EQUINE VETERINARY JOURNAL
25 I
E q r t i ~ ewf. J . (1992) 24 (4) 251-252
Editorials Equid herpesvirus abortion - another piece in the pathogenesis puzzle "Absence of evidence is not evidence of absence" (author unknown)
Abortion was the first clinical syndrome to be associated with equid herpesvirus 1 (EHV-I) infection (Dimock and Edwards 1936). The virus was later shown to cause respiratory and neurological disease with inhalation concluded to be the typical route of exogenous infection. In mares experimentally infected by the respiratory route, a leucocyte-associated viraemia was demonstrated, temporally preceding abortion. This finding suggested a likely mechanism whereby virus-infected cells could gain widespread access to tissues. including the foetus (Bryans 1969; Bryans and Prickett 1970). Virus-infected cells which traversed the vasculature of the epitheliochorial placenta could initiate fetal infection. In the past, only foetal pathology occurring as a result of virus infection could be consistently correlated with EHV-I induced abortions. However, an additional pathophysiological mechanism of EHV- I abortion involving exclusively maternal tissues is suggested by the work of Smith and coworkers reported in this issue (p 256). It has long been known that productive viral infection of the foetus with EHV-I results in fatal pathological changes and abortion. A causal relationship between foetal infection and subsequent abortion was established by direct foetal inoculation with infective virus (Doll and Bryans 1962). Foetuses aborted late in gestation may exhibit few or many of a constellation of gross lesions, most notably severe pulmonary oedema and the pathognomonic greyish white foci of necrosis in the liver (Jubb. Kennedy and Palmer 1985). Histologically. typical acidophilic intranuclear inclusion bodies can be seen in a variety of tissues, particularly lung. liver and lymphoreticular organs. These foetuses apparently die of suffocation and an undefined mechanism causes expulsion of a fresh appearing (nonautolysed) specimen (Bryans and Allen 1989). Foetuses infected with EHV-I very near term may be born alive but do not survive due to overwhelming respiratory pathology. Less frequently. ranipant viral damage results in abortion of an autolysed mid-gestational foetus. Postulated reasons for the different appearances of early and late abortions are the development of the foetal immune response or the gestational changes in the sensitivity of the equine placenta to injury (Prickett 1970; Edington, Smyth and Griffiths 1991). The report of Smith and co-workers describes the abortion
of virologically negative fetuses from Welsh pony mares experimentally infected with the AB4 strain of EHV-I. Neither live virus nor viral antigens were detected in 14 of 32 aborted foetuses. Gross and histological examinations of the virus-negative foetuses did not reveal pathological changes attributable to EHV- I . Evidence of viral infection was found, however, in the endometrium of one mare killed soon after abortion. Specifically, histological examination of multiple uterine sites revealed extensive vasculitis with thrombosis and many foci of necrosis. EHV-I antigen was demonstrated by immunostaining throughout the uterus and was localised primarily to endothelial cells. Virus was recovered from a single uterine site and viral DNA was demonstrated in two uterine tissue samples by the polymerase chain reaction (PCR). These findings corroborate the work of Edington el al ( 1991) who found immunohistochemical evidence of EHV- 1 antigen in the uterus of a pregnant pony mare that had been experimentally infected with the Army 183 strain of virus. Again, the virus was localised to the endothelial cells. No virus was detected in this mare's foetus. The evidence from both studies suggests that infection of the endometrium of the mare may, in some instances, play a major role in the pathogenesis of EHV- 1 abortion. These findings bear relevance for both equine clinicians and researchers. From a clinician's viewpoint, the diagnosis of a virus-negative aborted foetus must be viewed with increased caution. The virologically negative foetuses studied by Smith ef al represented early post-infection incidents. Because 9 of 13 mares tested were viraemic at the time of aborting a virologically negative foetus, the authors suggest that attempts to isolate virus from samples of heparinised blood in suspect cases of EHV-I abortion might be advised. Additionally. due to widespread EHV- 1 antigen in the single uterus examined, endometrial biopsy of aborting mares was suggested as a potential diagnostic tool. Time will tell whether these techniques prove useful; they certainly merit testing both in experimental and field situations. The role of maternal pathology in EHV- 1 abortions must now be seriously considered. Foetal infection can no longer be thought of as an absolute prerequisite for EHV-I induced abortion. It is becoming increasingly apparent that the pathogenesis of the several EHV-I clinical syndromes
252
