Untangling the stranglehold through mathematical modelling of Streptococcus equi subspecies equi transmission

Strangles, a disease caused by infection with Streptococccus equi subspecies equi (S. equi), is endemic worldwide and one of the most frequently diagnosed infectious diseases of horses. Recent work has improved our knowledge of key parameters of transmission dynamics, but important knowledge gaps remain. Our aim was to apply mathematical modelling of S. equi transmission dynamics to prioritise future research areas, and add precision to estimates of transmission parameters thereby improving understanding of S. equi epidemiology and quantifying the control effort required. A compartmental deterministic model was constructed. Parameter values were estimated from current literature wherever possible. We assessed the sensitivity of estimates for the basic reproduction number on the population scale to varying assumptions for the unknown or uncertain parameters of: (mean) duration of carriership (1∕γC), relative infectiousness of carriers (f), proportion of infections that result in carriership (p), and (mean) duration of immunity after natural infection (1∕γR). Available incidence and (sero-)prevalence data were compared to model outputs to improve point estimates and ranges for these currently unknown or uncertain transmission-related parameters. The required vaccination coverage of an ideal vaccine to prevent major outbreaks under a range of control scenarios was estimated, and compared available data on existing vaccines. The relative infectiousness of carriers (as compared to acutely ill horses) and the duration of carriership were identified as key knowledge gaps. Deterministic compartmental simulations, combined with seroprevalence data, suggest that 0.05 Collapse

Key Words

• Epidemiology
• Infectious disease modelling
• Streptococcus equi

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Detection and molecular characterization of Actinomyces denticolens causing lymph node abscessation in horses

Introduction

Abscessation of equine head lymph nodes can be caused by various bacteria, but Streptococcus equi subsp. equi is mainly involved. At our laboratory, samples of three unrelated horses with submandibular abscesses were found negative for S. equi, and further testing proved the presence of another genus. This raised the question for the exact identity of this pathogen and whether these isolates were epidemiologically related and it warranted further characterization with regards of virulence and resistance factors.

Methods

Culture followed by identification using MALDI-TOF MS, MIC testing and whole genome sequencing (WGS) was performed to characterize the bacteria.

Results

Bacterial culture and subsequent identification with MALDI-TOF MS resulted in the reliable identification of A. denticolens in two of the three cases. Final confirmation of A. denticolens for all three isolates was achieved by analysis of the WGS data, supported by multilocus sequence typing (MLST). The three isolates showed 95% nucleotide sequence identity. The number of single nucleotide polymorphisms (10,170 to 36,058) indicated that the isolates were not clonal, suggesting that these cases were epidemiologically unrelated. Only four known virulence related genes were detected. The absence of known antibiotic resistance genes was in line with the high susceptibility, as indicated by the susceptibility patterns obtained for two of the three isolates.

Conclusion

We conclude that A. denticolens should be included in the differential diagnosis of (submandibular) lymph node abscessation in horses, especially if strangles cannot be confirmed with laboratory diagnostics. Furthermore, we report the first draft genome of A. denticolens isolated from horses.

Estimation of the basic reproduction number for Streptococcus equi spp equi outbreaks by meta-analysis of strangles outbreak reports

BACKGROUND

Streptococcus equi spp equi (S. equi), the cause of strangles in horses, is considered a highly contagious pathogen affecting equines and the equine industry worldwide. Fundamental epidemiological characteristics of outbreaks, such as the basic reproduction number (R₀ ), are not well described.

OBJECTIVES

Estimate R₀ for S. equi in equine populations from outbreak data.

STUDY DESIGN

Systematic review and meta-analysis of published and unpublished data.

METHODS

A literature search for outbreak reports was carried out. Depending on data available in the reports, the early epidemic growth rate or final attack rate (AR) approach was used to estimate the basic reproduction number for that outbreak. Other recorded outbreak characteristics were the type of housing (group vs individual). An overall estimate for R₀ was computed by meta-analysis.

RESULTS

Data from 8 outbreaks were extracted from peer-reviewed publications. Data from two additional, non-published outbreaks was also included in the meta-analysis. A conservative estimate for R₀ was 2.2 (95% CI 1.9- 2.5). A less conservative estimate, including outbreaks with a 100% AR for which a lower limit R₀ was estimated, was 2.7 (95% CI 2.1- 3.3).

MAIN LIMITATIONS

Few papers describing longitudinal incidence data were found so most estimates were based on the outbreaks' final size. Several outbreaks had a 100% attack rate and could therefore only be included as a lower limit estimate in the meta-analysis. The reported result therefore may be an underestimation.

CONCLUSIONS

This estimate for R₀ for S. equi informs parameters for future mathematical modelling, quantifies desired preventive vaccine coverage and helps evaluate the effect of prevention strategies through future modelling studies.

