90

Guidance for Industry

EFFECTIVENESS OF ANTHELMINTICS: 

General Recommendations 

VICH GL7

FINAL GUIDANCE 

(THIS VERSION OF THE GUIDANCE REPLACES 

THE VERSION THAT WAS MADE AVAILABLE ON MARCH 26, 2001) 

This final guidance is intended to standardize and simplify methods used in the evaluation of 

new anthelmintics submitted for approval to the European Union, Japan, and the United States. 

Comments and suggestions regarding the document should be submitted to Division of Dockets 

Management (HFA-305), Food and Drug Administration, 5630 Fishers Lane, Rm. 1061, 

Rockville, MD 20852. Submit electronic comments to http://www.regulations.gov. All 

comments should be identified with the Docket No. 99D-2248. 

For questions regarding this document, contact Janis Messenheimer, Center for Veterinary 

Medicine, (HFV-135), Food and Drug Administration, 7500 Standish Place, Rockville, MD 

20855, 240-276-8348, e-mail: janis.messenheimer@fda.hhs.gov. 

U.S. Department of Health and Human Services 

Food and Drug Administration 

Center for Veterinary Medicine 

October 11, 2001
Page 2

VICH GL7 (ANTHELMINTICS GENERAL) 

EFFECTIVENESS OF ANTHELMINTICS: GENERAL RECOMMENDATIONS

Recommended for Consultation 

at Step 7 of the VICH Process 

on 16- 19 November 1999 

by the VICH Steering Committee 

THIS GUIDANCE HAS BEEN DEVELOPED BY THE APPROPRIATE VICH EXPERT WORKING GROUP AND 

IS SUBJECT TO CONSULTATION BY THE PARTIES, IN ACCORDANCE WITH THE VICH PROCESS. AT 

STEP 7 OF THE PROCESS THE FINAL DRAFT WILL BE RECOMMENDED FOR ADOPTION TO THE 

REGULATORY BODIES OF THE EUROPEAN UNION, JAPAN, AND USA. 
Page 3

EFFECTIVENESS OF ANTHELMINTICS: GENERAL RECOMMENDATIONS

Endorsed by the VICH Steering Committee at Step 7 of the VICH Process 

at its meeting from 16- 19 November, 1999 

This guidance represents FDA’s current thinking on this matter and does not create or 

confer any rights for or on any person and does not operate to bind FDA or the public. 

An alternative method may be used as long as it satisfies the requirements of applicable 

statutes and regulations. 

General Anthelmintic Guidance 

Two sections have been identified in the guidance’s general elements, and 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 and the definition of helminth claims. The Specific Evaluation Studies section 

describes dose determination, dose confirmation, field and persistent effectiveness 

studies. 

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 the 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. 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 generally are preferred to develop induced infections, although in 

some cases laboratory strains should be used (see glossary). Field strains are believed to 

reflect more accurately the current status of the parasite in nature. The characterization of 

each of the laboratory isolates used in the investigations should be included in the final 

report i.e. source, maintenance procedure, drug sensitivity, number of passages and 

expected establishment rates in the target host. For field isolates, characterization also 
Page 4

needs to include source, date, location of isolation, previous anthelmintic exposure and 

maintenance procedure. 

3 - Product Equivalence 

The principle of product equivalence should be used for alternative formulations of the 

same approved drug, when used at the same dose, by the same route of administration 

and in the same host. For a formulation change to a product with the same approved drug 

the pharmacokinetic attributes of the drug should dictate the study type that should be 

conducted for product equivalence. 

In either case 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, 2 dose 

confirmation studies using the dose-limiting parasite for therapeutic claims and/or 2 

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 faecal 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, it may be recommended that additional studies be conducted to 

increase the number of observations. 

4.1 Data Analysis Recommendations 

The recommendations for approval of an anthelmintic product should be based on 

significant statistical differences between the treated and control groups and on calculated 

percent effectiveness of 90% or more. 

