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Effectiveness of Anthelmintics: 
Specific Recommendations for Chickens 
Gallus gallus 
VICH GL21 
Guidance for Industry 
Draft Guidance 
This guidance document is being distributed for comment purposes only. 
This version of the guidance replaces the version made available June 2002.  This revision   
clarifies the definition of adequate infection in individual animals, updates considerations for 
field studies, and makes additional clarifying changes. 
Submit comments on this draft guidance by the dated provided in the Federal Register notice 
announcing the availability of the draft guidance.  Submit electronic comments to 
https://www.regulations.gov.  Submit written comments to the Dockets Management Staff 
(HFA-305), Food and Drug Administration, 5630 Fishers Lane, Rm. 1061, Rockville, MD  
20852.  All comments should be identified with docket number FDA-2022-D-1494. 
For further information regarding this document, contact Aimee Phillippi-Taylor, Center for 
Veterinary Medicine (HFV-114), Food and Drug Administration, 7500 Standish Place, Rockville 
MD 20855, 240-402-0601, email: mailto:aimee.phillippi-taylor@fda.hhs.gov. 
Additional copies of this draft guidance document may be requested from the Policy and 
Regulations Staff (HFV-6), Center for Veterinary Medicine, Food and Drug Administration, 
7500 Standish Place, Rockville MD 20855, and may be viewed on the Internet at 
https://www.fda.gov/animal-veterinary, https://www.fda.gov/regulatory-information/search-fda-
guidance-documents, or http://www.regulations.gov. 
U.S. Department of Health and Human Services 
Food and Drug Administration 
Center for Veterinary Medicine (CVM) 
August 2022  
 
 
VICH GL21 (ANTHELMINTICS: CHICKENS  GALLUS GALLUS) 
May 2022 
Revision at Step 9 
For consultation at Step 4 
 
 
 
 
 
 
Efficacy A :  
 OF  NTHELMINTICS  
S R  
PECIFIC  ECOMMENDATIONS
C  G  
FOR  HICKENS    ALLUS GALLUS
(R 1) 
EVISION 
 
 
 
 
 
Revision at Step 9 
Recommended for Consultation at Step 4 of the VICH Process 
in May 2022 
by the VICH Steering Committee 
 
 
 
 
This Guidance has been developed by the appropriate VICH Expert Working Group will be subject to 
consultation by the parties, in accordance with the VICH Process.  At Step 7 of the Process the final draft 
is recommended for adoption to the regulatory bodies of the European Union, Japan and USA. 
 
 
 
 
 
 
 
  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
Table of Contents 
 
I.  Introduction ............................................................................................................................. 1 
A.  General Elements .............................................................................................................. 2 
1.  The Evaluation of Effectiveness Data ....................................................................... 2 
2.  Use of Natural or Induced Infections ........................................................................ 2 
3.  Number of Infective Forms Recommended for Induced Infections ....................... 2 
4.  Recommendations for the Calculation of Effectiveness .......................................... 3 
4.1  Factors to Support a Claim ............................................................................... 3 
4.2  Number of Experimental Units (Dose Determination, Dose Confirmation, 
and Persistency Studies) .................................................................................... 3 
4.3  Adequacy of Infection ........................................................................................ 3 
4.4  Label Claims ....................................................................................................... 4 
5.  Treatment Procedures ................................................................................................ 4 
6.  Bird Selection, Allocation and Handling .................................................................. 4 
B.  Specific Evaluation Studies .............................................................................................. 4 
1.  Dose Determination Studies ....................................................................................... 4 
2.  Dose Confirmation Studies ........................................................................................ 5 
3.  Field Effectiveness Studies ......................................................................................... 5 
II.  Glossary ................................................................................................................................... 5 
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Effectiveness of Anthelmintics: Specific Recommendations for 
Chickens  Gallus gallus 
VICH GL21 
Draft Guidance for Industry 
This draft guidance, when finalized, will represent the current thinking of the Food and Drug 
Administration (FDA or Agency) on this topic.  It does not establish any rights for any person 
and is not binding on FDA or the public.  You can use an alternative approach if it satisfies the 
requirements of the applicable statutes and regulations.  To discuss an alternative approach, 
contact the FDA staff responsible for this guidance as listed on the title page. 
I.  Introduction
The present guidance for chickens was developed by the the Working Group established by the 
International Cooperation on Harmonisation of Technical Requirements for Registration of 
Veterinary Medical Products (VICH), Anthelmintic Guidances, and subsequently revised in 
2022.  It should be read in conjunction with Guidance for Industry (GFI) #90 (VICH GL7), 
Effectiveness of Anthelmintics: General Recommendations,1 which should be referred to for 
discussion of broad aspects for providing pivotal data to demonstrate product anthelmintic 
effectiveness.  The present document is structured similarly to GFI #90/VICH GL7 with the aim 
of simplicity for readers comparing both documents. 
The aim of the guidance for chickens is: (1) to be more specific for certain specific issues for 
chickens not discussed in GFI #90/VICH GL7; (2) to highlight differences with GIF #90/VICH 
GL7 on effectiveness data recommendations; and (3) to give explanations for disparities with 
GFI #90/VICH GL7.  We recommend that sponsors refer to the pertinent procedures described in 
detail in other published documents, e.g., World Association for the Advancement of Veterinary 
Parasitolgoy (WAAVP) guidelines for evaluating the efficacy of anthelmintics in chickens and 
turkeys. Veterinary Parasitology 116: 159-173, 2003, and updated versions as they are published.  
For other poultry, the principles outlined in this guideline should be used where applicable. 
The contents of this document do not have the force and effect of law and are not meant to bind 
the public in any way, unless specifically incorporated into a contract.  This document is 
intended only to provide clarity to the public regarding existing requirements under the law.  
FDAs guidance documents should be viewed only as recommendations, unless specific 
regulatory or statutory requirements are cited.  The use of the word should in Agency guidances 
means that something is suggested or recommended, but not required. 
1 https://www.fda.gov/media/70349/download 
1  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
 
