#96 
 
Effectiveness of Anthelmintics: 
Specific Recommendations for Ovines 
 
VICH GL13 
 
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Draft Guidance 
 
This guidance document is being distributed for comment purposes only.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 March 2001.  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Submit comments on this draft guidance by the dated provided in the Federal Register notice 
announcing the availability of the draft guidance.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: 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Additional copies of this draft guidance document may be requested from the Policy and 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 GL13 (ANTHELMINTICS: OVINES) 
May 2022 
Revision at Step 9 
For consultation at Step 4 
 
 
 
E A : 
FFICACY OF  NTHELMINTICS
S R  
PECIFIC  ECOMMENDATIONS
O  
FOR  VINES
(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Draft  Not for Implementation 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 Parasitic 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 Animals (Dose Determination, Dose Confirmation, and 
Persistency Studies) ........................................................................................... 4 
4.3  Adequacy of Infection ........................................................................................ 4 
4.4  Label Claims ....................................................................................................... 4 
5.  Treatment Procedures ................................................................................................ 5 
6.  Animal Selection, Allocation, and Handling............................................................. 5 
B.  Specific Evaluation Studies .............................................................................................. 6 
1.  Dose Determination Studies ....................................................................................... 6 
2.  Dose Confirmation Studies ........................................................................................ 6 
3.  Field Effectiveness Studies ......................................................................................... 6 
4.  Persistent Effectiveness Studies ................................................................................. 6 
 
 
i 
  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
Effectiveness of Anthelmintics: Specific Recommendations for 
Ovines 
 
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 
This ovine guidance was developed by the Working Group established by the International 
Cooperation on Harmonisation of Technical Requirements for Registration of Veterinary 
Medicinal 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 ovine guidance is: (1) to be more specific for certain specific ovine issues not 
discussed in GFI #90/VICH GL7; (2) to highlight differences with GFI #90/VICH GL7 on 
effectiveness data recommendations; and (3) to give explanations for disparities with the GFI 
#90/VICH GL7. 
 
It is also important to note that technical procedures to be followed in the studies are not the aim 
of this guidance.  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 
Parasitology (W.A.A.V.P.) second edition of guidelines for evaluating the efficacy of 
anthelmintics in ruminants (bovine, ovine, caprine). Veterinary Parasitology 58: 181-213, 1995, 
and updated versions as they are published. 
 
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 are recommended both for the 
dose determination and dose confirmation studies, since critical tests generally are not 
considered to be reliable for ruminants.  Egg counts/larval identification is the preferred method 
to evaluate the effectiveness in field studies.  Long-acting or sustained-release products should 
be subject to the same evaluation procedures as other therapeutic anthelmintics.  Adequate 
parasite infection should be defined in the protocol according to regional prevalence or historical 
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.  Limited experience exists with induced infections of 
Protostrongylidae, cestodes, and Dicrocoelium spp.  For these parasites the use of natural 
infections instead of induced infections may be justified. 
 
Dose confirmation studies should be conducted using naturally-infected animals; however, 
induced infections or superimposed induced infections can also be used.  This procedure will 
allow a wide range of parasites to be present.  For claims against 4th stage larvae, induced 
infections should be used.  For claims against hypobiotic larvae, natural infections should be 
used.  Sponsors should aim for a maximum period of accumulation of hypobiotic larvae for the 
particular parasite species being targeted in trial animals.  This will be area or regionally 
dependent.  Specific details on area or regional situations should be obtained from experts on a 
case-by-case basis.  In all cases, animals should be housed (to preclude reinfection) for a 
minimum of 2 weeks before treatment. 
 
Persistent effectiveness studies should be conducted using induced infections with recent field 
isolates. 
 
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 
The number to be used is approximate and will depend on the isolate that is used.  The final 
number of larvae used in the infection should be included in the final report.  Table 1 shows the 
range of numbers recommended for parasites with existing infection models. 
 
