I have seen chickens and other livestock that had worms and coccida even after being feed a lifetime of garlic.
Garlic Oil (the oil that comes from pressing garlic) is good for a lot of things including cleaning the gut after worming and medicating but a wormer it is not.
I definitely agree that it is not a dewormer! More like support for the immune system to cope with parasitism. Except for it's role as a prebiotic, I imagine most of the beneficial effects are mainly concentrated in the first part of the digestive system in a preventive role.
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So, I promised a sampling of studies I looked at:
Dietary fructooligosaccharides and potential benefits on health.
Sabater-Molina M1, Larqué E, Torrella F, Zamora S.
http://www.ncbi.nlm.nih.gov/pubmed/20119826
Probiotics and prebiotics: A brief overview.
Chow J1.
http://www.ncbi.nlm.nih.gov/pubmed/11953920
In vitro efficacy of allicin on chicken Eimeria tenella sporozoites.
Alnassan AA1, Thabet A, Daugschies A, Bangoura B.
http://www.ncbi.nlm.nih.gov/pubmed/26264230
Drug resistance evaluation of some commonly used anti-coccidial drugs in broiler chickens.
Harfoush MA1, Hegazy AM, Soliman AH, Amer S.
http://www.ncbi.nlm.nih.gov/pubmed/21246941
Effects of feed additives on ileal mucosa-associated microbiota composition of broiler chickens.
Ruiz R, Peinado MJ, Aranda-Olmedo I, Abecia L, Suárez-Pereira E, Ortiz Mellet C, García Fernández JM, Rubio LA.
http://www.ncbi.nlm.nih.gov/pubmed/26440010
The effects of increasing levels of dietary garlic bulb on growth performance, systolic blood pressure, hematology, and ascites syndrome in broiler chickens.
Varmaghany S1, Karimi Torshizi MA2, Rahimi S3, Lotfollahian H4, Hassanzadeh M5.
http://www.ncbi.nlm.nih.gov/pubmed/26049796
In vivo antimicrobial potentials of garlic against Clostridium perfringens and its promotant effects on performance of broiler chickens.
Jimoh AA1, Ibitoye EB1, Dabai YU2, Garba S3.
http://www.ncbi.nlm.nih.gov/pubmed/24517015