Introduction
Abortion associated with equine herpesvirus type 1 (EHV1) has occurred sporadically in horses in Australia since 1977. Prior to then only the respiratory form of the disease has been recognised in this country. Abortions due to EHV1 are common in England and the USA and the virus is found worldwide. Cases of EHV abortion have been confirmed in Queensland in mares allegedly coming from southern states where other cases of abortion due to EHV1 have been diagnosed.
While there are many causes for abortion in mares, EHV1 is a serious contagious disease and one that horse breeders should be aware of. Basic information on the disease is provided below to assist breeders to institute sensible precautions to prevent the introduction of the disease and to recognise and manage the disease should it occur on their farm.
In Queensland, EHV abortion is a notifiable disease and must be reported to the Department of Primary Industries and Fisheries.
The cause
There are five herpes viruses that cause disease in horses and donkeys in Australia. Of the equine herpesviruses types 1 to 5, only EHV1 causes outbreaks of abortion. EHV1 is also a common cause of respiratory disease in young horses and has occasionally caused neurological disease with incoordination, paralysis and subsequent death. EHV3 causes equine coital exanthema which is a mild vaginal infection spread venereally. EHV4 is a major cause of respiratory infection (usually called rhinopneumonitis) and EHV2 and EHV5 have been associated with mild respiratory disease. There is no evidence that any of these equine viruses will affect humans.
The signs
Abortion in horses due to EHV1 usually occurs between 7 and 11 months of gestation, and occasionally, as early as 4 months in gestation. The incubation period is highly variable with abortions occurring from 10 days to 12 weeks after virus infection. The mare seldom shows signs of respiratory infection or other illness.
Abortion occurs suddenly with no warning signs and the foetus is expelled sometimes still covered with the placenta.
Mares infected late in pregnancy may have a live foal that may be weak, jaundiced and have difficulty breathing, seldom surviving more than a few days.
Sources of infection
EHV1 virus is endemic in Australia. It is one of the common causes of 'colds' in young horses and most have antibodies to it by the time they are 2-3 years old. Older horses seldom show clinical signs of infection. Once infected, horses carry the virus for life and when they are stressed, the virus is reactivated and excreted.
It is unclear whether there is a separate strain of EHV1 virus that causes abortion or whether there is only one strain which, for some unknown reason, moves beyond the respiratory system.
Infection leading to both respiratory disease and abortion occurs mainly by inhalation but also by ingestion. The aborted foetus, foetal membranes and fluids and uterine discharges from the mare contain large amounts of infective virus. Nasal and eye discharges and aerosols from the respiratory tract of a mare that has aborted, clinically affected young horses and carrier horses are also sources of virus infection. Virus contamination of pasture, feed, feed bins, halters, rugs, bedding, transport vehicles and staff clothing can lead to infection. Weak EHV1 infected foals can be a source of infection to other foals that were born healthy.
After aborting, the virus in the mare's genital tract disappears in a few days, but she may shed virus through the respiratory route for up to 2 weeks. Virus may remain infective in the environment and on horse hair for up to 2 weeks in cool moist conditions if cleaning and disinfection are not adequate.
Actions if an aborted foal is found
Until the cause of the abortion is known, it is prudent to take action to reduce the potential risk of spread of EHV1 infection to mares in close contact with the aborted mare. Initial measures are:
* Isolate the aborted mare from contact with all other horses
* Leave her halter and lead rope with her in quarantine
* Call your veterinary surgeon
* Wear gloves to pick up the foetus and placenta and put it in a strong plastic bag
* Keep the foetus and placenta cool (not frozen) for the veterinarian to examine and to preserve it until it, or samples from it, can be sent to the laboratory
* If pasture has been contaminated with foetal fluids, the area should be treated with lime and fenced off
Anyone who has had contact with the aborted mare or the foetus or placenta should
* Disinfect hands and boots with a good surface disinfectant e.g. iodophors (Iovone Scrub), hexachlorophenes (Hibitane, or other chlorhexidine disinfectant) or phenols (Polyphen Polyphenolic Microbiocide)
* Shower and change clothes. Washing and sun drying clothes will effectively kill the virus
* Disinfect the vehicle used to transport the aborted mare to quarantine (inside and wheels) and anything that has come in contact with the foetus or foetal fluids or discharge from the mare
* Keep the mare completely isolated until a diagnosis has been made. If possible avoid entering the quarantine yard at all or use a 5% formalin footbath outside the yard to disinfect footwear
* Provide the quarantined mare with her own feed bin and other equipment and do not use them for any other horse
* The quarantined mare should be cared for by someone who does not work with other horses or who disinfects, showers and changes before they do
Treatment if EHV1 is confirmed
There is no specific treatment for EHV1 infection in mares. Most foals infected with EHV1 before birth succumb within a few days despite nursing and antibiotic administration.