 
 
 
 
 
2 
  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
 
Table 1. Number of Infective Stages Recommended to 
Produce Adequate Infections in Sheep for Anthelmintic 
Evaluation 
Parasite Anatomical Location  Range of 
Genus Species  Eggs/Larvae 
Abomasum   
Haemonchus contortus  400  4,000 
Teladorsagia circumcincta  6,000  10,000 
Trichostrongylus axei  3,000  6,000 
Intestines   
Cooperia curticei  3,000  6,000 
T. colubriformis & T. vitrinus  3,000  6,000 
Nematodirus spp.  3,000  6,000 
Oesophagostomum spp.  500  1,000 
Chabertia ovina  800  1,000 
Bunostomum trigonocephalum  500  1,000 
Strongyloides papillosus  80,000 
Gaigeria pachyscelis  400 
Trichuris spp.  1,000 
Lungs   
Dictyocaulus filaria  1,000  2,000 
Liver   
Fasciola hepatica (metacercaria)   
     Chronic  100  200 
     Acute  1,000  1,500 
 
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 
non-medicated animals (control group) in each study.  The infection of animals in the 
study will be deemed adequate based on historical, parasitological, and/or statistical 
criteria; 
 
3 
  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
b.  The differences in parasite counts between treated and control animals 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 Animals (Dose Determination, Dose Confirmation, and Persistency Studies) 
The minimum number of animals used per experimental group is a critical point.  Although the 
number of animals 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 animals in each experimental group is a minimum. 
 
In cases where there are several studies none of which has six adequately infected animals in the 
control group (for example, important rare parasites), the results obtained could be pooled to 
accumulate 12 animals in the studies and statistical significance calculated.  If the differences are 
significant (p < 0.05), effectiveness may be calculated and if the infection is deemed adequate, 
the claim may be granted.  Sampling techniques and estimation of worm burden should be 
similar among laboratories involved in the studies to allow adequate and meaningful 
extrapolation of the results to the population. 
 
4.3  Adequacy of Infection 
The minimum adequate number of helminths in individual control animals 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.  
The range of ovine helminths (adults) that has been considered adequate to grant a claim varies 
according to the species.  Generally, a minimum of 100 nematodes in individual control animals 
is considered an adequate infection.  Lower individual counts are to be expected with 
Bunostomum spp., Oesophagostomum spp., Trichuris spp., Gaigeria pachyscelis, and 
Dictyocaulus filaria.   Generally, for Fasciola spp., minimum counts of 20 adults are considered 
adequate. 
 
4.4  Label Claims 
For adult claims as a general rule, the treatment should not be administered earlier than 21 to 25 
days after infection; optimum for most species is 28 to 32 days.  Major exceptions are: 
Oesophagostomum spp. (28 to 41 days), Bunostomum spp. (52 to 56 days), Strongyloides 
papillosus (14 to 16 days), and Fasciola spp. (8 to 12 weeks). 
 
For L4 claims, treatments should be given on the following days after infection: 3 to 4 days for 
Strongyloides papillosus; 5 to 6 days for Haemonchus spp., Trichostrongylus spp., and Cooperia 
spp.; 7 days for T. (O.) circumcincta; 8 to 10 days for Nematodirus spp. and D. filaria; and 15 to 
17 days for Oesophagostomum spp.  The term immature on the labeling is not recommended 
for these claims.  
 
 
4 
  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
For early immature Fasciola spp., treatments should be given 1 to 4 weeks after infection and for 
late immatures, at 6 to 8 weeks. 
 
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.  It is advisable to consider the 
weather and animal relationship with regard to effectiveness of topical formulations.  Slow-
release products should be tested over the entire proposed effective time unless additional 
information suggests that this is unnecessary, e.g., blood levels demonstrate steady state at all 
points of the proposed therapeutic period. 
 
When the drug is to be administered in the water or in a medicated feed, it should be done 
following the labeling recommendations.  Palatability studies may be required 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.  For products used topically, the 
impact of weather (e.g., rainfall, UV light) and fleece length should be included in the evaluation 
of the effectiveness of the product. 
 
6.  Animal Selection, Allocation, and Handling 
Test animals 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, the animals should be ruminating and 
older than 3 months of age.  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.  
 