Ovine and Bovine Congenital Abnormalities Associated With Intrauterine Infection With Schmallenberg Virus

In December 2011, a previously unknown congenital syndrome of arthrogryposis and hydranencephaly in sheep and cattle appeared in the Netherlands as an emerging epizootic due to Schmallenberg virus (SBV). Gross lesions in 102 lambs and 204 calves included porencephaly, hydranencephaly, cerebellar dysplasia and dysplasia of the brainstem and spinal cord, a flattened skull with brachygnathia inferior, arthrogryposis, and vertebral column malformations. Microscopic lesions in the central nervous system showed rarefaction and cavitation in the white matter, as well as degeneration, necrosis, and loss of neurons in the gray matter. Brain and spinal cord lesions were more severe in lambs than in calves. Ovine and bovine cases examined early in the outbreak showed encephalomyelitis. SBV infection was confirmed by real-time quantitative reverse transcription polymerase chain reaction (RT-qPCR) in brain samples in 46 of 102 lambs (45%) and in 32 of 204 calves (16%). Immunohistochemistry, performed on tissue samples from 18 RT-qPCR-positive lambs, confirmed the presence of bunyaviral antigen in neurons of the brain in 16 cases. SBV antibodies were detected by enzyme-linked immunosorbent assay in fetal blood in 56 of 61 sampled ovine cases (92%). In a virus neutralization test, all tested dams of affected newborns, 46 ewes and 190 cows, were seropositive. Compared with other teratogenic viral infections, the pathogenesis and lesions of SBV in sheep and cattle fetuses are similar to those of other ruminant orthobunyaviruses. However, the loss of spinal ventral motor neurons and their tracts, resulting in micromyelia, distinguishes SBV infection from other viral central nervous system lesions in newborn ruminants.

Schmallenberg virus outbreak in the Netherlands: routine diagnostics and test results

At the end of 2011, a new Orthobunyavirus was discovered in Germany and named Schmallenberg virus (SBV). In the Netherlands malformations in new-born ruminants were made notifiable from the 20th of December 2011. After a notification, malformed new-borns were necropsied and brain tissue was sampled for reverse transcription-polymerase chain reaction (RT-PCR). In addition, blood samples from mothers of affected new-borns were tested for antibodies in a virus neutralization test (VNT). The aim of this study was to summarize and evaluate the diagnostic data obtained and to gain insight into the possible regional differences. In total 2166 brains were tested: 800 from lambs, 1301 from calves and 65 from goat kids. Furthermore 1394 blood samples were tested: 458 from ewes, 899 from cows and 37 from goats. Results showed that 29% of the lamb brains, 14% of the calf brains, and 9% of the goat kid brains were RT-PCR positive. The number of malformed and RT-PCR positive lambs decreased over time while the number of malformed and RT-PCR positive calves increased. In the VNT 92% of the ewes, 96% of the cows and 43% of the goats tested positive. Combining RT-PCR and VNT results, 18% of all farms tested positive in both the RT-PCR and VNT. The relative sensitivity and specificity of the RT-PCR are 19% and 97% respectively, and of the VNT 99% and 6%. The results show a widespread exposure to SBV and the regional evaluation seems to indicate an introduction of SBV in the central/eastern part.

Prevalence of Coxiella burnetii infections in aborted fetuses and stillborn calves

Coxiella burnetii infections are mostly subclinical in cattle, but can occasionally be associated with abortion. In the present study, 100 aborted fetuses or stillborn calves that were submitted for postmortem examination between September 2007 and March 2008 were examined for infection with C burnetii. Samples of both pooled fetal tissues and placental cotyledon were tested using a real-time PCR assay. In addition, the sections of placental cotyledon were examined using immunohistochemistry (IHC). The IHC of four placentas was positive. The PCR results of the IHC-positive placentas were high positive (HP); the PCR results of the organs of these four fetuses and calves varied from low positive (LP) to HP. The four IHC-positive fetuses had a gestation length of seven to nine months. All four placentas had histological signs of inflammation, but only one of four placentas had gross pathological signs of inflammation possibly due to a concomitant infection with Bacillus licheniformis. Five other IHC-negative placentas had (high) positive PCR results; the PCR results of the organs of these fetuses were LP or negative. The present study indicates that C burnetii infections are detected in a limited percentage of aborted fetuses and stillborn calves by IHC. To assess the importance of placentas with PCR-positive and IHC-negative test results, more research is needed.

Lawsonia intracellularis-associated proliferative enteritis in weanling foals in the Netherlands

Equine proliferative enteropathy (EPE) is an emerging infectious enteric disease caused by the obligate intracellular gram-negative bacterium Lawsonia intracellularis. EPE was tentatively diagnosed in six weanling foals, aged between 5 and 7 months. Clinical signs included depression, anorexia, ventral oedema, and weight loss. Plasma biochemistry consistently revealed severe hypoproteinaemia. The ante-mortem diagnosis of EPE was based on clinical signs, hypoproteinaemia (6/6), the detection of moderate-to-high titres of L. intracellularis antibody (6/6), and severe thickening of the small intestinal wall on ultrasonography (2/2), or L. intracellularis detected in faeces by PCR (I/2). The first foal died despite treatment and at post-mortem examination the tentative diagnosis was EPE. Three foals from the same farm, which showed similar clinical symptoms were treated with azithromycin and rifampicin; two survived. Post-mortem examination of the foal that died confirmed the tentative clinical diagnosis of EPE on the basis of the lesions found and the detection of L. intracellularis--DNA in the ileum and jejunum. The fifth foal died despite intensive treatment and the post-mortem examination revealed lymphohistiocytic enteritis, typhlitis, and widespread thrombosis in several organs. The sixth foal recovered completely after treatment. This report confirms the presence of clinical L. intracellularis infection in weanling foals in the Netherlands and shows the difficulty in reaching a definitive ante-mortem diagnosis.

