It’s All Foreign to Me

Find out which foreign animal diseases North American horse owners need to keep an eye on and why.
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Here are the foreign animal diseases North American horse owners need to keep an eye on and why.

Many Americans still vividly remember the turmoil both in the horse industry and the nation at large after West Nile virus (WNV) first surfaced here in 1999. The sudden death of local crows and exotic birds in a New York City zoo, increased numbers of encephalitis cases in horses, and the same among elderly people, led doctors to recognize the first WNV cases in the Western Hemisphere.

Fifteen years later, WNV is considered endemic (native) in this country, and widespread vaccination against the mosquito-borne disease, along with vector control measures, is commonplace (and recommended). In a perfect world we’d keep such foreign animal diseases at bay in the first place, so veterinarians in a number of sectors work to safeguard our country’s horses. But what is a foreign animal disease (FAD) in the first place? In this article we will define these pathogens and describe some of the concerns associated with introducing new, or transboundary, diseases to the U.S. horse population. We will also summarize the initial steps to take if faced with a potentially foreign disease.

What Exactly is an FAD?

“From our point of view, a foreign animal disease is one that does not exist in the United States,” says Rory Carolan, DVM, equine specialist with the USDA-Animal and Plant Health Inspection Services (APHIS) Veterinary Services’ National Equine Programs, in Riverdale, Maryland.

An FAD’s emergence could have multiple repercussions, including equine illness or death and economic losses from movement or event restrictions.

“Specific to horses, a 2009 contagious equine metritis outbreak was estimated to cost the industry between $11 million and $16 million, not including lost market opportunities due to export restrictions,”Carolan says. “Also in 2009 an outbreak of equine piroplasmosis resulted in $3 million in direct costs to a single equine operation, not including lost market opportunities due to export restrictions.” Veterinary officials still consider both of these diseases FADs.

In addition, as was the case with WNV, some FADs can sicken humans, giving health officials extra incentive to keep track of confirmed infection in mosquitoes, birds, and horses and, thus, public health risk.

Avoid These Like the Plague

The list of diseases and infections that horses can pick up worldwide is extensive. So how do veterinary officials decide which ones are serious potential threats to North American horses?

The OIE (World Organization for Animal Health) plays an integral role in monitoring animal disease and providing transparency between partner countries. As such, the organization constantly updates its lists of notifiable animal diseases. The OIE’s 2014 “listed diseases and infections in horses” include the following: 

  • Contagious equine metritis
  • Dourine
  • Western equine encephalomyelitis
  • Equine infectious anemia
  • Equine influenza
  • Equine piroplasmosis
  • Glanders
  • African horse sickness
  • Equine herpesvirus-1 (EHV-1)
  • Equine arteritis virus
  • Venezuelan equine encephalomyelitis

Of these, Carolan says the following three foreign diseases are of particular interest to the U.S. equine community:

African horse sickness (AHS) Biting midges (Culicoides) spread the fatal African horse sickness virus to horses and other animals, such as camels and dogs. Once the virus infects a horse, one of four versions of the disease can develop: the peracute (pulmonary) form, the subacute edematous (cardiac) form, the mild fever form, and the acute (mixed) form. Signs vary, depending on which form the horse develops, but fever is common and death comes quickly in all but the mild form. Horses are far more susceptible to AHS than other equids. As the name implies, AHS occurs most frequently in Africa, but cases have occurred elsewhere, including Europe. A vaccine against this disease is available, but there is currently no treatment for any form but the mild. 

Glanders This disease is caused by Burkholderia mallei, a bacterium transmitted by contact with infected equids’ respiratory and skin secretions (e.g., through skin abrasions or mucous membranes, such as the lips and gums). Animals can also become infected when they ingest B. mallei in contaminated food or water. The bacteria can pass from horse to horse via aerosol transmission (when infected animals sneeze and cough) and fomites such as harnesses, grooming tools, food, and water troughs. Different forms of the disease impact horses, depending on the route of infection: nasal, pulmonary, or cutaneous. Veterinarians commonly note abscesses in all three forms. While many other countries have eradicated the disease by testing for and euthanizing positive animals (there is currently no vaccine or treatment), glanders remains endemic in many areas of the world, including Asia, Africa, the Middle East, and South America. The last known U.S. case was reported in the 1940s.

Dourine This venereal disease is caused by the protozoon Trypanosoma equiperdum and affects horses, donkeys, and mules. It is spread primarily during breeding, but the parasite can also pass from mare to foal during birth or via colostrum. Affected horses have swollen genitals, skin lesions (often referred to as silver dollar plaques), and neurologic signs. Many countries have limited their dourine occurrence by testing for and euthanizing positive animals (again, no vaccine or treatment), and the disease primarily exists in areas without testing policies (such as Africa, Asia, and Russia).

