Serum Hepatitis in the
Horse & When to Use
TETANUS ANTITOXIN vs. TETANUS TOXOID
J Berrier, DVM
There is a disease syndrome called
“serum hepatitis” that affects horses.
This is a very rare event and is linked to
administration of equine serum origin products. About
20% of horses with idiopathi (unknown cause) acute hepatic
disease (IAHD) show clinical signs of
liver failure (anorexia, lethargy, jaundice) within 4 – 10
weeks after receiving an equine origin biologic – hence the
name “serum” hepatitis 1. One
of the most common equine serum origin products used in the
field today is tetanus antitoxin.
Many other equine serum products, including normal
horse plasma, have also been linked to serum hepatitis 2,
3, 4. This
link between equine serum origin products and hepatitis has
been well documented for almost 90 years 6.
The cause of serum hepatitis is not known 2,
mentioned earlier, it is a very rare event (incidence is
~1:500,000 doses sold - based on Colorado Serum data), but
“outbreaks” 1, 4 have been reported every few
years with multiple horses in the same and sometimes different
geographic areas involved.
Some horses will develop hepatitis having never
received an equine serum product before 1, 5.
Some toxic plants, moldy corn and blue-green algae can
also cause hepatitis 1. There
is not an equine specific virus that causes hepatitis and
there has been no causative agent identified in cases of serum
hepatitis despite repeated attempts to do so 2, 5.
Attempts to transmit the disease experimentally have
also failed 2, 4, 5.
The most plausible causative explanation/theory is a
type III hypersensitivity reaction 1, 7
which is a type of allergic reaction where
antigen-antibody complexes form in the liver which results in
hepatitis and has a mortality rate of 50 – 83% once symptoms
“antigen” in these cases is speculated to be some inherent
equine protein that is present in the serum product.
This is not a “contaminant” but instead would be a
protein found in horse serum products that apparently can
induce an allergic reaction (which results in hepatitis) in a
very select few recipient horses and/or under select
conditions and is impossible to predict.
Because of this link, there is a warning regarding
serum hepatitis in every equine serum origin product sold by
A prudent measure to avoid the risk of
serum hepatitis is to vaccinate your horse with tetanus toxoid
on a regular basis. By
vaccinating your horse as a foal at 5 or 6 months of age with
a booster 30 days later, followed by annual boosters (with
pregnant mares getting their annual booster 2 - 4 weeks before
foaling) the need to use tetanus antitoxin is eliminated.
In events when a vaccinated horse (vaccinated ≤12
months prior) receives a wound then all that is necessary is a
tetanus toxoid booster, along with antibiotics.
Tetanus antitoxin should be used (along with
antibiotics) in wounded horses that have no previous history
of tetanus toxoid vaccination, or are overdue (>12 months)
for a tetanus toxoid vaccination, or in cases of treatment for
tetanus disease, in which case your veterinarian should be
treating the animal with adjunctive therapy and very large
doses of tetanus antitoxin.
Tetanus antitoxin is usually a very safe
biologic that has its place in the equine world, but its use,
like all biologics, is not without risk and serum hepatitis is
a unique and very rare risk associated with equine serum
this reason it is wise for horse owners and veterinarians to
be aware of these risk factors when using biologics and to
understand when tetanus antitoxin is indicated vs. a tetanus
toxoid booster. Colorado
Serum Company also manufactures an economical single fraction
Veterinary Merck Manual;
6th edition, pages 285 – 286, 289 - 291, 2344, 2506.
2. Panciera, Roger J., Serum Hepatitis in the Horse, JAVMA,
Vol. 155, No. 2
3. Aleman, M., Nieto, J., Carr, E., Carlson, G.; Serum
Hepatitis Associated with Commercial Plasma Transfusion in
Horses, J. Vet. Intern. Med., 2005; 19: 120-122.
4. Thomsett, L.R., Acute Hepatic Failure in the Horse, Equine
Veterinary Journal, 1970; 2: 15-19.
5. Divers, T.J., Acute Hepatic Failure (Thieler’s Disease);
Current Vet Therapy in Equine Medicine 2, 1987.
6. Theiler, A., Acute Liver Atrophy and Parenchymatous
Hepatitis in Horses; U. of S. Africa, 5th & 6th reports to
The Director of Veterinary Research, Dept. of Agriculture;
1918: 7 – 164.
7. Tizard, IR. An Introduction to Veterinary Immunology.
: WB Saunders, 197: 110-112.