Page 702 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Respir atory system: 3.3 Medical conditions of the upper respir atory tr act 677

VetBooks.ir show severe disease due to a viral pneumonitis and and specific. qPCR supersedes PCR testing and
EHV-1 or EHV-4 infected neonatal foals can
virus antigen detection by ELISA or indirect fluo-
liver disease. Respiratory distress, jaundice and high
mortality are classical features of EHV infection in rescent antibody test, although these are still offered
by some laboratories. Confirming the diagnosis in
neonates. horses that do not exhibit detectable shedding of
EHV-2 and EHV-5 have been detected in nasal infectious virus is challenging, but seroconversion
swabs and tracheal aspirates from many clinically on the CFT provides good evidence of infection and
normal horses, especially in young horses. Both is considered diagnostic even if qPCR tests are nega-
viruses have been suggested to be linked to poor tive. Latently-infected horses and viraemic horses
performance, but other studies could not demon- can be identified by qPCR detection of virus DNA in
strate this relationship. It is possible that even if leucocytes and estimates of virus load help to differ-
they are subclinical on their own, they could pre- entiate lytic (high virus load) from latent infection
dispose to other infections. Furthermore, they have cycles (low virus load). In horses with EHV-1-
been detected in gastric mucosa and uterine flush- associated neurological disease, xanthochromia in
ings, raising also questions about their role in gas- a cerebrospinal fluid sample is suggestive. Infected
tric and in uterine disease. EHV-2 has been linked tissues, such as placental or central nervous system
to outbreaks of respiratory disease in young horses tissue, can be tested by PCR, virus isolation and
and also to keratoconjunctivitis. EHV-5 has been histopathology. Virus antigens can also be detected
linked to equine multinodular pulmonary fibrosis in infected tissues by immunocytochemistry, a par-
(see p. 718). ticularly useful test for placental and fetal tissues to
confirm EHV abortion. Virus isolation (demonstra-
Differential diagnosis tion of cytopathic effect on susceptible cell lines) can
Differential diagnoses for EHV respiratory tract be performed but is slow and has yielded frequent
disease include all other causes of infectious URT high false-negative results.
disease with or without abortion, especially influ-
enza virus and EVA. Management
Affected horses should be separated from the herd
Diagnosis and maintained in strict barrier housing conditions
Definitive diagnosis of EHV infection requires until 14–21 days after resolution of respiratory or
laboratory investigations (serology, virus isolation, neurological clinical signs. Very strict hygiene must
detection of virus antigen or DNA) as EHV respira- be maintained around aborted mares for 28 days
tory disease closely resembles other causes of infec- because uterine fluids are highly infectious. Horses
tious URT disease. Demonstration of increased can be confirmed to no longer be shedding virus
SAA, a leucopenia and lymphopenia followed by a by qPCR testing if required, and this can help with
leucocytosis and lymphocytosis on recovery is sug- decision making at the end of outbreaks. EHVs are
gestive of a viral infection, but is not diagnostic for easily inactivated by many disinfectants, including
EHV infection. Concurrent abortions or neurologi- 1% bleach, 70% ethanol, iodine-based disinfectants,
cal disease should greatly raise the index of suspi- quaternary ammonium disinfectants, peroxygen dis-
cion. A singe high (>1:80), or rising complement infectants and phenolics.
fixation titre (CFT) against EHV-1 or EHV-4 on Horses with uncomplicated URT disease simply
paired serum samples 10–14 days apart, is diagnostic. require rest and stabling in a clean, dust-free envi-
Elevated virus neutralising antibody titres are also ronment. NSAIDs can be used to control pyrexia.
diagnostic but, because these remain elevated for Broad-spectrum antimicrobials are often adminis-
months after infection, do not necessarily indicate tered, but unnecessary antimicrobial use should be
recent infection. Detection of virus DNA on nasal or avoided, unless bacterial super-infection has been
nasopharyngeal swabs using (real time) qPCR is the diagnosed. Other respiratory medicines (e.g. muco-
preferred diagnostic test because it is rapid, sensitive lytics and beta-2 agonists) are also not required.