Keeping An Eye On Emerging Diseases

While these diseases haven’t reached U.S. shores, federal officials are monitoring them.

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Emerging and re-emerging equine diseases appearing around the globe this year may prove problematic for U.S. horses. Examples include the Hendra virus, which led to several equine deaths in Australia, and African horse sickness, which has killed more than 2,000 horses on the African continent.

While these diseases haven’t reached U.S. shores, federal officials are monitoring them.

“We do constant surveillance for emerging and re-emerging diseases,” says Tracey Lynn, DVM, MS, Dipl. ACVPM, director of the Center for Emerging Issues at the USDA’s Animal and Plant Health Inspection Service.

“We are monitoring trends based on case occurrence,” Dr. Lynn says. “We are looking for those times when a disease or disease agent might fit into our criteria. We look at what’s happening in all parts of the world from an emerging disease surveillance perspective.”

She says her agency monitors and assesses disease pathways and introduction into the U.S. Regarding African sickness and Hendra virus, she says the current risk of infection among U.S. horses is relatively low.

“Hendra virus has never been identified outside the continent of Australia. Right now it seems to be a fairly geographically isolated situation, but of course we’re watching.”

African horse sickness is being watched, and Lynn says “a number of quarantine measures” are in place. She says that with the existing quarantine restrictions and minimal equine trade between the U.S. and Africa, at this point the disease risk is low here.

Official Definitions

According to the federal Centers for Epidemiology and Animal Health, an equine disease is considered to be “emerging” when it meets at least one of three general criteria:

  • It is identified for the first time.
  • It evolves and changes in virulence, host type or other pathogen behavior.
  • A change occurs in the disease’s geographic range or incidence within a range.

A re-emerging disease is classified as a previously known disease that shifts in its geographical distribution, expands its host range or significantly increases its prevalence.

Currently, equine herpes myeloencephalopathy (EHM), the neurologic form of equine herpesvirus-1 (EHV-1), is an emerging equine disease in the U.S. as identified by the Center for Emerging Issues.

In January 2007, the agency issued an “Emerging Disease” info sheet that suggested EHM met the criteria for an emerging infectious disease. In 2006, 11 EHM outbreaks were reported nationwide; from Jan. 1, 2007, through March 31, 2007, 11 outbreaks were reported.

While case numbers continue to rise, there is concern over accuracy. Variations in EHV-1 reporting requirements and case definitions make it difficult to accurately identify the true number of cases. EHV-1 is enzootic throughout the world and almost all horses older than 2 have been exposed.

Unfortunately, there isn’t sufficient epidemiologic data to fully  explain why some horses get EHM and others do not during an EHV-1 outbreak.

According to recent research studies, a PCR test has been reported to distinguish between the non-neuropathogenic and neuropathogenic strains of EHV-1 by detecting the single nucleotide polymorphism (SNP) genetic marker. Currently, however, no stall-side tests exist.

Vaccines help protect against respiratory disease and abortion associated with EHV, but no vaccine makes label claims to protect against the myeloencephalitic form of the disease.

Monitoring in the U.S.

All equine diseases keep Lynn’s agency on watch, but some have officials there on alert. Methicillin-resistant staphylococcus aureus (MRSA) is a good example.

“We are tracking and gathering more information. It has been an emerging and growing health problem,” Lynn says. “What we’re seeing now, as people gather more information about it, is that there are additional considerations that may classify it as an emerging disease.”

Methicillin is an antibiotic first introduced in human medicine in the 1950s for treating penicillin-resistant staphylococci. Within a few years, methicillin-resistance was identified and has been a problem ever since.

MRSA is becoming an increasingly important pathogen in horses worldwide, including the U.S., according to a research paper co-authored by J. Scott Weese, DVM, DVSc, Dipl. ACVIM. Dr. Weese, an assistant professor of clinical studies at Ontario Veterinary College, says he believes that the commonly found strain of MRSA originally jumped from human to horse and has adapted to thrive in equines.

Vets at Risk

“Infection with MRSA (in horses) can be life-threatening and difficult to treat,” Weese and fellow researcher Maureen E.C. Anderson write. “Additionally, veterinarians are at increased risk for zoonotic infection. Proper identification of MRSA infections and MRSA-colonized horses is critical for the treatment and control of this pathogen as well as the prevention of zoonotic spread to horse personnel.”

The researchers write that MRSA can cause diseases in horses ranging from “superficial soft-tissue infections to life-threatening bacteremia and toxic shock syndrome.”

Laboratory identification of S. aureus is relatively straightforward, but identification of MRSA requires additional testing.

Once properly diagnosed, “Treatment is based on general principles of treatment of the specific condition (i.e., pneumonia or incision infection) and consideration of antimicrobial-susceptibility testing results and pharmacokinetic data,” the researchers write.

Concurrent application of infection-control practices is critical to reduce the likelihood of re-infection or infection of other animals or people. This is of particular concern in veterinary hospitals because of the number of different animal contacts and contact with presumably high-risk animals (immuno-suppressed, antimicrobial-treated, and surgically incised animals).”

EMPF Also on Radar

Equine multinodular pulmonary fibrosis (EMPF) is also being closely monitored.

“It’s a disease that we are tracking, but we have not made the determination that it meets all of our criteria for making the case of an emerging disease,” Lynn says.

