Garlic is a common plant dewormer that is easy to find. It is known to be active against, among others, Ascaris, Enterobius and, of particular interest for ruminants, against lungworm in general1. It must be used, however, as prevention (prophylaxis) rather than as treatment or with other products. In fact, garlic does not prevent the production of eggs but prevents the eggs of certain parasites from developing into larvae5. In the ninth century, in Persia, Avicenne recommended the use of garlic as an additive rather than as a dewormer alone. Garlic is incorporated into certain commercial homeopathic or allopathic dewormers, but always with other plant-derived substances. The numerous therapeutic properties of garlic come mainly from its high sulphur content.

Garlic can be administered in several ways:

Fresh: Fresh minced garlic proved to be clearly more efficient than garlic extracts for controlling internal parasites in carp30. Using fresh garlic is ideal although not necessarily the most practical on a day-to-day basis. The leaves and bulbs may also be used. If the animals do not want to eat the leaves whole, they may be cut into small pieces, mixed with molasses and bran, and shaped into small balls. The bulbs may be grated and mashed with molasses or honey and flour. Garlic may also be planted directly in the pastures in such way that the animals have access to it as needed.
Powder: The most practical way to administer garlic is undoubtedly to add powdered garlic to animal feed. Powdered garlic can be bought at a reasonable cost in bulk from major food manufacturers (e.g. McCormick, Quest International, Griffith Laboratories, etc.).
Pills: This is a method that is useful only for very small herds. Two or three pills of four grains is the required daily dosage for one sheep.
Juice: British herbalist Grieve16 suggested using garlic juice or garlic milk as a dewormer. Garlic milk is made by boiling bulbs mashed in milk. Some researchers recommend, however, not boiling garlic as this reduces its effectiveness against parasite eggs and larvae.
Mother tincture: Garlic mother tincture is given in dosages of 20 drops/day/10 kg of live weight.
In the case of dairy animals, it is preferable to feed them garlic during or immediately after milking so that the milk does not pick up the taste.
Do not add the garlic juice to the sheep's watering trough. It will become rancid in a short time, additionally the natural oils in the garlic juice will rise to the top of the water and the rest of the juice will mix in the water. Both the juice and garlic oils are essential in treating parasites. Adding to water is not a recommended method.
Many Farm Managers are using a method much like this:
Mix the garlic juice with vegetable oil (which is available in the grocery store cooking oil section along with soybean oil) in a mixture much like this:
One gallon of cooking oil to 16 ounces (one pint) of Garlic Juice - stir well. 16 ounces of garlic juice is enough to give one dose to 96 sheep. (If some sheep get more garlic it is alright).
Get enough kelp or other leafy food that sheep will readily eat (alfalfa, clover, etc.) for 96 sheep to eat.
Spray the oil/garlic juice mixture thoroughly over the kelp (or other greenery) and coat the kelp thoroughly.
Once the kelp is coated with the cooking oil and garlic mixture - get enough salt (sea salt is preferred) and coat all the kelp that you have just mixed with the cooking oil and garlic mixture. Do this only in the shade - not in sunlight. Feed this to the sheep. They will normally only take in enough salt that they feel comfortable with and not over eat.
Sometimes the sheep are initially reluctant to eat the garlic/oil mixture and you may have to squirt some in their mouths to get them accustomed to the garlic - after that taste they generally just gobble up the kelp and oil/garlic mix.
When using a power syringe for dosing the sheep with Garlic Juice, your spray nozzle may clog up because very small bits of "garlic dust" in the juice. We recommend you either remove the filter from the spray nozzle, or strain the garlic juice through a strainer or through cloth to remove the "garlic dust".

You can be creative with administering Garlic Juice to Sheep:

Garlic and garlic juice is know in many countries to be an excellent dewormer. It is administered to sheep in many creative ways: Added to kelp, added to dry feed, mixed with molasses and salt, mixed with bread-molasses-milk and salt, etc. The main idea is to be sure that each animal gets at least a teaspoonful at proper intervals, you can give 2 teaspoonfuls if you feel the worm problem merits it. You can be creative in how you administer the garlic juice.
Spray or heavily dab our garlic juice on top of the area damaged by screw worms. Sometimes you can immediately see the worms leave. You may have to repeat the treatment. Generally, one treatment does the job and the sheep's skin will become normal again.