosecurity measures to stop transmission are indicated. Broad-spectrum antibiotics are often administered, but seldom indicated to prevent secondary antimicrobial infections; clenbuterol stimulates mucociliary clearance, but is usually not required; and dembrexine acts as a mucolytic, but is usually not required. Respiratory disease is preventive when stress or mixing has to take place and there is potential value in immunostimulants, such as parapox ovis. Abortion There is no evidence treatment of in-contact mares with antiviral agents (for example, nucleoside analogues) prevents abortions. Ensure the whole placenta has been passed (this is usually the case); in the rare event this does not occur, treat for retained fetal membranes. Treat with rigorous biosecurity (see further on). 4 / 14

5 EHM 12 For EHM, provide adequate bedding to prevent trauma and a quiet environment to prevent excitement. Recumbent horses can be nursed successfully in slings (for example, an Anderson sling; Figure 3 ). Indwelling Foley catheter in horses with bladder paralysis, urinary retention and overflow. Apply petroleum jelly around the perineum, along with an extension line to direct urine away and prevent scalding. Maintain sterility, but cystitis is a common complication and antimicrobial therapy is often indicated. Faecal evacuation if there is faecal incontinence. Fluids and indwelling feeding tubes. NSAIDs: for example, flunixin meglumine 1.1mg/ kg IV, twice a day. Corticosteroids: for example, sodium phosphate dexamethasone 0.1mg/kg IV, once a day. Antioxidants: vitamin E, dimethyl sulfoxide (DMSO) and thiamine. Nucleoside analogues ( Figure 4 ): acyclovir: 10mg/kg orally, five times daily. There is poor bioavailability and questionable efficacy with EHV1; valaciclovir: 20mg/kg to 40mg/kg orally, three times a day. There is better bioavailability and efficacy than acyclovir, but it is more expensive; and ganciclovir: 2.5mg/kg IV, every eight hours for the first 24 hours, then orally every 12 hours. Provides the best in-vitro testing of all nucleoside analogues, but is expensive. Prevention of spread of disease EHV1 disease control programmes have the following three goals (see the Horserace Betting Levy Board [HBLB] code of practice at Prevention of disease entry to premises Prevention of disease entry is difficult because the majority of horses carry latent EHV infections. 5 / 14

6 To reduce the risk of disease entry, new arrivals should ideally have been vaccinated before arrival and be kept isolated from other horses until sufficient time has passed for disease to become apparent. On studs, newly arrived and walk-in mares should be kept strictly separate from resident in-foal mares for 56 days after covering. Mares arriving at studs to foal should be transported at least 28 days before the foaling due date. Horses that have arrived from sales or markets are high risk and should have more stringent isolation and biosecurity measures. In yards with no pregnant mares, an isolation period of at least 21 days is advised because viral shedding may occur after reactivation of the virus induced by the stress of moving. Minimising stress in resident horses including while being transported, disruption of established social groups and at weaning should assist in reducing the frequency of reactivation of the virus from the latent state. Limiting the spread and severity of disease To limit the spread and severity of disease, different age groups should not be mixed and group size should be kept as small as is practical. Pregnant mares should be separated from other horses and kept in small groups to minimise the risk of a large-scale outbreak. Mares in their last trimester should be ideally housed and managed individually. Isolation areas should be geographically separate and be rigorously maintained. If a suspect case occurs, the horse should be isolated immediately and appropriate samples taken. Any in-contacts should be isolated and monitored for disease (take twice-daily temperatures). If the in-contact group is large, and it is practical to do so, it should be subdivided. In terms of environmental contamination, the virus can survive for limited periods, depending on the surface and prevailing weather conditions. All discharges from affected horses should be removed and the area disinfected with approved disinfectant. Bedding should be burned. Stop all movements by isolating aborted mares for 28 days and not mixing them with pregnant mares for 56 days. EHM horses should be kept isolated for minimum of 14 days 18 and sometimes up to 28 days to account for the maximum possible duration for viraemia. Testing of viral shedding can be undertaken to shorten the isolation period. Movement of all horses on and off the premises should stop for a period of 28 days. Limiting spread to adjacent properties To limit any spreading to adjacent properties, there should be efficient communication between 6 / 14