A.  General Elements 
1.  The Evaluation of Effectiveness Data 
Only controlled tests based on parasite counts of adults/larvae should be acceptable both for the 
dose determination and dose confirmation studies, since critical tests generally are not 
considered to be reliable for chickens.  Egg counts with identification of the genus should be the 
preferred method to evaluate the effectiveness in field studies.  Adequate parasite infection 
should be defined in the protocol according to regional prevalence or historic data and/or 
statistical analysis. 
 
2.  Use of Natural or Induced Infections 
Dose determination studies generally should be conducted using induced infections with either 
laboratory strains or recent field isolates. 
 
Dose confirmation studies should be conducted using naturally-infected birds which can have 
superimposed induced infections.  This procedure should allow a wide range of parasites to be 
present in the experimental birds.  Also, induced infections in one of the studies should be 
acceptable.  Studies for larval stages should be conducted with induced infections. 
 
The history of the parasites used in the induced infection studies should be included in the final 
report. 
 
3.  Number of Infective Parasitic Forms Recommended for Induced Infections 
Table 1 indicates the number of eggs/cysticercoids recommended to be used and will depend on 
the isolate that is used. The final number of eggs/cysticercoids used in the infection should be 
included in the final report. 
 
Table 1. Range of Infective Stages Recommended to 
Produce Adequate Infections in Chickens for Anthelmintic 
Evaluation 
Parasite  Range  
Ascaridia galli  200  500 
Capillaria obsignata  100  300 
Heterakis gallinarum  200  300 
Raillietina cesticillus   50  100 
Syngamus trachea  200  600 
 
Some factors to consider for induced infections in chickens are: 
a.  Young birds should be used in the studies; 
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b.  To maximize the establishment of adequate infections, it is recommended to use low 
numbers of infective stages; 
c.  Stress (e.g., poor diets) is not recommended to generate helminth infections; and 
d.  Housing conditions should not allow accidental infections. 
4.  Recommendations for the Calculation of Effectiveness 
4.1  Factors to Support a Claim 
To support a claim the following pivotal data should be included: 
a.  Two dose confirmation studies conducted with a minimum of six adequately infected 
experimental units (individual birds or pens, see section II. Glossary) in the non-
medicated control group in each study.  The infection of the experimental units in the 
study will be deemed adequate based on historical, parasitological, and/or statistical 
criteria; 
b.  The differences in parasite counts between treated and control experimental units 
should be statistically significant (p  0.05); and 
c.  Percent effectiveness should be 90% or higher and calculated and interpreted using 
the procedures described in section A.4.2. Calculation and Evaluation of Percent 
Effectiveness of GFI #90/VICH GL7. 
4.2  Number of Experimental Units (Dose Determination, Dose Confirmation, and 
Persistency Studies) 
The minimum number of experimental units used per experimental group is a critical point.  
Although the number of experimental units will depend on the possibility to process the data 
statistically according to the adequate statistical analysis, it has been recommended, to achieve 
harmonization, that the inclusion of at least six experimental units in each experimental group is 
a minimum. 
 
4.3  Adequacy of Infection 
The minimum adequate number of helminths in individual control birds should be defined in the 
protocol.  However, final conclusions regarding adequacy of infection will be made as part of the 
final report based on statistical analysis, historical data, literature review, or expert testimony.  If 
the experimental unit is a pen, an adequately infected pen should be defined by a minimum 
number of adequately infected birds out of the total number of birds in that pen (i.e., percentage 
of adequately infected birds in the pen).  The range of chicken helminths (adults) considered 
adequate to grant a claim will vary according to the species.  Generally, a minimum of 20 A. 
galli in individual control birds is considered to be an adequate infection.  Lower individual 
counts may be expected with H. gallinarum, C. obsignata, and R. cesticellus.  Necropsies should 
be conducted within 10 days of treatment.  
 
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4.4  Label Claims 
Generally, for adult claims, the treatment should not be administered earlier than 28 days after 
infection.  It is recommended to include at least six sentinel birds for helminth characterization 
and quantification before treatment is initiated.  Generally, for L4 claims, treatments should be 
given 7 days after infection, except for A. galli and H. gallinarum, which should be 16 days after 
infection. 
 
