The large roundworm or ascarid primarily affects horses less than two years of age. Ascarids are the most significant parasite in young horses because they are such large worms and can quickly develop into life threatening numbers. Ascarids have the potential to cause liver damage, which is repairable, and lung damage, which is permanent.
Each female ascarid can lay 100,000 to 200,000 eggs each day. The eggs pass out with the horse’s faeces. Infective larvae develop within coated eggs, which are not affected by adverse weather conditions and can remain viable for up to 10 years. When the egg is ingested its coating is digested in the horse’s stomach. As the eggs reach the small intestine they hatch and the larvae immediately penetrate the lining of the intestinal tract, beginning a 30 day migration. It is important to remember that faecal egg tests do not detect migration of parasite larvae within the horse.
The larvae travel by the hepatic vein to the liver, where they eat their way around the liver for seven to ten days. Fortunately, the liver is a very resilient organ and can regenerate itself. Permanent damage to the liver from ascarid larval migration is seldom seen.
The larvae then travel to the lungs and continue their migration for 14 to 21 days, again eating their way around lung tissue. Unlike the liver, damage done to the lungs is permanent as lungs heal by scarring and so do not regenerate. After the ascarids mature and are ready to complete migration they burrow from the blood side of the lung into the air side.
A microscopic look at normal lung tissue shows empty air spaces, the alveoli, where oxygen is exchanged for CO2. When migrating ascarid larvae are present, the immune system violently reacts to the foreign protein and destroys the alveoli. Such damage predisposes foals to pneumonia and may result in pulmonary haemorrhaging in a horse that intended to become an athlete. The worms then crawl from the alveoli into the bronchioles, to the bronchi and then into the trachea. They cause enough irritation to elicit a cough so that they are coughed to the back of the throat and are re-swallowed as mature larvae. As adults they swim upstream in the small intestine, robbing the horse of nutrition. Once the larvae reach the small intestine for the second time, their presence there is of relatively little consequence to the horse unless they are allowed to develop into large numbers that can cause life threatening blockages of the intestines, especially in foals.
When a horse is just a few months old it has all the lung tissue it is ever going to have. Because lung tissue heals by scarring, damage to these sensitive structures is permanent with less functional lung available for the horse to utilise. Horses whose lungs have been damaged by ascarid larval migration may have to breathe harder and faster to meet their oxygen demand as they develop and are asked to perform.
Ascarid larval migration can also lead to other diseases. It reduces overall thriftiness in foals and can be related to pneumonia in foals. Ascarid larvae may have an immunosuppressive effect in the lung, reducing the ability of the horse’s immune system to respond to foreign invaders like bacteria and viruses.
Proper product selection is paramount in areas where resistance is already present and equally important in preventing future development of resistance. Macrocyclic lactones or mectins on their own are no longer recommended for treatment of young horses. It is essential that all horses less than two years of age include a THP (pyrantel) in their worming program because mectins and BZs used on their own are not reliable enough against ascarids. Combination anthelmintics can slow the development of resistance and are more useful in the face of pre-existing resistant populations (Wilkes et al, 2017). EQUIMAX® ELEVATION and STRATEGY-T® are unique combination products highly suitable for the current resistance status of roundworms in Australian foals.
In summary, to protect young horses from potential resistance issues, it is recommended that a combination product registered to treat mectin resistant ascarids, such as EQUIMAX ELEVATION or STRATEGY-T, be used. Young horses should be wormed regularly with STRATEGY-T in spring and summer and EQUIMAX ELEVATION in autumn and winter from eight-twelve weeks of age until they are two years old. It is also recommended to begin a FEC monitoring program for yearlings to help guide treatment frequency. Refer to our targeted strategic worming guide for your foal program below.
Choose the correct worming strategy