Horses exhibiting sudden and severe neurological symptoms consistent with a diagnosis of EHV-1 pose a definite risk to the surrounding population of horses. Consequently, early intervention to prevent the spread of infection is required. Disease control measures based on established medical practices and recent successful experiences to control the spread of EHV-1 indicate the following measures to be reasonable.
Isolation of Sick Horses
To prevent an infected horse from having any further contact with other horses in the stable environment, any individual horse with clinical signs consistent with neurological EHV-1 infection should be removed immediately from the area of other horses and placed in a separate enclosure designated for infectious disease isolation. This isolation stall or enclosure should be located well away from high traffic areas associated with other barns or training areas. Animal caregivers should take precautionary measures to ensure that they do not transmit disease to other horses through contaminated hands, clothing, equipment or tack. It is essential that the isolation facility have supervised oversight by an individual knowledgeable in disease control and quarantine procedures to avoid the possibility of contamination.
Segregation of Exposed Horses
Horses known to have had intimate contact with the diagnosed clinical case of EHV-1 should be maintained in their existing barns and segregated from other horses during exercise periods until the sick horse has been confirmed to have EHV-1 by PCR testing.
Once the confirmation of EHV-1 is made, appropriate focal quarantine measures to restrict the movement of all potentially exposed horses will be necessary to prevent the possible spread of disease to other locations. These procedures may begin with initial restrictions such as the quarantine of those individuals in the immediate area of exposure (i.e., a single barn or other unit of housing) within a horse facility. Horses in the immediate contact area of the clinically affected individual should be monitored closely but tested by PCR for EHV-1 only if they exhibit fever or clinical signs consistent with EHV-1 infection. Since stress may play a role in eliciting the onset of clinical signs, horses stabled in areas of known exposure should not be subjected to strenuous physical exercise or long-distance transport until their health status can be determined.
Expanding the Quarantine Area
If new clinically ill or EHV-1 positive horse(s) are identified in other locations within the equine facility, an additional focal quarantine of exposed horses should be instituted at that location. The area under quarantine may be expanded to include other affected barns. If multiple cases are identified or suspected, the entire stable area may need to be quarantined. The optimum strategy should be the prudent imposition of a series of focal quarantine procedures using an expanding series of "concentric rings" of disease control.
Individual animals that have tested positive for EHV-1 within the designated quarantine area, whether symptomatic or not, should be retested periodically until disease is confirmed or eliminated based upon both a negative PCR test and a lack of clinical signs for the disease. Quarantine measures should be maintained until an absence of further clinical cases and positive tests suggest no new appearance of disease is occurring. At that time, a gradual drawdown of these procedures can be applied. Areas of the facility under focal quarantine may have their restrictions rescinded in a reverse of the concentric-ring approach.
Horses Outside the Quarantine Areas
Since the positive predictive value* of PCR-based tests for EHV-1 in asymptomatic horses is uncertain at this time, horses outside of quarantine areas or in unexposed stables should not be tested on a random basis. The finding of a positive PCR test result in an asymptomatic horse does not provide conclusive evidence of either active infection or the potential for disease transmission, because low levels of nonreplicating virus may be the source of the viral DNA detected. This means that the detection of virus through PCR analysis does not provide a diagnosis in the absence of clinical signs and/or of other corroborating information. Therefore, the use of this technology as a screening tool for EHV-1 in clinically normal, nonexposed horses is considered inappropriate.
* The positive predictive value of any medical diagnostic test is the relationship between those individuals who test positive to the number of those testing positive that actually develop clinically significant disease.