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Effectiveness of Anthelmintics: 
Specific Recommendations for Equines 
VICH GL15 
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Draft Guidance 
This guidance document is being distributed for comment purposes only.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Submit comments on this draft guidance by the dated provided in the Federal Register notice 
announcing the availability of the draft guidance.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: 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Additional copies of this draft guidance document may be requested from the Policy and 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 GL15 (ANTHELMINTICS: EQUINES) 
May 2022 
Revision at Step 9 
For consultation at Step 4 
 
 
 
E A : 
FFICACY OF  NTHELMINTICS
S R  
PECIFIC  ECOMMENDATIONS
E  
FOR  QUINES
(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Draft  Not for Implementation 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 ....... 3 
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) ........................................................................................... 3 
4.3  Adequacy of Infection ........................................................................................ 4 
4.4  Label Claims ....................................................................................................... 4 
5.  Treatment Procedures ................................................................................................ 4 
6.  Animal Selection, Allocation, and Handling............................................................. 4 
B.  Specific Evaluation Studies .............................................................................................. 5 
1.  Dose Determination Studies ....................................................................................... 5 
2.  Dose Confirmation Studies ........................................................................................ 5 
3.  Field Effectiveness Studies ......................................................................................... 5 
4.  Persistent Effectiveness Studies ................................................................................. 6 
5.  Egg Reappearance Period (ERP) Studies ................................................................. 6 
 
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Effectiveness of Anthelmintics: Specific Recommendations for 
Equines 
 
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 equine 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 equine guidance is: (1) to be more specific for certain specific equine 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 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 (WAAVP): second edition of guidelines for evaluating the efficacy of equine 
anthelmintics. Veterinary Parasitology 103: 1-18, 2002, 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 
 
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A.  General Elements 
1.  The Evaluation of Effectiveness Data 
Controlled tests are recommended both for the dose determination and dose confirmation studies. 
Critical tests also can be used for certain adult large nematodes, e.g., Parascaris equorum and 
Oxyuris equi.  Long-acting products 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. 
 
In the case of Strongyloides westeri, the evaluation of effectiveness data may be based on egg 
counts (at least two field effectiveness studies are recommended).  The justification for this is the 
fact that S. westeri is mainly observed in young animals.  At this age, few other helminths have 
matured and use of young animals in terminal tests is inappropriate from an ethical perspective. 
 
2.  Use of Natural or Induced Infections 
Because of the difficulties involved in carrying out induced infections in worm-free equine, most 
studies can be carried out in naturally-infected animals. 
 
Dose determination studies generally should be conducted using natural or induced infections 
with either laboratory strains or recent field isolates. 
 
Dose confirmation studies against adult stages for a wide range of parasites should be conducted 
using naturally-infected animals, which may be superimposed with induced infections of recent 
field isolates.  Induced infections with recent field isolates are also acceptable.  For claims 
against hypobiotic larvae (early L3 of small strongyles), natural infections should be considered.  
In these cases, animals should be housed for a minimum of 2 weeks before treatment to preclude 
unintended reinfection. 
 
To determine the number of hypobiotic larvae, digestion of the large intestinal mucosa is 
recommended; the number of intramucosal developing stages (late L3/L4 of small strongyles) 
should be determined by using both the digestion technique and the transillumination technique 
due to the inherent limitation of each technique in isolation. 
 
Persistent effectiveness studies should be conducted using induced infections with recent field 
isolates and using young equine, i.e., < 12 months of age. 
 
The history of the parasites used in the induced infection studies should be included in the final 
report. 
 
 
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3.  Number of Infective Parasitic Forms Recommended for Induced Infections 
As the use of induced infections in equines is not common (see above), only limited data on the 
number of infective larvae to administer are available.  The following range of infective 
larvae/eggs to be administered can be recommended: 
Parascaris equorum      100 - 500 
Trichostrongylus axei      10,000 - 50,000 
Strongylus vulgaris      500 - 750 
Small strongyles (Cyathostominae)  100,000 - 1,000,000 
 
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; 
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. 
 
 
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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 equine 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 cestodes 
(e.g., Anoplocephala perfoliata, minimum number of 10), trematodes (Fasciola spp.), Parascaris 
equorum, and Dictyocaulus arnfieldi. 
 
4.4  Label Claims 
For adult and larval claims, treatment should correspond to life-cycle timing appropriate for the 
species claimed.  In the case of small strongyles, distinction should be made between early 
(hypobiotic) L3 stages, (developing) intramucosal L4 stages, luminal L4 stages, and adults.  The 
term immature on the labeling is not recommended. 
 
Parasite identification should determine the type of claim proposed on the labeling.  A species 
claim is highly recommended.  For the small strongyles, a genus claim should be acceptable on 
the assumption that, generally speaking, there is more than one species in that genus and the 
study was conducted with a mixed larval population. 
 
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 this is unnecessary, e.g., for systemic acting compounds, blood levels 
demonstrate steady state at all points of the proposed therapeutic period.  When the drug is to be 
administered in the water or via a medicated feed, it should be done as much as possible 
following the labeling recommendations.  Palatability studies may be advisable for medicated 
feed.  Samples of medicated water or medicated feed should be collected to confirm drug 
concentration.  The amount of medicated product consumed by 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 coat 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 3 to 12 months 
of age and raised helminth-free, if induced infections are used, because there is no guarantee that 
pre-existing infections can be removed.  For natural infections, animals between 12 to 24 months 
are preferred (except for S. westeri) and to reduce individual variations in worm counts, it can be 
 
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useful to graze the equids for at least 5 months together on the same infected pasture.  
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 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. 
 
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.  For additional descriptions of the procedures refer to GFI 
#90/VICH GL7. 
 
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 
 
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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 
These claims should only be determined on the basis of actual worm counts and not on eggs per 
gram of feces to demonstrate drug effectiveness. 
 
A persistent effectiveness claim (for each duration and helminth claim) should include two trials 
(with worm counts), each with a non-treated and one or more treated groups.  At least six 
animals in the control group (of the same age) should be adequately infected.  Persistent 
effectiveness claims should only be granted on a species-by-species basis, genus-by-genus in the 
case of small strongyles. 
 
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 consistency of 
interpretation of results, a standardized study design is recommended using multiple daily 
challenges, as this most closely mimics what occurs in nature. 
 
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. 
 
5.  Egg Reappearance Period (ERP) Studies 
ERP only relates to strongyles.  ERP is a pasture contamination management tool and is not 
intended to be used to measure individual animal strongyle burdens.  It is a tool to manage 
equine strongyles on a herd basis focusing on pasture contamination management.  Claims for 
egg reduction during a certain period after treatment should be acceptable if the reduction in 
treated animals is at least 90% compared to pretreatment egg counts.  In these studies, animals 
should remain on infected pastures.  Two studies should be the minimum needed to determine 
the ERP.  At least one of the two studies should be conducted in the geographical location where 
 
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registration is being pursued.  These studies should be conducted so that they are sufficiently 
representative of the various conditions under which the product will be authorized. 
 
 
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