Oh lord the great worming debate, :lol: I worked very closely with my vet to establish a rigerous worming program five years ago after discovering that a yearling filly had a severe encysted worm burden. Her grazing was basically shared with other tennants in a boarding situation and regulary used strongid double dose for tape twice annually and panacur paste as a regular wormer, as did most of the residents not realising that neither treated for encysted worm. 
After thoruough investigation and a shot of quest the problem was quickly cleared up and have since had no further problems, especially now that the horses are all wormed effectively with no boarders.
So this is the how I understand things. Bascially there are three chemical groups that can be used to treat for small red worm, roundworm, threadworm and the majority of other species of worms incluiding different larval stages that foals need to be protected against. I have noticed oxibendazole e.g Anethelicid as an option in the US where resistance to panacur is known and assuming it falls into the Tetrahydropirimidines (Benzimidazole) family? (There is actually a fourth group but prefer to leave it aside as this treats for tape + all other species in a single dose excluding encysted worms, so not recomended for pre-foaling treatment)
Tetrahydropirimidines (Pyrantel Embonate) e.g strongid
Macrocyclic Lactones (Ivermectin + Moidectin) e.g Generic Ivermectin
Tetrahydropirimidines (Benzimidazole & Fenbendazole) Panacur Paste
However it is very important to recognise that the only two chemicals as far as I know that can claim license to treat for encysted worms are Fenbendazole (five day treatment) and Moxidectin.
The general consensus therefore is to worm the mare four weeks prior to foaling with quest (moxidectin) or a five day treatment of panacur gaurd (Benzimidazole & fenbendazole) and repeat the treatment in the autumn[/B]. :yes:
Change the chemical group wormer Moxidectin to Fenbendazole in year two only or vice versa. Research has shown that there is yet no resistance to moxidectin in the 10 year study but rotation is still recomended to discourage it from happening.
This prefoaling treatment of the mare will remove all stages of small and large redworm, roundworm and thread worm including all stages of larvae, AND encysted worms as well as all other species except tape. Mares should then be wormed one week after with a tape wormer. e.g equitape
Worming the mare early enough will avoid the suckling foal ingesting any migrating dead worms passing from the mare and prevent the foal from any worm contamination via her milk, as she herself will be relatively worm free.
The four week rule prior to foaling is advised as this covers the eventuality that the mare foals early. Another reason for doing so is that a heavy burden of encysted worms may cause adversory effects on the mare, and we don’t want her feeling grumpy and uncomfortable at the time she is due to foal or has foaled. :no:
It is important to note that scouring foals during foal heat is an indication that there is a high worm presence of Strongyloides westeri (Intestinal threadworm) in the mare passing through her milk. Had always been told that it was caused by a change in hormones in the mare but beware, threadworm is a more likely cause! :yes:
Foals usually develop an immunity to threadworm as they mature hence the reason the scouring can pass without intervention, but prevention is better than cure, as a newborn foal can quickly become dehydrated and lead to death, so treating the mare prefoaling will erradicate any burden of thread worm. Having followed this program now for three years we no longer see the scouring of foals during foal heat.
Encysted worms, can become active at any given time, so assuming that the mare has only been covered by a basic ivermectin wormerprefoaling with no previous annual treatment of encysted worms, does not assure that she does not carry a burden. :no:
Failing to cover the mare with a wormer that caters for encysted worm can lead to complications of a worm burden in the foal if the mare heavily contaminates the field in which the foal grazes. She may appear in good health and condition and not affected but the foal does not have this same resistance and will be exposed to the worms that she sheds. If your mare suddenly sheds weight and many will claim “oh she gives everything to her foal” it is important to bear in mind that it is perhaps more likely that she has an encysted worm burden that has not been appropraitely dealt with. The same reasoning is often given by mare owners whos mares shed weight rapidly in the autumn and the growing foal is often held responsible for the mares loss of condition despite all the “feeding” she receives to compensate for the foals demands!
Fortunately foals will not neccesarily be affected by encysted worms until much later in in the season, normally his first autumn and foals are unlikely to be affected by tape under two months of age. However be carefull, if the classic, pot bellied weanling does not pick up after regular basic ivermectin treatment as this can suggest an encysted worm burden and should preferably be treated in the autumn at the same time as the mare with a five day treatment of panacur gaurd or equest and then treated one week later with a chemical group wormer that treats for tape.
A feacal count will not detect the presence of tape worm or encysted worm, the only reliable detection method is a blood test. So if in doubt have the foal blood tested and a feacal test carried out.
The mare should then be wormed a second time at six weeks after her prefoaling treatment. The foal can then be wormed one week after the mare and continue throughout the spring summer season. (this is merely to avoid overdosing of the chemical as some will pass through the mares milk if wormed at the same time) Choose one single chemical group wormer and stick with it, different brands of wormer from the same chemical group is fine
wormer groups:
- Macrocyclic Lactones e.g. Ivermectin
- Benzimidazole e.g Panacur
- Tetrahydropirimidines e.g Strongid
Strongid (Pyrantel Embonate Tetrahydropirimidines) and Ivermectin (Macrocyclic Lactone) are two different chemical groups
Fluctuating between different chemical groups eg ivermectin then strongid (Pyrantel Embonate) in any one year every four/six weeks is not recomended. :no:
Mares and foals should then be wormed every six weeks with the near as possible correct doseage throughout the spring summer months.
It is vitaly important that the correct worming doseage is established by measuring your foal with a weight measuring tape. Underworming the foal can lead to a burden and over worming with unescesary chemicals can lead to resistance.
If a foal appears not to be thriving, a feacal count should be done at the end of the six week period. A feacal count done too early will produce a false negative result. If the feacal count reveals a heavy presence, then shortening of the worming period is advisable, If the count is minimal continue with the six week program of mare and foal.
As previously mentioned treat your foal for encysted worm in the autumn of his first year followed by a tape wormer one week later. Autumn/Winter worming should then continue as before every six weeks in the foals first year of life with the same chemical group wormer used throughout the spring summer. Start treating him with the second chemical group wormer in the spring of the following year with the wormer that treats for encysted worms and once again a week later for tape Consquently six weeks after his encysted treatment be sure to select a different chemical wormer for the control of small red worm roundworm etc for the six week worm control throughout the year. Repat in year three with the third chemical group for the treatment of regular worm species.
Worming really is easy and straight forwards but not fully understanding the basic chemical groups when to treat, with what, and what to treat for, can quickly lead to an incorrect worming program and chronic infestation and sadly foals and youngstock are more vunerable.
Other factors can help reduce worm infestations, regulary harrowing pasture in very hot dry weather will kill off eggs in manure. Poop picking and cross grazing with sheep and cattle will also help to keep pastures clean and to conclude never assume that it’s because you have regularly wormed a foal you have erradicated all possibilites of their being a problem, if an animal is not thriving then get peace of mind and blood test in conjunction with a feacal count at the end of a worming period. 
Yearlings are frequently excused from looking their best because they all go through ugly phases when in reality they may be thing and scawny because there is an underlying encysted worm problem that has never been dealt with because owners are not aware that there are only two drugs that can treat the problem!
happy worming