5.  Treatment Procedures 
The method of administration (oral, parenteral, topical, slow release, etc.), formulation, and 
extent of activity of a product will influence the protocol design. 
 
When the drug is to be administered in the water or in a feed, it should be done as much as 
possible following the labeling recommendations.  Palatability/consumption studies may be 
recommended for medicated feeds.  Samples of medicated water or medicated feed should be 
collected to confirm drug concentration.  The amount of medicated product provided to each 
animal should be recorded to ensure that the treatment satisfies the label recommendations. 
 
6.  Bird Selection, Allocation, and Handling 
Test birds should be clinically healthy and representative of the age, sex, and class for which the 
claim of the test anthelmintic is to be made.  In general, birds should be young and from a breed 
that is susceptible to helminth infections.  If birds are housed in pens (e.g., cages or floor pens), 
the birds should be randomly assigned to each pen.  The experimental units should also be 
randomly assigned to each treatment group.  Randomization to treatment group should be 
performed using an adequate method that should be described in the protocol and final report.  
Blocking is only recommended if it is expected to reduce residual error in the study.  If blocking 
is used, blocks should be included as a random effect in the statistical model.  Nevertheless, 
blocking is not always the most appropriate method for reducing residual error.  Alternative 
methods may therefore be considered, e.g., a suitably selected covariate. 
 
Animal housing, feeding, and care should follow recommendations for welfare, including 
vaccination according to local practices.  This information should be provided in the final report. 
A minimum 10-day acclimatization period is recommended.  Housing and feed/water should be 
adequate according to the geographical location. Birds should be monitored daily to determine 
adverse reactions. 
 
B.  Specific Evaluation Studies 
1.  Dose Determination Studies 
If the treatment recommends extended administration, one or more studies are recommended to 
determine the minimum treatment period for effectiveness. 
 
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2.  Dose Confirmation Studies 
No species-specific recommendations. 
 
3.  Field Effectiveness Studies 
Depending on the facilities available, the experimental unit may be the animal, pen, or the 
shed/house (see section II. Glossary).  The design of the field studies should represent current 
commercial conditions and should be replicated in different geographic locations and in different 
production class(es), depending on the indication being pursued.  Housing will differ based on 
the production class under investigation (e.g., layers vs. broilers).  The protocol should state the 
number of experimental units per treatment group (sample size), describe allocation (proportion) 
to treatment groups, and include a brief description of how the sample size was determined.  
Regardless of whether one or multiple parasites are being evaluated within a study, an 
appropriate sample size calculation or justification is necessary prior to study conduct. 
 
When commercial facilities (or similar) are used, the shed/house should be subdivided, when 
possible, to allow for sufficient replication to enable a statistical analysis.  If the shed/house is 
the experimental unit and there is only one replicate per treatment group at a study site, the study 
may need to utilize additional sites with the same housing conditions to achieve sufficient 
replication and enable a statistical analysis.  Otherwise, a study with one replicate can only be 
summarized using descriptive statistics and may not provide sufficient inferential value.  
 
Effectiveness should be assessed by the reduction of worm counts in all birds or in representative 
birds as determined by comparing the treated and control groups.  If representative birds are used 
for worm counts, the protocol should describe procedures for random selection of animals 
(number and percentage) to be necropsied.  Fecal egg counts may be used to establish pre-
treatment infection levels and parasite species present.  A comparison of pre- and post-treatment 
fecal egg counts may be included but is not required.  If fecal egg counts are evaluated, fresh, 
clean droppings should be collected immediately before treatment, and at 7 to 14 days after 
treatment.  The fecal sampling method, number of pens/animals sampled, and egg counting 
technique should be defined in the protocol.  Standard, well accepted techniques should be used 
and fully described in the protocol and final report.  
 
Clinical observations, production variables, and records of culls and mortality should be 
maintained and compared to control birds and historical data of the commercial establishment.  If 
birds are processed at the end of the study, slaughterhouse inspection reports with final 
observations regarding possible abnormalities, which are collected per the standard practices of 
the slaughterhouse, should be included in the final report.  However, if the study duration does 
not coincide with or include slaughterhouse processing, these data are not required. 
II.  Glossary 
EXPERIMENTAL UNIT: The entity (e.g., individual animal, cage, pen, or shed/house) which 
can be independently and randomly assigned to a treatment, and whose response to the assigned 
treatment can be independently evaluated.  The experimental unit is the basic unit for the 
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statistical analysis.  The experimental unit may be the individual bird or the pen/shed/house, 
depending on the circumstances of the study. 
1)  The pen/shed/house is the experimental unit in the analysis if all birds in a 
pen/shed/house are provided the same treatment through medicated feed or water; or 
2)  The individual bird is the experimental unit in the analysis if the treatment can be 
individually administered, the treatments are randomly assigned to birds within a 
pen/shed/house, and the endpoint can be evaluated independently for each bird in a 
pen/shed/house. 
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