The statistical analyses should be determined by the sponsor in the protocol stage prior to 

any data analyses. Nonparametric or parametric procedures are recommended. If the 

sponsor is able to demonstrate significant statistical differences between the treated and 

control groups, then percent effectiveness would be calculated using geometric means. 

For a product to be acceptable, the calculated percent effectiveness should be at least 

90% (see Standards of Effectiveness). 

4.2 Geometric Versus Arithmetic Means 

Differences in effectiveness may be seen whether geometric or arithmetic means are 

used. However, in the context of harmonization, recommendations are needed for one 

method of calculating the means. Log-transformed parasite counts or egg-counts tend to 

follow a normal distribution more closely than do non-transformed parasite counts. The 

geometric mean is therefore a more appropriate estimate of central tendency and has less 

potential for misinterpretation. The use of arithmetic means to evaluate effectiveness has 

been considered to be a more stringent criterion reflected in a more conservative 

estimation of therapeutic activity of the product and may be acceptable in certain 

circumstances only. 
Page 5

For the calculation of percent of effectiveness geometric means are recommended for 

dose determination, dose confirmation, field trials and persistent effectiveness studies. In 

certain circumstances there may be conditions acceptable for the use of arithmetic means. 

 

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, the 

inclusion of at least 6 animals in each experimental group is a minimum 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 below) have been conducted and the majority of individual studies provide 

90% or greater efficacy, i.e. minimally three studies with at least two of these 

demonstrating efficacy of 90% or greater are recommended to pool data. The overall 

efficacy of the pooled studies should demonstrate efficacy 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 are not 

recommended. 

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 

addressed, 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. 

In cases where all animals in the control group are infected, then one possible statistical 

method involves the use of calculating the lower 95% confidence limit of the control group 

geometric mean burden. If this value is greater than 10% of the control group geometric 

mean burden, then the infection can be said to be adequate. In the case where some of 

the animals in the control group are not infected (counts=zero) then the geometric means 

should be replaced by the median and the 95% confidence limit should be based on the 
Page 6

control group median burden. However, whatever statistical method may be 

recommended, adequate infections are still recommended in (a minimum of) six control 

animals. 

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, based on calculation of geometric means 

using pooled data (when appropriate), 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, especially when the aim for drug effectiveness is focussed 

specifically on preventing pasture contamination. These will be covered in the individual 

host species guidances. Effectiveness below 90% may be adequate when the claimed 

parasites do not have any other effective treatment. 

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 should be acceptable on the 

assumption that generally speaking 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. 

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 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 
Page 7

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, 

groups of treated and untreated controls should consist of at least six (= recommended) 

adequately infected animals, but if there is any doubt about the level of infection then the 

number should be increased accordingly (see data analysis). 

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 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 strains of parasites. 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 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. Since the sensitivity of 

the parasites to treatment should be independent of the geographical location, additional 

confirmation studies from local regions are not justifiable. Such considerations should be 

addressed in the confirmatory field trials. 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 animals per treatment group 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 recommendations for the 

calculation of effectiveness). 
Page 8

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 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 

Modern 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 faeces. Claims of activity of less than seven 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. 
Page 9

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. 

The effectiveness of the compound is calculated as follows: 100 [(GM of No of parasites in 

control group) - (GM of No parasites in treated animals)] divided by [GM of No of parasites 

in control animals] is equal to % effectiveness for the parasite or life stage (GM = 

geometric mean). 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 are 

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)] X100 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 doselimiting 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% on the basis of the 

calculation of geometric means 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. 
Page 10

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 

effectiveness tests isolated from the field less than 10 years ago. The helminths are 

considered representative of current parasitic infections in the field and have been 

characterised (source, date, location, previous anthelmintic exposure and maintenance 

procedures).

LABORATORY STRAIN: A sub-population of helminths that has been isolated from the 

field at least 10 years ago, characterised and has been segregated in the laboratory based 

on a particular property which makes it unique for research areas such as resistance to 

certain antiparasitic compounds. 

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: Veterinary International Cooperation on Harmonization.