This fact sheet was developed by students enrolled in Purdue's ANSC 442 Sheep Management course in Spring 2007, as a semester project. These fact sheets provide useful information on various topics related to sheep. View the list of fact sheets.

Parasitic Diseases in Sheep

Erin Barnes, Maria Cooper, Samara Varner
Department of Animal Sciences, Purdue University

Caring for your sheep and making sure that they are healthy should be the primary concern for any sheep owner, regardless of your level of sheep experience. Whether you are raising sheep to make a profit, showing them in 4H, or merely owning them as a hobby, their health can make or break your success. Parasites, both internal and external, can cause damage to the animal's wool, meat, and can result in death. Keep reading and you will find some examples of the parasites that most commonly affect sheep, as well as their treatment and prevention.


Life Cycle of Internal Parasites
The life cycle and acquisition of internal parasites is often complicated. The common mature worms residing in the sheep may shed eggs that reach the pasture through fecal contamination. Other sheep can then ingest the eggs or hatched larvae while grazing, and they may mature in the GI tract. This common cycle is illustrated in the diagram below.

Life cycle of internal parasites

Symptoms and Diagnosis:
First and foremost, it is important to know that many of the signs of internal parasitism are also shared with other diseases. Keeping that in mind, it is wise to consult a veterinarian before making your own diagnosis.

Common signs include:

Prevention and Treatment
Sheep do have the ability to develop some degree of immunity to parasites, however control programs including a routine deworming agent recommended by a veterinarian for your environment is important. The use of drugs that kill or stun parasites (anthelmintics) is very important, but the extent of worm control is enhanced by considering management techniques as well. These techniques include:

Additionally, knowing when to treat your sheep is essential. Some key times to consider are:

Anthelmintics come in many forms including oral drenches, injections, and continuous feed additives. There are advantages and disadvantages to each type. Oral drenches tend to be most popular. Continuous low- level feeding of dewormers may encourage the development of parasites that are resistant to the dewormer.

Dewormers used in sheep include (under their generic names) levamisole, thiabendazole, ivermectin, fenbendazole, morantel, and albendazole. Your veterinarian may be able to recommend an appropriate agent and control program that best fits your needs, flock, and environment.


Ectoparasites, or commonly called external parasites, are insects that live on or within the skin and wool of the sheep.   Some of the most common ectoparasites include keds, the blow fly, the bot fly, mites, lice, and ticks.  

In general, sheep contract these ectoparasites when they have dirty, manure covered wool or when they have open wounds.   The parasites can be passed between sheep through close contact or bedding.  

The Sheep Ked

The sheep ked is a wingless fly that spends its entire lifecycle on the sheep itself and is transferred by direct contact between animals.  

The Sheep Ked(Lehane, 253)


  1. Female deposits the living larvae on the wool of the sheep
  2. The larvae encases itself in a red puparium
  3. 21 days in the puparium case until maturity
  4. Leaves case and begins to feed on blood of the sheep
  5. Lifecycle begins again

The most damage to sheep by keds is caused when the sheep are already in a bad nutritional state.   The sucking of blood can cause anemia (low red blood levels) which cause the sheep to be irritable, show signs of dizziness, and fatigue.   This results in poor quality products and loss of income because the sheep tend to bite, rub, scratch, and chew at the skin and will consequently damage wool and pelt.

Treat immediately after shearing with an insecticide and repeat two weeks later.   A common insecticide is Pyrethrin because it is safe, effective, and can be used on pregnant ewes.   Treatments come in sprays, dips, and dusts.

The Blow Fly and other fly strikes

Blow flies and many other flies can cause fly strikes, wool maggots, and myiasis.  

Blow Fly(Triplehorn et al, 730)


  1. Fly lands on wound and lays eggs in dirty wool near the crotch region or a wound area
  2. Maggots spread and eat the skin surface
  3. Maggots develop into flies and cycle starts again

Fly strikes and maggots can cause the sheep to be extremely restless, stamp its feet, and bite at the site of the maggot infestation.   This will cause damage to the pelt and possible scarring if the wound is unable to heal.

Blow fly strikes commonly occur in the summer so it is best to remove dirt and manure from the sheep, shear them early, dock tails when born, and medicate any wounds they get to prevent flies from laying eggs in the open wound.  

The Bot Fly

The bot fly is a common problem in November and December and may be confused with an animal having a cold due to the snotty nose symptoms shown.

Bot Fly and Larva (Miller, 130)


  1. Female Bot fly deposits living larvae in the nostrils
  2. The larvae (maggots) migrate to the head sinuses and develop
  3. The adult bot fly then migrates back out and drops to the ground
  4. Lifecycle begins again

The migration pattern within the nostril can cause secondary infections and irritation to the nasal membranes.   This can cause the sheep to head bang on walls, feed bunks, fences, and rub their noses into the ground.   Healthy sheep survive and can usually fight off secondary infections, however severely infested and weak sheep can die.

The only treatment for bot flies is Ivomec (ivermectin) at 0.08 % AI oral drench.   Read the label carefully, there is an 11 day treatment to slaughter interval that must take place.


Other common ectoparasites include ticks, mites, and lice.   All three of these insects are blood sucking, can cause anemia, transmit other diseases, and induce scratching, rubbing, etc that can decrease the value of the wool and pelt.   Oral drenches, dips, sprays, and dusts can be used to treat these ectoparasites.   Prevention of ticks can be helped by not allowing sheep to graze close to wooded areas or areas of tall grasses.

We hope that this information will be helpful to you in your sheep raising needs. Remember that the health of your sheep is essential to their success. Make sure to treat problems as soon as they become apparent. In most cases prevention is the best way to ward off these parasitic pests. A healthy sheep is a happier sheep.     


Campbell, John B.

Lehane, M.j. The Biology of Blood-Sucking in Insects. 2nd ed. Cambridge UP, 2005. 253.

Miller, David. Common Insects in New Zealand. Wellington: A.H. and a.W. Reed, 1971. 130.

Schoenian, Susan

Triplehorn, Charles A., and Norman F. Johnson. Borror and DeLong's Introduction to the Study of Insects. 7th ed. Belmont, CA: Thomson Brooks/Cole, 2005. 730.

W. Dee WhittierAnne Zajac, and Steven H. Umberger. Control of External Parasites.Virginia cooperative extension, publication 410-027, October 2003