October 31, 2002

United States

Department of

Agriculture

Marketing and 

Regulatory 

Programs 

Animal and Plant 

Health Inspection 

Service 

Veterinary Services 

Center for Veterinary

Biologics 

Suite 104 

510 South 17th Street 

Ames, IA 50010

(515) 232-5785 

FAX (515) 232-7120 

CENTER FOR VETERINARY BIOLOGICS NOTICE NO. 02-25 

Subject: Conditional Licenses for Products Containing 

Clostridium perfringens Type A 

To: Biologics Licensees, Permittees, and Applicants 

Directors, Center for Veterinary Biologics 

The purpose of this notice is to inform interested parties of an Animal and Plant Health 

Inspection Service (APHIS) policy regarding license requirements for a claim of 

protection against the alpha toxin of Clostridium perfringens. 

I. BACKGROUND 

Despite numerous attempts by researchers in industry and academia, the disease 

syndrome associated with C. perfringens Type A has not been successfully reproduced 

using artificial methods. In the past, the Center for Veterinary Biologics (CVB) has 

rejected proposals by vaccine manufacturers to use the demonstration of production of 

anti-C. perfringens alpha toxin antibodies or the demonstration of protection of animals 

from an intravenous injection of C. perfringens alpha toxin to support an efficacy claim 

for “aids in the protection against disease caused by C. perfringens Type A.” However, 

an increasing amount of anecdotal and experimental evidence supports the association of 

C. perfringens alpha toxin with various disease entities in several species. In addition, 

these disease entities appear to be increasing in prevalence in recent years. 

II. GUIDELINES 

The CVB will consider conditional licensure of a C. perfringens Type A toxoid or 

antitoxin under the following conditions: 

A reasonable expectation of efficacy for a toxoid may be demonstrated by the 

development of a serum antibody concentration of at least 4 international antitoxin units 

per ml in at least 80 percent of vaccinated animals, which were seronegative prior to 

vaccination. If that antibody level is not achieved postvaccination, a reasonable 

expectation of efficacy may be demonstrated if at least 80 percent of vaccinated animals 

are able to survive an intravenous toxin challenge equal to 10 host animal LD50 of 

C. perfringens alpha toxin. Host animal toxin titration data verifying the challenge dose 

must be submitted. The 10 LD50 toxin dosage should be calculated as ml of toxin per kg 

body weight of the host animal. All host animal toxin titrations must be conducted using 

standard C. perfringens alpha toxin obtained from the Center for Veterinary BiologicsLaboratory. 

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Center for Veterinary Biologics Notice No. 02-25 2 

2. A reasonable expectation of efficacy of an antitoxin may be demonstrated if treated 

animals meet either of the two criteria outlined above for toxoids. 

3. Conditionally licensed C. perfringens Type A toxoids or antitoxins must be 

monovalent. No combinations with fully licensed fractions will be allowed until full 

licensure may be granted for the C. perfringens Type A fraction. Definitive efficacy 

studies may be either in the form of a field efficacy study or may utilize an acceptable 

animal challenge model which reflects the pathogenesis of the disease under field 

conditions, if such a model can be developed. Intravenous administration of toxin will 

not be allowed to establish efficacy for full licensure of a C. perfringens Type A toxoid 

or antitoxin. 

4. The label claim of conditionally licensed C. perfringens Type A toxoid or antitoxin 

will be “as an aid in the control of disease syndromes caused by the alpha toxin of 

Clostridium perfringens”. The labels must state that the product is conditionally licensed, 

efficacy studies are in progress, and, as applicable, that potency test development is in 

progress. 

5. Rabbit serum-toxin neutralization tests, similar to the tests used to measure the 

potency of C. perfringens beta and epsilon toxoids [Supplemental Assay Methods 

(SAMs) 201 and 203], will be considered for potency tests of C. perfringens Type A 

toxoids. The assay must be validated for sensitivity, specificity, and repeatability in a 

manner acceptable to APHIS. The release titer for toxoids must be based on the results 

of at least three independent tests of the product that were used to demonstrate efficacy. 

6. The release titer and dose volume for antitoxins must be based on the number of 

international antitoxin units required to establish efficacy. The potency of antitoxins 

should be determined in international antitoxin units per milliliter of serum using a 

serum-toxin neutralization test. A test similar to those used for C. perfringens beta and 

epsilon antitoxins (SAMs 202 and 204), which is acceptable to the CVB, may be used. 

/s/ Richard E. Hill, Jr. 

Richard E. Hill, Jr. 

Director 

Center for Veterinary Biologics