#90 
Effectiveness of Anthelmintics: 
General Recommendations 
VICH GL7 
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Draft Guidance 
This guidance document is being distributed for comment purposes only.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 October 2001.  This revision 
updates data analysis and isolate characterization recommendations, outlines how to approach 
new indications, 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Submit comments on this draft guidance by the dated provided in the Federal Register notice 
announcing the availability of the draft guidance.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Additional copies of this draft guidance document may be requested from the Policy and 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 GL7 (ANTHELMINTICS: GENERAL) 
May 2022 
Revision at Step 9 
For consultation at Step 4 
 
 
 
E A : 
FFICACY OF  NTHELMINTICS
G R  
ENERAL  ECOMMENDATIONS
(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Draft  Not for Implementation 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 
II.  General Anthelmintic Guidance ......................................................................................... 1 
A.  General Elements ........................................................................................................... 2 
1.  Good Clinical Practice ............................................................................................. 2 
2.  The Evaluation of Effectiveness Data, Use of Natural or Induced Infections, 
Definition of Laboratory and Field (Helminth) Strains ....................................... 2 
3.  Product Equivalence ................................................................................................ 2 
4.  Recommendations for the Calculation of Effectiveness ....................................... 3 
4.1  Data Analysis Recommendations .................................................................. 3 
4.2  Calculation and Evaluation of Percent Effectiveness .................................. 3 
4.3  Number of Animals (Dose Determination, Dose Confirmation, and 
Persistency Studies) ........................................................................................ 6 
4.4  Pooling Data .................................................................................................... 6 
4.5  Adequacy of Infection ..................................................................................... 7 
4.6  Aliquot size ...................................................................................................... 7 
5.  Standards of Effectiveness ...................................................................................... 7 
6.  Definition of Helminth Claims ................................................................................ 8 
7.  Approach to New Indications ................................................................................. 8 
B.  Specific Evaluation Studies ........................................................................................... 8 
1.  Dose Determination Studies .................................................................................... 8 
2.  Dose Confirmation Studies ..................................................................................... 9 
3.  Field Effectiveness Studies .................................................................................... 10 
4.  Persistent Effectiveness Studies ............................................................................ 10 
III.  Appendix 1. Glossary ......................................................................................................... 11 
IV.  Appendix 2. Effectiveness Decision Criteria for Dose Determination, Dose 
Confirmation, and Persistent Effectiveness Studies ....................................................... 13 
 
i  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
Effectiveness of Anthelmintics: General Recommendations 
 
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 objective of this guidance is to provide study design recommendations that will facilitate the 
universal acceptance of the generated effectiveness data to fulfill the national/regional 
requirements for anthelmintic drugs in veterinary species.  This guidance was prepared after 
consideration of the current national/regional requirements and recommendations for evaluating 
anthelmintic drugs in the International Cooperation on Harmonisation of Technical 
Requirements for Registration of Veterinary Medicinal Products (VICH) regions.  
 
This guidance provides general recommendations for the design of effectiveness studies for 
anthelmintics, applicable to the most common veterinary species.  Additional guidance for 
individual species-specific recommendations is provided in Guidance for Industry (GFI) 
#95/VICH GL12 (bovine); GFI #96/VICH GL13 (ovine); GFI #97/VICH GL14 (caprine); GFI 
#109/VICH GL15 (equine); GFI #110/VICH GL16 (swine); GFI #111/VICH GL19 (canine); 
GFI #113/VICH GL20 (feline); and GFI #114/VICH GL21 (chicken). 
 
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. 
 
II.  General Anthelmintic Guidance 
The guidance includes two sections: II.A. General Elements and II.B. Specific Evaluation 
Studies.  The General Elements section includes: good clinical practice, evaluation of 
effectiveness data, types of infection and parasite strains, product equivalence, recommendations 
for the calculation of effectiveness, standards of effectiveness, the definition of helminth claims, 
and an approach to new indications.  The Specific Evaluation Studies section describes: dose 
determination, dose confirmation, field, and persistent effectiveness studies. 
 
1  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
A.  General Elements 
1.  Good Clinical Practice 
The principles of Good Clinical Practice (GCP) should apply to all clinical studies and sponsors 
should work within the principles of GCP recommendations.  Non-GCP studies are considered as 
non-pivotal studies and may be used as supporting data. 
 