The seroprevalence of Lawsonia intracellularis in horses in The Netherlands

Equine proliferative enteropathy caused by Lawsonia intracellularis is an emerging disease of weanling foals and affects their growth and development. The prevalence of Lawsonia intracellularis in The Netherlands is not known. The aim of the study was to investigate the seroprevalence of Lawsonia intracellularis in horses in The Netherlands. Blood samples were taken from healthy foals before and after weaning and from healthy yearlings and mature horses on farms throughout The Netherlands. These samples were analysed for the presence of Lawsonia intracellularis-specific antibodies with a blocking ELISA. White blood cell count, packed cell volume, and total protein concentration were also measured in all foals. Information regarding housing, pasture access, and contact with pig manure on the premises was obtained for all animals. The prevalence of Lawsonia intracellularis antibodies in foals increased significantly from 15% before weaning to 23% after weaning (p = 0.019); it was 89% in yearlings and 99% in horses older than 2 years. There was no significant difference in seroprevalence between the pasture-kept and stable-confined adult horses (97% and 100%, respectively), and there was no significant influence of contact with pig manure. None of the sampled animals showed clinical disease. In conclusion, the results suggest that Lawsonia intracellularis is widespread in The Netherlands and that seropositivity is not necessarily associated with clinical problems. The high seroprevalence in adult horses suggests long-term persistence of antibodies against Lawsonia intracellularis or constant exposure to the bacterium.

Aspects of the epidemiology, research, and control of lentiviral infections of small ruminants and their relevance to Dutch sheep and goat farming

In 1862, the veterinarian Loman reported the first sheep in The Netherlands with symptoms associated with lentiviral infection, although at the time the symptoms were ascribed to ovine progressive pneumonia. In the following century, similar cases were reported by South African, French, American, and Icelandic researchers. Extensive research into the pathology, aetiology, and epidemiology of this slowly progressive and ultimately fatal disease was initiated in several countries, including the Netherlands. Studies of the causative agents--maedi visna virus (MVV) in sheep and caprine arthritis encephalitis virus (CAEV) in goats, comprising the heterogeneous group of the small ruminant lentiviruses (SRLV)--prompted the development of diagnostic methods and the initiation of disease control programmes in many European countries including the Netherlands, as a pioneer in 1982, and in the U.S.A. and Canada.

Evaluation of an indirect ELISA for detection of antibodies in bulk milk against bluetongue virus infections in the Netherlands

After the introduction of bluetongue virus serotype 8 (BTV-8) in western Europe in 2006, an indirect ELISA for detection of serogroup-specific antibodies against BTV in serum samples was validated for individual milk samples by the Central Veterinary Institute and the Animal Health Service in the Netherlands (Kramps et al., 2008). In order to develop a cost-effective monitoring tool, we now have evaluated this ELISA also for use in bulk milk. Therefore, bulk milk samples and individual milk samples were collected from 92 herds in the affected southern region in the Netherlands in 2007, before the start of the vaccination campaign. In addition, bulk milk samples collected from 88 herds before the bluetongue introduction in 2006 ("historically negative" samples) have been tested. With these results ROC analyses were performed and herd specificity and herd sensitivity of the bulk milk ELISA were estimated. All "historically negative" bulk milk samples were negative in the ELISA, with a mean S/P ratio of 10 ± 0.8%. The herd sensitivity and herd specificity of the ELISA in bulk milk samples depend on the cut-off that is chosen. In order to detect a within-herd-prevalence of 1%, the optimal cut-off S/P ratio 13% was found. A few herds with one or two milk-positive animals would then be missed. The specificity will be 100%. A within-herd-prevalence of 10% can be detected with 100% sensitivity at a cut-off S/P ratio of 96%. In conclusion, the indirect ELISA in bulk milk samples is a very specific and sensitive test which can be implemented in monitoring and surveillance systems in unvaccinated populations.

Evaluation of Ziehl-Neelsen stained faecal smear and ELISA as tools for surveillance of clinical paratuberculosis in cattle in the Netherlands

Testing cattle suspected of clinical paratuberculosis is an important element of surveillance of paratuberculosis. The aim of this study was to evaluate the diagnostic-test characteristics of microscopic examination of Ziehl-Neelsen stained faecal smears for acid-fast Mycobacteria (ZN-test) and serum-ELISA in cattle suspected of clinical paratuberculosis in the Netherlands. Results of all samples submitted for ZN-test and serum-ELISA between April 2003 and April 2006 to our laboratory were retrieved. Results from cattle for which both tests were performed were analysed using two Bayesian latent-class models for evaluation of diagnostic tests in two populations without a gold standard, assuming (a) conditional independence of tests, or (b) conditional dependence of tests in both infected and non-infected cattle. Sampled cattle were divided into two populations in different ways using four known risk factors for clinical paratuberculosis: region, soil type, clinical signs, and age. For 892 cattle suspected of clinical paratuberculosis, both ZN-test and serum-ELISA results were retrieved: 250 ZN-positive and ELISA-positive, 12 ZN-positive and ELISA-negative, 260 ZN-negative and ELISA-positive, and 370 ZN-negative and ELISA-negative cattle. With priors based on the available literature, the posterior estimates of sensitivity, specificity, and positive and negative predictive values of the ELISA were always higher than those of the ZN-test. Furthermore, lower limits of the 95% credibility intervals of the posterior positive predictive values of the ELISA were >or=99.7%, and of the negative predictive values of the ELISA >or=56.4%. We conclude that the ELISA is preferred to the ZN-test to confirm the presumptive diagnosis of clinical paratuberculosis in the Netherlands. Little diagnostic information can be gained by performing the ZN-test in addition to the ELISA.

