An Equine Coronavirus Associated Epidemic of Infectious Pyrexia in Iceland

Vilhjálmur

Svansson1, Sigríður Björnsdóttir2, Eggert Gunnarsson1, Constance Smits3 and Kees van Maanen3

1 Institute for Experimental Pathology, University of Iceland, Keldur, Reykjavík, Iceland

2 Icelandic Food and Veterinary Authority, Selfoss, Iceland

3 Royal GD, Deventer, the Netherlands

Introduction

An epidemic of infectious pyrexia of unknown aetiology swept through the isolated, native Icelandic horse population in 1998 (figure 1). Despite extensive restrictions, the disease spread from the primary foci all over the country during February – October 1998. Most horses were mildly affected with slightly elevated body temperature and reduced appetite. Clinical signs and negative bacterial isolation, together with orofecal transmission strongly pointed towards a viral infection. However, despite extensive investigations, no viruses known at that time to infect horses, could be connected to the disease.

Objective

Since 2010, several cases of equine coronavirus (ECoV) infections have been reported in adult horses from the United States, Europe and Japan with similar clinical signs. To determine if this epidemic of infectious pyrexia in horses was caused by equine coronavirus (ECoV) a retrospective, immuno-epidemiological study was conducted.

Materials and methods

Serum samples from the Icelandic Horse Biobank, collected in the period 1990-2020 were tested, employing a recently developed ELISA for detection of antibodies to equine ECoV together with an ECoV virus neutralisation test (Zhao et al. 2019). A total of 817 equine serum samples were included in this study. Additionally, 18 and 6 paired (acute- and convalescent-phase) serum samples were tested that were respectively collected from individual cases of infectious pyrexia during the 1998 epidemic and in 2021.

Results

Figures 2a and 2b show the seroprevalence point estimates in consecutive years as determined by ELISA without and with confirmation by VNT, respectively. ECoV antibody seroprevalences in sample sets retrieved before, during and after the epidemic show a strong increase in seroprevalence in 1998. In the period 1990 – 1997 the seroprevalence as determined by ELISA after confirmation by VNT was 0-2% with 0% in the three years preceding the epidemic, whereas in 1998 the seroprevalence increased significantly to 55%. In 1998 the seroprevalence showed a steep rise from 0% in sera collected before the export ban to 96% in sera collected after the export ban.

Antibody responses against ECoV in paired serum samples collected from 18 affected horses during the epidemic showed, both in ELISA and VNT, seroconversions or highly significant increases in antibody levels (Figures 3a and 3b). Antibody responses against ECoV in paired serum samples collected from 6 horses with similar clinical signs collected in 2021 also showed highly significant increases in ELISA antibody levels, providing evidence for ECoV still circulating in the population and causing infectious pyrexia.

Conclusion

This study strongly indicates that the epidemic of infectious pyrexia in native Icelandic horses in 1998 was caused by introduction of ECoV into an immunologically naïve population. The current seroprevalence seems to prevent serious outbreaks.

References ban, respectively.

Zhao S, Smits C, Schuurman N, Barnum S, Pusterla N, Kuppeveld FV, Bosch BJ, van Maanen K, Egberink H. Development and Validation of a S1 Protein-Based ELISA for the Specific Detection of Antibodies against Equine Coronavirus. Viruses. 2019 Nov 30;11(12):1109.

2b ECoV antibody seroprevalence as measured with an S1 recombinant ECoV ELISA and an ECoV VNT confirmation test in samples retrieved from the Iceland biobank from years between 1990 and 2020. Samples 1998a and 1998b were collected before and after the export ban, respectively.

Fig. 3a Antibodies responses against ECoV from 18 horses during an epidemic of infectious pyrexia. Boxplots show the ELISA reactivities of 18 horses sampled during the acute and the convalescent phase of the disease. The cut-off is indicated by the dashed line

Fig. 3b Antibodies responses against ECoV from 18 horses during an epidemic of infectious pyrexia. Boxplots show the virus neutralisation reactivities of 18 horses sampled during the acute and the convalescent phase of the disease. The cut-off is indicated by the dashed line