Viruses associated with respiratory disease of horses in New Zealand: an update

Detection and Nucleotide Sequencing of a DNA-Packaging Protein Gene of Equine Gammaherpesviruses

In previous studies, novel putative viral pathogens designated that asinine herpesvirus 4 (AsHV4) and asinine herpesvirus 5 (AsHV5) were associated with fatal interstitial pneumonia in donkeys ( Equus asinus). Nucleotide sequence analysis of a portion of the DNA polymerase gene identified these putative pathogens as herpesviruses and possibly as members of the Gammaherpesvirinae subfamily. Although similar to equine herpesvirus 2 (EHV2) and equine herpesvirus 5 (EHV5), sequence diversity was observed among the detected viruses. In this study, novel sequence is reported for a DNA-packaging protein gene of EHV5 plus AsHV4, AsHV5, and a newly described putative pathogen herein designated asinine herpesvirus 6 (AsHV6). Phylogenetic analysis of these sequences suggested that the equine gammaherpesviruses may form a separate clade within the Gammaherpesvirinae subfamily. Based on the sequence of EHV2 and the novel sequences reported in this study, a PCR assay was developed to detect equine gammaherpesviruses. Products of the predicted size were produced after amplification of DNA from EHV2, EHV5, AsHV4, AsHV5, and AsHV6. This nonnested assay was shown to consistently amplify approximately 10 genomic copies of EHV2. Amplification products were not produced from DNA template of other alpha- and gammaherpesviruses. Because the role of gammaherpesviruses has not been well defined in equine disease, it is envisioned that a single, sensitive PCR assay to detect these potential pathogens will facilitate further assessment of their role in disease.

A survey of respiratory viruses in New Zealand horses

AIMS

To determine which viruses circulate among selected populations of New Zealand horses and whether or not viral infections were associated with development of respiratory disease.

METHODS

Nasal swabs were collected from 33 healthy horses and 52 horses with respiratory disease and tested by virus isolation and/or PCR for the presence of equine herpesviruses (EHV) and equine rhinitis viruses.

RESULTS

Herpesviruses were the only viruses detected in nasal swab samples. When both the results of nasal swab PCR and virus isolation were considered together, a total of 41/52 (79%) horses with respiratory disease and 2/32 (6%) healthy horses were positive for at least one virus. As such, rates of virus detection were significantly higher (p<0.001) in samples from horses with respiratory disease than from healthy horses. More than half of the virus-positive horses were infected with multiple viruses. Infection with EHV-5 was most common (28 horses), followed by EHV-2 (27 horses), EHV-4 (21 horses) and EHV-1 (3 horses).

CONCLUSIONS

Herpesviruses were more commonly detected in nasal swabs from horses with respiratory disease than from healthy horses suggesting their aetiological involvement in the development of clinical signs among sampled horses. Further investigation to elucidate the exact relationships between these viruses and respiratory disease in horses is warranted.

CLINICAL RELEVANCE

Equine respiratory disease has been recognised as an important cause of wastage for the equine industry worldwide. It is likely multifactorial, involving complex interactions between different microorganisms, the environment and the host. Ability to control, or minimise, the adverse effects of equine respiratory disease is critically dependent on our understanding of microbial agents involved in these interactions. The results of the present study update our knowledge on the equine respiratory viruses currently circulating among selected populations of horses in New Zealand.

First genetic characterization of equine adenovirus type 1 (EAdV-1) in Turkey

Equine adenovirus type 1 (EAdV-1) is a cause of repiratory tract infection in equids. In present study for the first time in Turkey, the prevalence of EAdV-1 in nasal swab samples obtained from horses showing respiratory symptoms was investigated by polymerase chain reaction (PCR), and molecular characterization of the hexon gene detected in the Turkish (TR) strain was performed. Overall, the prevalence of EAdV-1 was found low (1.4%) as indicated by a positive PCR reaction from the nasal swab extracts tested. Phylogenetic analysis based on the partial sequences of the hexon gene of a TR-EAdV-1 strain with those of previously isolated AdVs from different mammals and an EAdV-1 M1 strain showed that the EAdV-1 strains were placed into a unique cluster. Although the TR-EAdV-1 strain was closely related to CAV-1, CAV-2 and bat adenovirus reference strains, larger-scale studies are necessary to better understand the molecular epidemiology and population structure of EAdV-1 in Turkey.

