S. equi ELISA Testing Recommendations
Streptococcus equi, a group C streptococcus, causes equine
strangles, a highly contagious purulent lymphadenitis and pharyngitis.
A nonvariant, clonal pathogen derived from a closely related ancestral
strain of Streptococcus zooepidemicus, it has a worldwide
distribution, facilitated by the mobility of its host and the fact
that some horses become long-term asymptomatic carriers,
intermittently shedding bacteria from the nose. Established outbreaks
may last from months to years; 70–75% of recovered horses develop
long-term immunity. Vaccines are available in the form of injectable
extracts or as live, attenuated organisms given intranasally. Their
use is somewhat controversial and indicated primarily for at-risk
horses, such as those living on or moving onto a premise with a
current incidence or past history of strangles. Vaccination of a horse
with an existing high level of S. equi-specific antibody may
cause an immune-mediated vasculitis. The ACVIM has published a consensus statement entitled “Streptococcus equi infections in horses: guidelines for treatment, control, and prevention of strangles.” (J. Vet. Inter. Med. 2005;19:123–134.) Specific recommendations regarding the use of culture, PCR and ELISA testing can be found in this article, which is available at www.acvim.org.
ELISA Testing
The immune response to the major virulence factor of S. equi,
the SeM protein, can be measured by our proprietary ELISA assay
developed in the laboratory of Dr. John Timoney, a renowned expert in
equine streptococcal disease. This quantitative test, performed
primarily on serum, is useful for diagnosing recent (but not
necessarily current) S. equi infection, determining the need
for booster vaccination, and as an aid in the diagnosis of purpura
hemorrhagica and metastatic abscesses (Timoney, 1997;
Sheoran et al, 1997). It does not distinguish between
vaccine and infection response. Comparison of titers obtained from
sequential samples may provide an indication of exposure/infection
status. As with any serologic test, variations in time of onset and
amplitude of the immune response among individual horses must be kept
in mind when interpreting results.
Antibody levels, reported as titers, are categorized below
Negative: (<1:200)
No SeM-specific antibodies detected.
This result may occur in a horse with recent exposure (<7 days
postexposure).
Weak Positive: (1:200–1:400)
SeM-specific antibodies detected at
a very low level. This is an equivocal result and repeat testing is
recommended in 7–14 days to confirm a presumptive diagnosis of
strangles or exposure to S. equi.
Moderate Positive: (1:800–1:1,600)
SeM-specific antibodies
detected at an intermediate level. This level may occur in a horse at
2–3 weeks postexposure or when the infection occurred six months
to two years previously.
High Positive: (1:3,200–1:6,400)
SeM-specific antibodies detected at
a high level. High levels are found 4–12 weeks post-infection or
following intranasal vaccination with live, attenuated organisms.
Vaccination with injectable extracts results in high levels in
1–2 weeks. Vaccination is contraindicated in horses with existing
high levels of antibody.
Very High Positive: (≥1:12,800)
SeM-specific antibodies
detected at a very high level. These levels are often found in horses
with a metastatic abscess or purpura hemorrhagica (immune-mediated
vasculitis) following exposure to strangles vaccine or S. equi.
Vaccination is contraindicated in horses with existing high levels of antibody.
Interpretation of ELISA results from foals less than six months of age must take into consideration that all or part of the SeM antibodies may have been passively acquired. Foals receive crucial antibody from colostrum, but eventually become susceptible as their passively acquired IgG levels decline with time.
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References:
- Timoney JF. Serology of Streptococcus equi
infection: uses and limitations. Equine Disease Quarterly.
1997;5(3):2–3.
- Timoney JF, Artiushin SC. Detection of Streptococcus
equi in equine nasal swabs and washes by DNA amplification. Veterinary
Record. 1997;141(17):446–447.
- Sheoran AS, Sponseller BT, Holmes MA, Timoney JF.
Serum and mucosal antibody isotype responses to M-like protein (SeM)
of Streptococcus equi in convalescent and vaccinated
horses. Vet Immunol. Immunopathol.
1997;59(3–4):239–251.
- Newton JR, et.al. Control of strangles outbreaks by
isolation of guttural pouch carriers identified using PCR and
culture of Strptococcus equi. Equine Veterinary Journal.
2000;32(6):515–526.
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