|The bloodworm, Strongylus vulgaris, belongs to a group of intestinal worms of the horse commonly referred to as the large strongyles. The bloodworm is by far the most dangerous of the large strongyles, and indeed of all horse parasites, being responsible for illness, colic and poor performance in all types of horses.
The adult bloodworm lives in the large intestine of the horse, including the caecum. The two centimetre long worms are pinkish grey in colour and are found attached to the lining of the intestine. They are occasionally seen with the naked eye in the droppings of infected horses. The effects of the adult worms in the intestine of the horse are variable, and generally far less severe than those of the migrating larvae, Nevertheless, the adult worm does suck the blood of the horse, and heavy infections can cause illness, periodic diarrhea, colic and even anemia (particularly in young or aged horses).
The life cycle of the bloodworm is illustrated above.
Adult female bloodworms in the horse's large intestine lay about 30,000 eggs per day, which pass out in the horse's droppings. On the ground they go through a development stage until larvae hatch out, still on the ground. Larvae emerge and move away from the dung pat in moisture in the soil and on the herbage. These larvae moult twice and become infective larvae - that is, they cannot infect the horse until they have developed to this stage. The climatic conditions which influence the development of these ground-dwelling stages of the parasite are complex.. Essentially, the eggs and larvae require mild to hot conditions and adequate moisture to thrive. Therefore, horses will be at greater risk from heavy infection from the pasture when the weather is hot and wet or humid - under these conditions eggs can produce infective larvae within seven days of being deposited.
The infective larvae are taken in by a grazing horse, and swallowed with each mouthful of grass. Under moist conditions, the larvae actually 'swim' up the water film on blades of grass and will congregate in droplets on the pasture.
Once inside the horse's intestine, the larvae emerge from their protective coats or sheaths and burrow into the lining of the intestine. Here they enter the arteries, which branch off, from the mesenteric artery, the main arterial blood supply to the intestines. They burrow under the lining of the artery and gradually work their way up the artery, in the opposite direction to the flow of blood, until they congregate under the lining of the main trunk of the Mesenteric artery. They reach this point in approximately 3-4 weeks, but they remain here for a further 3 or 4 months where they grow in size and develop into immature adult worms. Frequently bloodworm also infects and damages the aorta or the major arteries supplying the legs in a similar manner.