Equine Herpes Virus
The first thought that comes to most people when the word "herpes" is mentioned is one of those nagging pain-in-the-neck cold sores, one type of which is caused by a herpes virus, and the venereal disease "herpes simplex." The word "herpes" refers to a family of viruses that actually cause a variety of diseases in people and animals. Chickenpox in people, as well as the related disease "shingles," is caused by a herpes virus. In addition, there is a herpes virus that causes neurologic disease in pigs (a disease called pseudorabies) and one that causes respiratory disease in cattle. So, the term "herpes infection" actually represents a number of diseases caused by a virus that is a member of the herpesvirus family.
The herpes viruses are unique and are well known for qualities that allow them to be successful pathogens. The first of these qualities is an uncanny ability to evade the immune system (see article on the immune system in The Horse of August 1996, page 34). The herpes viruses have been shown to produce substances that decrease the immune response against them. In addition to actively evading the immune system, the herpes viruses also are well known for possessing the characteristic of "latency." Latency is the state in which a virus can lie dormant and then recrudesce (recur) during times of stress. During latency, the virus produces very few proteins that would alert the immune system to its presence within the body, and thus it exists with little opposition from the immune system.
In horses, eight different types of herpes virus have been identified, with three of them currently being recognized as major players in disease. The herpes viruses that affect equidae are named equine herpes virus (EHV) 1-8; the viruses are numbered in their order of discovery. Equine herpes virus 1-5 have been demonstrated in horses and EHV 6-8 have been demonstrated in donkeys. Equine herpes virus 6-8 have also been called asinine herpes virus 1-3.
As previously mentioned, not all of the equine herpes viruses are associated with clinical disease. Equine herpes virus 1, 3, and 4 are the most clinically relevant and will be discussed here. An extremely comprehensive review of the equine herpes viruses by Eileen N. Ostlund, DVM, MS, PhD, of the Department of Veterinary Science, Gluck Equine Research Center, University of Kentucky, Lexington, Ky., was published in a 1993 edition of Veterinary Clinics of North America: Equine Practice and was used as a general source of information for this story.
Equine Herpesvirus 1
Infection of a horse with EHV-1 can result in a variety of clinical problems ranging from subclinical (presence of infection in absence of distinct clinical signs) disease with shedding of virus or the development of respiratory disease, abortion, fatal neonatal illness, or neurologic syndromes. In contrast to other diseases, where infection and survival lead to lifelong immunity, horses which have been infected with EHV-1 may be re-infected multiple times throughout their life.
EHV-1 Respiratory Disease
It is thought that the natural spread of EHV-1 is mainly via the respiratory tract. Infection of young horses with either
EHV-1, EHV-4 (to be discussed later), or both, generally occurs within the first year
of life. Respiratory disease caused by either EHV-1 or EHV-4 often is referred to as "equine rhinopneumonitis" (inflammation of the upper airways and lungs). Clinical signs of disease can vary de-pending on many factors, such as: The age of the animal (is this the first exposure, which can cause worse disease, or does the animal have some immunity); the vaccination status of the animal (vaccinated animals can become infected if the circumstances are right); and the individual strain of virus.
The virus is reported to have a two- to-10-day incubation period, after which the prominent clinical signs are a fever and nasal discharge. The fever can cause depression and inappetence, but many horses can have a significant fever and appear to be fairly normal with respect to these signs. The fever is said to be biphasic--a biphasic fever is one that is inconstant in that there is fever for a period of time shortly after infection, then a period of normal temperature, then a second fever episode. The importance of this, obviously, is that if your horse is a little off or has a nasal discharge but a normal temperature, you might be in the falling temperature period of a biphasic fever-- so take its temperature every six hours to be sure. The typical fever spikes noted with this disease are between 102-106° F (38.9-41.1° C). The characteristic nasal discharge starts out as thin and clear, then progressively gets thicker and milky in color. A cough also is a prominent clinical sign, but is not always present with the herpesvirus respiratory diseases.
Special blood tests can give an index of suspicion of EHV infection, but they must be interpreted carefully as many normal horses will have a positive blood test. The only definitive way to make the diagnosis is to grow the virus from the blood or a swab of the nasal passage of an infected horse.