2.  The Evaluation of Effectiveness Data, Use of Natural or Induced Infections, Definition 
of Laboratory and Field (Helminth) Strains 
The evaluation of effectiveness data should be based on parasite counts (adults, larvae) in dose 
determination and dose confirmation studies.  Egg counts/larval identification is the preferred 
method to evaluate the effectiveness in field studies.  Controlled and critical tests are acceptable 
both for the dose determination and dose confirmation studies (critical tests are not used for 
those drugs that destroy the parasites body).  However, controlled tests are preferable, and the 
option to utilize critical tests should be supported with an explanation from the sponsor. 
 
The use of natural or induced infections in effectiveness studies should be determined by the 
type of parasite and the claim proposed by the sponsor.  In some rare, but epizootiologically 
important parasites, the use of induced infections is the only solution. 
 
Recent field isolates are generally preferred to develop induced infections, although in some 
cases laboratory strains should be used (see Appendix 1. Glossary).  Field isolates are believed to 
reflect more accurately the current status of the parasite in nature.  The characterization of each 
of the laboratory strains used in the investigations should be included in the final report, i.e., 
source, acquisition date, location of isolation, maintenance procedure, drug sensitivity profile, 
number of passages (including anthelmintic exposure during passage), and expected 
establishment rates in the target host.  For field isolates, characterization should include source, 
acquisition date, location of isolation, previous anthelmintic exposure, maintenance procedure, 
and number of passages. 
 
In certain circumstances, such as for studies using product containing a previously approved 
active ingredient or an active ingredient within the same class as a previously approved drug, 
characterization of the field isolate prior to its use in a study may include an evaluation of the 
sensitivity/resistance of the isolate to previously approved drugs and/or the proposed drug 
product, but is not required.  If multiple candidate field isolates are characterized, the 
justification for field isolate selection should be determined a priori based on the study 
objectives.  Any sensitivity/resistance characterization performed on field isolates (e.g., number 
of field isolates examined and results of sensitivity/resistance characterization) should be 
described in the final report.  As for natural infections, induced-infection studies should use field 
isolates that reflect the current status of infection in the field. 
 
3.  Product Equivalence 
The principle of product equivalence may be used for certain formulation changes that do not 
significantly affect the safety or effectiveness of the same approved drug, when used at the same 
dose, by the same route of administration, and in the same host.  For all formulation changes, the 
2  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
significance of the change (dose, route of administration, host, etc.), the pharmacokinetic 
attributes of the drug and the predilection site of the targeted parasites should dictate the study 
type that should be conducted for product equivalence. 
 
For absorbed drugs that can be measured in the blood plasma and for which a relationship with 
effectiveness can be correlated with pharmacokinetic parameters, a blood level bioequivalence 
study may be used.  Alternatively, and particularly where pharmacokinetic parameters cannot 
demonstrate a relationship with effectiveness, two dose confirmation studies using the dose-
limiting parasite for therapeutic claims and/or two persistence effectiveness studies per species 
claimed may be needed. 
 
4.  Recommendations for the Calculation of Effectiveness 
The analysis of parasite data in support of effectiveness uses estimations of several 
parasitological parameters, including fecal egg counts and worm counts, which may be a 
reflection of the success of the treatment.  In most natural infections, and less in induced 
infections, large variations in data values between similarly treated animals have been observed.  
In this case, additional studies may be recommended to increase the number of observations. 
 
4.1 Data Analysis Recommendations 
For data analysis, either parametric or non-parametric procedures are acceptable.  However, the 
statistical analyses process should be described in the protocol prior to any data analyses.  
Parametric methods preserve the magnitude of observed parasite burdens and their biological 
interpretability.  Parametric analysis also accommodates random effects (as needed) in the 
statistical model and provides an analysis that facilitates both group comparisons and an 
estimation of the means of the parasite counts for use in the calculation of percent effectiveness.  
Non-parametric tests are appropriate when parametric methods are not applicable due to 
computational issues or the distribution of the count data. 
 
If the results demonstrate significant statistical differences between the treated and control 
groups, then the next steps in the effectiveness evaluation should be performed as described in 
section A.4.2. below. 
 
