Page 850 - The Veterinary Care of the Horse

and under their chest and abdomen. The skin may lose its pigment. When the microfilariae

occur in the eye, the conjunctiva may become inflamed and develop raised lesions. They are
VetBooks.ir considered to be one of the causes of equine recurrent uveitis.

Diagnosis

A skin biopsy may confirm the presence of microfilariae.

Treatment

The numbers of microfilariae may be reduced by regular use of ivermectin, but it does not
kill the adult worms. There is sometimes a flare-up of signs after treatment as the horse has a
reaction to the dying larvae. If this is severe, systemic corticosteroids may help to control the

irritation. It is sometimes necessary to use corticosteroid drops in the eyes.

Control

Reducing the horse’s exposure to biting midges may help to prevent the spread of the
parasite. Stabling the horse at dawn and dusk and the use of fly repellents can be helpful.

HABRONEMIASIS (SUMMER SORES)

Habronema muscae, Habronema majus and Draschia megastoma are worms that live in the
stomach of horses. They rarely cause problems unless the larvae are deposited by flies in the
medial canthus of the eye, in wounds or on the male genitalia. These worms are 1–2 cm ( −1

in) long and white in colour.

Life cycle

The adult worms in the horse’s stomach lay eggs which quickly hatch into larvae. The eggs
and larvae pass out of the horse in the droppings. The house fly (Musca domestica) and the
stable fly (Stomoxys calcitrans) lay their eggs on the droppings and these develop into larvae

(maggots) which eat the stomach worm larvae. When the adult fly develops, it can contain
infective Habronema larvae in its mouth parts. When it feeds around the horse’s lips and

nostrils, the larvae crawl onto the skin and are swallowed. Alternatively, the horse may ingest
the flies containing the larvae. Once in the horse’s stomach, the larvae mature into adult

worms, so completing the life cycle. The infected flies also feed and deposit larvae on moist