Thursday, March 25, 2010

4/25/2010 - How Diseases Spread

Vet tip of the Day: Infectious Disease Control/Equine Herpes Virus
Key Words: EHV-1, EHV-4, neurologic disease, upper respiratory virus

This story is loosely based on actual events that took place in 2007-2008, spanning from Europe to New York, then south to Florida, and west to California. The goal is not to alarm you, or make you an expert on Equine Herpes Virus type-1 (EHV-1), but to provide guidelines on how to protect your horse from infectious diseases. Developing an increased awareness of how diseases can change and spread rapidly will help you to improve the care and well-being of your equine companions. Our story:

An eight-year-old horse that received excellent care in Germany was purchased by a person in the United States. He was vaccinated and dewormed regularly, and received appropriate booster vaccinations three weeks before his transport by air to New York. Upon arrival he was quarantined according to federal regulations. Once released from quarantine, he was transported by truck to Kentucky with one group of horses, and then shipped on to Florida with a different group. When he arrived in Florida 10 days after leaving New York, a handler noticed he seemed depressed and unsteady on his feet. The owner was alerted and a veterinarian was contacted to evaluate the horse.

How are diseases spread?

How and why did the horse become ill? We know the horse received excellent management with booster vaccinations for appropriate infectious diseases three weeks before shipping overseas. The immune system takes at least 10 days to fully respond to a booster vaccine, so ideally they should be given two to four weeks before shipping or change of environment.

It would have been advisable to rest the horse in New York following the flight and quarantine before transporting to Florida, and to avoid the mixing of different groups of horses during travel. Research has shown that horses subjected to long-distance transport are at significantly greater risk of infection with respiratory tract pathogens compared to similar non-transported horses.

The veterinarian exam revealed the horse had a 102.5 ºF fever and neurologic signs manifested as weakness and incoordination of the hind limbs, and a rectal palpation determined the horse’s bladder was distended with urine. The owner was told that a neurologic form of EHV-1 was suspected and immediate isolation was recommended until a diagnosis could be confirmed because a new, highly contagious form of this disease had been reported in other U.S. locations.

What is EHV-1?

Equine Herpes Virus Type-1 (EHV-1) is an old and common disease in horses, most commonly causing an upper respiratory infection in young horses, but also responsible for late term abortion and a sporadic neurologic disease. Luckily, this veterinarian was aware of outbreaks in recent years of neurologic disease affecting multiple horses caused by EHV-1; most notably one in a university hospital in which 46 of 135 in-house patients developed neurologic signs, leading to death in 12 of those horses.

It was discovered that the EHV-1 virus had undergone a mutation in which a single element in the viral DNA code was altered. The resulting strain was more virulent, contagious, and specifically attacked the horse’s neurologic system. Therefore it was named neuropathogenic EHV-1. With the emergence of this mutated virus, the clinical form of EHV-1 associated with neurologic signs was becoming more common and more. This exemplifies how a tiny change in the genetic code of a virus can have far-reaching and dangerous effects.

In Florida, a nasal swab confirmed the diagnosis of neuropathogenic EHV-1. This first horse was treated and recovered. Meanwhile, another horse off-loaded from the original truck in Kentucky was arriving in Southern California, and a third horse that had accompanied our horse by van to Florida was taken to a farm, and later to a horse show facility nearby. The horse in California walked off the truck with clinical signs similar to the first index horse in Florida, and was quickly assessed and isolated. The third horse never showed signs of illness, but 10 days after arriving at the show grounds, other horses began to develop signs of neurologic disease. Subsequently, the presence of the neuropathogenic form of EHV-1 was confirmed, leading to an extended quarantine of the show grounds. No other cases were detected in Southern California after isolation of the index case, but within the month, a horse was confirmed infected at Golden Gate Race Track. There have been isolated confirmations of neuropathogenic EHV-1 in California horses since, but no multiple case outbreaks have occurred.

Disease control and prevention among horses

How could the spread to the show grounds have been prevented? First, when a contagious disease is suspected, the affected horse should be isolated and other in-contact horses should be quarantined and monitored for signs of disease for an appropriate duration. This would have meant immediately contacting the shipping company and identifying all the horses transported with the original horse in Florida with clinical signs. In the case of EHV-1, the incubation period is typically two to eight days, but can be as short as 24 hours – showing how quickly this becomes a logistical nightmare, and how critical every hour becomes. The horse in Southern California was traced back to the horse in Florida, which was connected to the air transport from Europe – tedious tracking, but important to successfully contain infectious diseases.

Second, rapid diagnosis is critical. Our astute veterinarian in Florida submitted the nasal swab for a state-of-the-art diagnostic technique called PCR (polymerase chain reaction). This test amplifies specific DNA segments in the sample and allows the detection and identification of minute amounts of viral DNA, confirming that this particular viral strain was present in the affected horse.

Finally, how can you as a horse owner reduce the spread of infectious disease? Your first goal is to protect your horse against infection by consulting with your veterinarian to ensure that your horse is appropriately vaccinated. Learn all you can about the safe and comfortable transporting of horses. With these good management techniques, you’re supporting your horse’s immune system and reducing the odds that he will succumb to an infectious disease, even in the face of exposure. Use common sense during competitive events: do not share water, tack, or grooming equipment. EHV-1 is spread primarily by horse to horse contact and by virus transported on equipment and the clothing, hands, and shoes of people moving between horses. If your horse is exposed to fewer viral particles and has a healthy immune system, illness is less likely.

When your horse is at high risk of contracting an infectious disease such as EHV-1, even the best management may not protect him completely. If he does become infected with a contagious disease, your goal is to protect him and other horses with early detection. Fever is the earliest sign of most viral diseases. The single most important thing you can do is to monitor your horse’s rectal temperature twice daily at shows, before and after shipping, and when new horses come on the property. Whenever your horse’s rectal temperature is 102.2 ºF or higher, contact your veterinarian and follow their recommendations.

Don’t abandon your equestrian activities for fear of infectious disease. Just stay informed and use common sense to guide you. Your veterinarian is your best resource for advice on the prevention and control of infectious diseases. Don’t hesitate to ask questions and continue your life-long education as a responsible and caring horse owner.

More Information on EHV-1

This story first appeared in Petfolio magazine, published in Reno, Nevada.

I believe that education is the key to evolution.
I believe that animals are the key to compassion.
I believe the learning never stops.

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