Management if EHV1 is confirmed
* The aborted mare must be isolated for at least 30 days. After that (provided there are no further abortions) she can be treated as a dry mare and bred.
* In contact pregnant mares should be isolated until they foal and for 30 days after foaling or if another mare aborts, 30 days after the last abortion
* In contact mares include those in the same paddock as the aborted foetus and those in direct contact with the mares in the same paddock e.g. sharing a water trough or in direct contact over a single fence line
* Consult your veterinarian about a vaccination program. A combined EHV1/EHV4 vaccine (Duvaxyn EHV 1,4) is available. While this does not induce total immunity to these viruses, it may prevent some abortions and it will reduce the risk of spread of infection.
* Communication with other stud farms is essential. Simple precautions will reduce the risk of disease spread to other farms and minimise disruption to horse movements usual during the stud season.
Where there is only one case of abortion and the mare has been rigorously isolated and thorough disinfection has occurred, there is minimal risk of spread of infection and the management of the unexposed mares can continue as normal.
The long term prospects for the mare that aborted are good. Her conception rate is not adversely affected and it is rare her to abort again due to EHV1.
Preventative management
Disease introduction can be prevented and disease spread restricted by good management practices such as the following:
* Keep weanlings and yearlings and non-breeding stock in a separate part of the farm with their own facilities and staff
* Separate pregnant mares from non-pregnant mares and from mares with foals
* Keep resident mares separated from mares visiting only for the stud season
* Isolate visiting mares on arrival at stud for 3-4 weeks. Keep them in small groups of 2 or 3 which can be combined if they remain disease free
* Minimise stress factors for pregnant mares such as overcrowding and transport in the last 2 months of gestation and provide access to good quality feed and ensure adequate parasite control
* Establish isolation and quarantine facilities and equip it with gloves, plastic bags, disinfectant etc.
* Clean and disinfect horse floats after each journey
* Train staff so they understand about quarantine and the principles of preventing disease spread
* Keep accurate records of mares (e.g. date of arrival, where from, previous stud details) and of movements of horses on the farm. These are essential for identifying the source of infection
* Vaccination of pregnant mares in the 5th, 7th and 9th months of pregnancy will help reduce the risk of abortion
Abortion associated with equine herpesvirus type 1 (EHV1) has occurred sporadically in horses in Australia since 1977. Prior to then only the respiratory form of the disease has been recognised in this country. Abortions due to EHV1 are common in England and the USA and the virus is found worldwide. Cases of EHV abortion have been confirmed in Queensland in mares allegedly coming from southern states where other cases of abortion due to EHV1 have been diagnosed.
While there are many causes for abortion in mares, EHV1 is a serious contagious disease and one that horse breeders should be aware of. Basic information on the disease is provided below to assist breeders to institute sensible precautions to prevent the introduction of the disease and to recognise and manage the disease should it occur on their farm.
In Queensland, EHV abortion is a notifiable disease and must be reported to the Department of Primary Industries and Fisheries.
The cause
There are five herpes viruses that cause disease in horses and donkeys in Australia. Of the equine herpesviruses types 1 to 5, only EHV1 causes outbreaks of abortion. EHV1 is also a common cause of respiratory disease in young horses and has occasionally caused neurological disease with incoordination, paralysis and subsequent death. EHV3 causes equine coital exanthema which is a mild vaginal infection spread venereally. EHV4 is a major cause of respiratory infection (usually called rhinopneumonitis) and EHV2 and EHV5 have been associated with mild respiratory disease. There is no evidence that any of these equine viruses will affect humans.
The signs
Abortion in horses due to EHV1 usually occurs between 7 and 11 months of gestation, and occasionally, as early as 4 months in gestation. The incubation period is highly variable with abortions occurring from 10 days to 12 weeks after virus infection. The mare seldom shows signs of respiratory infection or other illness.
Abortion occurs suddenly with no warning signs and the foetus is expelled sometimes still covered with the placenta.
Mares infected late in pregnancy may have a live foal that may be weak, jaundiced and have difficulty breathing, seldom surviving more than a few days.
Sources of infection
EHV1 virus is endemic in Australia. It is one of the common causes of 'colds' in young horses and most have antibodies to it by the time they are 2-3 years old. Older horses seldom show clinical signs of infection. Once infected, horses carry the virus for life and when they are stressed, the virus is reactivated and excreted.
It is unclear whether there is a separate strain of EHV1 virus that causes abortion or whether there is only one strain which, for some unknown reason, moves beyond the respiratory system.