4.2 Calculation and Evaluation of Percent Effectiveness 
The choice of mean to estimate the central tendency of parasite or egg counts (e.g., geometric or 
arithmetic mean) may result in differences in the calculated percent effectiveness.  However, 
generally the measure of central tendency should be derived from the statistical analysis that is 
consistent with the distribution of the data.  In the context of harmonization, recommendations 
are needed for how and when to use geometric or arithmetic means.  Log-transformed parasite or 
egg counts in untreated animals tend to follow a normal distribution more closely than do non-
transformed parasite or egg counts.  The geometric mean is therefore chosen as the initial 
estimate of the central tendency of parasite or egg counts for most dose determination, dose 
confirmation, and persistent effectiveness studies.  The log transformation includes the choice of 
a constant (e.g., c = 1) added to the parasite or egg counts, which should be pre-defined and 
justified in the protocol. 
 
3  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
For dose determination, dose confirmation, or persistent effectiveness studies in which adequate 
infections are established in the control group and a statistically significant difference was 
demonstrated between the groups, the percent effectiveness should be calculated and evaluated 
using the following steps in order (as also shown by the decision tree in Appendix 2. 
Effectiveness Decision Criteria for Dose Determination, Dose Confirmation, and Persistent 
Effectiveness Studies).  The process starts with calculation of effectiveness based on geometric 
means which, if effectiveness is  90%, is then complemented by calculation of effectiveness 
based on arithmetic means.  When effectiveness based on arithmetic means is below 90%, a 
secondary assessment is applied to provide a predictable and harmonized approach to the 
evaluation of the biological relevance of such results.  Such discrepancies between the % 
effectiveness calculated based on geometric or arithmetic means typically occur when wide 
variations in worm counts are observed in the treated group at necropsy. 
 
Steps in the interpretation of percent effectiveness:  
 
a.  Calculate percent effectiveness for the parasite or life stage using geometric means as 
follows:  
 
100 x ((Geometric mean for parasite count in control group  Geometric mean for parasite 
count in treated group) / Geometric mean for parasite count in control group)  
 
The geometric means should be calculated by back-transforming the least squares means 
estimated from a parametric model analysis of the log-transformed parasite counts, then 
subtracting the constant (e.g., c = 1).  If non-parametric methods are used for group 
comparison, the geometric means can be calculated directly from the observed values 
(parasite counts).  If the experimental unit is a pen, rather than an individual animal, the 
initial calculation of effectiveness should be performed by first computing pen averages 
(arithmetic mean of parasite counts in the pen) and then using these pen averages in the 
analysis to derive the geometric means.  In situations where each experimental unit includes 
the same number of animals, pen totals may be used instead of pen averages. 
  
b.  Perform one of the following steps depending on the results from Step a. above. 
  
1.  If the % effectiveness based on geometric means is < 90%, no further calculations or 
secondary assessment is performed.  The % effectiveness does not support a conclusion 
of effectiveness.   
 
2.  If the % effectiveness based on geometric means is  90%, calculate % effectiveness 
using arithmetic means as shown below, where the arithmetic mean is computed as the 
average of parasite counts over all animals in each group:   
 
100 x ((Arithmetic mean for parasite count in control group  Arithmetic mean for 
parasite count in treated group) / Arithmetic mean for parasite count in control group)   
 
If the experimental unit is a pen, rather than an individual animal, the secondary 
calculation of effectiveness should be performed by first computing pen averages 
(arithmetic mean parasite counts in the pen) and then using these pen averages to 
4  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
compute the average parasite count in each treatment group.  In situations where each 
experimental unit includes the same number of animals, pen totals may be used instead of 
pen averages. 
 
Following the calculation of % effectiveness based on arithmetic means, proceed to Step 
c. below.  
 
c.  Perform one of the following steps depending on the results of Step b.2. above:  
 
1.  If the % effectiveness based on arithmetic means is  90%, no further assessment is 
necessary.  The % effectiveness supports a conclusion of effectiveness.    
 
2.  If the % effectiveness based on arithmetic means is < 90%, a secondary assessment of the 
parasite counts of the experimental units (animal or pen) in both the treated and control 
groups should be performed.   
 
The methods used in the secondary assessment assume the use of appropriate animal (and 
pen, if applicable) selection and randomization procedures to minimize differences 
between treated and control groups.  The control animal (or experimental unit) with the 
highest worm burden is used as the basis for estimating the proportion of treated animals 
that likely had at least a 90% reduction in worm counts to minimize the chance of 
overinterpreting higher worm burdens in the treated group as potential treatment failures.    
 
