Ostertagia ostertagi, commonly known as the medium stomach worm or brown stomach worm, is a parasitic nematode (round worm) of cattle. O. ostertagi can also be found to a lesser extent in sheep, goats, wild ruminants, and horses. It causes ostertagiosis, which is potentially fatal in cattle. It is found worldwide and is economically important to cattle industries, particularly those found in temperate climates.
The abomasal nematode O. ostertagi is a clade V nematode of the order Strongylida, the family Trichostrongylidae and genus Ostertagia. Ransom first described the genus Ostertagia in 1907, which currently contains approximately 15 species. All species of the genus Ostertagia infect domestic or wild ruminants. These species form a large and complex group, the taxonomy of which has not been fully elucidated.
Life cycle
O. ostertagi has a life cycle which consists of two stages: the free-living, pre-parasitic stage and the parasitic stage in the host (e.g. cattle). Eggs from mature females in the abomasum are passed in the faeces. These eggs hatch in the faeces to first-stage larvae (L1). The L1 larvae grow and moult to second-stage larvae (L2), which then moult to become the infective third-stage larvae (L3). The L3s retain the cuticle from the second stage (L2) as a protective sheath, and can survive for long periods within the faeces. The time taken to develop into infective larvae is dependent on favourable stimuli such as temperature (approximately ) and humidity. Typically, development takes 10 days to 2 weeks.
The parasitic stage of the life cycle begins when warm moist conditions cause the L3 larvae to migrate onto herbage surrounding the fecal pat, which are ingested during grazing. In the host's rumen, the L3 larvae shed their protective sheath and pass into the abomasum, where they penetrate the gastric glands. After exsheathment and penetration into the gastric glands the L3 moult into L4 and subsequently to L5 larvae. The young adult worms then emerge from the gastric glands and continue their maturation on the mucosal surface of the abomasum. Once fully mature, sexual reproduction begins and eggs are produced, thus completing the life cycle.
The normal prepatent period for O. ostertagi is 21 days. However, under certain circumstances, ingested L3 larvae can suspend their maturation and become dormant as inhibited L4 larvae inside the gastric glands. This arrested development is called hypobiosis, and can last for up to six or seven months. Studies suggest that the inhibition process in O. ostertagi depends on the weather conditions to which the infective L3 larvae are exposed on pasture, and the length of time that L3 larvae spend on pasture. In temperate regions of the northern hemisphere, hypobiosis occurs in autumn and early winter, but in the southern hemisphere there is evidence that this phenomenon occurs in spring and early summer.
Morphology
O. ostertagi adults are slender reddish-brown worms. Adult males are long, adult females long, and the eggs long. Identification of adult medium stomach worms is based on the structure of the bursa, genital cone, and spicules in males and on the dimensions of the oesophageal valve and the configuration of the synlophe in males and females. The cuticle in the anterior region is striated transversely whereas the rest of the body is unstriated and bears around 36 longitudinal ridges. The Ostertaginae are characterized by a reduced buccal capsule and square head, and well-developed copulatory bursa in the male. The genital cone ventrally bears two small slender papillae and dorsally bears two long slender papillae surrounded by an accessory bursal membrane. The lateral rays of the copulatory bursa are in a pattern of 2-1-2 or 2-2-1 pattern. The key to morphologically identifying O. ostertagi males is a prominent proconus and the remainder of the genital cone is not prominent. The spicules are of equal length and shape, tapering towards the distal end. The key to morphologically identifying O. ostertagi females is the lateral synlophe with one pair of ridges ending next to the lateral ridge between cervical papilla and the posterior end of the oesophagus. The cervical papillae are prominent and thorn-like. A large number of surface cuticular ridges (synlophe) are perpendicular to the surface.