Experimental infection with neuropathogenic equid herpesvirus type 1 (EHV-1) in adult horses

Equid herpesvirus type 1 (EHV-1)-associated myeloencephalopathy (EHM) may follow an infection with the virus in horses. This study tested three hypotheses: (1) a large inhaled dose of a neuropathogenic EHV-1 strain would induce a cell-associated viraemia in all infected horses; (2) neurological disease will only occur in viraemic horses, and (3) the cerebrospinal fluid (CSF) composition following EHV-1 viraemia will be an indicator for EHM. Four EHV-1 seronegative horses were inoculated with EHV-1 by inhalation. Three developed clinical signs of neurological disease, which were mild in two horses and lacking typical EHM histopathological findings, but moderately severe in the third horse. This latter animal was the only one found to be viraemic, with xanthochromic CSF and spinal cord histopathology findings characteristic of EHM. This study showed that cell-associated viraemia was not guaranteed, despite a large-dose inoculation with EHV-1, yet viraemia was probably a pre-requisite for subsequent development of EHM. The histopathological changes used to confirm EHM may be predicted from CSF analysis.

Evaluation of five enzyme-linked immunosorbent assays and an agar gel immunodiffusion test for detection of antibodies to small ruminant lentiviruses

In the framework of the Dutch control program for small ruminant lentiviral (SRLV) infections, too many drawbacks were encountered with respect to serological testing. To improve the quality of testing, five enzyme-linked immunosorbent assays (ELISAs) and an agar gel immunodiffusion test (AGIDT) were evaluated. The focus was on the sensitivity, specificity, and variances of the commercially available tests. Clear differences were found among the tests in analytical and diagnostic sensitivity and overall diagnostic performance, whereas no significant differences in specificity were found. For serodiagnosis of sheep with clinical symptoms of maedi-visna virus (MVV) (histopathologically confirmed), one ELISA was significantly more sensitive than the other ELISAs and than the AGIDT, while for asymptomatic sheep originating from infected flocks, three ELISAs and the AGIDT demonstrated similar performance. The diagnostic performance appeared to be related to animal species and virus infection (MVV or caprine arthritis encephalitis virus [CAEV]) as well as the phase of infection/progression of disease. Receiver operating characteristic analysis, demonstrating the diagnostic potential of tests irrespective of defined cutoffs, again revealed clear differences between tests with respect to diagnostic performance for detection of antibodies against CAEV or MVV. An indirect ELISA, of which the solid phase is sensitized with a combination of the core protein p27 of MVV produced in Escherichia coli and a peptide derived from the transmembrane protein gp46, appeared to be the test of choice for serodiagnosis of SRLV infections in sheep and goats.

Evaluation of nephelometry for albumin measurement in serum and cerebrospinal fluid: experiences with an indwelling subarachnoidal catheter system for repetitive cerebrospinal fluid collection in horses

The measurement of albumin concentrations in cerebrospinal fluid (CSF) and serum for albumin quotient (AQ) calculations in normal horses was performed by 2 methods: 1) total protein measurement, followed by electrophoresis of the samples to obtain an albumin percentage; and 2) albumin immunoprecipitation quantitated by nephelometry. The results of both methods correlated well, and nephelometry was chosen to determine the albumin concentrations in CSF samples obtained from an indwelling subarachnoidal catheter for daily sampling. Because the use of an indwelling catheter to collect repetitive CSF samples is a novel technique, routine cytological CSF analysis was performed along with daily clinical evaluation to ascertain the well-being of the horses. The catheters were placed in 2 horses for periods of 14 and 17 days. One horse exhibited pleocytosis on cytological evaluation of CSF on 2 occasions for a 1-2-day duration; however, the AQ showed a significant increase on only 1 occasion. The other horse had a normal cell count in CSF but showed 2 sudden changes in the AQ value; however, these values remained within the 95% confidence interval for AQ in horses. Albumin quotient values of the second horse were consistently below the lower range of the confidence interval. Results from this study indicate that nephelometry can be used for albumin determination in serum and CSF samples from horses. Furthermore, an indwelling subarachnoidal catheter system can provide serial CSF samples in horses, thus obviating the need for repetitive centesis for serial CSF sampling.