Infection leading to both respiratory disease and abortion occurs mainly by inhalation but also by ingestion. The aborted foetus, foetal membranes and fluids and uterine discharges from the mare contain large amounts of infective virus. Nasal and eye discharges and aerosols from the respiratory tract of a mare that has aborted, clinically affected young horses and carrier horses are also sources of virus infection. Virus contamination of pasture, feed, feed bins, halters, rugs, bedding, transport vehicles and staff clothing can lead to infection. Weak EHV1 infected foals can be a source of infection to other foals that were born healthy.
After aborting, the virus in the mare's genital tract disappears in a few days, but she may shed virus through the respiratory route for up to 2 weeks. Virus may remain infective in the environment and on horse hair for up to 2 weeks in cool moist conditions if cleaning and disinfection are not adequate.
Actions if an aborted foal is found
Until the cause of the abortion is known, it is prudent to take action to reduce the potential risk of spread of EHV1 infection to mares in close contact with the aborted mare. Initial measures are:
* Isolate the aborted mare from contact with all other horses
* Leave her halter and lead rope with her in quarantine
* Call your veterinary surgeon
* Wear gloves to pick up the foetus and placenta and put it in a strong plastic bag
* Keep the foetus and placenta cool (not frozen) for the veterinarian to examine and to preserve it until it, or samples from it, can be sent to the laboratory
* If pasture has been contaminated with foetal fluids, the area should be treated with lime and fenced off
Anyone who has had contact with the aborted mare or the foetus or placenta should
* Disinfect hands and boots with a good surface disinfectant e.g. iodophors (Iovone Scrub), hexachlorophenes (Hibitane, or other chlorhexidine disinfectant) or phenols (Polyphen Polyphenolic Microbiocide)
* Shower and change clothes. Washing and sun drying clothes will effectively kill the virus
* Disinfect the vehicle used to transport the aborted mare to quarantine (inside and wheels) and anything that has come in contact with the foetus or foetal fluids or discharge from the mare
* Keep the mare completely isolated until a diagnosis has been made. If possible avoid entering the quarantine yard at all or use a 5% formalin footbath outside the yard to disinfect footwear
* Provide the quarantined mare with her own feed bin and other equipment and do not use them for any other horse
* The quarantined mare should be cared for by someone who does not work with other horses or who disinfects, showers and changes before they do
Treatment if EHV1 is confirmed
There is no specific treatment for EHV1 infection in mares. Most foals infected with EHV1 before birth succumb within a few days despite nursing and antibiotic administration.
Management if EHV1 is confirmed
* The aborted mare must be isolated for at least 30 days. After that (provided there are no further abortions) she can be treated as a dry mare and bred.
* In contact pregnant mares should be isolated until they foal and for 30 days after foaling or if another mare aborts, 30 days after the last abortion
* In contact mares include those in the same paddock as the aborted foetus and those in direct contact with the mares in the same paddock e.g. sharing a water trough or in direct contact over a single fence line
* Consult your veterinarian about a vaccination program. A combined EHV1/EHV4 vaccine (Duvaxyn EHV 1,4) is available. While this does not induce total immunity to these viruses, it may prevent some abortions and it will reduce the risk of spread of infection.
* Communication with other stud farms is essential. Simple precautions will reduce the risk of disease spread to other farms and minimise disruption to horse movements usual during the stud season.
Where there is only one case of abortion and the mare has been rigorously isolated and thorough disinfection has occurred, there is minimal risk of spread of infection and the management of the unexposed mares can continue as normal.
The long term prospects for the mare that aborted are good. Her conception rate is not adversely affected and it is rare her to abort again due to EHV1.
Preventative management
Disease introduction can be prevented and disease spread restricted by good management practices such as the following:
* Keep weanlings and yearlings and non-breeding stock in a separate part of the farm with their own facilities and staff
* Separate pregnant mares from non-pregnant mares and from mares with foals
* Keep resident mares separated from mares visiting only for the stud season
* Isolate visiting mares on arrival at stud for 3-4 weeks. Keep them in small groups of 2 or 3 which can be combined if they remain disease free
* Minimise stress factors for pregnant mares such as overcrowding and transport in the last 2 months of gestation and provide access to good quality feed and ensure adequate parasite control
* Establish isolation and quarantine facilities and equip it with gloves, plastic bags, disinfectant etc.
* Clean and disinfect horse floats after each journey
* Train staff so they understand about quarantine and the principles of preventing disease spread
* Keep accurate records of mares (e.g. date of arrival, where from, previous stud details) and of movements of horses on the farm. These are essential for identifying the source of infection
* Vaccination of pregnant mares in the 5th, 7th and 9th months of pregnancy will help reduce the risk of abortion
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