Horses, like people, can contract a viral infection, and there are hundreds of different viruses around that can infect animals.
A common statement often used is “my horse has a virus”, but what does this really mean? Horses, like people, can contract a viral infection, and there are hundreds of different viruses around that can infect animals. But often it’s a case of a virus being blamed for any illness or malaise that can’t be diagnosed at the time. So why is “the virus” blamed?
A virus is something we know causes a problem, is difficult to diagnose, and relies largely on the animal itself to recover. Therefore, it can be blamed for many things that aren’t specifically diagnosed.
Viruses are very small pieces of genetic material contained within a protein envelope that are so small they require an electron microscope to be seen. Depending on the type of virus, these microorganisms are transferred from horse to horse either by direct contact or via a third-party vector such as a mosquito or tick. Viruses require living cells to replicate and multiply and therefore rely on infecting a host to be able to survive. They can reside in a population of horses or insect vectors where they live happily without causing significant illness, until a new animal is introduced into the population and the virus resurfaces.
Viruses, unlike bacteria, do not respond to antibiotics, rather the immune defence system of the infected animal must mount a response and, where possible, eliminate the virus from the body. In humans there are several antiviral medications, but these are not used commonly in the equine world due to their cost and availability. Instead, we rely on the use of vaccinations to help boost a horse’s natural immunity to fight off some of the more common viruses or fight the viruses that have a major impact economically or on human health.
We rely on the use of vaccinations to help boost a horse’s natural immunity to fight off some of the more common viruses.
“There are several
different ways viruses
are transferred.”
Examples of these vaccines are the EHV-1,4 vaccine against the herpesvirus that can cause respiratory, abortive or neurological symptoms in affect horses, the Equine Influenza vaccine (only used in Australian horses intended for travel overseas) and Hendra vaccine, used in horses to reduce the risk of Hendra occurring in humans that contact infected horses. There are many other vaccines against equine viruses around the world, but as Australia is free of many exotic diseases or if they occur, it is in such small numbers their use in Australia is very reduced.
MUTATING AND CHANGING
One of the interesting characteristics of viruses that help perpetuate their existence in the environment and limit the success of vaccination is mutation. Many viruses can change the nucleic sequence in their DNA to enable them to become resistant to the immune response that the horse has developed with time to fight off infection. When a horse is vaccinated or had a previous infection, the immune system produces antibodies or cell defences that directly act on the virus and help the body recover from infection. By mutating and changing the DNA sequence, some viruses can avoid detection and elimination because the body does not immediately recognise the virus as the same virus to which it has produced antibodies too.
There are several different ways viruses are transferred, depending on the type of the virus and whether a third party, called a vector, is required. Perhaps the most common route of transmission is via aerosol, meaning the horse breathes in an infective viral particle. This is common for all the respiratory viruses that we see with millions of viral particles unleashed into the environment when a horse coughs. It is the reason why multiple horses become infected within a stable block once the first horse is infected and becomes ill.
One of the most common routes of transmission is via aerosol, meaning the horse breathes in an infective viral particle.
THE WORST THING
A virus can still be transmitted from horse to horse in a paddock, but the number of viral particles inhaled is usually greater when horses are confined to an area where airflow is reduced and therefore the concentration of viruses is higher within a defined area. For this reason, the worse thing an owner can do when their horse is sick is to put the horse in a barn with other horses and keep the door and windows locked to “keep the horses warm”. This serves to increase the dosage of infective particles that other horses in contact with it encounter.
The number of viral particles inhaled is usually greater when horses are confined to an area where airflow is reduced, such as in a stable.
“Not all infections are
going to result in disease.”
Why is this important? It is important because not all infections are going to result in disease. As mentioned earlier, a virus relies on the body’s own immune system to remove it from the horse’s system, and in many cases where the challenging dose is only small, the body can remove the threat before the virus is able to get hold of the horse and cause disease.
In situations where the horse is challenged with very large doses of a virus, the immune system is unable to rid the body of the virus before it can take up a site within the cells and multiply, producing symptoms of the disease and putting the horse’s health at risk. Examples of respiratory transmitted viruses are the Equine Herpesviruses, Equine Influenza, Equine Rhinitis Virus and possibly Hendra.
Viruses can be transmitted by the faecal-oral route, meaning horses can ingest viral particles that have been excreted in manure. These include Equine Rotavirus and Equine Coronavirus — viruses that cause diarrhoea in foals and are often spread quickly between foals in high density settings. Like the situation with respiratory viruses in confined spaces, the more foals that are infected within a small area, the higher the concentration of viral particles in the environment and therefore the higher the challenge to the horse’s immune system. This is why stud farms can have “outbreaks” of foal diarrhoea that affect a large portion of their foal crop and rely heavily on management to help rid the foal of disease.
In some scenarios where there has been an ongoing outbreak of diarrhoea every year, the causative virus is identified through sample testing and then the stud vaccinates the pregnant mares to help with passive transfer of helpful antibodies to the newborn foal. Vaccines for Rotavirus (and Salmonella, which is a bacteria) are used on many stud farms.
Viruses can spread quickly between foals in high density settings.
There are viruses that can be transmitted sexually such as Equine Viral Arteritis (EVA), and this virus damages blood vessels, causing a variety of symptoms including oedema, haemorrhage, respiratory discharges, conjunctivitis and short-term infertility. It can also be transmitted via the respiratory route, showing that some viruses can be spread in different ways.