Vertical transmission of bovine viral diarrhoea virus (BVDV) in mousedeer (Tragulus javanicus) and spread to domestic cattle

This study investigates the transmission of bovine viral diarrhoea virus (BVDV) 1f from a persistently infected (PI) lesser Malayan mousedeer to two bovine calves. Different contact routes to two calves were analysed: 1) aerosol contact between two adjacent pens without physical contact; 2) indirect contact by use of common utensils; 3) direct nose-to-nose contact for 30 seconds. One of the calves was infected either by aerosol or indirect contact. The virus sequence in 247 nucleotides in the 5'-UTR was 100% identical in mousedeer and calf. To elucidate the distribution of BVDV within the affected mousedeer family the captive population in a Zoo was analysed. The maternal line of PI animals was maintained, whereas a PI male was able to reproduce and have a non-PI calf. As a consequence of this, six female PI mousedeer were killed; subsequent autopsies did not reveal any lesions. Sequencing mousedeer BVD virus in the E2 region (420 nucleotides) through 4 generations showed only 7 mutations, which were maintained from mother to offspring.

Equine herpesvirus type 1-associated myeloencephalopathy in The Netherlands: a four-year retrospective study (1999-2003)

Equine herpesvirus type 1(EHV-1)-associated myeloencephalopathy (EHM) is a rare disease affecting the central nervous system and, in particular, the spinal cord of horses. Epidemiologic data are limited, and usually are restricted to the description of a single outbreak. During an observational period of 4 years, we evaluated 9 outbreaks of EHM in The Netherlands. A risk factor analysis was done on the data from 6 outbreaks. Findings were: regular appearance of EHM outbreaks in The Netherlands (2-3/y); strong association of EHM with season; and risk factors, such as breed, sex, age, and fever. Female horses, aged horses, and specific breeds were at greater risk to develop severe neurologic disease. Other breeds and horses <3 years old were never observed to suffer from EHM during these outbreaks. It was concluded that breed variation, in addition to the presence of a specific EHV-1 strain, and environmental factors influence an outbreak of EHM on a premise.

Supranational comparison of Neospora caninum seroprevalences in cattle in Germany, The Netherlands, Spain and Sweden

Herd, within-herd and animal prevalences for Neospora caninum in beef and dairy cattle were compared between four countries. In randomly selected herds from regions of Germany, The Netherlands, Spain and Sweden that were representative for the cattle production of these countries, all animals > or = 2 years were examined serologically by enzyme-linked immunosorbent assays (ELISAs) with high test specificity (> 98.0%). In a previous study, the ELISAs had been validated against each other. Single reacting animals within a herd were confirmed by immunobloting. At the time of sampling, animal (age, breed, herdtype, sex, lactation stage) and herd data (region) were collected. Considerable differences in N. caninum herd, within-herd, and overall animal prevalence estimations were observed between countries, regions, herdtype, age categories and breeds. Herd prevalences, based on confirmation of single reactors, for dairy herds were estimated to be 16% (95%CI: 10-24%) in Sweden, 49% (95%CI: 39-59%) in Germany, 63% (95%CI: 57-69%) in Spain and 76% (95%CI: 67-84%) in The Netherlands and for beef herds 41% (95%CI: 31-50%) in Germany, 46% (95%CI: 41-51%) in Spain and 61% (95%CI: 50-72%) in The Netherlands. No beef herds were examined in Sweden. The lowest animal true prevalence was estimated in dairy cattle in Sweden (0.5% (95%CI: 0.1-0.8%)) while the highest animal true prevalence was estimated for dairy cattle in Spain (16.2% (95%CI: 14.9-17.5%)). Within-herd prevalences varied greatly, with very few farms in Sweden having more than 10% seropositive animals while in Spain more than 10% of the herds had within-herd prevalences between 50 and 100%. Seropositivity was significantly associated with herdtype (beef versus dairy), age, breed and region within countries. The results of this supranational comparative study showed that the importance of N. caninum infection varied greatly within in Europe. Estimates of prevalence can be used to calculate the economic impact of N. caninum infection as well as to evaluate the effect of prevention and control strategies over time.

Persistent BVDV infection in mousedeer infects calves

Bovine virus diarrhoea virus (BVDV)-1f was isolated from a Lesser Malayan Mousedeer in Copenhagen Zoo during a routine screening. Analysis of animals related to the Copenhagen mousedeer revealed that its mother and all siblings were virus positive, a pattern also seen for persistently infected (PI) cattle. BVDV could be transmitted from the PI mousedeer to a calf after indirect contact. The host spectrum for BVDV seems to be even wider than expected; the implications for BVDV control are discussed.

Evaluation of three enzyme-linked immunosorbent assays for detection of antibodies to Neospora caninum in bulk milk

Three ELISAs for the detection of antibodies against Neospora caninum in bulk milk were evaluated in 162 Dutch dairy herds. The first ELISA was the Dutch Animal Health Service (AHS) in-house ELISA, developed from the routine in-house serum ELISA. The other two ELISAs were commercial milk ELISAs from IDEXX and LSI. Blood samples of all lactating cows in 162 dairy herds were tested using the AHS in-house serum ELISA. Based on previous studies in the Netherlands a within-herd N. caninum seroprevalence of 15% was associated with increased risk for reproductive losses. This percentage was therefore used as positive seroprevalence cut-off value. Repeatability of the ELISAs was evaluated by testing on three different days. The AHS in-house ELISA lacked specificity, probably due to use of a different batch of antigen on the second and third test-day. Cut-off values were determined using misclassification costs term calculations. At cut-off values 0.6 for the IDEXX and 0.2 for the LSI, a herd sensitivity of 61% (95% CI: 49--73%) and 47% (95% CI: 35--60%) was estimated. Herd specificity at these cut-off values was 92% (95% CI: 87--98%) for the IDEXX and 94% (95% CI: 90--99%) for the LSI ELISA. The positive and negative predictive values were 84% (95% CI: 68--100%) and 86% (95% CI: 79--94%) for the IDEXX ELISA, and 85% (95% CI: 67--100%) and 82% (95% CI: 74--90%) for the LSI ELISA. The agreement between all possible combinations of test-days was expressed by kappa values. These were found to be slightly higher for the IDEXX than for the LSI ELISA. It is concluded that both commercial ELISAs performed satisfactorily to detect a within-herd seroprevalence of N. caninum in lactating cows of at least 15%.

