Dichloromethane (DCM) extract of garlic (
Tick bites and tick-borne diseases resulting from encounters with ticks are widespread; for example, in South Africa, there are reports of ixodids such as
Many reasons have been put forward to explain this surprising lack of progress towards development of plant-based tick repellent products. There are considerable knowledge gaps on the repellent activities of botanicals against ticks, for example, the study of Stjernberg and Berglund (
Host-seeking (3–4 weeks old)
Fresh garlic (
The Type A tick climbing repellency bioassay described in this study or its variations have been used successfully by workers in our laboratory to evaluate tick repellency of essential oils and crude extracts of plants and DEET (N,N-diethyl-methyl-m-toluamide) (Mkolo & Magano
The positions of ticks were recorded after an hour following the start of the experiment. Ticks that were found on upper filter paper were considered not repelled. Those on the bottom filter paper, naked part of the glass rod and on the platform were considered repelled. Ticks that moved into water were dried and replaced or re-introduced onto the platform (Nchu
Basically, this bioassay is very similar to bioassay A except for slight modifications. The previous bioassays showed limitations, since discrimination between the effects of the stimulus that produces avoidance in ticks and deterrent stimulus could not be established. This behavioural bioassay provides clarity on the nature (volatiles or fixed oils) of the plant responsible for tick repellency. Only the highest concentration, that is, 1.4% w/v, for DCM extract of garlic was used for treatments on the filter paper strips. DCM only was used for control. Adult ticks were placed one after the other on each platform and allowed to climb the glass rod. A total of 50 ticks (five replications of ten ticks each in both control and test) were used. This was done to minimise the amount of volatiles lost to the surrounds during the duration of the experiment. However, the outcome for each tick was considered as independent. Since adults of
For the Type A repellency bioassay, data are presented as percentage repellency and proportion of ticks repelled according to sex. Data were transformed to arc sin square root of the proportion of ticks repelled prior to subjecting it to one-way independent Analysis of variance (ANOVA) (Hammer
In the Type A repellency bioassay, fewer ticks were observed on test filter papers than on control filter papers at all concentrations of DCM extract of garlic (
Relationship between % repellency and concentrations of dichloromethane extract of
Effective dose needed to repel 50% of ticks in the Type A repellency bioassay following exposure of adults
Effective dose needed to repel 50% of ticks (ED50) | Lower confidence limit (CI) | Upper confidence limit (CI) |
---|---|---|
0.45% w/v | 0.29% w/v | 0.58% w/v |
At concentration of 1.4% w/v, the DCM extract of garlic bulbs produced a high repellency index of 87% (male ticks) and 87.5% (female ticks) in the Type A repellency bioassay, whereas only 4% avoidance of male ticks or female ticks was recorded in the Type B repellency bioassay (
Summary of the results obtained in the Type B repellency bioassay using dichloromethane extracts of
DCM extract of |
Number of ticks used |
Number of ticks avoiding extract |
% avoidance for a maximum of 45 seconds |
|||
---|---|---|---|---|---|---|
M | F | M | F | M | F | |
1.4 | 50 | 50 | 2 | 2 | 4 | 4 |
0 | 50 | 50 | 2 | 2 | 4 | 4 |
DCM, Dichloromethane; Conc., concentration; M, male; F, female.
No statistical significance (
This study was approved by the Animal Ethics Committee, University of Limpopo, Medunsa campus (now Sefako Makgato Health Sciences University) and rabbits used were treated humanely.
Generally, DCM extract of garlic showed positive repellent effects on ticks at all concentrations tested in the Types A and B repellency bioassays with a dose-dependent response in the Type A bioassay and an EC50 of 0.45% w/v. Previously, Mkolo and Magano (
Discrepancies in the results obtained in the two bioassays used in this study can also be associated with the differences in the tick bioassays and the corresponding tick behaviours that were assessed. At times, bioassays that differ in seemingly minor ways can yield surprisingly different results (Carroll
The use of the word repellent may in some instances lead to confusion. It is important to clearly explain or define what is meant by a repellent and clearly distinguish contact repellent from vapour repellent (Bissinger & Roe
In both repellency bioassays, initial kinesis, coupled with the natural tendency of ticks to climb (Carroll, Klun & Schmidtmann
DCM extracts of garlic repelled adults of
The National Research Foundation, South Africa, provided funding for the project to J.N. Eloff and S.R. Magano. MEDUNSA (now Sefako Makgato Health Sciences University) and the University of Pretoria provided laboratory equipment. Colleagues in the Department of Biology, Sefako Makgato Health Sciences University are thanked.
The authors declare that they have no financial or personal relationships that may have inappropriately influenced them in writing this article.
F.N., S.R.M. and J.N.E. made conceptual contributions and edited the final manuscript. F.N. performed the experiments and wrote the first draft of the manuscript.