Perform the secondary assessment as follows:  
 
Calculate the proportion of animals/experimental units in the treated group that appear to 
have at least a 90% reduction in parasite burden based on the highest parasite count 
within the experimental units of the control group.  
 
For sample sizes between 6 and 12 animals/experimental units:   
 
  If the proportion of experimental units in the treated group estimated to have a  
90% reduction in parasite burden is at least 80%,1 effectiveness is supported.   
 
 
1 The 80% proportion cut-off was selected based on the typical sample sizes seen in these types of studies (6-12 animals), 
the assumption that parasite counts in the treated and control groups are similar before treatment, and a concern for 
protecting against overinterpretation of treated animals with positive parasite counts after treatment.  The proposed cut-off 
allows one or two animals in the treated group to be potential treatment failures, with a potential treatment failure defined 
as an individual animal that does not have a  90% reduction in worm count when compared to the control animal with the 
highest worm count.  This method helps to distinguish whether the cause of the lower % efficacy based on arithmetic 
means is due to one or two animals with higher-than-expected worm counts or a more widespread issue that may reflect a 
true effectiveness of < 90%.  The secondary assessment method was tested using historical data sets from over 100 studies 
submitted to regulatory authorities (multiple animal host species and more than one jurisdiction represented) to confirm 
that it could identify studies with high parasite counts in the treated group that were likely of biological concern without 
being overly conservative.  
5  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
  If the proportion of experimental units in the treated group estimated to have a  
90% reduction in parasite burden is less than 80%, the results do not support a 
conclusion of effectiveness for the study.   
 
See Tables 1-4 in Appendix 2. Effectiveness Decision Criteria for Dose Determination, 
Dose Confirmation, and Persistent Effectiveness Studies for specific examples of this 
secondary assessment.  
 
For studies with sample sizes greater than 12 animals/experimental units, the threshold 
proportion of animals/experimental units with at least a 90% reduction in parasite burden 
used to support effectiveness should be justified in the protocol.   
 
Due to the differences in parasite detection methods, animal species husbandry, and other 
factors, there is not a single harmonized recommendation for calculating percent effectiveness 
from field studies.  Furthermore, new endpoints and analysis methods for evaluating field 
effectiveness should be considered as they are developed and generally accepted by experts in 
veterinary parasitology.  
 
4.3 Number of Animals (Dose Determination, Dose Confirmation, and Persistency Studies) 
The minimum number of animals used per experimental group is a critical point.  The number of 
animals will depend on the type of statistical analysis used; however, a minimum of at least six 
animals in each experimental group is recommended. 
 
4.4 Pooling Data 
Pooling data is allowed when certain criteria are taken into account.  For sponsors intending to 
pool data, it is important to ensure that a general protocol is standardized for each type of study 
proposed, that is, dose confirmation, field, and persistency studies.  There should be similarity 
among numbers of animals/group numbers of parasites, type of animals, and experimental 
conditions.  Where pooled data are used, any aberrant result should be explained to the 
regulatory authorities. 
 
Pooling of data should only be considered where more than two studies (as defined in section 
B.2. Dose Confirmation Studies below) have been conducted and the majority of individual 
studies provide 90% or greater effectiveness following the procedure described in section A.4.2. 
Calculation and Evaluation of Percent Effectiveness, i.e., minimally three studies with at least 
two of these demonstrating effectiveness as described in section A.4.2. Calculation and 
Evaluation of Percent Effectiveness are recommended to pool data.  The overall effectiveness of 
the pooled studies should demonstrate effectiveness of 90% or greater. 
 
In the case of rare parasites, an alternative approach should be used (i.e., more trials may be 
recommended). 
 
The geometric means should be calculated based on all control values, i.e., dropping zero counts 
in control groups and a corresponding number of zero-treated animals is not recommended. 
 
6  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
4.5 Adequacy of Infection 
A universal definition of adequacy of infection should not be formulated because of the diversity 
of genera, species, and strains of helminths subject to evaluation.  Furthermore, each strain under 
test may have unique characteristics of infectivity and pathogenicity.  However, in the 
development of study protocols the adequacy of infection should be defined, especially in terms 
of the statistical, parasitological, and clinical relevance of the infection level in individual control 
animals, as well as the number of control animals in which infections are established.  The level 
of infection, and its distribution, among control animals should be adequate to permit the 
appropriate standards of effectiveness to be met with acceptable statistical and biological 
certitude/confidence.  Multiple infections are acceptable; however, each helminth species should 
reach acceptable minimums of infection.  For some parasite species, low worm counts are 
expected and should be accounted for in the definition of adequate infection in the study 
protocol.  If inadequate infections in a significant number of individual study animals are 
expected, increasing the number of animals in the study groups to achieve six adequately-
infected control animals should not, by itself, be considered an appropriate modification to the 
study design.  In such cases, a statistical method of evaluating adequacy of infection, based on 
worm count distributions, may be needed in addition to the minimum requirement of six 
adequately-infected animals as outlined in the relevant species-specific guidelines. 
  