can be related to virally infected endothelial cells. Infection of this cell type was recognised long ago (Anderson and Goodpasture 1942; Westerfield and Dimock 1946). Infected endothelial cells appear to be central to the vasculitis, thrombosis, and associated necrosis characteristic of EHV- 1 lesions in nervous tissue. A similar mechanism may be operative in the uterus of some infected mares. As yet, we have little information which can explain the various outcomes of EHV-I infection. We do not know if the pathological uterine changes reported commonly accompany early post-infection, virus-negative abortions because the sample sizes examined to date have been small. Nor can we state what influences the extent of viral replication in the uterus, the timing or mechanism of placental transfer, or what triggers the abortion event. It will be fascinating to follow further reports. predicated by the pivotal observations of workers in the UK. EILEEN N. OSTLUND Department of Veterinary Science, Gluck Equine Research Center, University of Kentucky, Lexington, Kentucky 40546, USA
EQUINE VETERINARY JOURNAL
References Anderson, K. and Goodpasture, E.W. (1942) Infection of newborn syrian hamsters with the virus of mare abortion. Am. .I. Path. 4.555-559. Bryans. J.T. (1969) On immunity to disease caused by equine herpesvirus 1. J . Am. vet. med. Ass. 155. 294-300. Bryans, J.T. and Allen, G.P. (1989) Herpesviral diseases of the horse. In: Herpesviral Diseases of' Cattle. Horses and Pigs. Ed: G. Wittmann. Kluwer Academic Publishers, Boston. pp 176-229. Bryans. J.T. and Prickett, M.E. (1970) A consideration of the pathogenesis of abortigenic disease caused by equine herpesvirus 1. In: Proc. 2nd Int. Cortj: Eyrtine Infectious Diseases. Paris 1969. Karger, Basel. pp 34-40. Dimock, W.W. and Edwards, P.R. (1936) ,The differential diagnosis of equine abortion with special reference to a hitherto undescribed form of epizootic abortion of mares. Cornell Vet. 26, 23 1-240. Doll, E.R. and Bryans, J.T. (1962) Incubation periods for abortion in equine viral rhinopneumonitis.J . Am. \'el, met/. Ass. 141, 351-354. Edington, N., Smyth. B. and Griffiths, L. (1991) The role of endothelial cell infection in the endometrium, placenta and foetus of equid herpesvirus I (EHV-I) abortions. J . comp. Ptrth. 104,379-387. Jubb, K.V.F., Kennedy, P.C. and Palmer, N . (1985) Putholo.qy o j Domestic Aiiinuls. Academic Press, Inc., San Diego. p 361. Prickett. M.E. (1970) The patholngy of disease caused by equine herpesvirus I . In: Proc. 2nd I n / . Cot$ Eqrtine /rrfec.tiousDiseases. Paris 1969. Karger, Basel. New York. pp 24-33. Westerfield. C. and Dimock, W.W. (1946) The pathology of equine virus abortion.J . Am. vet. men. Ass. 109, 101-11 1 .
EQUINE VETERINARY JOURNAL E9ttine
i
~ J . (1992) . 24
(4) 252-2S3
Videoendoscopic hysteroscopy : advanced technology in practice and research THE ABILITY to look directly into the interior of body organs has always attracted the interest of clinicians, and the first attempts to do so were reported as long ago as the middle of the last century (Aubinais 1864). Collaboration between doctors and physicists subsequently led to the development of flexible fibreoptic endoscopes. These have revolutionised the approach to understanding the physiological and pathological function of organ systems as diverse as the respiratory, gastrointestinal and urogenital tracts and even such small tubular structures as blood vessels. Although originally restricted to human medicine, the fibreoptic endoscope quickly became an essential tool in equine diagnostic medicine, especially to evaluate the horse's respiratory tract (Cook 1974). Its advent was acclaimed as providing "a clear view of a confused field" (Lane 1984). In women, endoscopic examination of the uterus (hysteroscopy) is now a routine diagnostic procedure in infertility investigation (Benedetto, Zona and Sobbrio 1990). Further, the amazing photographs of pre-natal human life in situ obtained by Nilsson (1990). using laparoscopic hysteroscopy, have fascinated both professional and lay people; and they have added notably
to our understanding of the processes involved in the development of a new being. In mares, hysteroscopic examinations have been performed using rigid (Leidl and Schallenberger-Pottiez 1976) and flexible fibreoptic endoscopes (Brandt and Manning 1969; Wilson 1983). mainly as an adjunct to the fertility evaluation of barren mares or to examine visually the effects of intrauterine treatments (Mather et al 1979). However, despite favourable reports on the value of this technique, it has not yet found wide application in routine stud medicine. With the increased economic pressure on the horse breeding industry, a barren mare at the end of the mating season represents a significant financial loss. Therefore, the ability to identify a broodmare's fertility problem, and to be able to assess her breeding potential, are of prime importance to the breeder. A series of three papers by one group of workers on this subject are published in this issue (pp 274, 279, 285). The first two of these describe the considerable technological advances provided by the modern strobe-light electronic imaging videoendoscope when examining barren mares. In Paper I , the hysteroscopic appearance of the normal equine uterus at all stages of the oestrous and annual breeding