As with most viral respiratory diseases (the common cold or flu, for example), there is no specific treatment. In most uncomplicated cases, complete recovery can be expected within several weeks. Your veterinarian might choose to use antibiotics should there be a concern over secondary bacterial pneumonia developing and/or prescribe something to reduce fever should it be very high and a source of depression. My own personal preference is not to reduce a fever chemically unless it is causing significant depression. The reason for this is twofold: First, the development of fever is part of the immune system's defense mechanism; second, the monitoring of a fever is a useful indicator of the natural progression and, hence, resolution of the disease. In addition, persistence of the fever beyond an expected time frame can be an indicator of complications.
Reducing stress in the horse's life is extremely important. Overall stress reduction can reduce the convalescent time and might reduce the chance of secondary complications. A good rule of thumb, as presented in Ostlund's review, which I feel is very appropriate, is "One week of postinfection rest per day of elevated rectal temperature during the acute phase of the disease."
With a somewhat unpredictable nature, the time interval between EHV-1 infection and abortion can be less than two weeks or up to several months. The typical scenario is a situation in which a late gestational mare (usually in the seventh to eleventh month), which has had a clinically silent EHV-1 respiratory infection, has a sudden abortion. The abortion can be a single isolated event or the beginning of a so-called "abortion storm" where a large number of pregnant mares in the same herd abort over a period of several weeks. In these EHV-1 abortions, the fetus has been infected with the virus and has characteristic pathology re-lated to this disease. An aborted fetus should be evaluated by a veterinary pathologist in an effort to determine if herpesvirus was involved. The fetal tissues can be evaluated for the presence of the herpesvirus.
At the time of abortion, it is unknown for a particular animal if there is shedding of the virus via the respiratory tract, as infection in that animal might have occurred several months prior to the abortion. However, because of the possibility, it is generally recommended not to rebreed for 30 days after the abortion (more on control later.)
EHV-1 Neurologic Syndrome
In addition to respiratory disease and abortion, EHV-1 can cause a neurologic syndrome characterized by paresis (weakness), which starts in the hind legs. There might or might not be a history of respiratory disease in that animal or in other animals from the same environment. The weakness is acute and rapidly progressive. These horses often are noted to be uncoordinated behind (see below photo) and can rapidly progress to recumbency within 48 hours of onset.
Other clinical signs include a weak and floppy tail and urinary incontinence. These horses cannot urinate freely and will have a greatly distended bladder, then start to "dribble" or "squirt" urine as the bladder becomes over-distended. The diagnosis is usually made by evaluating history, clinical signs, and results of a spinal tap. These horses might or might not be shedding the virus, so the results of nasal swabs or blood cultures can be variable.
There is no specific treatment for the neurologic form of EHV-1. Supportive therapy and nursing care can be extremely intensive, but are vitally important to an animal's potential survival. Many of these horses are down and cannot support weight, necessitating the use of a sling. Various anti-inflammatory agents are used in an effort to minimize damage to the spinal cord. Should these horses survive, recovery can take several months. Non-recumbent animals often appear to have a complete recovery, whereas animals which were down often have some degree of residual neurologic deficit. Even severely affected horses have been reported to have complete recoveries.
EHV-1 Neonatal Disease
When EHV-1 infects a near-term fetus, the outcome is often the birth or premature birth of a very sick baby. These foals can have severe respiratory pathology and liver damage as a result of the viral infection. Typically, despite significant advances in neonatal care (including ventilatory support), these foals have an extremely guarded to grave prognosis and often die within several days of birth.
Equine Herpesvirus 4
From a clinical standpoint, EHV-1 and EHV-4 respiratory disease are indistinguishable. The clinical signs associated with EHV-4 are the same as those described previously for EHV-1, with the severity of disease in an individual animal being subject to the same set of variables. If one were going to wish a herpes virus on his or her horse, EHV-4 would be a lesser evil than EHV-1 becaues EHV-4 has not been shown to cause neonatal disease, the neurologic syndrome, and it is a much less significant cause of abortion. It can, on the other hand, be a significant contagious respiratory path-ogen of young horses that can have a negative impact on sales, training, and performance activities of an entire stable. The rumor of a barn's having a herpes outbreak can set off paranoia, panic, and pandemonium as well as just plain ruining somebody's day. But, knowing that the outbreak is the EHV-4 type instead of EHV-1 can make things a bit more tolerable, especially if there are pregnant mares around.
