Imagine this scenario: you are reading through the worming advice being given by pet owners in a Facebook group, who say that garlic can be used as an alternative to conventional anthelmintics. Some owners are recommending its routine use to prevent dogs or cats becoming parasitised, and others are recommending it if the pet is known to have an intestinal worm burden. What is the evidence base for this recommendation?
No papers were identified that addressed the use of garlic as a prevention from parasitism of intestinal worm species. Three papers were identified that either fully or partially investigated the use of garlic for reducing intestinal worm burden. Two of the studies focused on dogs (Bastidas, 1969; Andrei et al., 2011), while the other focused on cats (Ronagh et al., 2015). All studies were clinical trials that either used the animal as its own control or allocated the animals to separate treatment groups.
The study by Andrei et al. (2011) used garlic in conjunction with other herbs in a tincture, meaning any potential anthelmintic effect of the garlic would be confounded with the other components. The tincture was associated with a greater than 90 percent reduction in eggs per gram of faeces for all studied worm species (Toxocara canis, Ancylostoma spp., Trichocephalus spp.). This was found for both population studies – shelter dogs and owned dogs. The authors aimed to investigate the efficacy of the worming preparation overall, rather than the effect of garlic in isolation as an anthelmintic, so it is unclear what the relative contribution of the garlic was. The study finished immediately after the end of the tincture dosing period and therefore did not measure any long-term outcomes.
The other canine study (Bastidas, 1969), with a sample size of one, demonstrated that larvae count of Ancylostoma caninum decreased during daily dosing with garlic, but eggs per gram of faeces remained similar following a five-day dosing period. Additionally, there was rapid recovery to pre-dosing levels observed two days following treatment cessation. While presence of adult female worms was not an outcome measure of the study, these post-treatment changes in larvae count suggest that the addition of garlic to the diet (at this dosage and dosing period) did not affect adult female worm mortality or longer-term fecundity rates (the number of eggs produced by a female adult worm) over the dosing period studied. With a study size of one, the findings could potentially be explained by undefined confounding effects.
The final study found that cats dosed with garlic showed a numerical reduction in Toxocara cati eggs and a numerical reduction in fecundity rate (Ronagh et al., 2015). No such reduction was observed for control cats. However, the cats were euthanised at the end of the study and the authors did not measure faecal egg counts for a few days post-treatment cessation. Therefore, while the egg counts were lower – likely due to a reduction in the number of eggs produced by each viable female – it is not known whether any inhibitory effect of the garlic is temporary or more permanent. It is unclear whether discontinuing the garlic treatment would lead to an increased egg production again, or whether the garlic provided an increased morbidity or mortality rate of adult female worms.
In summary, garlic may have a temporary inhibitory impact on larvae and/or egg production of the intestinal worm species studied. However, no studies directly studied the effect of garlic as an adulticide, which remains an important practical limitation in the use of these findings. All the studies considered demonstrated limited data handling and limited statistical analysis, and had small sample sizes. The relative lack of studies addressing the query means not all known intestinal worms in the UK were studied. Thus, any positive anthelmintic effects at the level of the individual species may still limit clinical use to the practitioner or owner seeking an anthelmintic effective against a broad range of intestinal worms.
In light of these limitations, and with consideration of the toxicity levels of garlic, clients should be advised that garlic is not proven as an effective anthelmintic (against multiple species or a single species) for use in dogs and cats to prevent or treat an intestinal worm burden.
The full Knowledge Summary can be found online.