Equine gammaherpesviruses: pathogenesis, epidemiology and diagnosis

Equine gammaherpesviruses (γEHV) have been widely studied over the past 45 years and many isolates have been characterised. Despite this, the diagnosis of γEHV infection remains difficult to establish as its clinical manifestations lack specificity, ranging from mild respiratory signs in a small number of animals to outbreaks in large groups of young horses. This review focuses on the epidemiology, pathogenesis, clinical manifestations and diagnosis of equine herpesvirus (EHV)-2 and -5 infections, as well as on the genetic variation of these viruses. Study of these variations has resulted in hypotheses relating to viral re-infection and re-activation. Interestingly, the viruses were found to contain genetic sequences identical to those of eukaryotic cells which are considered central to the development of viral latency through interfering with host immune and inflammatory responses. Future molecular biological studies will further elucidate the virulence mechanisms of these equine pathogens.

Prevalence of serum neutralising antibody to equine rhinitis A virus (ERAV), equine rhinitis B virus 1 (ERBV1) and ERBV2

The objective of this study was to determine the incidence of serum neutralising (SN) antibody to ERAV, ERBV1 and ERBV2 in a population of horses from birth to 22 years of age. The prevalences of ERAV, ERBV1 and ERBV2 SN antibodies in 381 sera obtained from 291 horses were 37%, 83% and 66%, respectively. ERAV, ERBV1 and ERBV2 maternal antibody was present in foals 12 h postsuckling but by 10-12 months, ERAV SN antibody was not detected in any of the horses, while ERBV1 and ERBV2 SN antibodies were common (83% and 100%, respectively). Sera were obtained from 44 Thoroughbred horses when they were newly introduced into a training centre when their average age was 23 months and a second sample was obtained approximately 7 months later. ERAV SN antibody was present in 8 (18%) when first bled and in 27 (61%) when tested 7 months later. Accordingly 19 of the 44 horses (43%) seroconverted to ERAV within 7 months of entering the training stable. Among all the horses the average ERAV SN antibody titre was relatively high (3796) and in contrast, ERBV1 and ERBV2 titres were relatively low (average 84 and 45, respectively) and often fell to below detectable levels over time and at a rate comparable to new seroconversions in the same group of horses.

EHV-1 and EHV-4 infection in vaccinated mares and their foals

A silent cycle of equine herpesvirus 1 infection was described following epidemiological studies of unvaccinated mares and foals on a Hunter Valley stud farm. Following the introduction of routine vaccination with an inactivated whole virus equine herpesvirus 1 (EHV-1) and equine herpesvirus 4 (EHV-4) vaccine in 1997, a subsequent study identified excretion of EHV-1 and EHV-4 in nasal swab samples tested by PCR from vaccinated mares and their unweaned, unvaccinated foals. The current sero-epidemiological investigation of vaccinated mares and their young foals found serological evidence of EHV-1 and EHV-4 infection in mares and foals in the first 5 weeks of life. The results further support that EHV-1 and EHV-4 circulate in vaccinated populations of mares and their unweaned foals and confirms the continuation of the cycle of EHV-1 and EHV-4 infection.

Viruses associated with outbreaks of equine respiratory disease in New Zealand

AIM

To identify viruses associated with respiratory disease in young horses in New Zealand.

METHODS

Nasal swabs and blood samples were collected from 45 foals or horses from five separate outbreaks of respiratory disease that occurred in New Zealand in 1996, and from 37 yearlings at the time of the annual yearling sales in January that same year. Virus isolation from nasal swabs and peripheral blood leukocytes (PBL) was undertaken and serum samples were tested for antibodies against equine herpesviruses (EHV-1, EHV-2, EHV-4 and EHV-5), equine rhinitis-A virus (ERAV), equine rhinitis-B virus (ERBV), equine adenovirus 1 (EAdV-1), equine arteritis virus (EAV), reovirus 3 and parainfluenza virus type 3 (PIV3).

RESULTS

Viruses were isolated from 24/94 (26%) nasal swab samples and from 77/80 (96%) PBL samples collected from both healthy horses and horses showing clinical signs of respiratory disease. All isolates were identified as EHV-2, EHV-4, EHV-5 or untyped EHV. Of the horses and foals tested, 59/82 (72%) were positive for EHV-1 and/or EHV-4 serum neutralising (SN) antibody on at least one sampling occasion, 52/82 (63%) for EHV-1-specific antibody tested by enzyme-linked immunosorbent assay (ELISA), 10/80 (13%) for ERAV SN antibody, 60/80 (75%) for ERBV SN antibody, and 42/80 (53%) for haemagglutination inhibition (HI) antibody to EAdV-1. None of the 64 serum samples tested were positive for antibodies to EAV, reovirus 3 or PIV3. Evidence of infection with all viruses tested was detected in both healthy horses and in horses showing clinical signs of respiratory disease. Recent EHV-2 infection was associated with the development of signs of respiratory disease among yearlings [relative risk (RR)=2.67, 95% CI=1.59-4.47, p=0.017].

CONCLUSIONS

Of the equine respiratory viruses detected in horses in New Zealand during this study, EHV-2 was most likely to be associated with respiratory disease. However, factors other than viral infection are probably important in the development of clinical signs of disease.

