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Neck Threadworm

Neck Threadworm   Onchocerca Cervicalis , O.retuculata

These worms infect the neck, skin, eyes, tendons and suspensory ligaments.  They are transmitted to the horse from the culicoides midges.

Size Appearance

5cm – 30cm long

Location In Horse

O.cervicalis lives in the main ligament in the neck.
O.reticulata lives in the nodules, tendons and suspensory ligaments, usually in the front legs.


O.retuculata causes swelling or lameness in the front legs.
Adult O.cervicalis do not cause problems however the microfilariae can cause skin lesions.
Diagnosis is difficult due to other parasites causing skin lesions.  A biopsy will be performed on a small skin sample.

Life Cycle



There is no drug treatment for the adult worm in the tendons or neck.  However the microfilariae in the skin can be killed with ivermectin or moxidectin.
This treatment could make the condition temporarily more severe, and so if a horse has visible lesions anthelmintics should only be given under veterinary supervision.

Preventative Measures

  • Use sprays to prevent the horse being bitten by the Culicoides midges, and transmitting the disease to horses.