Frequently Asked Questions

IDEXX Streptococcus equi ELISA Testing

S. equi ELISA Testing

1. What are the best uses of this test?
2. Does this ELISA distinguish between a vaccine response and natural infection?
3. Is it possible for a vaccinated horse to have a negative titer (<1:200)?

1. What are the best uses of this test?
   The SeM ELISA measures IgG antibodies against the M protein of S. equi. It yields an end point titer (up to 1:25,600 dilution) and the results are given as a titer accompanied by a descriptive interpretation. IgG titers are useful for the following situations:

   • Determination of vaccination status by measuring the current level of antibody
   • Diagnosis of aberrant S. equi abscesses
   • Assessment of predisposition to development of purpura hemorrhagica

   IgG titers are not useful to diagnose current infection. Samples drawn prior to production of IgG antibodies by the immune system (<21 days post-exposure) may have titers of <1:200 (negative).

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2. Does this ELISA distinguish between a vaccine response and natural infection?
   No, the M protein is present on the strains of S. equi used to make both of the vaccines currently available in the United States. Vaccination and natural infection elicit indistinguishable IgG antibody responses to the M protein. How can the titer be used to make vaccination/booster decisions? 1:1600 is the cutoff titer for vaccination. Horses with titers of less than or equal to 1:1600 and are at risk for S. equi exposure should be vaccinated. It is accepted that vaccination is not necessary and would be contraindicated in horses with titers greater than 1:1600.

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3. Is it possible for a vaccinated horse to have a negative titer (<1:200)?
   Yes, there are some instances in which a titer for a vaccinated horse will be negative.

   • Samples should not be collected until at least 21 days postvaccination to give the immune system time to develop an IgG response.
   • Improper administration of the vaccine may not illicit an immune response.
   • Some animals are “nonresponders” and may not generate an antibody response to the vaccine for currently unknown reasons.
   • If boosters are not administered, the antibody level will decrease over time due to natural degradation.

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