Equine rhinopneumonitis refers to the respiratory disease caused by the herpesviruses EHV-1 and EHV-4. The two viruses are very closely related but cause diseases of different severity. Clinical signs for EHV-4 are typically mild and it rarely causes abortion in pregnant mares. EHV-1 infection can lead to outbreaks of abortion in stud farms and it may also lead to paralytic disease.
EHV-1 infection starts with replication in the respiratory tract and virus can be detected during the early phase in nasal secretions. The quantitative PCR tests our Diagnostic Services Laboratory uses for diagnosis are very sensitive and can discriminate between EHV-1 and EHV-4.
Following replication in the respiratory epithelium, EHV-1 can cross the basement membrane and infect the white blood cells, to cause a cell-associated viraemia. This means the virus can be transported around the body of the horse and this is an important step for the development of more serious clinical signs such as abortion and paralysis. To detect viraemia, we use virus isolation from whole blood samples.
To determine whether a horse has been recently exposed to EHV-1, we carry out serological assays. It is difficult to develop sensitive tests that discriminate between EHV-1 and EHV-4 because the viruses are so similar. One of the most useful tests for diagnosis is the complement fixation assay, because antibodies rise quickly and fall away within a few months of infection. However, this test is not type-specific so it can’t distinguish between EHV-1 and EHV-4 infection.
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