Neurological syndromes among horses in The Netherlands a 5 year retrospective survey (1999–2004)

The presence of toxins or infectious agents combined with environmental factors in combination with a susceptible host can be the cause for neurological disease in groups of horses. During a 5 year observational period outbreaks of neurological diseases among horses were evaluated. Causes of occurring neurological diseases were equine botulism, lolitrem intoxications, equine herpesvirus type 1-associated myelo(encephalo)pathy, and encephalitis caused by (disseminated) Streptococcus equi subspecies equi infection. This article focuses on the first three syndromes because of their predominant influence on locomotion. The pathogenesis of each disease is presented, followed by a description of a general presentation of the diseases as encountered under Dutch circumstances.

An interlaboratory comparison of immunohistochemistry and PCR methods for detection of Neospora caninum in bovine foetal tissues

Seven European laboratories contributed to a multi-centre evaluation of detection techniques for Neospora caninum in bovine foetuses. Six laboratories participated in immunohistochemistry (IHC) testing. All seven laboratories participated in PCR testing, but the results from one laboratory were not included in the analysis, because of contamination problems in the preparation of the samples. A coded panel of tissue sections from 36 infected and non-infected foetuses was used to evaluate the IHC detection of parasites. A coded panel consisting of 44 homogenized foetal brain samples from natural bovine abortion cases and 32 spiked samples were used to evaluate the PCR methods. Inclusion of a duplicate dilution series of spiked samples was used to evaluate detection limits and repeatability. IHC methods had a relatively low sensitivity, but a high specificity. There was considerable variation in IHC results between participating laboratories, which may be partly explained by examination practices that depended on the experience of the operator. In addition, the use of different antibody reagents, different antibody dilutions, and different enzymatic treatments of tissues may have contributed to the observed variation. PCR methods generally had a higher sensitivity than IHC methods and also a high specificity. The agreement between the majority scores of IHC and PCR methods was low. False positive PCR results indicated contamination problems in some instances. Agreement between the PCR results of the various laboratories was better, compared with the IHC results. There appeared to be no clear relationship between the PCR format (i.e. single or nested) and diagnostic sensitivity. Consequently, an improvement of diagnostic performance of PCR might possibly be achieved by optimizing DNA extraction methods.

[Clinical signs and diagnosis of a severe primary infection with BVDV subtype 1b in a dairy herd]

As a result of a BVDV infection in a herd consisting of 95 adult cattle 17 cows aborted their calves within a period of 3.5 months, one third got severe diarrhoea, 3 cows died and an increased percentage of the cattle got lochiometra after calving or abortion. The disease was diagnosed by paired serological testing of cattle with diarrhoea or abortion and post mortem examination of several aborted calves. From one foetus BVDV virus was isolated and subsequently subtyped by sequencing. Of aborting cattle, the testing results were influenced by the interval between infection and abortion. These results indicate that a primary infection with BVDV subtype 1b can cause severe clinical symptoms in a dairy herd.

Comparison and standardisation of serological methods for the diagnosis of Neospora caninum infection in bovines

Various existing serological tests were compared with a standard panel of 523 sera in a multicentred study across Europe. Well characterised sera from animals that were experimentally or naturally infected with Neospora caninum as well as sera from cattle deemed uninfected with N. caninum were provided by the participants of the study and analysed in several commercial (CHEKIT Dr. Bommeli/Intervet, CIVTEST BOVIS NEOSPORA Hipra, Cypress Diagnostics C.V., Herd Check IDEXX, Mastazyme MAST Diagnostics, P38-ELISA Animal Welfare and Food Safety GmbH (AFOSA)) as well as in-house assays (five ELISAs and one IFAT). Most tests showed a high level of agreement in the interpretation of the test results (positive or negative). A further distinct increase in agreement between tests was obtained after the application of standardised cut-offs offered by a two-graph receiver operating characteristic analysis. This procedure allows a standardised interpretation of results obtained with different tests used in independent, parallel seroepidemiological studies.

