Ascarids - Roundworm

Symptoms of roundworm infection

The most common visual indicators include debilitation, poor growth (weigh your foals/youngsters regularly), lethargy, weight loss, coughing, rough coat and a “pot belly”.

Internally, infections will manifest themselves as damage to the liver and lungs which are both sites of larval stages of a roundworm life-cycle. The larval damage to the liver is though to be reversible as the liver is a regenerative organ so permmanent damage is not often seen. The same cannot be said of damage to the lungs. The lungs heal by scarring and so any damage is permanent and not reversible- damage may well lead to issues during performance activities even after the infection has been successfully treated. Essentially the damage means there will less functionally lung for the horse to utilise. The lung damage can also predispose foals to pneumonia and can results in pulmonary haemorrhaging.

Adult horse often exhibit no obvious signs of infection i.e. they are asymptomatic, but they can carry a burden and act as a reservoir for the infection of foals.

The clincal signs exhibited during ascaris infection correspond to the life-cycle stage of the parasite:

Clincal Signs Versus Stage of Infection

• When migrating to the lungs a horse may well show signs of respiratory problems- it may well have a nasal discharge or a cough and possibly a fever.  Likewise, larval stage breaking into the lungs may well cause haemorrhage.
• A heavy adult burden in the gut will likely exhibit as classic worm symptoms- poor coat, loss or weight or failure to gain weight, a pot-bellied appearance and general lethargy.

Ascarids do present a notable danger of colic.  Their physical size, and numbers, is such that collectively they have the ability to block or obstruct the intestinal tract of young horses and therefore tigger a bout of colic.

Somewhat ironically this colic can actually be tiggered by the use of wormer- its not caused by the wormer itself- but as the adult worms are killed by the treatment and fall away from the intestinal lining they form a mass and create a blockage.  This is the primary reason that you not only need to treat young horse but treat them regularly to ensure a potentially fatal build of mature ascarids in the gut is not allowed to occur.

Roundworm Life-cycle

The roundworm life-cycle is interesting and has migratory stages within the host. A simplified cycle is presented below:

1. Adult female worms within the horse produce upwards of 100 000 eggs/day which are excreted in the faeces and dropped on to the pasture.
2. The eggs are thick-shelled and can remain dormant, but viable, for many months if not years.
3. Within the eggs the larval stages develop with the 2nd stage infective larvae developing usually within 7-14 days.
4. The infective larvae are then inadvertanly ingested by the horse during normal grazing.
5. Once in the horses digestive tract the larvae begin a migration from the small intestine to the liver. Once at the liver they feed for ca 7-10 days and molt to a further stage.
6. After the liver the larvae begin their next migration to the lungs via the circulatory system. They are though to soend ca 14-21 days within the lungs. The overall migratory phase totals some 30 days.
7. The immature worms move from the blood side to the air side of lungs where they cause the host to cough. The coughing allows the worms to be re-swallowed and re-enter the digestive tract. 
8. The worms follow the routine digestive tract and return to the small intestine. Once here the worms mature and reproduction begins again