IN-DEPTH: INFECTIOUS DISEASE OUTBREAK MANAGEMENT


quarantine, in most cases this is not necessary and we tend to jointly respond to regulatory diseases using the state’s quarantine authority. Some of the resources that the federal animal health authority (USDA-APHIS–Veterinary Services) brings to a re- sponse effort include field personnel, such as veter- inary medical officers and animal health technicians, laboratory diagnostic support through the National Veterinary Services Laboratories and the National Animal Health Laboratory Network, subject matter experts and epidemiological support, and trade support and negotiation. Overall, it is the quick identification and rapid response involving strong collaboration between state animal health officials, federal animal health officials, private practitioners, academia, laboratories, and the equine industry that results in the most successful responses to regulatory disease outbreaks.


3. Recent High-Impact Equine Disease Outbreaks


A “high-impact” or “high-consequence” equine dis- ease is a disease that has one or more of the follow- ing characteristics:


● Results in high morbidity or high mortality ● Has the potential for human health implications


● Is listed as a foreign-animal disease ● Domestic disease with new increased/unex- pected virulence


● Disease with limited intervention options ● Disease that has severe or debilitating trade ramifications


● Results in an outbreak that affects a large number of equids/owners/premises


● Any disease that elicits a palpable level of con- cern or panic in the equine industry


Unfortunately, the United States has experienced a number of equine disease outbreaks in recent years that could be considered to fit into this “high-impact” category. The following includes summary infor- mation and lessons learned on some of the most significant high-impact equine disease outbreaks encountered.


4. Contagious Equine Metritis


In December 2008, an active breeding stallion in Kentucky was found to be culture positive for Tay- lorella equigenitalis, the causative agent of conta- gious equine metritis (CEM), when he was sampled to qualify for semen export. Given that CEM is a foreign-animal disease in the United States, an ex- tensive regulatory investigation ensued during the next 2 years in which 1005 horses (278 stallions and 727 mares) were determined to have been directly exposed in 48 states. Diagnostic testing of the ex- posed horses eventually led to the finding of 22 stal- lions, one gelding, and five mares being chronic carriers of Taylorella equigenitalis at the time they were tested.3 It is likely that many more mares


may have actually become infected immediately af- ter their exposures to positive stallions in the out- break; however, due to the ability of many mares to clear the infection on their own with only a small percentage resulting in a chronic carrier state, it is unknown just how many mares were actually previ- ously infected in the incident. Epidemiologic anal- ysis suggested that all of the positive horses were linked to a single common source, most likely a Fjord stallion imported into the United States in 2000.3 The investigation also showed that stallion-to-stal- lion transmission occurred through contaminated fomites at breeding facilities such as inadequately cleaned and disinfected artificial vaginas, wash buckets, breeding dummies, hands, or other sources. Transmission to mares occurred through both live- cover breeding and artificial insemination even when semen extenders containing seemingly appro- priate antibiotics were used. All exposed horses were quarantined and could


not be used for breeding activities until completion of the regulatory testing protocol yielded all nega- tive results. This was an expensive process espe- cially for stallion owners given the official protocol, based on our rigorous U.S. CEM-import protocol, required test breeding of two qualified mares and a sampling period over approximately 30 days to be completed. Although some cost sharing was pro- vided in which USDA-APHIS covered the cost of the sample shipping and diagnostic testing and state animal health officials provided personnel resources and oversight, the exposed horse owner had to cover the cost of sample collection by their private practi- tioner. This burden ended up being cost prohibi- tive for some stallion owners at the time and those stallions were forced to stay under quarantine until the sampling and testing could be completed. All of the infected horses were ultimately success-


fully treated, confirmed as negative on retesting, and released from quarantine with the response to the incident completed in late 2010. Although this outbreak was the largest CEM outbreak in U.S. history both in geographic scope and numbers of horses involved, other smaller outbreaks of CEM have occurred since with most cases traced back to some involvement with previously imported horses. This leaves concern as to how imported horses in- fected with T. equigenitalis are slipping though our import-testing protocols and whether exemption from CEM testing on horses imported from certain countries might be contributing to the problem. In addition, as was the case in the 2008–2010 out- break, most of the subsequent index-infected stal- lion cases in the United States were identified on routine culture for qualification to export semen to another country. The absence of adequate domes- tic surveillance for T. equigenitalis in the U.S. active breeding stallion population almost guarantees that CEM cases will not be detected quickly and, there- fore, may eventually lead to another outbreak of size


AAEP PROCEEDINGS  Vol. 62  2016 343


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