7 attending vets, premises owners and other parties working with the affected premises, as well as care with personnel and fomites, where easy and clear biosecurity measures should be implemented. Vaccination: pros and cons There are two EHV vaccines licensed in the UK. The one used presently is an inactivated vaccine containing EHV1 and EHV4. The other (currently off the market) is also an inactivated vaccine containing EHV1 and EHV4 and inactivated influenza virus. Worldwide, there are 10 killed commercial EHV vaccines available (eight in the United States and two in Europe) and two live vaccines (one in the United States and one in Europe). None of these vaccines induce a perfect sterile immunity (complete clinical and virological protection) that prevents all disease syndromes and the development of a more effective EHV vaccine is a priority worldwide in EHV research. However, it is important to be clear that vaccination with current vaccines has an important role to play in reducing the risk and impact of clinical disease. The UK vaccines induce high titres of complement fixating and virus neutralisation antibodies. They reduce the duration and titre of nasal virus shedding of virus, but there are contradictory reports of the ability of killed vaccines to reduce viraemia and hence stop abortion 19, 20. With regards to abortion, the field data is unclear. The introduction of vaccination in the early 1960s coincided with a marked reduction in EHV1 abortion rates across the world 21 ; however, simultaneously vigorous hygiene measures were introduced and the relative impact of both is impossible to assess in light of the lack of randomised, controlled field studies. Current vaccines do not make any claims for efficacy against EHM because the syndrome is difficult to reproduce experimentally and there are no reliable field data on the effect of vaccination of prevention of EHM because the syndrome is uncommon and largescale outbreaks are infrequently reported in the UK. The reasonable expectation of current vaccines should not be to produce sterile immunity, but rather to reduce the severity of clinical disease (respiratory) and limit virus shedding from infected horses, thus reducing contagion. Vaccines should therefore be used to supplement hygiene control measures, which, as previously discussed, have a central role in reducing exposure to the virus. The use of vaccination in prevention of EHM is even less clear as this is, thankfully, rare and assessing vaccine efficacy is more difficult. Vaccinated horses do get EHM and none of the current UK vaccines are licensed to protect against EHM. The case for vaccination is further complicated by the initial analysis of a large outbreak in 2003 at Ohio State University, which seemed to show an increase risk of EHM in vaccinated horses / 14

8 This also seemed to support the then theory of immune-mediated pathogenesis for EHM. We now know EHM is caused by vascular endothelium cell death and local ischaemia causing neuronal cell death. We also know old horses (greater than 15 years old) are a potential experimental model for EHM. When age is accounted for in the Ohio data, the increase in risk of vaccination is removed and, therefore, the direct causality of EHV vaccination and EHM is unclear and could easily be accounted for by age. The number of times a horse is vaccinated is only one factor that affects the horse immune system, and it may be that age-related changes (immunosenescence) have an important role to play. Further investigation is required to elucidate this interaction further. It is the author s opinion that general vaccination at herd level (that is, all horses on a yard) is beneficial in decreasing nasal shedding of the virus and, therefore, environmental contagion. Yard vaccination in the face of an EHM outbreak is contra-indicated as the state of infection/immune response of the individuals will be unknown. Vaccination of closely associated horses (for example, geographically), but separate from the present outbreak is potentially beneficial and should be judged on an individual basis in consultation with the testing laboratory and/or epidemiological input. However, the fact remains vaccination is a valuable tool used appropriately to reduce environmental contagion and disease incidence integrated into biosecurity and hygiene measures. Note some drugs mentioned within this article are unlicensed for use in horses and are used under the cascade. Acknowledgement Thanks to J Slater for reading this manuscript. References 1. Patel J R and Heldens J (2005). Equine herpesviruses 1 (EHV-_) and 4 (EHV-4) epidemiology, disease and immunoprophylaxis: a brief review, Vet J 170(1): Lunn D P, Davis-Poynter N, Flaminio M J et al (2009). Equine herpesvirus-1 consensus statement, J Vet Intern Med 23(3): Daly P and Doyle S (2003). The development of a competitive PCR-ELISA for the detection of equine herpesvirus-1, J Virol Methods 170(2): Perkins G A, Goodman L B, Dubovi E J, Kim S G and Osterrieder N (2008). Detection of equine herpesvirus-1 in nasal swabs of horses by quantitative real-time PCR, J Vet Intern Med 22(5): 1,324-1, / 14