Identifying and Controlling Disease

Carolan says APHIS Veterinary Services’ primary mission is to protect American animal health and economic viability. “Perhaps one of the most important aspects of that mission would be preparing for, conducting surveillance for, and responding to an FAD,” he says.

For ease of discussion, Scott Weese, DVM, DVSc, Dipl. ACVIM, a professor in the Department of Pathobiology at the University of Guelph’s Ontario Veterinary College, in Canada, suggests that foreign diseases can be categorized into one of three classes:

  1. Known diseases Equine infectious anemia is an example of a disease we know occurs in the United States and Canada, but veterinarians keep it at very low levels via testing and control programs. 
  2. Foreign diseases that could be introduced An example of a foreign disease that has the potential to establish in North America is AHS. The insect vector capable of spreading AHS exists in North America, which means it could spread and would likely be difficult and costly to eradicate if introduced. Veterinary officials are monitoring for AHS carefully not only in North America but also in other AHS-free countries, such as the United Kingdom.  
  3. Newly identified (emerging) diseases The Hendra virus in Australia is an example of an emerging disease. Veterinarians identified it in 1994, and it currently only occurs in Australia. 

“For any of these diseases—be they existing, new, or foreign—prevention and control methods should rely on science-based information along with risk assessments,” Carolan says.

“We can’t specifically target unknown bugs, but we can have surveillance and reporting mechanisms in place to allow for rapid identification and response,” Weese adds. “That’s really of relevance for any new disease threat.”

Recognizing the importance of real-time tracking, Weese has launched an online disease mapping system to identify and track selected infectious diseases (Wormsandgermsmap.com).

“Data are entered by veterinarians and/or technicians using specific case definitions,” Weese explains. “It also includes ‘unknown’; causes of disease outbreak. This program will let us track the usual suspects, but also document unclassified outbreaks (which can sometimes be the first sign of something suspicious), and we can add any disease at any time, should a new threat emerge.”

Responding to an FAD

In the event of a possible FAD case, your veterinarian, acting either as a private practitioner or as a federally accredited vet, will report clinical signs to the state veterinarian and/or federal officials. 

“This is one of the first and most important steps in responding appropriately to an FAD,” says Carolan. “An investigation will likely be conducted, and samples may be sent to a laboratory in either Ames, Iowa, or Plum Island, New York (for testing).”

What happens after veterinarians identify an FAD varies, depending on the disease and the extent of the outbreak. 

“In general, the goal is to contain the disease and limit spread by (implementing) quarantine, testing, and removing infected animals,” Carolan says.

Vaccination can also play an important role in disease control—even in outbreaks of newly introduced diseases. In the case of WNV, the pharmaceutical company Fort Dodge responded rapidly, making a vaccine available by the summer of 2001. As a result of many individuals’ concerted efforts to recognize the new threat and respond appropriately with measures from elimination of mosquito-breeding grounds to preventive spraying, West Nile virus-related deaths have decreased in multiple species. And the number of equine cases has declined overall: According to USDA-APHIS, 15,257 cases were reported in 2002 and approximately 377 in 2013.

The Economic Impact of FADs

In the case of WNV, veterinarians rapidly identified the FAD and manufacturers promptly produced a vaccine; however, thousands of horses died because of the disease, and manufacturing the vaccine was expensive, as was educating veterinarians and the public about WNV. The price tag attached to controlling and eliminating disease can be hefty. Doctors successfully eradicated smallpox in humans worldwide, for example, but that effort cost $300 million. For rabies, the U.S. Centers for Disease Control and Prevention (CDC) reports, “The estimated public health costs associated with disease detection, prevention, and control have risen, exceeding $300 million annually. These costs include the vaccination of companion animals, animal control programs, maintenance of rabies laboratories, and medical costs, such as those incurred for rabies postexposure prophylaxis.”

The CDC also indicates that only two or three human cases of rabies are reported annually, so, “The cost per human life saved from rabies ranges from approximately $10,000 to $100 million.”

In addition to the expense, there are a number of reasons why diseases are difficult (and in some cases impossible) to eradicate from a country. For example, many working horses live in remote areas that might not be amenable to implementing changes to animal health care. Additionally, political insecurity in some countries and inadequate funding can severely limit efforts to control equine diseases.

Take-Home Message

Foreign animal diseases pose a real risk to the health of North America’s entire horse population (and the industry overall). Horse owners and veterinarians serve as the front line of defense against FADs because we are often the first to notice abnormal behaviors or clinical signs of disease in our horses. Be sure to call your veterinarian when concerned about your horse’s health to expedite the swift diagnosis and containment of any FAD that enters the country.

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Written by:

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she’s worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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