Haemonchosis

Sheep, cattle, and goats are all affected by species of the nematode genus Haemonchus. H. contortus is the species most commonly found in sheep and goats, but H. placei is the usual species in cattle. Molecular studies have confirmed that these are distinct taxa. Even so, cross infection may occur when small ruminants and cattle graze together but the infestations are usually of lesser severity.


Life Cycle

H. contortus inhabits the abomasum. It is easily seen because it is 1 to 2.5 cm long and a little stouter than most other trichostrongylids. Adult males are homogeneously red but the females have a spiral red and white appearance as the intestine and uterus intertwine. Adult H. contortus are prolific egg layers. Egg production increases until maximum output is reached 25 to 30 days after infection, after which individual females lay up to 10,000 eggs per day for several months. The egg hatches and passes through two noninfective larval stages in 4 days under optimal conditions, but in less suitabl environments this period may be prolonged. Infective larvae migrate away from fecal pellets, some traveling 90 cm in 24 hours. More than 90%, however, are found within 10 cm of the fecal mass. Motility is greatest in hot, moist conditions.

Larvae are susceptible to desiccation and do not withstand cold temperatures well. Where hostile external conditions occur regularly, such as winter in temperate regions or the dry season in some tropical climates, H. contortus larvae on pasture are conditioned at the appropriate time to become hypobiotic after uptake by a host. Transmission occurs when the host ingests infective larvae while grazing. After developing through the fourth larval stage in the abomasal glands, the adult worms emerge to commence egg laying in about 18 days after infection. Hypobiotic larvae resume their development so that egg laying is coordinated with the start of spring or the wet season.

The life cycle of H. placei is similar except that the first eggs do not appear in the feces of cattle until the 26th day after infestation, rising to a peak at 6 to 7 weeks and declining rapidly to low levels by 11 to 14 weeks.


Epidemiology

When conditions are favorable, large numbers of infective larvae can accumulate very rapidly on pasture. Opportunities for transmission are, however, restricted by the susceptibility of the larvae to desiccation and cold. The disease is uncommon in semiarid regions unless there are opportunities for transmission to occur, for example, in irrigation schemes.

Hemonchosis is an important disease of sheep, goats and cattle. The greatest economic impact is seen in sheep in tropical and warmer temperate regions. In sheep, losses occur mostly in lambs, especially those recently weaned, but yearlings and mature sheep may also be affected. Goats of any age are susceptible but their browsing habit may protect them from the heaviest sources of contamination. Dairy calves are the most commonly involved group among cattle but steers and other young cattle up to 3 years of age may also be affected.

Predisposing causes for hemonchosis include overcrowding, lush pasture, hot and humid climatic conditions, and a low plane of nutrition.


Pathogenesis

Vigorous bloodsucking by both fourth-stage larvae and adults is the main factor differentiating the pathogenesis of H. contortus from that of other abomasal nematodes.

The hemorrhagic anemia that evolves is caused by the daily loss of around 0.05 mL of whole blood sucked per worm.

In continuing infections, the increased rate of red cell production is maintained at the expense of the animals’ iron stores and a state of iron deficiency occurs. Death may be acute and result purely from blood loss or may be more gradual and accompanied by weight loss, anemia, and hypoproteinemia.


Clinical Findings

Hemonchosis causes heavy losses because of animal deaths and reduced production.

Animals may be found dead without premonitory signs. The mucosae and conjunctivae of such animals are always extremely pale. More chronic cases show lethargy and muscular weakness, pallor of the mucosae and conjunctivae, and edema, particularly under the lower jaw and to a lesser extent along the ventral abdomen. Grazing animals lie down a good deal of the time, often around the water troughs; the energy needed to walk and eat appears to be lacking.

In calves, the disease is characterized clinically by severe anemia and anasarca.


Necropsy Findings

Gross necropsy findings include severe anemia, gelatinization of fat deposits, general anasarca, and the presence of large numbers of readily visible H. contortus or H. placei in the abomasum. If the cadaver is fresh, the worms may still be attached or swimming actively in the ingesta. The abomasal wall is hyperemic and blood clots may be present in the mucosa. Small ulcerations may be present in which adult worms have been attached. The abomasal contents usually have a distinct brownish color caused by the presence of free blood.


Treatment

Sheep

• Ivermectin, doramectin or moxidectin (0.2 mg/kg SQ or PO)
• Monepantel (2.5 mg/kg PO)
• Combination of derquantel (2 mg/kg PO) and abamectin (0.2 mg/kg PO)
• Albendazole (7.5 mg/kg PO)
• Febantel (5 mg/kg PO)
• Fenbendazole (5 mg/kg PO)
• Netobimin (7.5 mg/kg PO)
• Oxfendazole (5 mg/kg PO)
• Mebendazole (15 mg/kg PO)
• Levamisole (7.5 mg/kg PO)

Goats

• Ivermectin, doramectin, or moxidectin (0.2 mg/kg SQ or PO).
• Albendazole (10 mg/kg PO)
• Fenbendazole (5 mg/kg PO)
• Levamisole (12 mg/kg PO)

Cattle

• Ivermectin, doramectin, or moxidectin (0.2 mg/kg SQ)​
• Albendazole (10 mg/kg PO)​
• Febantel (7.5 mg/kg PO)​
• Fenbendazole (5 mg/kg PO)​
• Netobimin (7.5 mg/kg PO)​
• Oxfendazole (4.5 mg/kg PO)​
• Levamisole (7.5 mg/kg PO)​
• Netobimin (7.5 mg/kg PO)​
• Oxfendazole (4.5 mg/kg PO)​
• Levamisole (7.5 mg/kg PO)​

Control

Methods to control H. contortus must attempt the break the life cycle of the worm; whether through anthelmintics, animal management, or pasture management