Ostlund's review referenced studies that demonstrated that "EHV-4 has been responsible for most of the herpesvirus respiratory disease outbreaks among young horses in Kentucky, including those in early race training." EHV-1 and EHV-4 can be distinguished via laboratory testing.
Equine herpesvirus 4 occasionally can cause abortion. The nature of EHV-4 abortion is sporadic; there have been no reported cases of "abortion storms" resulting from EHV-4 infection. The fetal pathology caused by EHV-4 is very similar to that caused by EHV-1, but the specific cause can be determined by special testing. In Ostlund's review, it is reported that "in Kentucky, between 1983 and 1992, EHV-4 accounted for less than 1% of the herpesvirus abortions."
Control of Equine Herpesvirus 1 & 4
As with all diseases, the most effective way to control them is to prevent them. This is easier said than done with regard to the herpesviruses. The first step is tight control of animals coming and going from your barn. All new animals coming on the farm should be separated and quarantined (as best as your situation permits) for at least three weeks prior to being allowed contact with the general population. During this time, they should have their temperature taken daily and be monitored for the development of disease. If you have a larger farm, it helps to group the horses by age, i.e., keep the yearlings away from the pregnant mares. If there is show stock coming and going all the time, keep them away from the horses which never leave the farm--even if they are just at opposite ends of the barn. Making some effort to separate them is better than not making any effort.
You should be strict regarding sanitation. Disinfection of stalls between new horses and the act of hand washing between horse-to-horse contact can have significant benefit. If you are traveling with your horse, disinfection of the stalls at the show, training barn, track, etc., prior to placing your horse in them also is beneficial.
Should a horse on the farm develop respiratory disease or have an abortion, it should be isolated until a diagnosis has been made and a contagious disease ruled out. Remember, the placental membranes, fetal fluids and fetus of a herpesvirus abortion are sources of the virus (wear gloves and wash hands). The placenta and fetus should be sealed in a plasticbag, the area where the abortion occurred thoroughly disinfected, and the contaminated bedding disposed of properly (burning is good). The placenta and fetus should be saved for veterinary examination.
I'll review the different vaccination options here, but your veterinarian should be consulted regarding a vaccination program that is tailored to your stable's individual needs (especially with regards to the herpes vaccines). The first comment is that the vaccination will not prevent the disease. The risk can be reduced, but depending on the time since vaccination, the individual strain of virus to which your animal was exposed and the "dose" of virus, disease can develop. Reducing the risk of exposure and utilizing vaccination should be used together in the effort to prevent herpesvirus infection.
There are separate EHV-1 and EHV-4 and combination vaccines available. The typical protocol is two initial vaccinations several weeks apart, then subsequent boosters every three months to once a year, depending on the individual situation. The standard protocol for prevention of abortion is to use one of the vaccination products approved by the Federal Drug Administration, for the protection against abortion from EHV-1, administered during the fifth, seventh, and ninth month of gestation. Again, it is best to seek veterinary consultation for assistance with these vaccination protocols.
Equine Coital Exanthema
Equine coital exanthema is veneral disease of horses caused by EHV-3. As can be deduced by the name, the disease is primarily transmitted by sexual intercourse, but it can be transmitted via contaminated examination instruments, breeding tools, and, of course, hands.
In mares, the clinical signs develop four to seven days after contact with the virus. The initial appearance consists of small (2mm diameter) circular bumps on the vulva and surrounding skin. These small bumps then progressively develop into a blister and rupture, leaving raw ulcerated areas up to a half-inch in diameter; the affected areas are very painful to the touch. These lesions slowly heal over two to three weeks, leaving unpigmented scars behind.
The disease has a similar course in the stallion, with the lesions being on the shaft of the penis and the prepuce. During the acute phase, stallions are, as would be expected, reluctant to mate--this can be the handler's first hint of a problem.
Occasionally there are lesions on the lips and nasal mucosa, although these generally are uncommon. The disease is contagious during the acute stages when the fluid inside of the blisters is a source of the EHV-3 virus. The treatment consists of three weeks of sexual rest, which allows time for the ulcers to heal and prevents further spread of the disease. Should secondary bacterial infection develop, the use of an antibiotic ointment or cream might be necessary to aid treatment. On a final EHV-3 note, although latency of the herpes viruses is one of the hallmarks, a carrier or latent state for this disease has, to my knowledge, never been reported.
About the Author
Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.
Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.