Assessment of Passive Immunity

The acquisition of immunoglobulins is entirely by passive transfer in foals. The speed with which this occurs following birth, the amount and types of specific antibodies in the colostrum, and the amount and types absorbed are factors in the prevention of infection. Good management may be just as important as increasing amounts of immunoglobulins in preventing infections [1].

I. Time Factors

1. General Considerations

1. Tests of serum IgG taken before 18 hours of age may indicate lower levels of IgG than will be attained at 24 hours of age, due to ongoing transport of immunoglobulins into blood via the lymphatics and continued gastrointestinal absorption.

2. Early Testing of Foal Serum IgG

1. Foal's serum IgG tested at 8-12 hr of age.
2. IgG has been nearly completely absorbed in most foals by this time.
3. Most foals should have at least 400 mg IgG/dl serum at this time.
4. If IgG is < 400 mg/dl at 8-12 hours, then previously harvested colostrum can be supplemented.
5. This minimizes the incidence of FPT, decreases the time period of immunocompromise associated with low immunoglobulin levels, and reduces the need for plasma transfusions.

3. Late Testing of Foal Serum IgG

1. 18-24 hours of age.
2. If FPT is detected at this time plasma therapy is the only means to raise serum IgG.
3. Delay in detecting a very low serum IgG may increase risk of acquiring infection.

II. TESTS OF SERUM IgG [2-4]

Editor's Comment - Standards for the reference test of single radial immuno-diffusion (SRID) vary with the SRID system. Previously the same serum sample sent to different laboratories could read double the amount of IgG in one lab compared to another. New standards have been developed; in the meantime, several tests have proven reliable and consistent, and can be useful.

1. Single radial immuno-diffusion (SRID)

1. IgG moves through gel to meet antibody and forms visible circles.
2. Requires 24 hours.
3. Measures 0-3000 mg/dl.
4. Kits available: VMRD, Inc; P.O. Box 502, Pullman, WA 99163 www.vmrd.com and Equine-RID, Plasvacc, Inc. www.plasvaccusa.com Phone 800-654-9743.

2. Glutaraldehyde Coagulation [5]

1. Chemical grade 25% glutaraldehyde solution.
2. Forms insoluble complexes with basic proteins.
3. Make 10% solution with deionized water.
4. Use serum in test.
5. Add 5 ml of serum plus 50 ul of glutaraldehyde and check for coagulation. Measure ≤ 400 ≥ 800 mg/dl

1. 800 mg/dl observed in 10 minutes.
2. Reaction in 60 minutes ≥ 400 mg/dl.

6. Inexpensive - Approx. 0.25 cents/test.
7. Hemolysis may result in overestimated IgG.
8. Test is reliable and accurate [5]
9. GAMMA-CHECK-C for colostral IgG testing commercial kit from Plasvacc Inc. (www.plasvaccusa.com).

3. Snap Test

1. Measures < 400, 400-800 and > 800 mg IgG/dl.
2. Enzyme combines with IgG for color change.
3. Serum or plasma preferred; can use whole blood.
4. 8-10 minutes for result.
5. Accurate and reliable especially in lower (< 400 mg/dl) and higher range (> 800 mg/dl) [6,7]
6. Manufactured by Idexx Inc. www.idexx.com

4. Zinc Sulfate Turbidity [8]

1. Method correlates well with SRID but can occasionally over-estimate amounts of immunoglobulin in the 400 mg/dl range [9].
2. Preparation of solutions - For 1 liter: dissolve 250 mg of ZnSO4.7 H2O granules (Sigma Chemical Co.) in 1 liter of distilled water (boiled to remove all the CO2). Inject 6 ml of the zinc sulfate solution into a 10 ml red top vacutainer tube using a syringe and small needle and preserve the vacuum in the tube. Shelf life is long if protected from air.
3. Modified zinc sulfate test for foals [10]

1. Controls - Obtain pre-suckle serum and adult serum samples and store in the freezer in small aliquots for thawing to run with a test.
2. Bleed foals between 8 and 18 hrs of age. Separate serum.
3. Bring zinc sulfate solution and serum to room temperature.
4. Measure 0.1 ml (100 μl) of serum and control sample. If using a 1 cc TB syringe, fill to 0.3 and express to 0.2 and add to zinc sulfate tube.        
5. Rotate for 15 seconds.
6. If a precipitate forms there is > 400 mg/dl IgG, if hemolysis is present in serum sample results will be erroneously high.
7. Potential problems

1. Old solution - Rapidly decomposes (max shelf life 3 months) when exposed to small amount of room air and may give false results
2. Temperature for test should be run at room temperature.
3. Hemolysis - Gives false positive values.

III. Definitions of Passive Transfer [11]

Editor's Comment - Keep in mind measurements below represent total IgG; specific antibodies against a particular pathogen may be more important than the total IgG. New levels and definitions have risen what is considered the normal IgG level.

1. < 200 mg/dl serum of IgG is complete failure of passive transfer.
2. 200-800 mg/dl is partial failure of passive transfer.
3. ≥ 800 mg/dl is normal or adequate passive transfer.

Editor's Comment - Some confusion has occurred because 800 mg/dl has been recommended for infected foals or high risk foals. The ≥ 400-600 mg/dl level is still adequate for healthy low risk foals in my opinion.

IV. Interpretation of Values - (IgG) (Early or Late Testing)

1. ≤ 200 mg/dl is failure of passive transfer - Recommend I.V. plasma if > 18 hours old.
2. 200-800 mg/dl is partial failure of passive transfer.

1. Treat with colostrum if < 18 hours old.
2. If < 400 mg/dl and > 18 hours old treat with plasma or serum products.
3. If 400-800 mg/dl, excellent management and no risk factors identified treatment is not necessary; otherwise treat with plasma and antibodies.

3. ≥ 800 mg/dl - Normal - No treatment unless a high risk foal or sepsis is present.

V. Most Septic foals have IgG Serum < 400 mg/dl [12,13]

VI. Newborn foals have reduced neutrophil function prior to ingestion of colostrum [14]