Diagnostic methods applied to analysis of an outbreak of equine influenza in a riding school in which vaccine failure occurred

An outbreak of equine influenza H3N8 in a riding school is described retrospectively with emphasis on diagnosis and putative vaccine failure. In March 1995 an outbreak of equine influenza occurred among 11 horses in a riding school, where most horses had received basic primary immunizations and several booster vaccinations against influenza. Six of the 11 diseased horses had received their last booster vaccination within 5 months of the outbreak. Nevertheless, the influenza infection spread rapidly and clinical manifestations were prominent with frequent, harsh, dry coughing often accompanied by high fever. Nasal swabs were taken from 11 diseased horses. Influenza A virus of the equine H3N8 (equi-2) subtype was isolated from five nasal swab extracts. Stored nasal swab extracts were also retrospectively investigated in two different enzyme immunoassays designed to detect the type-specific conserved nucleoprotein of influenza A viruses, and in a single-tube reverse transcription-PCR (RT-PCR) using a set of primers based on highly conserved regions of the matrix gene of influenza A viruses. Five nasal swab extracts were found positive in a DAS-ELISA and seven in the Directigen((R)) Flu A (DFA) assay, respectively. Two nasal swab extracts from which virus was isolated did not give a positive result in the DAS-ELISA, and one of these also did not give a positive result in the DFA assay. Nine nasal swab extracts were found positive by RT-PCR. Moreover, all virus isolation and/or ELISA positive nasal swab extracts were confirmed by RT-PCR. Three nasal swab extracts were negative by virus isolation, PCR and ELISA. A significant rise in HI titre against influenza A/eq/Miami/63 (H3N8) virus was detected in seven of the nine paired sera available. In acute phase serum samples from 10 horses, SRH antibody levels varied widely. However, some horses with high, or at least putatively clinically protective SRH antibody levels, showed clinical signs and infection was confirmed. Antigenic analysis of two isolates showed that A/eq/Holland/1/95 (H3N8) and A/eq/Holland/2/95 (H3N8) cluster with the UK isolate Osgodsby/92, the Swedish isolate Borlänge/91 and some other European isolates, with H/2/95 identical in reactivity to Borlänge/91 and H/1/95 more similar in reactivity to Osgodsby/92 than H/2/95. Nucleotide and deduced amino-acid sequences showed large differences of both isolates as compared with Miami/63, Fontainebleau/79 and Kentucky/81, the influenza A H3N8 subtype strains incorporated in the vaccines used in this riding school. The role of antigenic drift in vaccine breakdown is discussed in the light of evidence for vaccine breakdown in the UK in 1989, Sweden in 1991 and in the USA since 1991.

Equine influenza virus infections: an update

Equine influenza is one of the most economically important contagious respiratory diseases of horses. In this paper the current state of knowledge of equine influenza virus and the most important aspects of these virus infections, e.g. epidemiology, clinical aspects, pathogenesis and pathology, immunity, diagnosis, treatment, management and vaccination, are reviewed with an emphasis on epidemiology, diagnosis and vaccinology. Many questions have remained and with the advent of improved technology new questions have arisen. Consequently, research priorities should be set in an attempt to answer them. Therefore, this review ends with some personal recommendations for important priorities for future research.

Equine herpesvirus 1 and 4 infections: an update

Equine herpesvirus 1 (EHV1) and equine herpesvirus 4 (EHV4) are important ubiquitous equine viral pathogens, causing much damage to the horse industry. EHV1 strains are associated with respiratory disease, abortion, and paresis/paralysis, whereas EHV4 strains are predominantly associated with respiratory disease. In the past decades much research effort has been put into improving knowledge about these viruses. In this paper the current state of knowledge of these viruses and the most important aspects of these virus infections, e.g. epidemiology, clinical aspects, pathogenesis and pathology, immunity, diagnosis, preventive management and management in the course of an outbreak and vaccination, is reviewed. Because we performed some research ourselves in the areas of diagnosis, epidemiology and vaccinology these aspects are reviewed in more depth than the other aspects. Still many questions have remained and new questions have risen. Consequently, research priorities should be made in an attempt to answer these questions. Therefore, this review ends with some personal recommendations for important priorities for future research.

Evaluation of two absorbed enzyme-linked immunosorbent assays and a complement fixation test as replacements for fecal culture in the detection of cows shedding Mycobacterium avium subspecies paratuberculosis

Control of paratuberculosis in dairy herds is based on preventing the transmission of Mycobacterium avium subsp. paratuberculosis (Mptb) from cows to calves by management measures, supported by removal of cows excreting these bacteria by the fecal route (Mptb shedders). Fecal culture is the most accurate test for identifying Mptb shedders, but this technique is expensive and takes up to 16 weeks for results to be available. Serologic tests are inexpensive, rapid, and easy to perform. Of serologic tests, the complement fixation test (CFT) and absorbed enzyme-linked immunosorbent assay (ELISA) are the serologic tests used most frequently; the CFT is considered less accurate than the ELISA with respect to sensitivity and specificity. The commonly accepted absorbed ELISA is from the Australian Central Serum Laboratory. However, a European supplier has marketed a second ELISA that is supposed to be more sensitive in detecting Mptb shedders. These 2 absorbed ELISAs, designated ELISA-A and ELISA-B, and an in-house CFT were compared with data from 2 serum panels. The Mptb shedding panel consisted of sera from 198 culture-positive cows from 53 infected herds. The method used for culture of fecal samples was a modified Jørgensen method on individual samples. The Mptb shedder detection rate by the 3 serologic tests ranged from 29.8% to 39.4%. Detection rate for ELISA-A was lower than that for ELISA-B and CFT. For all 3 tests, detection rate was dependent on the level of Mptb shedding and the age of the animals. Detection rates increased as cattle age increased to 4 years. The specificity panel was initially composed of sera from 811 cows randomly selected from 41 herds without clinical paratuberculosis that were negative for Mptb based on whole-herd fecal culture. The modified Jørgensen method for culture was used on pooled fecal samples. Serologic test specificity ranged from 93.4% to 99.8%. The specificity of ELISA-A was higher than that of ELISA-B and CFT. Specificity of ELISA-B between herds was 75-100%. Specificity of CFT between herds was 62-100%. The low specificity of ELISA-B and CFT could not be explained by a higher sensitivity for Mptb-infected cows before onset of shedding, because in the 19 herds with 8 more subsequent negative whole-herd fecal cultures in the 4 years after sampling, specificity was not improved. The insufficient specificity of ELISA-B was not corrected sufficiently by heightening the cutoff value because Mptb shedder detection rate was lowered to 28.9%, equal to that of ELISA-A, and specificity only rose to 97%, much lower than that of ELISA-A. Taking into account the different test characteristics, serologic tests are a cost-effective alternative to fecal culture in high-prevalence herds. For certification programs, only ELISA-A is recommended because in a large number of nonsuspect herds specificity remained almost 100%.

