"When you hear hoofbeats on the covered bridge, please think about the zebra!" said Corrie Brown, DVM, PhD, Dipl. American College of Veterinary Pathology, vice chair of the Cornell Committee for Animal Diseases. Her point was that when veterinarians are making diagnoses, they need to consider foreign animal diseases. "The amount of damage that a foreign animal disease will cause is directly proportional to the time between introduction and accurate diagnosis," she said. "In other words, we have to accurately recognize a foreign animal disease at first blush if we are to implement effective control measures. It is imperative that practitioners consider foreign animal diseases in their diagnostic rule-outs."

Among the diseases that Brown discussed was African horse sickness, glanders, Hendra virus, epizootic lymphangitis, trypanosomal diseases, and Venezuelan equine encephalitis.

She noted that the United States "remains at risk" for incursion of African horse sickness. While it is unknown if our insect species in the Culicoides family can transmit the virus (the research has never been done), she said that, "it seems likely that the virus could be transmitted by our indigenous Culicoides species." She detailed the severe impact on the Iberian peninsula (including Spain) from 1987-1990. It was imported in zebras, which can have a subclinical infection. During the four-year period, hundreds of horses succumbed to the disease or were euthanized. The mortality rate in horses is 70-95%.

"The U.S. quarantine restrictions regarding equine species are designed to protect against introduction of an infected equine," noted Brown. "Consequently, the most likely route of entry would be from an infected insect that comes in through air travel." She added that this is a very difficult disease to control once it gets in the equine and mosquito populations of an area. There is a vaccine.

"Glanders used to be a low priority, but with bioterrorism, the threat (it could be seen in the United States) ratcheted it up," noted Brown. She said during World War I, glanders was used to maliciously to infect horses of enemy troops. The disease was eradicated from the United States in 1937, but was developed as a biological weapon during the cold war era.

The causative bacteria is Burkholderia mallei, which can also cause disease in humans. Brown said the incubation period can be from weeks to months. In non-endemic areas, glanders could be mistaken for strangles, rhodococcal pneumonia, or ulcerative lymphangitis. Affected animals usually are destroyed. Treatment is not recommended, and there is no vaccine.

There were no human cases recorded in the literature after 1949 until an accident in a federal biodefense laboratory in 2000, when a researcher became infected (but recovered).

Hendra Virus
Brown said this is an important emerging disease which was first seen in Australia in 1994. It caused illness and death in horses and humans. It was again seen in incidents in 1995 and 1999. To date, she said, there have been 17 equine and two human deaths from the newly described virus. Fruit bats are the hosts, and they are not clinically affected by the virus.

"The probability of Hendra virus entering the United States is minimal," noted Brown. "However, the same urbanization forces at work in Australia (that brought the bats in closer contact with humans and horses) are also operating on our continent. It is estimated that we know less than 10% of all the bacteria and viruses present in any of our animal species—and normal flora in one when transmitted to another may cae an emerging disease. We are constantly placing new species into evolutionarily incompatible zones and are always at risk of the development of new disease."

Brown added, "We haven't heard the last of this disease."

Epizootic Lymphangitis
Brown considered this rare, chronic disease a low threat to the United States. However, she said, a horse with the disease could be imported. The causative fungus usually manifests itself as chronic ulcerative dermatitis, most commonly on the extremities, when the animal comes into contact with the fungus in contaminated soil (although biting flies have been incriminated).

The disease gets its name because the lymphatics frequently become filled with inflammatory exudates and cells and can rupture along the tracts or "cords" that are formed in the lymphatic system. Brown said that infrequently, the horse can develop pneumonia. "If diagnosed in the United States, prompt and radical attention should be paid to the environment to prevent this fungus from becoming established."

Trypanosomal Diseases
The three specific diseases she discussed in this section were dourine, surra, and nagana. Dourine, a venereal disease that can become systemic, has been eradicated from the United States. It can cause central nervous system problems such as weakness, incoordination, and paralysis. Treatment usually isn't undertaken because a carrier state can be induced.

"The most likely mode of entry for dourine into the United States would be in an infected animal," said Brown. "Because all imported animals are examined for signs of disease, incursion is highly unlikely."

The non-specific nature of surra makes it a difficult disease to diagnose. Clinical signs can include weakness, malaise, anemia, and ultimately death. Without treatment, the mortality rate is 100%.

"Introduction of Trypanosoma evansi (the causative agent) into the United States would most likely be through an infected animal," said Brown. "Because this is a vector-borne disease, immediate recognition is imperative to institute sufficient control measure to keep flies from becoming infected and disseminating the disease."

Nagana, or African animal trypanosomiasis (AAT), is transmitted by biting insects, including the tsetse fly. Primarily a disease of cattle in Africa, AAT can infect and cause disease in horses and people. Clinical signs of the three primary trypanosome species viruses are weakness, lethargy, and anemia.

"Introduction of any of these agents of AAT into the United States is unlikely," she said. "Establishment of the disease within our borders is even less likely because of the absence of the primary fly vector (tsetse fly)."

Venezuelan Equine Encephalitis
"Of the three encephalitides—Eastern, Western, and Venezuelan—only the latter is a foreign animal disease," explained Brown. However, Venezuelan equine encephalitis (VEE) is a big risk for the United States.

The last known cases of VEE in the United States occurred in 1971, but there are continued outbreaks in Mexico and the northern part of South America. Morbidity (illness) is high, and mortality ranges from 40-90%. There are vaccines against the encephalitides.

One major problem with VEE is that the horse is an amplifying host for the causative virus, which is transmitted by mosquitoes. VEE also can affect humans.

If the disease were introduced into the United States, it would be very important to effectively quarantine affected horses to decrease spread, noted Brown.

In conclusion, she stated, "There are two ways a veterinarian can become 'famous' when a foreign animal disease enters the neighborhood. One way is to diagnose it; the other is to miss it!"


About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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