Chabertia ovina
Chabertia ovina
is commonly called the large-mouthed bowel worm. In domestic animals, its predilection
site is the colon of sheep and goats and it is occasionally seen in cattle. It has a
worldwide distribution but it tends to be more common in temperate areas of the world. In
the United States, Chabertia ovina is relatively uncommon but is found primarily
in more temperate areas of the north.
Chabertia ovina is one of the easiest ruminant nematodes to
identify because of its size (1-2cm long), location (colon) and its prominent, curved,
bell-shaped buccal capsule which lacks teeth. The image on the right shows the head end of
Chabertia ovina with its buccal capsule curved anteriorly and ventrally.
Life cycle
The life cycle is direct with the preparasitic phase similar to the
Trichostrongyles. After ingestion by the final host, L3s exsheath in the small intestine,
penetrate the mucosa and molt to L4s. These emerge, gather in the cecum, molt to immature
adults (L5s) and pass on the colon to mature. The prepatent period is approximately 6
weeks.
Pathogenesis
Pathogenic effects are caused by the feeding adults which
become attached to the mucosa and draw a plug of mucosa into the buccal capsule which is
then digested. This results in area of mucosal ulceration and local hemorrhage with
protein loss into the gut through these lesions. In heavy infections, the feeding effects
of 200-300 adult worms results in a colon that is edematous and thickened with local
areas of hemorrhage where the worms were attached. Some reports claim that larvae and
immature adults are blood suckers.
Epidemiology
Infective third stage larvae will survive mild winters on pasture.
Hypobiosis is also an important winter survival mechanism in the life cycle of this
nematode with L4s being the hypobiotic stage in the mucosa of the small intestine or the
cecum.
In most parts of the world, Chabertia is not a primary parasite in terms
of disease. Its effects are usually additive to more important pathogens such as Ostertagia
and Haemonchus. However, in Australia and South Africa it has been recorded
as a primary pathogen in sheep.
Clinical signs
Diarrhea is the usual clinical sign in Chabertia infections where it is
seen as a primary pathogen. Otherwise, infected sheep may lose weight and condition and
may become anemic. In heavy infections, clinical signs may occur during the prepatent
period since immature adults are aggressive feeders. In these cases eggs will be absent
from feces but since these are strongyle-type, they cannot be distinguished from the eggs
of many other trichostrongyle and strongyle nematodes infecting the guts of ruminants.
Diagnosis
A specific diagnosis is not usually possible in live animals for the
reasons mentioned above. At necropsy the worms are readily identified from their location,
size and shape of the buccal capsule.