Equine viral arteritis control scheme: a brief review with emphasis on laboratory aspects of the scheme in New Zealand

AIM

To review laboratory aspects of the equine viral arteritis (EVA) control scheme in New Zealand between 1989 and 2002.

METHODS

The optimisation and performance of the virus neutralisation test (VNT) for equine arteritis virus (EAV) antibody, and the cell culture test to detect EAV in semen were analysed. Laboratory data and control scheme results were reviewed.

RESULTS

Using optimised tests, it has been shown that antibody prevalence in Standardbred horses has steadily declined from 54% to <20%. Prevalences in Thoroughbred horses have remained at a low level of around 3%. The number of horses shedding EAV (all Standardbreds) has steadily declined from a maximum at any one time of 20 to the current figure of three.

CONCLUSION

Eradication of EVA from the horse population in New Zealand is achievable in the near future.

Equine respiratory viruses in foals in New Zealand

AIMS

To identify the respiratory viruses that are present among foals in New Zealand and to establish the age at which foals first become infected with these viruses.

METHODS

Foals were recruited to the study in October/ November 1995 at the age of 1 month (Group A) or in March/ April 1996 at the age of 4-6 months (Groups B and C). Nasal swabs and blood samples were collected at monthly intervals. Nasal swabs and peripheral blood leucocytes (PBL) harvested from heparinised blood samples were used for virus isolation; serum harvested from whole-blood samples was used for serological testing for the presence of antibodies against equine herpesvirus (EHV)-1 or -4, equine rhinitis-A virus (ERAV), equine rhinitis-B virus (ERBV), equine adenovirus 1 (EAdV-1), equine arteritis virus (EAV), reovirus 3 and parainfluenza virus type 3 (PIV3). Twelve foals were sampled until December 1996; the remaining 19 foals were lost from the study at various times prior to this date.

RESULTS

The only viruses isolated were EHV-2 and EHV-5. EHV-2 was isolated from 155/157 PBL samples collected during the period of study and from 40/172 nasal swabs collected from 18 foals. All isolations from nasal swabs, except one, were made over a period of 2-4 months from January to April (Group A), March to April (Group B) or May to July (Group C). EHV-5 was isolated from either PBL, nasal swabs, or both, from 15 foals on 32 occasions. All foals were positive for antibodies to EHV-1 or EHV-4, as tested by serum neutralisation (SN), on at least one sampling occasion and all but one were positive for EHV-1 antibodies measured by enzyme-linked immunosorbent assay (ELISA) on at least one sampling occasion. Recent EHV-1 infection was evident at least once during the period of study in 18/23 (78%) foals for which at least two samples were collected. SN antibodies to ERBV were evident in 19/23 (83%) foals on at least one sampling occasion and 15/23 foals showed evidence of seroconversion to ERBV. Antibodies to ERAV were only detected in serum samples collected from foals in Group A and probably represented maternally-derived antibodies. Haemagglutination inhibition (HI) antibody titres 1:10 to EAdV-1were evident in 21/23 (91%) foals on at least one sampling occasion and 16/23 foals showed serological evidence of recent EAdV-1 infection. None of the 67 serum samples tested were positive for antibodies to EAV, reovirus 3 or PIV3. There was no clear association between infection with any of the viruses isolated or tested for and the presence of overt clinical signs of respiratory disease.

CONCLUSIONS

There was serological and/or virological evidence that EHV-1, EHV-2, EHV-5, EAdV-1 and ERBV infections were present among foals in New Zealand. EHV-2 infection was first detected in foals as young as 3 months of age. The isolation of EHV-2 from nasal swabs preceded serological evidence of infection with other respiratory viruses, suggesting that EHV-2 may predispose foals to other viral infections.

Association of two newly recognized herpesviruses with interstitial pneumonia in donkeys (Equus asinus)

Over a period of 6 years, antemortem and postmortem examinations were performed on a number of donkeys suffering from respiratory disease. For many cases, initial diagnostic efforts failed to identify an etiology consistent with the pathologic findings. However, retrospective examination of these cases using consensus primer polymerase chain reaction, designed to recognize herpesviruses from all 3 subfamilies of the Herpesviridae, amplified a fragment of the highly conserved herpesvirus DNA polymerase gene from a number of these animals. Two novel herpesviruses, herein designated asinine herpesvirus 4 (AHV4) and asinine herpesvirus 5 (AHV5), were consistently detected in lung tissue from donkeys in which the histopathology was characterized by interstitial pneumonia and marked syncytial cell formation but not in lung tissue from donkeys with evidence of bacterial or verminous pneumonia. Nucleotide sequence and phylogenetic analysis places these new viruses within the Gammaherpesvirinae subfamily and indicates that they are most closely related to the recently identified zebra herpesvirus and wildass herpesvirus as well as equine herpesviruses 2 and 5.

