What is the most common disease that affects sheep?

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Worms and More Worms

Parasitized lamb
A severely parasitized lamb that died


The most common health problem of domestic sheep, especially young lambs, is internal parasites (worms). There are many different types of worms that can cause problems to sheep, but stomach worms are the most common. Stomach worms cause many symptoms in sheep and lambs and can result in death if the symptoms go undetected. The worm that causes the most losses in sheep flocks is the barber pole worm.

The Barbor Pole Worm

Haemonchus contortis, better known as the barber pole or wire worm, is a blood sucking parasite that cause blood and protein loss (anemia) in the sheep. It pierces the lining of the sheep's abomasum (or true stomach). It causes anemia as evidenced by pale mucous membranes, shown in the picture below (left). It can also result in a condition called "bottle jaw," (below, right) which is an accumulation of fluid under the sheep's jaw.

Pale Eyelids
Pale mucous membranes

Lamb with Bottle Jaw
"Bottle Jaw"
Sheep become infected with worms when they graze. After they consume the infective larvae on the pasture, the larvae develop into adult worms and lay eggs inside of the sheep's digestive tract. The eggs are deposited on the pasture when the sheep poops. Once on pasture, the eggs develop into infective larvae that are consumed by the sheep and and the cycle starts all over again unless the sheep are treated with an effective drug, then moved to a place where there aren't any worm eggs to be re-ingested. The environmental conditions which are most conducive to the parasite life cycle are warm and moist.

Katahdin sheep grazing
Katahdin Sheep grazing

Sheep vs. Cows 

Sheep are more susceptible to worms than cattle because they graze closer to the soil surface. As animals get older, they begin to develop immunity to parasites, but it takes longer in lambs than calves. When a ewe lambs, her immunity to parasites is temporary compromised.

Drenching is when you give
worming medicine to the sheep.

 Controlling Worms

In the past, worms were effectively controlled with anthelmintics, drugs that kill parasites. But nowadays, the worms have become resistant to most of the drugs, making parasite control more difficult. Scientists are working hard to find new ways to control parasites in sheep and goats.

The Eyes Have It 

Scientists in South Africa developed a system called FAMACHA© (named for its originator Dr. Faffa Malan) whereby you compare the color of the sheep's lower eyelid to a score card to determine its "eye score" and need for deworming. This way you only treat the animals with clinical signs of parasitism instead of treating all of the animals in the flock. This helps to slow down the rate by which the worms become resistant to the drugs.


Examining eyelid
A red eye lid indicates a healthy sheep.


worm egg
A stomach worm egg.

 Fecal Testing

A microscopic fecal test can also be used to pinpoint the need for deworming. However, unlike fecal tests for dogs and cats, the mere presence of worm eggs does not mean a sheep needs treated for worms. It is normal for sheep to have parasites. Rather it is the number of parasite eggs that are found in a gram of feces. The number of eggs gives an indication of how much that sheep is contaminating the pasture with eggs that have the potential to develop into infective larvae and infect other sheep and lambs. The purpose of deworming in this case would be to reduce the source of contamination.

Go BACK to Zoonotic Diseases.

. . . New Words . . .

Anemia - deficiency in red blood cells.

Bottle jaw - an accumulation of fluid under the jaw.

Larvae - immature stage for worms.

Immunity - natural or acquired resistance provided by the immune system to a specific disease.

Anthelmintic - a substance with the property to destroy or expel intestinal worms.

Deworm - give medicine to kill worms.

Drench - force to drink.

Fecal - relating to feces or "poop."


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Last updated 14-Nov-2006 by Susan Schoenian.


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