The adequacy of infection in at least six individual animals, as defined in each of the species-
specific guidances, is intended to provide recommendations for when adequacy of infection 
should be considered acceptable without additional justification.  However, if a study fails to 
meet the pre-defined adequacy of infection levels, investigators should consider the scientific 
validity of the model and investigate and discuss the reason for failing to meet expected infection 
levels in the study.  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.  
Justification for including the study to support effectiveness should also be included as part of 
the submission file, as described above. 
 
4.6 Aliquot Size 
Aliquot size to determine parasite burdens should be at least 2%.  Smaller aliquot size may be 
used with justification. 
 
5.  Standards of Effectiveness 
A compound should be declared effective only when effectiveness against each parasite declared 
on the labeling stands at 90% or above, as described in section A.4.2. Calculation and 
Evaluation of Percent Effectiveness, using pooled data (when appropriate), provided the control 
group was adequately infected with this parasite and there is a statistically significant difference 
in parasite numbers between control and treated animals.  However, there are regional 
differences where the epizootiology of certain parasitic infections may recommend higher 
minimal effectiveness.  These will be covered in the individual host species guidances (e.g., 
zoonotic infections, Dirofilaria spp.).  Effectiveness below 90% may be adequate when the 
claimed parasites do not have any other effective treatment. 
 
7  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
6.  Definition of Helminth Claims 
Parasite identification will determine the type of claim proposed on the labeling.  A species claim 
is highly recommended.  However, a genus claim may be acceptable if the parasites cannot be 
speciated and there is more than one species in that genus.  If species claims are to be made, then 
the presence of each should be confirmed including two dose confirmation studies for each 
parasite. 
 
7.  Approach to New Indications 
For new parasite indications (not currently addressed in VICH guidances), the following items 
should be taken into account according to the requirements of, or in collaboration with, the 
appropriate regulatory bod(ies): 
  Number and type of studies proposed: defined based on objective (e.g., dose 
determination, dose confirmation, or field trial) and type (e.g., laboratory vs. field; if 
laboratory, natural vs. induced); 
  Justification for any deviations from GFI #90/VICH GL7 recommendations; 
  Availability of different parasitic isolates; and 
  If available, justification of the model, which may include how the experimental model 
was developed, details of its conduct, and how well the model reflects natural infection or 
if the use of the model may impact the inference of the results when considering the 
broader population: 
  Method of determining eligibility of animals for inoculation (e.g., age); 
o
  Method of inoculation of test animals/ relevance of inoculate concentration to 
o
worm burden of naturally infected animals; 
  The selection of the time between treatment and necropsy; 
o
  The selection of the time between infection and treatment; and 
o
  Minimum number of parasites to determine an adequate infection 
o
Generally, the parasite should be present in the target animal species and in the geographic 
region in which registration is sought.  Additionally, zoonotic parasitic diseases may have 
implications for study design, which should also be addressed. 
 
B.  Specific Evaluation Studies 
Three types of studies should be used in the evaluation of all new anthelmintics: dose 
determination, dose confirmation, and field effectiveness studies.  Special studies may also be 
used to determine the persistent effectiveness of an anthelmintic. 
 
1.  Dose Determination Studies 
Dose titration trials should from now on be referred to as dose determination studies, their 
purpose being to determine the dose rate to be recommended for the particular target animal.  
The studies may or may not be conducted using the final formulation.  However, if not, any 
8  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
changes in the formulation should be scientifically justified.  Some regulatory authorities may 
waive the requirement for a dose determination study where alternative data are presented to 
support the intended dosage.  For generic products, where the optimum dose of the active 
ingredient has already been generally adopted, dose determination studies are not necessary. 
 
