9 Most Common Internal Horse Parasites

Pinworms are found worldwide and any horse is susceptible. Horses ingest the egg which then hatches and invades the intestinal wall which doesnt result in clinical disease. However after maturation worms migrate and lay their eggs on the perianal skin which becomes immensely itchy for the horse. This results in tailrubbing and loss of tail hair. Diagnosis can be made by looking for the worms on a piece of cellophane tape after it has been stuck to the tail headthe worms are thin and white.
Treatment consists of washing the tail and perianal area and a dewormer recommended by your veterinarian.
Bots
Bots are the larval stage of the equine botfly and are extremely common. The fly lays its eggs on the horses legs which look like small yellow dots. These eggs can be difficult to .  remove and sometimes clipping the limbs or a specialized tool called a bot knife is required. If eggs are allowed to hatch the horse will ingest the larva when selfgrooming or grooming pasture mates. These larvae then migrate to the stomach where they may live for several months before being passed in feces to mature into flies.
Bots rarely cause disease aside from mild irritation and are extremely susceptible to ivermectin a commonly used 

dewormer.
Equine Stomach Worm
While the stomach worm can cause
irritation of the stomach wall termed gastritis the problem most associated with Habronema is the summer sore. In a normal life cycle adult house or horse flies unknowingly transport stomach worm larvae from manure to the area around the horses mouth where the larvae are ingested mature and lay eggs to be passed with manure. However if the fly lands on a wound the larvae cant be ingested and complete their lifecycle and so they migrate within the wound. This causes inflammation and irritation with nonhealing of the wound compounded by the horse trying to relieve the itchiness associated with the sore.

These summer sores can be very difficult to treat and often require multiple approaches including deworming with ivermectin or moxidectin paste. Fly control is the best preventative. 16
Lungworm
Lungworm eggs are passed in feces usually passed from donkeys to horses. These eggs hatch and become larvae before being ingested. They then migrate from the intestine to the lungs where they mature leading to bronchitis or even pneumonia. Affected animals often have a cough and unthriftiness. Infection with lungworm once leads to partial or total immunity from future infections.
Common dewormers such as moxidectin and ivermectin are effective.
Threadworm
The equine threadworm is transmitted in a mares milk to her offspring or by penetration of the foals skin through bedding and is quite commonly found in foals one to two weeks of age. The primary medical problem for a foal with threadworm infection is diarrhea which can lead to dehydration. Deworming the mare with an anthelmintic effective against strongyloides within 24 hours of foaling will reduce the transmission of this parasite to the newborn 7 However clinical disease associated with the threadworm is rare and natural immunity is developed with age.