Page 708 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Respir atory system: 3.3 Medical conditions of the upper respir atory tr act 683

VetBooks.ir Mild ‘atypical’ disease the resultant high rate of false-negative results.
Culture has been superseded by ‘real time’ qPCR
This form is transient and self-limiting. It resembles
URT viral infectious disease, with pyrexia, depression,
qPCR is at least three times more sensitive than
lymph node enlargement and nasal discharge, and is as the method of choice for detecting S. equi.
frequently only recognised as S. equi infection if sam- culture, offering sensitivities and specificities
ples are collected for microbiology. The more serious greater than 95%. The high sensitivity and speci-
sequelae associated with ‘classical’ disease (lymph node ficity of qPCR testing means there is no advantage
abscesses, guttural pouch empyema, metastatic absces- in combining qPCR testing with culture, as was
sation and purpura haemorrhagica) do not develop. previously recommended for PCR testing. qPCR
However, these horses are epidemiologically important can be performed on nasal swabs, nasopharyngeal
in an outbreak because they shed bacteria, are a source swabs or lavages or pus aspirates from abscesses.
of contagion to others and may become carriers. Nasopharyngeal swabs or nasal lavages are the
preferred sample for initial diagnosis of clinical
Carriers cases, although nasal swabs are usually adequate
Clinical signs for carriers are variable. In most cases, in acute clinical cases. Where lymphadenopathy
guttural pouch carriage is subclinical, although or lymph node abscessation is present, a needle
sometimes there is intermittent uni- or bilateral aspirate from a lymph node is the optimal sample.
nasal discharge. Carrier horses usually have some Cases should still be considered to have strangles
guttural pouch pathology, although this may be if they present with the typical clinical signs, even
subtle, and obvious empyema is less common. if samples are negative for S. equi, because shed-
Pouches that appear normal on endoscopy may still ding of bacteria can be intermittent resulting in
be infected and the only way of identifying carriers single samples from infected horses yielding nega-
is to determine whether persistent pouch infection is tive test results.
present by sampling and testing the pouches. If the Serology using the dual antigen (SEQ2190/‘Antigen
paranasal sinuses are the sites of carriage, there may A’ and SeM/‘Antigen C’) duplex iELISA is a very sen-
be clinical signs of sinus disease. sitive and specific means of demonstrating exposure
to S. equi, provided the horse has not been vaccinated.
Differential diagnosis Titres remain elevated for several months, making the
Other causes of upper respiratory infectious disease test very useful at the end of an outbreak for identifica-
should be considered, including equine influenza tion of horses that have been exposed to infection and
virus, EHV-1 and -4, equine rhinovirus and EVA so may be carriers. The test is also used for screening
virus. Abscesses may be caused by other bacteria, horses before entry into stables to identify recent infec-
particularly S. equi subsp. zooepidemicus. Enlarged tion or carrier animals and can therefore be of use in
lymph nodes can also be encountered by other types screening of new arrivals in combination with quaran-
of inflammatory, infectious or neoplastic processes. tine. It may take up to 2 weeks for seroconversion to
occur and so horses with intermediate, or ‘grey zone’,
Diagnosis titres should have a second sample collected 10–14 days
For classical disease, history and clinical findings of later to confirm their serological status. Singlex ELISA
an outbreak of URT infectious disease with abscesses serological tests measuring antibody to SeM as a sin-
are characteristic of S. equi infections. However, his- gle antigen are not sufficiently reliable to be used for
tory and clinical examination findings are not diag- clinical diagnosis because of cross reactivity between
nostic for the milder (atypical) disease because this M proteins from S. equi and S. zooepidemicus. SeM
resembles other types of URT infectious disease. antibody titres are raised in cases of purpura haemor-
Confirmation of strangles can be obtained by rhagica and metastatic abscessation, and can be used to
demonstrating the presence of S. equi. According identify horses at risk for purpura haemorrhagica but
to the ACVIM consensus statement 2018, micro- are not a measure of protective immunity.
biological culture is no longer considered the gold Endoscopy confirms guttural pouch abnormali-
standard because of the low recovery of S. equi and ties and is useful for visualising retropharyngeal