For induced infections, the use of helminth-naive animals is recommended.  Animals not raised 
in a helminth-free environment should be treated with an approved anthelmintic drug, chemically 
not related to the test drug, to remove pre-existing infections followed by fecal examination to 
determine that the animals are helminth-free. 
 
Animal housing, feeding, and care should follow strict requirements of welfare, including 
vaccination according to local practices.  This information should be provided in the final report.  
A minimum 7-day acclimatization period is recommended.  Housing, feed, and water should be 
adequate according to the geographical location.  Animals should be monitored daily to 
determine adverse reactions. 
 
 
5 
  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
B. Specific Evaluation Studies 
1.  Dose Determination Studies 
No species-specific recommendations. 
 
2.  Dose Confirmation Studies 
Confirmation studies are recommended to support each claim: adult, larvae, and, when 
applicable, hypobiotic larvae.   
 
3.  Field Effectiveness Studies 
The field studies should be replicated in different geographic locations and in animal/production 
class(es) that represent the conditions of use for the indication being pursued.  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.  The protocol should also describe procedures for random selection of 
animals (number and percentage) to be sampled (if fecal samples will not be collected from all 
available animals in the study), as appropriate, and the methods to be used for both fecal 
collection and examination.  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. 
  
Effectiveness against adult nematodes can be assessed by the reduction of fecal egg counts and 
should be performed using samples from the same animal before and after treatment in both 
study groups (control and treated).  Post-treatment counts are generally made 10-14 days after 
treatment, but the timing of post-treatment counts will depend on the parasite species and class of 
anthelmintic evaluated.  For example, due to the known effects of macrocyclic lactones on 
nematode egg suppression, post-treatment counts should be delayed until at least 14 days or 
longer.  Effectiveness should be calculated using post-treatment fecal egg counts from the treated 
and control (typically placebo or untreated control) groups.  Additionally, a calculation of 
effectiveness using pre- and post-treatment fecal egg counts may provide further information on 
field effectiveness.  Furthermore, additional endpoints for evaluating field effectiveness should 
be considered as they are developed and generally accepted by experts in veterinary parasitology. 
 
See also sections A.4.1. Data Analysis Recommendations and A.4.2. Calculation and Evaluation 
of Percent Effectiveness of GFI #90/VICH GL7. 
 
4.  Persistent Effectiveness Studies 
Two basic study designs have been used to pursue persistent effectiveness claims:  one using a 
single challenge, another using multiple daily challenges following treatment.  For both 
procedures, no standardized protocols have been developed.  When conducting studies, protocol 
details should include, among other things: determination of larval viability throughout the study, 
rationale for larval challenge, and justification for slaughter time.  Parasite-naive sheep are 
 
6 
  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
recommended for the studies.  A study design is recommended using multiple daily challenges, 
as this most closely mimics what occurs in nature. 
 
Two studies (with parasite counts), each with a non-treated and one or more treated groups, are 
recommended for a persistent effectiveness claim (for each duration and helminth claim).  At 
least six animals in the control group should be adequately infected.  Persistent effectiveness 
claims should only be granted on a species-by-species basis.   
 
In the protocol using multiple daily challenges, different groups of animals should be treated and 
exposed to a daily natural or induced challenge for 7, 14, 21, or more days after the treatment, 
then at approximately 3 weeks after the last challenge (or earlier) the animals are examined for 
parasite burden.  The challenge interval and schedule may vary for longer-acting products and 
should take into consideration the pharmacological properties of the product. 
 
Persistent effectiveness claims should be supported by a minimum 90% effectiveness at each 
time point, and calculated and interpreted using the procedures described in sections A.4.1. Data 
Analysis Recommendations and A.4.2. Calculation and Evaluation of Percent Effectiveness of 
GFI #90/VICH GL7.  Persistent effectiveness claims should be granted for the longest period 
between treatment and the last challenge where effectiveness criteria are met, and all preceding 
time points tested meet the criteria as well.  
 
 
7