When broad spectrum activity is claimed for an anthelmintic preparation, dose determination 
studies should contain a dose-limiting species within the claimed spectrum and should be 
independent of whether the dose-limiting species is a high or a low (= rare) prevalence species.  
The sponsor should select the parasites taking into consideration their impact on animal health.  
Confirmation of effectiveness against the species for which a claim is made should be completed 
in the dose confirmation studies. 
 
When only one parasite is claimed (e.g., Dirofilaria immitis), the discussion on the number of 
species and the dose limiter becomes irrelevant. 
 
One internationally accepted design includes a minimum of three groups receiving different 
levels of anthelmintic treatment (e.g., 0, 0.5, 1, and 2x the anticipated dose together with a group 
of untreated controls).  It is suggested that the range of doses should be selected on the basis of 
preliminary studies to encompass the approximate effective dose.  The reason for the dose 
selected should be explained.  For each selected parasite, there should be at least six (= 
recommended) adequately infected control animals, but if there is any doubt about the level of 
infection, then the number should be increased accordingly (see section A.4.1. Data Analysis 
Recommendations). 
 
This phase of the testing should be conducted using adult parasites unless there is information 
that larvae of a particular parasite could be a dose-limiting stage or the proposed product claim is 
only targeting a specific parasite at the larval stage (e.g., Dirofilaria immitis).  Dose 
determination studies may be conducted using natural infections; however, induced infections 
are preferred.  Both laboratory strains and recent field isolates (see Appendix 1. Glossary) can be 
used to develop induced infections. 
 
2.  Dose Confirmation Studies 
These studies should be conducted using the final formulation of the drug to be commercialized. 
The dose confirmation work should not be conducted on known drug-resistant parasites, unless 
justified by the objectives of the study.  To investigate effectiveness against adult parasites, 
naturally-infected animals are preferred.  However, induced infections using recent field isolates 
in one of the studies are acceptable.  For rare parasite species, laboratory strains may be used and 
they may be conducted outside the geographic location in which the product will be authorized 
for marketing.  Dose confirmation for larval stages should be conducted using induced 
infections.  The sponsor should explain deviations from this recommendation.  Against inhibited 
stages, only natural infections are recommended. 
 
At least two controlled or, when appropriate, critical dose confirmation studies per individual 
claim are recommended (single or multiple infections).  Two studies are the minimum suggested 
to verify that effectiveness can be achieved against various helminth strains in animals raised in 
disparate regions and climates and under respective husbandry conditions.  At least one of the 
9  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
studies should be conducted in the geographic location where registration is being pursued and 
both studies should be conducted under conditions that are sufficiently representative of the 
various conditions under which the product will be authorized.  In the event that in certain 
locations parasites are particularly rare, then two trials from outside the location should be 
acceptable.  A dose determination study can be used in place of one of the confirmation studies if 
the final formulation was used and administered under label recommendations. 
 
For each study, it is recommended that at least six control animals should be adequately infected.  
The adequacy of the infection should be defined in the protocol phase.  A sufficient number of 
infected animals should be examined before treatment to ensure that at least 6 (= recommended) 
adequately-infected animals for the parasite or life stage of a parasite are present at the start of 
the trial (see section A.4.2. Calculation and Evaluation of Percent Effectiveness). 
 
3.  Field Effectiveness Studies 
These studies should be conducted using the final formulation of the drug product to be 
commercialized to confirm effectiveness and safety.  The number of field trials to be conducted 
and animals involved in each trial should depend on: (1) the animal species, (2) the geographic 
location, and (3) local/regional situations.  The controls, i.e., non-treated animals or animals 
treated with a registered anthelmintic with a known profile, should equal a minimum of 25% of 
the treated animal numbers.  Local/regional implies within a country and/or association with a 
climatic and/or management area (see also Appendix 1. Glossary).  To achieve the requested 
numbers, it is also acceptable to conduct multi-center studies with sub-trials in each local/region.  
The request for additional (or fewer) studies, and/or animals (animal welfare considerations) by 
local regulatory authorities should be fully justified.  The product should always be tested in the 
breed/age range/class/production type of animal intended to be treated as indicated on the 
labeling. 
 
4.  Persistent Effectiveness Studies 
Broad spectrum anti-parasitic compounds may show persistent effectiveness due to the presence 
of residual activity of either the parent compound, or the metabolites, in the treated animal.  
These claims should be based on actual worm counts and not on number of eggs per gram of 
feces.  Claims of activity of less than 7 days should not be considered a persistent effect and 
claims should mention persistent effectiveness for a certain number of days.  The type of 
protocol depends on the animal species and will be discussed under the specific target species 
guidance. 
 