25 I
E q r t i ~ ewf. J . (1992) 24 (4) 251-252
Editorials Equid herpesvirus abortion - another piece in the pathogenesis puzzle "Absence of evidence is not evidence of absence" (author unknown)
Abortion was the first clinical syndrome to be associated with equid herpesvirus 1 (EHV-I) infection (Dimock and Edwards 1936). The virus was later shown to cause respiratory and neurological disease with inhalation concluded to be the typical route of exogenous infection. In mares experimentally infected by the respiratory route, a leucocyte-associated viraemia was demonstrated, temporally preceding abortion. This finding suggested a likely mechanism whereby virus-infected cells could gain widespread access to tissues. including the foetus (Bryans 1969; Bryans and Prickett 1970). Virus-infected cells which traversed the vasculature of the epitheliochorial placenta could initiate fetal infection. In the past, only foetal pathology occurring as a result of virus infection could be consistently correlated with EHV-I induced abortions. However, an additional pathophysiological mechanism of EHV- I abortion involving exclusively maternal tissues is suggested by the work of Smith and coworkers reported in this issue (p 256). It has long been known that productive viral infection of the foetus with EHV-I results in fatal pathological changes and abortion. A causal relationship between foetal infection and subsequent abortion was established by direct foetal inoculation with infective virus (Doll and Bryans 1962). Foetuses aborted late in gestation may exhibit few or many of a constellation of gross lesions, most notably severe pulmonary oedema and the pathognomonic greyish white foci of necrosis in the liver (Jubb. Kennedy and Palmer 1985). Histologically. typical acidophilic intranuclear inclusion bodies can be seen in a variety of tissues, particularly lung. liver and lymphoreticular organs. These foetuses apparently die of suffocation and an undefined mechanism causes expulsion of a fresh appearing (nonautolysed) specimen (Bryans and Allen 1989). Foetuses infected with EHV-I very near term may be born alive but do not survive due to overwhelming respiratory pathology. Less frequently. ranipant viral damage results in abortion of an autolysed mid-gestational foetus. Postulated reasons for the different appearances of early and late abortions are the development of the foetal immune response or the gestational changes in the sensitivity of the equine placenta to injury (Prickett 1970; Edington, Smyth and Griffiths 1991). The report of Smith and co-workers describes the abortion
of virologically negative fetuses from Welsh pony mares experimentally infected with the AB4 strain of EHV-I. Neither live virus nor viral antigens were detected in 14 of 32 aborted foetuses. Gross and histological examinations of the virus-negative foetuses did not reveal pathological changes attributable to EHV- I . Evidence of viral infection was found, however, in the endometrium of one mare killed soon after abortion. Specifically, histological examination of multiple uterine sites revealed extensive vasculitis with thrombosis and many foci of necrosis. EHV-I antigen was demonstrated by immunostaining throughout the uterus and was localised primarily to endothelial cells. Virus was recovered from a single uterine site and viral DNA was demonstrated in two uterine tissue samples by the polymerase chain reaction (PCR). These findings corroborate the work of Edington el al ( 1991) who found immunohistochemical evidence of EHV- 1 antigen in the uterus of a pregnant pony mare that had been experimentally infected with the Army 183 strain of virus. Again, the virus was localised to the endothelial cells. No virus was detected in this mare's foetus. The evidence from both studies suggests that infection of the endometrium of the mare may, in some instances, play a major role in the pathogenesis of EHV- 1 abortion. These findings bear relevance for both equine clinicians and researchers. From a clinician's viewpoint, the diagnosis of a virus-negative aborted foetus must be viewed with increased caution. The virologically negative foetuses studied by Smith ef al represented early post-infection incidents. Because 9 of 13 mares tested were viraemic at the time of aborting a virologically negative foetus, the authors suggest that attempts to isolate virus from samples of heparinised blood in suspect cases of EHV-I abortion might be advised. Additionally. due to widespread EHV- 1 antigen in the single uterus examined, endometrial biopsy of aborting mares was suggested as a potential diagnostic tool. Time will tell whether these techniques prove useful; they certainly merit testing both in experimental and field situations. The role of maternal pathology in EHV- 1 abortions must now be seriously considered. Foetal infection can no longer be thought of as an absolute prerequisite for EHV-I induced abortion. It is becoming increasingly apparent that the pathogenesis of the several EHV-I clinical syndromes
252