Fortunately, whilst we have EVA in Australia, the clinical disease is extremely rare here, indicating that the viral strain of EVA we have has a low virulence compared to other countries around the world where clinical disease is more common. Horses going to New Zealand from Australia require a blood test to identify any horse that may be carrying EVA and if identified further tests might be required to ascertain if the horse is a risk for transmitting the virus to other horses.
TRANSMISSION BY INSECTS
Viruses transmitted via insects are common around the world with mosquitoes, biting flies and midges some of the more common vectors. Mosquitos are known to carry and transmit some of the neurological viruses such as Murray Valley Encephalitis Virus, Ross River Virus and Japanese Encephalitis Virus. These viruses tend to cause disease at certain times of the year, particularly when the weather is warm and wet, which correlates to increased numbers of mosquitoes. Fortunately, we don’t currently have some of the severe diseases carried and transmitted by insects such as African Horse Sickness, Vesicular Stomatitis and the Equine Encephalomyelitis and this largely because Australia has strong biosecurity measures in place to help prevent these diseases entering Australia.
Hendra virus is a well-known Australian virus capable of infecting and killing horses. It requires a vector for transmission to the horse — but with this virus, fruit bats are the natural hosts. Horses become infected with Hendra following contact with contaminated bat excretions (urine, saliva, birth products) and develop severe but vary variable symptoms including fever, depression, respiratory ailments, neurological signs and sudden death.
Viruses transmitted via insects are common around the world.
RISK TO HUMANS
Horse-to-horse spread can occur when there are overwhelming amounts of virus-contaminated bodily fluids amongst horses in close contact. But the real concern with this virus is that Hendra can infect humans when there is very close contact with infected horses and to date seven people have been infected, with four dying from the disease, while three recovered. The transmission of a disease from animal to human is known as zoonosis. There is no cure for Hendra and the current recommendation is to vaccinate horses to help prevent infection in humans. Many veterinarians in Queensland and NSW will not attend an ill horse unless it has been vaccinated, as equine veterinarians have accounted for four of the seven people infected.
Most viral infections are confirmed using either a blood sample or tissue swabs that undergo testing using several different methods. Sometimes virus isolation is used prior to identification. This involves taking a sample of body fluid or tissue and inoculating this sample into cell cultures that allow the virus to replicate and multiply to a level that allows easier identification of the pathogen.
One of the more common techniques used to identify the type of virus is PCR analysis. PCR stands for Polymerase Chain Reaction, which is a test that detects small amounts of genetic material in body tissue samples, allowing the pathologist to identify what, if any, viruses are infecting the animal. PCR tests are very useful as they are sensitive and rapid, allowing the vet to obtain a diagnosis in a timely manner, which is very important for diseases such as Equine Influenza, Equine Herpes Virus 1 and EHV 4. This test can also be used to differentiate between EHV1 and EHV4. This is very important in the management of a disease outbreak where identifying infected animals is paramount to reducing the spread of an infection.
Nasopharyngeal swabs are commonly used to obtain samples for respiratory infections and faecal samples are used for PCR testing for equine coronavirus and equine rotavirus, which are known to cause enteritis and diarrhoea. Viruses that cause abortion are normally investigated by performing PCR on samples from the aborted foetus (liver, thymus, spleen and lung), the placental tissue, and the fluids (amniotic fluids) associated with these tissues. Other techniques available for virus identification include ELISA (enzyme-linked immunosorbent assay), immunofluorescence, neutralization tests and DNA probes.
Serology was used commonly several years ago to confirm a viral infection in a horse but is less desirable now because of the length of time taken to get a positive or negative diagnosis. This test uses serum (from the blood) to identify antibodies to a specific virus and then compares the level of antibodies in this serum to a follow-up serum sample taken approximately two to four weeks later to see if the levels have significantly risen. Two samples are required as it is necessary to show that the levels of antibodies in the blood have risen significantly due to a current infection. If the antibody levels are stable, it means there has been no current infection, and the antibodies present are due to either a previous infection or due to the horse having been vaccinated against the virus being investigated.
SUPPORTING THE HORSE
Treating horses that have a viral infection is usually based around supporting the horse, managing the symptoms and allowing the horse to generate sufficient antibodies or cell mediated defenses to overcome the infection. This can mean giving the horse anti-inflammatory medications to reduce inflammation and high temperatures, intravenous fluids in severely dehydrated horses, bandaging legs if there is distal limb swelling and administering antibiotics if secondary bacterial infections become an issue.
Depending on the disease and severity of the infection, some horses will require no treatment and overcome an infection in several days; in other circumstances infection can mean death despite everything done, as the virus causes too much destruction before the horse mounts a suitable response. This has been the case with several horses infected with the Hendra virus.
In areas where a virus is endemic in the population, keeping all horse updated with an appropriate vaccine is recommended. Viruses that rely on an insect vector to spread the disease can be managed by controlling the insect numbers, such as removing stagnant areas of water where mosquitoes can fester, and keeping horses boxed at dawn and dusk when the insects are more prevalent.
In the face of a viral outbreak, every effort should be made to keep horses separated or at the very least restricted in their movements both on the farm and between farms. Good hygiene, both personally and when attending the horse, should be strictly adhered to where possible. and any bodily fluids, including respiratory secretions, faeces and aborted products should be removed as quickly and efficiently as possible. EQ
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