Outbreak Investigations
Investigation of strangles outbreaks should begin by an
interview with horse owners to obtain a detailed history
and to evaluate the potential full extent of the disease prob-
lem. The review should identify affected groups of horses
and allow the geography of the premises and the manage-
ment practices to be assessed for further risks and future
opportunities for disease control.
A practical disease control strategy should then be agreed
and implemented. The general aims and measures for such128 Sweeney et al
Table 1. Aims and associated measures used to control transmission of S equi on affected premises.
Aim Measure
To prevent the spread of S equi infection to horses on
other premises and to new arrivals on the affected pre-
mises.
To establish whether convalescing horses are infectious
after clinical recovery.
To investigate all outwardly healthy horses in which S
equi is detected either by culture or PCR. To eliminate S equi infection from the guttoral pouches. To prevent indirect cross-infection by S equi from horses in the ‘‘dirty’’ area to those in the ‘‘clean’’ area of the
premises.
Stop all movement of horses on and off the affected premises immediately and
until further notice.
Horses with strangles and their contracts should be maintained in well-demar-
cated ‘‘dirty’’ (ie, S equi positive) quarantine areas.
Clustering of cases in groups should allow parts of the premises to be easily
allocated as ‘‘dirty’’ and ‘‘clean’’ areas.
At least 3 nasopharyngeal swabs or lavages are taken at approximastely weekly
intervals from all recovered cases and their contacts and tested for S equi by
culture and PCR.
Horses that are consistently negative are returned to the ‘‘clean’’ area.
Endoscopy of the upper respiratory tract and guttural pouches.
Healing of lesions through a combination of flushing and aspiration with saline
and removal of chondroids using endoscopically guided instruments.
Topical and systemic administration of antimicrobials to eliminate S equi infec-
tion.
Personnel should use dedicated protective clothing when dealing with infectious
animals and should not deal simultaneously with susceptible animals. If this
is unavoidable, infectious horses should be dealt with after susciptible ani-
mals.
Strict hygiene measures are introduced, including provision of dedicated cloth-
ing and equipment for each area, disinfection facilities for personnel, and
thorough stable cleaning and disinfection methods. When cost is not a factor,
consideration should be given to destruction of this equipment following
eradication of the infection.
Following removal of organic material from stables, all surfaces should be
thoroughly soaked in an appropriate liquid disinfectant or steam treated and
allowed to dry. This should be repeated if possible.
Manure and waste feed from infectious animals should be composted (inactiva-
tion of bacteria by heat) in an isolated location.
Pastures used to hold infectious animals should be rested for 4 weeks.
Care should be taken to disinfect water troughs at least once daily during an
outbreak. Horse vans should be hosed clean and disinfected after each use.
a strategy are outlined in Table 1. This outline strategy may
need to be adapted to the individual circumstances of spe-
cific premises and outbreaks. In summary:
● All movements of horses on and off the affected pre-
mises should be stopped and segregation and hygiene
measures implemented immediately.
● Cases of strangles and their contacts should be main-
tained in well-demarcated ‘‘dirty’’ (ie, S equi positive)
quarantine areas.
● Rectal temperatures should be taken at least once daily
during an outbreak to detect, promptly segregate, and
possibly treat new cases.
● The aim of the control strategy, following bacteriological
screening, is to move horses from the ‘‘dirty’’ to ‘‘clean’’
areas where nonaffected and noninfectious horses are
kept.
● Every care should be taken to ensure very high hygiene
standards throughout the premises and for the duration
of the outbreaks.
● Screening of all convalescing cases following clinical re-
covery and their healthy contacts should be conducted
using swab or lavage of the nasopharynx, with special
care taken to maintain good hygiene to avoid inadvertent
transmission between horses during sampling.
● Swabs or lavage fluid should be collected at weekly in-
tervals following recovery over several weeks and tested
for S equi by conventional culture and PCR.
● Because PCR can detect dead as well as living bacteria,
positive PCR results are regarded as provisional, subject
to further investigation.
● Because the vast majority of subclinical long-term car-
riage of S equi appears to occur in the guttural pouches
of recovered horses, endoscopy of the upper respiratory
tract and guttural pouches should be performed in all
outwardly healthy horses in which S equi is detected,
either by culture or by PCR.
● Lavage samples from guttural pouches should then be
tested for S equi by culture and PCR.
● Sites such as the cranial nasal sinuses or tonsils should
be considered in horses that continue to harbor S equi in
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