can be related to virally infected endothelial cells. Infection of this cell type was recognised long ago (Anderson and Goodpasture 1942; Westerfield and Dimock 1946). Infected endothelial cells appear to be central to the vasculitis, thrombosis, and associated necrosis characteristic of EHV- 1 lesions in nervous tissue. A similar mechanism may be operative in the uterus of some infected mares. As yet, we have little information which can explain the various outcomes of EHV-I infection. We do not know if the pathological uterine changes reported commonly accompany early post-infection, virus-negative abortions because the sample sizes examined to date have been small. Nor can we state what influences the extent of viral replication in the uterus, the timing or mechanism of placental transfer, or what triggers the abortion event. It will be fascinating to follow further reports. predicated by the pivotal observations of workers in the UK. EILEEN N. OSTLUND Department of Veterinary Science, Gluck Equine Research Center, University of Kentucky, Lexington, Kentucky 40546, USA
EQUINE VETERINARY JOURNAL
References Anderson, K. and Goodpasture, E.W. (1942) Infection of newborn syrian hamsters with the virus of mare abortion. Am. .I. Path. 4.555-559. Bryans. J.T. (1969) On immunity to disease caused by equine herpesvirus 1. J . Am. vet. med. Ass. 155. 294-300. Bryans, J.T. and Allen, G.P. (1989) Herpesviral diseases of the horse. In: Herpesviral Diseases of' Cattle. Horses and Pigs. Ed: G. Wittmann. Kluwer Academic Publishers, Boston. pp 176-229. Bryans. J.T. and Prickett, M.E. (1970) A consideration of the pathogenesis of abortigenic disease caused by equine herpesvirus 1. In: Proc. 2nd Int. Cortj: Eyrtine Infectious Diseases. Paris 1969. Karger, Basel. pp 34-40. Dimock, W.W. and Edwards, P.R. (1936) ,The differential diagnosis of equine abortion with special reference to a hitherto undescribed form of epizootic abortion of mares. Cornell Vet. 26, 23 1-240. Doll, E.R. and Bryans, J.T. (1962) Incubation periods for abortion in equine viral rhinopneumonitis.J . Am. \'el, met/. Ass. 141, 351-354. Edington, N., Smyth. B. and Griffiths, L. (1991) The role of endothelial cell infection in the endometrium, placenta and foetus of equid herpesvirus I (EHV-I) abortions. J . comp. Ptrth. 104,379-387. Jubb, K.V.F., Kennedy, P.C. and Palmer, N . (1985) Putholo.qy o j Domestic Aiiinuls. Academic Press, Inc., San Diego. p 361. Prickett. M.E. (1970) The patholngy of disease caused by equine herpesvirus I . In: Proc. 2nd I n / . Cot$ Eqrtine /rrfec.tiousDiseases. Paris 1969. Karger, Basel. New York. pp 24-33. Westerfield. C. and Dimock, W.W. (1946) The pathology of equine virus abortion.J . Am. vet. men. Ass. 109, 101-11 1 .
EQUINE VETERINARY JOURNAL E9ttine
i
~ J . (1992) . 24
(4) 252-2S3
Videoendoscopic hysteroscopy : advanced technology in practice and research THE ABILITY to look directly into the interior of body organs has always attracted the interest of clinicians, and the first attempts to do so were reported as long ago as the middle of the last century (Aubinais 1864). Collaboration between doctors and physicists subsequently led to the development of flexible fibreoptic endoscopes. These have revolutionised the approach to understanding the physiological and pathological function of organ systems as diverse as the respiratory, gastrointestinal and urogenital tracts and even such small tubular structures as blood vessels. Although originally restricted to human medicine, the fibreoptic endoscope quickly became an essential tool in equine diagnostic medicine, especially to evaluate the horse's respiratory tract (Cook 1974). Its advent was acclaimed as providing "a clear view of a confused field" (Lane 1984). In women, endoscopic examination of the uterus (hysteroscopy) is now a routine diagnostic procedure in infertility investigation (Benedetto, Zona and Sobbrio 1990). Further, the amazing photographs of pre-natal human life in situ obtained by Nilsson (1990). using laparoscopic hysteroscopy, have fascinated both professional and lay people; and they have added notably
to our understanding of the processes involved in the development of a new being. In mares, hysteroscopic examinations have been performed using rigid (Leidl and Schallenberger-Pottiez 1976) and flexible fibreoptic endoscopes (Brandt and Manning 1969; Wilson 1983). mainly as an adjunct to the fertility evaluation of barren mares or to examine visually the effects of intrauterine treatments (Mather et al 1979). However, despite favourable reports on the value of this technique, it has not yet found wide application in routine stud medicine. With the increased economic pressure on the horse breeding industry, a barren mare at the end of the mating season represents a significant financial loss. Therefore, the ability to identify a broodmare's fertility problem, and to be able to assess her breeding potential, are of prime importance to the breeder. A series of three papers by one group of workers on this subject are published in this issue (pp 274, 279, 285). The first two of these describe the considerable technological advances provided by the modern strobe-light electronic imaging videoendoscope when examining barren mares. In Paper I , the hysteroscopic appearance of the normal equine uterus at all stages of the oestrous and annual breeding