9 5. Hussey S B, Clark R, Lunn K F et al (2006). Detection and quantification of equine herpesvirus-1 viremia and nasal shedding by real-time polymerase chain reaction, J Vet Diagn Invest 18(4): Thomson G R, Mumford J A, Campbell J, Griffiths L and Clapham P (1976). Serological detection of equid herpesvirus 1 infections of the respiratory tract, Equine Vet J 8(2): Doll E R, Wallace M E, Bryans J T and Richards M G (1953). Complement-fixation antibody response following administration of equine virus abortion vaccine, Am J Vet Res 14(50): Doll E R, McCollum W H, Wallace M E, Bryans J T and Richards M G (1953). Complement-_ xation reactions in equine virus abortion, Am J Vet Res 14(50): Smith K C, Whitwell K E, Blunden A S et al (2004). Equine herpesvirus-1 abortion: atypical cases with lesions largely or wholly restricted to the placenta, Equine Vet J 36(1): Whitwell K E and Blunden A S (1992). Pathological findings in horses dying during an outbreak of the paralytic form of equid herpesvirus type 1 (EHV-1) infection, Equine Vet J 24(1): Whitwell K E, Gower S M and Smith K C (1992). An immunoperoxidase method applied to the diagnosis of equine herpesvirus abortion, using conventional and rapid microwave techniques, Equine Vet J 24(1): Goehring L S, Landolt G A and Morley P S (2010). Detection and management of an outbreak of equine herpesvirus type 1 infection and associated neurological disease in a veterinary teaching hospital, J Vet Intern Med 24(5): 1,176-1, Carmichael R J, Whit_ eld C and Maxwell L K (2013). Pharmacokinetics of ganciclovir and valganciclovir in the adult horse, J Vet Pharmacol Ther 36(5): Garré B, van der Meulen K, Nugent J et al (2007). In vitro susceptibility of six isolates of equine herpesvirus 1 to acyclovir, ganciclovir, cidofovir, adefovir, PMEDAP and foscarnet, Vet Microbiol 122(1-2): Garré B, Shebany K, Gryspeerdt A et al (2007). Pharmacokinetics of acyclovir after intravenous infusion of acyclovir and after oral administration of acyclovir and its prodrug valacyclovir in healthy adult horses, Antimicrob Agents Chemother 51(12): 4,308-4, Garré B, Gryspeerdt A, Croubels S, De Backer P and Nauwynck H (2009). Evaluation of orally administered valacyclovir in experimentally EHV1-infected ponies, Vet Microbiol 135(3-4): Garré B, Baert K, Nauwynck H, Deprez P, De Backer P and Croubels S (2009). Multiple oral dosing of valacyclovir in horses and ponies, J Vet Pharmacol Ther 32(3): Burgess B A, Tokateloff N, Manning S et al (2012). Nasal shedding of equine herpesvirus-1 from horses in an outbreak of equine herpes myeloencephalopathy in Western Canada, J Vet Intern Med 26(2): Bresgen C, Lämmer M, Wagner B, Osterrieder N and Damiani A M (2012). Serological responses and clinical outcome after vaccination of mares and foals with equine herpesvirus type 1 and 4 (EHV-1 and EHV-4) vaccines, Vet Microbiol 160(1-2): Goehring L S, Wagner B, Bigbie R et al (2010). Control of EHV-1 viremia and nasal 9 / 14

10 shedding by commercial vaccines, Vaccine 28(32): 5,203-5, Doll E R and Bryans J T (1963). A planned infection program for immunizing mares against viral rhinopneumonitis, Cornell Vet 53: Henninger R W, Reed S M, Saville W J et al (2007). Outbreak of neurologic disease caused by equine herpesvirus-_ at a university equestrian centre, J Vet Intern Med 21(1): Suggested further reading Lunn D P, Davis-Poynter N, Flaminio M J, Horohov D W, Osterrieder K, Pusterla N and Townsend H G (2009). Equine herpesvirus-1 consensus statement, J Vet Intern Med 23: Figure 1. A nasopharyngeal swab. 10 / 14

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12 Figure 2. Yellow discolouration of xanthochromic cerebrospinal fluid (CSF; right) in an EHM case. 12 / 14

13 Figure 3. An EHM case being managed in an Anderson sling. IMAGE: Courtesy of A Draper MRCVS. 13 / 14

14 Powered by TCPDF ( Figure 4. Valaciclovir treatment being used in a quarantine area to treat EHM cases. IMAGE: Courtesy of A Draper MRCVS. 14 / 14