Equine gammaherpesvirus 2 (EHV2) is latent in B lymphocytes

Peripheral blood leukocytes were collected from 5 Thoroughbred horses and examined for the presence of EHV2 in sub-populations of mononuclear cells. Peripheral blood mononuclear cells were separated on Percoll gradients and then enriched for plastic adherent cells (predominantly monocytes), surface immunoglobulin positive (sIg+) B lymphocytes and T lymphocytes, using panning techniques. The purity of each cell population was assessed by fluorescence activated cell scanning. In an infectious centre assay, each cell population was inoculated onto equine foetal kidney monolayer cell cultures which are fully permissive for the replication of EHV2. Only enrichment for sIg+ B lymphocytes resulted in a marked increase in the number of infectious centres, indicating that EHV2 is present in B lymphocytes. Freeze-thawing of sIg+ B lymphocytes, prior to inoculation onto EFK monolayer cell cultures, resulted in the complete abrogation of infectious centre formation, confirming that EHV2 is latent in B lymphocytes i.e., infectious free virus was not present in the cells. The number of EHV2 infected B lymphocytes varied considerably between horses from 4 to 780 per 10(6) cells. Evidence was also obtained that direct cell to cell contact between the epithelial cells and sIg+ B lymphocytes was necessary for the production of infectious centres. The data indicate that EHV2, like other members of the Gammaherpesvirinae, is latent within B lymphocytes.

Equine herpesvirus 5: comparisons with EHV2 (equine cytomegalovirus), cloning, and mapping of a new equine herpesvirus with a novel genome structure

A new equine herpesvirus, provisionally designated equine herpesvirus 5 (EHV5; Browning and Studdert (1987) J. Gen. Virol. 68, 1441-1447), was examined for the degree of genomic difference from equine herpesvirus 2 (EHV2) by Southern hybridizations. EHV5 and EHV2 whole genomic DNA probes were highly specific for homologous DNA only, indicating that significant genomic difference exists between the two viruses. Restriction endonuclease analysis of EHV5 strain 2-141 (EHV5.2-141) revealed that the genome is 179 kb and exists as a single isomer. Clones representing 82% of the genome were obtained and used to construct restriction maps for four restriction endonucleases. Hybridization experiments indicated that the EHV5.2-141 genome does not contain large terminal or internal repeats, although some evidence for very short repeated sequences in the genomic termini was obtained. Such a genome structure makes EHV5 unique among the equine herpesviruses but similar to the mouse, rat, and guinea pig cytomegaloviruses and the tupaiid herpesvirus. Sequence analysis of one of the genomic termini of EHV5.2-141 revealed the presence of a 30-bp sequence (pac-1; Deiss et al. (1986) J. Virol. 59, 605-618) which is highly conserved among herpesviruses.

Physical mapping of a genome of equine herpesvirus 2 (equine cytomegalovirus)

The genome of a low equine cell passage equine herpesvirus 2 was partially cloned and physical maps for the restriction endonucleases BamHI, EcoRI, HindIII, and SalI determined. The genome length was estimated to be 192 kilobase pairs (kbp) and no evidence of isomerization was found. Two separate repeat structures were detected: 18 kbp direct terminal repeats; and an unrelated second pair of short internal, indirect repeats at 0.20 and 0.75 map units. Such a genomic structure does not appear to have been reported amongst the herpesviruses--all the genomes that do not isomerize either have repeat structures only at the termini, or if present internally, have only direct repeats.

Asinine herpesvirus genomes: comparison with those of the equine herpesviruses

Two previously unknown and distinct herpesviruses were isolated from donkeys. One, with the characteristics of a betaherpesvirus, was isolated from the leukocytes of an apparently healthy donkey, while the second, an alphaherpesvirus, was recovered from the nasal cavity of donkeys given high doses of corticosteroids, and caused rhinitis in two seronegative weanling donkeys when they were intranasally infected. Few, if any, restriction endonuclease fragments were shared by the donkey betaherpesvirus, equine herpesvirus 2 (EHV 2) or EHV 5, a second distinctly different equine betaherpesvirus, nor by the donkey alphaherpesvirus, EHV 1, EHV 4, or EHV 3. In Southern blot analysis the donkey betaherpesvirus showed low levels of sequence similarity to both EHV 2 and EHV 5, while the donkey alphaherpesvirus and EHV 1 shared a moderate degree of sequence similarity, less similarity with EHV 4 and very low level of sequence similarity with EHV 3. These two isolates appear prototypic of two previously unrecorded herpesviruses for which the names asinine herpesvirus 2 and 3 are suggested for the betaherpesvirus and the alphaherpesvirus respectively.