Although we are always talking about killing worms, it might be interesting to inform you a bit about the “intelligence” of worms. Imagine if you end up in the middle of a body which is a million times bigger than yourself. How would you find the way? Some of the worms travel through the body and know exactly where to go and when to leave certain areas of the body. They penetrate through the wall of the intestinal tract, travel via blood vessels, stay in certain area’s move on to for instance the lungs, get coughed up, are swallowed again and travel out with the dung. I always find this amazing to realise. Nevertheless we would like to get rid of them as they can cause a lot of trouble for the horse and owner. 

As in most things in live; you have to know what your are up against to be able to win the battle. It is important to realise that every horse will carry worms. In the case of a mild infection this will not lead to problems. Heavy infections can lead to all sort of problems as loosing condition, not performing well, colic and sometimes death. 

The prevention of problems is focussed on maintaining a low grade infection. This is commonly done by (regular) treatment with anthelmintica (wormers). The drug and frequency are often the middle of the discussion. Before going into this I would like to inform you about the different kind of worms. 

Kind of worms 

Strongilidae
First of all there are the Strongylidae or redworms. Infections with these worms are most severe in the late summer and autumn. It is important to realise that a field that is infected will stay infectious till at lea next spring. 
The big red worm is mainly dangerous because of it’s travelling through the walls of the blood vessels. The clinical symptoms can be very severe. Heavy colic en death can occur after the first three weeks of infection. Old (infected) vessel wall can give long term problems as well. The most frightening is rupture of the damaged waal with acute bleeding to death as a result. This can happen years after heavy infection. 
The small redworms are mainly dangerous because of their ability of inhibition in the wall of the intestinal tract. The majority (80%) of the worm population will do this. When they activate again in the winter they can cause severe problems with lose of condition and severe diarrhoea. Very often horses will not survive this disease. It is important to worm these horses and threat symptoms as good and soon as possible. It is most often seen in horses younger then three years of age. Prevention of these worms is most effective if large quantities of eggs on the field are prevented by regular worm treatment. 

Parascaris worms.
 Infection with these worms is most often seen in foals and yearlings. Older horses will develop immunity. The worms travel through the body and end up in he lungs. Clinical symptoms as coughing, increased respiration, anorexia, loosing condition, etc. Eggs of these worms can stay infective for years! Infection form one year to the next year foals can occur. 

Strongyloides Westeri or Threadworms. 
The first infection will be through the milk of the mare. The foals will re-infect themselves after that. Heavy infections in the small intestine will cause severe diarrhoea, fever and eventually death. Prevention has to be focussed on hygiene in the stable. Removal of dung out of the stable on a daily base is not overdone.

Tapeworms
Heavy infectious with tapeworms are very rare. There is a worldwide discussion going whether tapeworms are a danger to horses or not. Although horses will be infected, this does not have to lead to clinical symptoms. As they can cause cramp and/or constipation colic it is probably wise to threat for tapeworms once a year. 

Gasterophilus or Stomach bots 
Flies will lay the eggs on the skin and the horse will lick them up. They will develop for approximately one month in the tongue, before they leave to the stomach. In the stomach they will anchor their mouth in the wall and spread their eggs in spring. Severe infections will lead to ulcers, colic, losing weight and anaemia. 

I think the above will give you enough information to take the treatment and prevention of worm infections very serious. 

Treatment and prevention of worms 

Every treatment of clinical symptoms caused by worm infections is a waste as the whole thing probably could have prevented! If treatment is necessary it is often a combination of anti inflammatory, antibiotics, fluids and worm treatment. Prevention is the key to the problem! 

First of all I would like to clear a couple of things:
1. There is no wormer that will work for months/year. A wormer will work on the day that it is given and will kill whatever it is effective against. There is no “depot” of drugs in the body. Some drugs will have a long term effect, but that is something different. There are basically three groups of drugs available. There is difference in the effectiveness of the different groups, but they all need regular application. 
2. There are no drugs registered/licensed as an injection for the treatment and/or prevention of worms in horses. Although effective and sometimes necessary they are not allowed to be used by legal terms. Injections can sometimes also be too effective. If a lot of worms are killed in a short period of time, this can lead to serious intestinal problems and sometimes death. In general one has to use the available oral dozers. 

Knowing the lifecycle of the different worms that like to spend their live in your horse there are general rules in the prevention of infection; 

• The less contact with old faeces, the less change of infection. Cleaning of stables, yards and fields is always wise/essential.
  
• If possible one can “clear” certain fields by grazing schedules. Problems are often he space and management of the fields.
  
• Several repeated treatments in a year will be necessary as the lifecycle of most worms is short.
  
• Always give for the proper weight. Never under dose, rather a bit on the high side.
  
•  Worm mares 4 weeks before expects date of foaling 
  
• Start with foals from 6 weeks old with suitable drug.
  
•  Effectiveness of worm programs need to be monitored as resistance for drugs by worms is possible. Testing of droppings on eggs and drug resistance should be carried out once a year.
  
• Realize that a horse will never be free from worms.We don’t want that either as we want the horse to have a light infection to keep up his or her immunity.
  

 Given the different worms, cycles, management of horses, drug resistance, etc., it is hard to give a solid schedule on worming programs, but there is a general advice:

• End of April
• 6 weeks later 
• End of September (including tapeworm) 
• End of October

Sometimes it is wise to treat one time extra in the spring. I always use the same group of drugs for the year and use the combination paste for tapeworms in September. In general Ivermectin is very reliable, and has proven to show thus far the least resistance. 

The earlier mentioned testing of droppings once a year might look a bit overdone, but is essential in a good prevention program. Treating with the wrong drug, or on the wrong times is dangerous and a waste of money. 

I hope the above will give you enough information to prevent problems with worms in your horse(s).