Recently, researchers reported that a subset of fibrotic lung disease in horses was associated with equid herpesvirus-5 (EHV-5) infection. They termed the disease “equine multinodular pulmonary fibrosis,” or EMPF. According to a 2007 paper released by the research group, “Pulmonary fibrosis and interstitial lung disease are poorly understood in horses; the causes of such conditions are rarely identified.”

The researchers say EHV-5 is  in the gammaherpesvirus family, which distinguishes  it  from better-known equid alphaherpesviruses such as EHV-1.

Horses with fibrotic lung disease of any type are typically treated supportively with anti-inflammatory drugs and antibiotics if secondary bacterial infection is suspected. The response to treatment depends on the severity of the disease.

Based on the association with EHV-5, EMPF treatment with antiviral drugs could be considered and researchers at the University of California, Davis’ Equine Viral Disease Laboratory are looking into the possibility.

Uncertainty Over EEEV

Regardless, the current prognosis for EMPF horses is guarded pending further discoveries about the pathogenesis of the disease.
Eastern equine encephalomyelitis virus (EEEV) is not considered emerging or re-emerging, but that may change. While EEEV has plagued the Eastern U.S. for many years, its incidence out West has only recently been reported.

For example, in 2006, Texas had just one EEEV case; in 2007 that number was 29. Wisconsin reported its first EEEV case this year. Additionally, Florida and Georgia both report numerous EEEV cases.

Florida agriculture officials counted four times as many cases in the first half of 2008 as in the same period in 2007.

“We are definitely monitoring it and will continue to do so,” Lynn says. When asked whether her group might at some point classify the disease as re-emerging, she explains, “We’ll have to wait and see if this is a typical pattern—if the virus waxes and wanes in some sort of defined period—or if there is a steady increase, and we’ll make a determination at that point.”

According to USDA officials, EEEV is thought to have affected North American horses since 1831, but the virus did not receive its official name until a major outbreak hit horses in the coastal areas of Delaware, Maryland, New Jersey and Virginia in 1933. Additional outbreaks occurred in Virginia and North Carolina in 1934 and 1935, respectively.

Mosquitoes were first determined to be potential carriers of EEEV in 1934. Since then, studies have shown that various mosquito species of Aedes and Culex could become infected with and transmit EEEV from one vertebrate to another.

It was during the 1935 outbreak that birds were considered a possible reservoir host for the virus, but it was not until 1950 that the first virus isolation was made from a wild bird proving that to be true. Subsequent studies have shown that many birds are susceptible to EEEV infection.

As a result of vaccination, severe EEEV epidemics no longer occur regularly in the U.S., but sporadic cases and small outbreaks are still seen. Because its incidence has been isolated, Equine piroplasmosis (EP) is not considered an emerging disease, but it has kept Florida state officials busy. After a Florida horse tested positive for EP in August, the Florida State Veterinarian’s Office began a full investigation.

EP is a blood-borne parasitic disease primarily transmitted by ticks or contaminated needles. Signs range from acute fever, inappetence and malaise to anemia and jaundice, sudden death, or chronic weight loss and poor exercise tolerance.

EP Returns

The disease was eradicated from Florida in the 1980s, and prior to the recent outbreak the U.S. has been free of EP for two decades.
Florida officials say  the recent EP outbreak investigation was near completion at press time. Throughout the investigation, about 200 horses were tested; horses confirmed positive for EP have been euthanized.

All the EP-positive horses have been directly linked to what is believed to be two horses that entered Florida from Mexico. The evidence indicates that the disease transmission was by management practices of shared needles, and not by ticks. About 50 ticks (Dermacentor variabilis) that have been shown to be experimental vectors for the parasitic organism were located in the outbreak areas and tested. All were EP negative.

Because of the recent EP outbreak, Canada has tightened import requirements for Florida horses. Horses shipped to Canada from states other than Florida need additional certification specifying that in the 21 days before exportation, a horse was not in Florida. These restrictions remain in effect.

Disease Front

Horse owners and equine practitioners are the first line of defense in mitigating the spread of any equine disease.
“Equine owners and their veterinarians will be the first to recognize a potential emerging disease,” says Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM, a professor of equine medicine at Colorado State University.

“The private equine veterinarian recognizes what’s usual versus what is atypical for horses in the area. A local equine veterinary practitioner has a perspective beyond an individual operation as well as having a historical perspective.

“When mare reproductive loss syndrome (MRLS) was evolving in Kentucky, it was a private practitioner who recognized a problem,” Dr. Traub-Dargatz notes.

She explains that the veterinarian, Thomas Riddle, DVM, saw an increase of foals dying in-utero over a very short time, more than he typically saw over an entire breeding season, which led him to contact the University of Kentucky Gluck Equine Research Center.

“The university was able to use its own expertise along with bringing in other  experts on the topic and a study was eventually done to determine  risk factors for MRLS,” Traub-Dargatz says. 

“The first veterinarian to recognize West Nile in horses was a private equine practitioner,” Traub-Dargatz adds. “He became concerned because there was a cluster of neurologic horses in his practice. Initially he thought equine protozoal myeloencephalopathy, but he sent in samples to the National Veterinary Services lab and the diagnosis of West Nile was made.”

Educate the Client

Future equine disease outbreaks are certain. Minimizing spread within a group of horses or between equine operations of facilities is key. One of the most important disease prevention strategies is to educate horse owners.

“The individual horse owner,” Dr. Lynn says, “can do quite a lot to mitigate or minimize the risk of something happening to their horse by following good vaccination practices and good biosecurity practices.”

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