Gastrointestinal parasitism of deer (roundworms)
Gastrointestinal parasitism by roundworms (nematodes) is a very common disease in deer, especially in farms or parks where the animals live in groups on pastures. These small worms live in the stomach (aboma) and intestine. They feed on blo...
What is it?
Gastrointestinal parasitism by roundworms (nematodes) is a very common disease in deer, especially in farms or parks where the animals live in groups on pastures.
These small worms live in the stomach (aboma) and intestine. They feed on blood or tissue, and disrupt digestion and absorption of nutrients.
Young deer, called fawns, are the most susceptible because their immune systems are not yet mature. Weakened or stressed adults are also at risk.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
The signs are often subtle at first, making detection difficult.
Progressive weight loss is frequently observed despite a sometimes preserved appetite.
The coat becomes dull and shaggy, the animal seems less lively than its peers.
Diarrhea or loose stools may occur, especially in young people.
In severe forms, we can see swelling under the jaw (called “bottle jaw” or submandibular edema), linked to a loss of proteins in the blood.
The mucous membranes (gums, inside of the eyelids) can turn pale in the event of blood-sucking parasites such as Haemonchus.
In fawns, marked growth retardation is an important warning sign.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
The disease is caused by several species of gastrointestinal nematodes, including genera similar to those found in sheep and goats: Haemonchus, Trichostrongylus, Ostertagia, Teladorsagia, Nematodirus and Cooperia.
These adult worms live in the abomasum or intestine and lay eggs which are passed in the feces.
These eggs turn into larvae on the pasture, larvae which are then ingested by deer during grazing.
The cycle then repeats itself, with self-perpetuating contamination on overstocked pastures.
Young animals (fawns) are particularly vulnerable, as are females at the end of pregnancy or lactation, a period when natural immunity decreases.
A high density of animals on the same plot encourages the accumulation of infesting larvae.
A mild, humid climate, such as in spring and autumn, is favorable for the development of larvae on grass.
Stress (transport, change of enclosure, undernutrition) weakens natural defenses and increases sensitivity.
Reused grazing without rotation or rest greatly increases the risk of massive infestation.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The veterinarian first relies on the history of the herd, the season and the body condition of the animals.
The definitive diagnosis is based on coproscopy, that is to say the microscopic analysis of stools to count parasite eggs.
A blood test may be useful to assess anemia or blood protein levels in severe cases.
In certain contexts, an autopsy on a deceased animal allows the precise identification of the species of worms involved.
Treatment
The treatment is based on the administration of antiparasitics (wormers) belonging to different chemical families, chosen by the veterinarian according to the results of the analyzes and the known resistance in the region.
Nutritional supplementation, particularly protein, may accompany treatment to help the animal recover.
In cases of severe anemia, additional support may be necessary.
It is essential to never treat blindly and repeatedly without veterinary advice, as this encourages the appearance of resistance to antiparasitic drugs.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Grazing management is essential: alternating plots, avoiding overgrazing and letting meadows rest helps reduce the larvae load.
Regular monitoring of the body condition and behavior of the herd helps to identify affected animals early.
Periodic stool tests (coproscopies) carried out by a veterinarian allow the level of infestation to be assessed before treating, rather than treating systematically.
A good, balanced and sufficient diet strengthens the natural resistance of animals to parasites.
Avoiding mixing young and adults on the same pastures limits transmission to the most fragile fawns.
Possible complications
A massive untreated infestation can lead to severe anemia, permanent growth retardation in the fawn, or even death in extreme cases, especially in association with other stress factors or undernutrition.
Weakened animals also become more susceptible to other infections.
When to see a veterinarian
Progressive weight loss despite a preserved appetite
Persistent diarrhea, especially in a fawn
Dull coat and listless animal
Swelling under the jaw
Paleness of the mucous membranes (gums, eyelids)
Visible growth delay in a young deer
Several animals in the group showing the same signs
Prognosis
The prognosis is generally good if the disease is detected early and treated by a veterinarian, with a return to normal body condition within a few weeks.
It becomes more reserved in heavily infested fawns or in the event of a delay in treatment, with a risk of after-effects on growth.
Frequently asked questions
Can deer transmit these worms to my sheep or goats?
Yes, some gastrointestinal parasites are shared between deer, sheep and goats. It is therefore preferable to avoid common or close rotation grazing between these species.
Should all deer be dewormed systematically?
No, systematic treatment without analysis promotes resistance. It is recommended to carry out regular coproscopies and to treat only according to the results and the advice of the veterinarian.
At what age are fawns most at risk?
Fawns become especially vulnerable after weaning, when they begin to graze alone and the immune protection provided by the mother diminishes.
Are these worms visible to the naked eye in the stool?
Generally no, eggs and larvae are only visible under a microscope. Only certain very large adult worms can sometimes be seen, but this remains rare.
References
ESCCAP — European Scientific Counsel Companion Animal Parasites
WOAH — Organisation mondiale de la santé animale (ex-OIE)
Content reviewed by the TunisieVet editorial team · Updated 15 September 2026
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