Equine respiratory viruses in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Massey University, Turitea, Palmerston North, New Zealand
Equine respiratory disease is a cause of wastage resulting in financial losses for the equine industry throughout the world. A serological and virological survey was conducted on samples collected from a total of 133 horses from different parts in New Zealand. Three groups of foals were sampled on a monthly basis, five outbreaks of respiratory disease were investigated, and samples were collected from 37 yearlings during and following the yearling sales. The only viruses isolated were equine herpesviruses (EHV) types 2, 5, and 4. EHV-2 was isolated from 99% of peripheral blood leucocyte (PBL) samples from foals sampled on a monthly basis and from PBL of 96% of horses from outbreaks and yearlings from the sales. Additionally, EHV-2, EHV-5 or both were isolated from nasal swabs of up to 100% of foals sampled on a monthly basis between March and July. The time of virus excretion from the nasal cavity varied slightly between the three groups. The rate of virus isolation from the nasal swabs was highest at the time when most foals from two of the groups experienced some respiratory signs. Foals from the remaining group, however, were healthy throughout the study period. Of horses from outbreaks and yearlings from the yearling sales, EHV-2, EHV-5, or both were isolated from nasal swabs of 35% of horses showing respiratory signs, 9.5% of healthy horses, and 37.5% of horses for which individual clinical data were not available. EHV-4 was isolated on only one occasion, from PBL of a foal with respiratory disease. There was serological evidence that EHV-1, equine adenovirus-1 (EAdV-1), and equine rhinoviruses (ERhV) types 1 and 2 are all present in New Zealand. The average antibody seroprevalence to these viruses was 67%, 61%, 78%, and 13%, respectively. All serum samples tested were negative for antibodies to equine arteritis virus, mammalian reovirus-3 and parainfluenza virus-3. Most of the foals sampled showed serological evidence of infection with EHV-1/4 (78%), EAdV-1 (61%), and ERhV-2 (65%) within their first year of life. There was no indication that any of the foals sampled became infected with ERhV-1 within the period of study. Samples for virus isolation and two blood samples for serology were collected from 54 of 82 (66%) horses sampled from outbreaks and yearlings from the sales for which individual clinical data were available. These included 35 horses showing signs of respiratory disease around the time of sampling and 19 healthy horses. For the remaining 28 horses, either individual clinical data were not available, or the second blood sample for serology was not collected. Recent viral infection was not associated with development of respiratory signs in yearlings from the sales when all viruses were considered, although this result was not statistically significant (adjusted OR 1.3, p = 0.5). Equine herpesvirus-2/5 and ERhV-2 infections appeared to be associated with development of clinical signs in yearlings from the yearling sales, although these results were significant only for EHV-2/5, and not ERhV-2. However, since none of the foals or horses sampled was examined endoscopically, it is possible that a number of lower airway infections were not recognised. The most common infection among horses with respiratory signs from outbreaks, for which paired serum samples were available, was EHV-2/5 infection (30.4%), followed by ERhV-2 (13.0%), ERhV-1 (4.3%), and EHV-1/4 (4.3%) infections. None of the 56 horses for which a full set of data were available showed serological evidence of recent EAdV-1 infection and only two horses showed serological evidence of recent ERhV-1 infection. Most horses with signs of respiratory disease that showed serological evidence of recent viral infection also yielded EHV-2 or EHV-5 from their nasal swabs, indicating that EHV-2/5 either predisposes to other infections, or that infection with other viruses re-activates latent EHV-2/5. During the survey, EHV-5 was isolated on 56 occasions. This represented the first isolation of this virus outside Australia. Representative New Zealand isolates were compared to the reference Australian strain by restriction digest of the cloned glycoprotein B gene. Restriction fragment length polymorphism (RFLP) profiles of all but one New Zealand isolate differed from the RFLP pattern of the prototype strain. With few exceptions, isolates from different horses showed different RFLP profiles. However, isolates from individual horses, collected either at different times, from different sites, or grown on different cells showed identical RFLP patterns. The effect of EHV-2 infection on gene expression in equine leucocytes was investigated by representational difference analysis of cDNA. The results suggested that EHV-2 infection of leucocytes down-regulates the expression of monocyte chemoattractant protein-1. This indicates that EHV-2 has the ability to modulate the chemokine environment of infected cells and may predispose to secondary infections. This work has contributed to the understanding of factors involved in equine respiratory disease in New Zealand. Although infection with none of the viruses was detected only in horses showing respiratory signs, the results suggest that EHV-2/5 and equine rhinoviruses may be more important than previously thought.