Neurological disease associated with EHV-1-infection in a riding school: clinical and virological characteristics

An outbreak of neurological disease caused by EHV-1 infection is described with emphasis on diagnosis and prognosis for recumbent horses. In April 1995, an outbreak of the neurological form of Equine herpesvirus type 1 (EHV-1) occurred in a well-managed riding school with 41 horses: 34 horses showed a temperature spike and 20 some degree of neurological signs, of which 10 were nursed intensively in the indoor arena of the riding school for 3 to 20 days, 8 having to be maintained in slings for 2-18 days, while 9 needed bladder catheterisation b.i.d. for 2-16 days. Within the first 3 days, one horse was subjected to euthanasia and another horse died. Postmortem examination revealed a mild vasculitis with perivascular mononuclear cuffing and axonal degeneration in the central nervous system. Clinical diagnosis was confirmed by serology and virology: 28 horses seroconverted in one or more tests during the outbreak, whereas 12 had already high CF and SN titres in the first sample, suggestive of recent infection. Virus was isolated from nasal swabs of 4 horses, and identified as EHV-1 with type-specific monoclonal antibodies. Restriction enzyme analysis revealed that the EHV-1 strains from this outbreak belonged to genome type EHV-1.IP. The electropherotypes were identical to those from another, epidemiologically unrelated, outbreak of neurological disease 2 months earlier. The timing of the temperature spikes and seroconversions indicated that the infection was probably introduced by a horse purchased 3 weeks before neurological signs occurred. At follow-up one year later, the 10 horses that showed mild neurological signs had recovered completely. Of the 8 horses that survived intensive care, 3 had returned to around their former performance level (2 of which had been in slings), while the other 5 had become pasture-sound. At follow-up 4 years later, all pasture-sound horses had been subjected to euthanasia because of persistent mild ataxia and incontinence. In conclusion, the prognosis for recumbent horses due to EHV-1 infection is grave. For virological diagnosis, extensive and strategic sampling of febrile in-contact horses is required, and the EHV-1-specific glycoprotein G (gG) ELISA is a valuable tool for specific serological diagnosis of EHV-1 infection causing neurological disease.

Airborne transmission of bovine herpesvirus 1 infections in calves under field conditions

A small scale transmission experiment was performed with bovine herpesvirus 1 (BHV1) in a cattle population under field conditions. 10 calves were housed under strict hygienic conditions, with a distance of 4m between each calf. Five calves were experimentally infected with BHV1, two calves with strain Harberink and three with strain Lam, respectively. Experimentally infected calves were placed at 4 m distance from five susceptible sentinel calves. Airborne transmission to sentinel calves was detected using virus isolation and BHV1 specific polymerase chain reactions in samples of nasal fluids, and BHV1 specific antibodies in serum samples. Strain Harberink was hardly transmitted to sentinel calves, whereas strain Lam was transmitted to all sentinels. Estimating the rate of transmission per day, the total number of calves infected by one (strain Lam) infected calf was 1.18. Comparing this estimated transmission ratio between cattle at a distance of 4 m to the estimated transmission ratio R of BHV1 in susceptible commingled cattle reported before, the effect of the factor distance on the transmission ratio could be calculated. Extrapolating these results, a distance of 4.4 m between cattle populations would be necessary to reduce transmission for this strain to R<1.

An equine herpesvirus 1 (EHV1) abortion storm at a riding school

An outbreak of EHV1 abortions occurred at a riding school in The Netherlands in 1991. Seven of twelve pregnant mares aborted, and another foal died at 8 days of age. Six abortions occurred within 12 days in March after an initial abortion on 8 February. Four mares delivered live foals. Virological examination of four aborted foals revealed an EHV1 infection. Serological results for paired sera from 17 horses suggested, that the initial abortion on 8 February was the index case, and probably caused the other six abortions. The index case could well have been caused by reactivation of latent virus induced by transport stress. The laboratory results are discussed in the light of the present knowledge of the pathogenesis and epidemiology of EHV1 abortion.