As described for dose confirmation, a persistence claim (for each duration and parasite claim) 
should include two trials (with worm counts), each with a non-treated and treated group.  It is 
recommended that at least six animals per treatment group should be adequately infected.  The 
adequacy of the infection should be defined in the protocol phase.  Persistence claims should be 
in terms of a species-by-species basis.  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. 
 
10  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
 
III.  Appendix 1. Glossary 
ADEQUATE INFECTION: Natural or induced infection level defined in the study protocol 
that will allow the evaluation of the therapeutic effectiveness of the drug when comparing 
parasitological parameters (e.g., number of parasites) in medicated and control animals. 
 
ALIQUOT SIZE: A sample (known volume) of gastrointestinal or other (lung, etc.) content 
collected to determine the number of parasites. 
 
CLAIM: A parasite species or genus (adult and/or larvae) listed on the labeling with proven 
susceptibility (90% or better effectiveness) to an anthelmintic drug. 
 
CONTROLLED TEST: A procedure to study the effectiveness of a drug using two groups: a 
control and at least one treated group of experimental animals.  Adequately-parasitized animals 
are included in each treated and control group; after a suitable period of time after treatment the 
animals are necropsied and the parasites are enumerated and identified.  This test is the most 
widely used and accepted when the sample size is the same. 
 
CRITICAL TEST: A procedure whereby the number of parasites recovered from an animal 
after the treatment is added to the number counted in the intestine at necropsy, which is 
considered to be the total number of parasites in the animal at the time of treatment.  The 
effectiveness is calculated as follows: [No of parasites expelled] divided by [(No of parasites 
expelled) plus (No of parasites remaining)] X 100 is equal to % effectiveness in the individual 
animal. 
 
DOSE CONFIRMATION STUDY: In-vivo study to confirm the effectiveness of a selected 
drug dose and formulation; may be conducted in the laboratory or in the field. 
 
DOSE DETERMINATION STUDY: In-vivo study conducted to determine the most 
appropriate dose or range of effectiveness of a veterinary drug. 
 
DOSE-LIMITING PARASITE: A parasite that will be identified during dose determination 
studies that will identify the dosage of the drug at which it shows 90% effectiveness.  Any lower 
concentration of the product will show an effectiveness below 90% for the dose-limiting parasite 
even though it will adequately treat other parasites (90% or better effectiveness) in the host. 
 
EFFECTIVENESS: The degree to which the manufacturers claims on the labeling have been 
supported by adequate data, i.e., providing control of at least 90% and meeting the criteria 
described in sections A.4.1. Data Analysis Recommendations and A.4.2. Calculation and 
Evaluation of Percent Effectiveness using pooled data from controlled studies. 
 
FIELD EFFECTIVENESS STUDY: Larger scale study to determine effectiveness and safety 
of a veterinary drug under actual use conditions. 
 
11  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
GCP: Good Clinical Practice: A set of recommendations intended to promote the quality and 
validity of test data.  It covers the organizational process and the conditions under which studies 
are planned, performed, monitored, recorded, and reported. 
 
GEOGRAPHICAL LOCATION: A subdivision where the guidances will be implemented: 
Japan, European Union, USA, and Australia/New Zealand. 
 
FIELD ISOLATE: A collection of a sub-population of helminths for the conduct of drug 
evaluation studies (see section B. Specific Evaluation Studies) and isolated from the field less 
than 10 years from the start of the study.  The helminths are considered representative of current 
parasite infections in the field and have been characterized (see section A.2. The Evaluation of 
Effectiveness Data, Use of Natural or Induced Infections, Definition of Laboratory and Field 
(Helminth) Strains). 
 
LABORATORY STRAIN: A sub-population of helminths isolated from the field, which has 
been characterized and segregated in the laboratory based on a particular property which makes 
it unique for research areas, such as resistance to certain antiparasitic compounds. 
Characterization should include the elements described in section A.2. The Evaluation of 
Effectiveness Data, Use of Natural or Induced Infections, Definition of Laboratory and Field 
(Helminth) Strains. 
 
RARE PARASITE: Low prevalence parasite species which may or may not be able to produce 
significant morbidity and clinical symptoms, usually limited to certain geographic locations. 
 
