Evaluation of an air tester for the sampling of aerosolised equine herpesvirus type 1.

EQUINE herpesvirus type 1 (EHV-1) is a major equine pathogen, responsible for well-documented syndromes of respiratory disease, abortion, neonatal foal death and myeloencephalopathy (Van Maanen 2002). Recent outbreaks at racetracks, riding schools and veterinary hospitals have provided ample documentation of the contagiousness of EHV-1 and highlight the importance of early detection (Kohn and others 2006, Henninger and others 2007). EHV-1 commonly spreads directly via horse-to-horse contact, as well as indirectly via contaminated fomites and personnel. Aerosolised droplets of respiratory secretions represent the primary route of transmission in horses with infection of the respiratory tract (Slater 2007). During recent outbreaks of neurological disease associated with EHV-1, the relative importance of direct contact and/or aerogenic transmission was not determined, due to a lack of temporal and spatial information for the affected horse populations. The titre of EHV-1 present in nasal mucus can be as high as 106 plaque-forming units (pfu) per swab following primary infection (Burrows and Goodridge 1972). However, viral loads in nasal secretions are lower (102 to 105 pfu/swab) in horses following either re-exposure or recrudescence of a latent state (Burrows and Goodridge 1975). The airborne transmission of herpesviruses, such as bovine herpesvirus type 1, has been shown to occur at distances of up to 3·85 m under experimental conditions (Wentink and others 1993, Mars and others 1999). However, such information is not available for EHV-1. Air-sampling devices are used routinely in the pharmaceutical, medical and industrial settings to investigate the presence of airborne pathogens. The objective of this study was to evaluate the capability of a commercially available air tester to sample aerosolised EHV-1 for detection by real-time PCR. The study was performed in rooms with similar air volumes of approximately 3426 m3. The doors and windows were kept closed during each experiment. A commercially available modified life virus vaccine (Rhinomune; Pfizer Animal Health) was used for the study. The concentration of the virus, determined via real-time PCR, was 8 x 109 EHV-1 gene copies per vaccine vial (total volume 1 ml). The vaccine virus was nebulised using a commercial ultrasonic nebuliser (Devilbiss; Sunrise Medical), according to the manufacturer’s guidelines, to deliver particles smaller than 5 μm. A total of four separate nebulisation events were performed, one week apart, with increasing doses of EHV-1 (8 x 106, 8 x 107, 8 x 108 and 8 x 109 gene copies). Each nebulisation event was performed in a different room. The study protocol was as follows. The nebuliser was placed in the middle of the room, at a height of 30 cm, and 10 ml of the EHV-1 solution was nebulised over a 10-minute period, followed by an additional five minutes to allow the air to settle. Thereafter, air samples (500 litres) were collected at a height of 30 cm using a portable air-monitoring system (M Air T; Millipore) according to the manufacturer’s guidelines, at 14·5, 9·6, 4·8, 1·5 and 0·5 m from the nebuliser. The portable air tester works on the basis of sampling a pre determined volume of air through a sieve directly onto a retrievable agar plate. Each sampling period lasted 3·5 minutes, and during each event, care was taken to follow a far-to-close samVeterinary Record (2008) 163, 306-308