Not quite, Ryle. A single dose of dewormer will not do it in this case. Yes it will clear out the parasites in their system, but it does NOT kill the ones in the pasture already. Which is why you have to continually treat them, because if they have a high count, and the pasture area is small, then there is little doubt that the other horses have them. (If, on the other hand, you have large pastures... you have a better chance of them not having such a high count.) This is an 'infestation', not a normal 8-week rotation of wormers.
Yes, a single dose will only kill the parasites the horse currently has and won't kill the ones in the pasture, bue deworming weekly is going to just be a waste of money because there is going to be little to nothing in the horse to kill each week. Even for an infestation you should treat at appropriate intervals based upon the last drug used.
That means deworming again 4 weeks after using pyrantel, 6-8 weeks after using ivermectin and 12 weeks after using moxidectin. This rotation will minimize pasture contamination by minimizing the number of ova that are redeposited into the pasture. This program has been used on many large farms with great success but it does take time. Any plan to reduce parasite loads on pastures takes time because the larva can survive on pastures for months after being shed. This is another reason why just deworming once a week for 4 weeks will not be effective at minimizing pasture contamination---you won't have ova shed for 4 weeks (wouldn't anyway for that period if you used a single effective dewormer at the beginning of that 4 weeks) but the larva and ova that are already in the pasture can survive there for months and thus serve as a continual source of reinfection.
There is no drug that you would have to deworm again one week after using to stop more eggs being shed into the pasture at that point. To prevent recontamination you deworm based upon the Egg Reappearance Period of the drug last used--none of the commonly used drugs have an egg reappearance period of 1 week unless you are using a drug that the strongyles are resistant too.
Another important point is that fenbendazole shouldn't even be included in a program targeted at strongyles because the parasites are resistant to that drug in more than 90% of areas studied. Pyrantel also has resistance issues in strongyles, but it's only around 50% of areas tested so there is less chance that the parasites in your area are resistant to it, but it's still probably not a great choice for treating a heavy strongyle infection. Ivermectin or moxidectin would be the best choice for dealing with a heavy strongyle infection as there has only been one study showing any evidence of strongyle resistance to ivermectin and none so far showing resistance to moxidectin.
BTW, the "every 8 week rotation deworming plan" is no longer recommended either. If you keep up with the changes in equine medicine and deworming you will find that strategic deworming is now recommended. And even if you don't want to go to the trouble of a strategic deworming program, deworming every 8 weeks (unless you are using only ivermectin) still allows for parasite recontamination of pastures due to deworming at incorrect intervals and likely due to using drugs that are not effective against strongyles. The Horse: Videos http://equistarvet.com/sitebuilderco...iles/deworming