REGION: An area within a geographical location defined by climatic conditions, target animal 
husbandry, and parasite resistance prevalence. 
 
VICH: International Cooperation on Harmonisation of Technical Requirements for Registration 
of Veterinary Medicinal Products 
12  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
 
IV.  Appendix 2. Effectiveness Decision Criteria for Dose Determination, Dose 
Confirmation, and Persistent Effectiveness Studies 
Step 1:   Assess adequacy of infection.  If adequate infections are confirmed in the control 
group, proceed to Step 2.  If adequate infections are not confirmed, do not proceed.  
Step 2:   Perform the appropriate statistical analysis.  If p  0.05, proceed to Step 3.  If p > 0.05, 
do not proceed, study does not support effectiveness.  
Step 3:   Calculate % Effectiveness using Geometric Means.  If % effectiveness is  90% (GM), 
proceed to Step 4.  If % effectiveness is < 90%, do not proceed, study does not support 
effectiveness.  
Step 4:   Calculate % Effectiveness using Arithmetic Means.  If % effectiveness is  90% (AM), 
the study supports effectiveness.  If % effectiveness is < 90% (AM), proceed to Step 5.  
Step 5:   Perform a secondary assessment comparing the worm counts in individual treated 
animals to the counts in the control group.  See section A.4.2. Calculation and 
Evaluation of Percent Effectiveness, Step c. for details on this assessment, and 
examples in Tables 1-4 below.  
 
 
 
 
13  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
 
Examples:  
 
Table 1  
Animal Number  Treated  Control 
1  1,700  15,880 
2  13,240  740 
3  0  25,300 
4  5,200  17,600 
5  13,540  22,200 
6  20  21,620 
 
In this example, the experimental unit is the animal.  The % effectiveness based on the GM (c = 
1) is 95.1%.  The % effectiveness based on the AM is 67.4%.  The highest control animal is 
25,300 worms.  If this animal were to have 90% reduction in worm burden, the worm count 
would be 2,530; therefore, there are 3/6 animals that are considered failures (only 50% meet the 
secondary criterion) and the conclusion is that the study does not support effectiveness.  
 
 
Table 2   
Animal Number  Treated  Control 
1  2,900  8,250 
2  1,700  7,950 
3  1,400  9,360 
4  400  15,250 
5  2,700  15,800 
6  600  6,000 
7  350  28,000 
8  350  5,800 
9  300  8,700 
10  2,300  17,270 
 
In this example, the experimental unit is the animal.  The % effectiveness based on the GM (c = 
1) is 91.6%.  The % effectiveness based on the AM is 89.4%.  The highest control animal is 
28,000 worms.  If this animal were to have 90% reduction in worm burden, the worm count 
would be 2,800; therefore, there are 1/10 animals that are considered failures (90% meet the 
secondary criterion) and the study would support effectiveness. 
 
 
 
14  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
 
Table 3  
Animal Number  Treated  Control 
1  0  350 
2  71  95 
3  37  10 
4  0  6 
5  1  35 
6  2  22 
7  0  2 
8  0  27 
9  0  67 
10  1  4 
  
In this example, the experimental unit is the animal.  The % effectiveness based on the GM (c = 
1) is 92.0%.  The % effectiveness based on the AM is 81.9%.  The highest control animal is 350 
worms.  If this animal were to have 90% reduction in worm burden, the worm count would be 
35; therefore, there are 2/10 animals that are considered failures (80% meet the secondary 
criterion) and the study would support effectiveness.  
  
 
Table 4   
In Table 4, each pen has 10 animals.  The pen parasite counts listed are the pen averages 
(arithmetic mean pen counts).  The experimental unit is the pen. 
Treated mean parasite  Control mean parasite 
Pen number 
count  count 
1  5.7  11.7 
2  0.3  75.6 
3  5.6  25.6 
4  0.5  35.7 
5  2.2  69.2 
6  19.7  28.4 
7  2.5  21.3 
8  0  45.6 
 
In this example, the % effectiveness based on the GM (c = 1) is 90.0%.  The % effectiveness 
based on the AM is 88.3%.  The highest average worm burden in any of the control pens is 75.6 
worms.  If this pen were to have 90% reduction in worm burden, the worm count would be 7.6; 
15  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
therefore, there are 1/8 pens that are considered failures (> 80% of pens meet the secondary 
criterion) and the study would support effectiveness. 
16