Sunday, August 14, 2022

Nodular worm: One of the most damaging parasites in sheep

Estimated reading time: 4 minutes

The nodular worm (Oesophagostomum spp) (Afrikaans = knoppieswurm) is a parasitic roundworm. The adults occur in the caecum and colon, while immature stages are found in nodules in the wall of the small and large intestines of sheep, goats, cattle and pigs. These worms can be found worldwide but are more prevalent in regions where livestock is present and in areas with warmer weather. An infection caused by the nodular worm is called oesophagostomiasis.

Life cycle and transmission

One characteristic of nodular worms is that they have a direct life cycle, which means there is no intermediate host involved and infection takes place per mouth. The life cycle begins with the female roundworm that lays her eggs in the large intestine of the host. The eggs are excreted through the faeces. When they are exposed to outside conditions, the L1-larvae are released and develop into infective larvae on the pasture. Livestock become infected when grazing on these pastures.

Indigested infected larvae penetrate the submucosa of the large intestine and cause cysts, with further development of the larvae that takes place within these cysts. Afterwards they migrate back to the lumen of the large intestine where they develop into egg-producing adults.

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Clinical signs and symptoms

When nodular worms infect livestock, they cause severe caseous lesions in the wall of the gut. In serious cases these lesions cause inflammation in the peritoneum, known as peritonitis. Even when livestock is infected with low numbers of the parasite, the result is still devastating and leads to severe weight loss, anorexia, anaemia and diarrhoea. Anaemia is caused by the adult worms that feed on the intestinal mucosa.

The wool of sheep becomes dry, brittle and yellow in colour. Infected animals are also weak and may exhibit a hunched-up posture.

Diagnosis

Diagnosis of nodular worm infestation is difficult because most gastrointestinal worms cause similar symptoms. Diagnosis is based on clinical signs, but can only be confirmed with faecal examinations and necropsy. Acute disease occurs within the prepatent period (the period between infection with a parasite and the demonstration of the parasite in the body) and parasite eggs may therefore not be seen in the faeces.

Differential diagnosis

Any cause of diarrhoea such as coccidiosis and nutritional causes should be taken into account for Oesophagostomum.

Economic importance

The nodular worm has a huge economic impact because it causes serious losses for the meat industry and sheep producers. Sheep infected with nodular worms become stunted and produce less wool, as well as wool of poorer quality.

The worms also cause abscess-like swellings/nodules in the wall of the large intestine, which gives rise to the condition known as nodular disease, pimply gut or knotty gut. The development of nodules causes gritty and grainy masses in the wall of the large intestine, leading to the intestine being unfit for the use of sausage casings, for the manufacturing of catgut to make internal surgical sutures, as well as for the extraction of heparin.

Heparin can be extracted from the lining of the intestine of sheep to function as an anticoagulant to prolong the clotting time of blood. It can also be used to thin blood and prevent blood clotting during surgery and organ transplants.

Read more about the importance of biosecurity here.

Treatment and control methods

An effective drench once the cold season has started is the most effective therapy for nodular worm, because if the winter is cold enough, hardly any eggs and larvae will survive, reducing reinfection rates of livestock after treatment. The infective stage of nodular worms cannot survive cold weather, unlike other gastrointestinal worms. Although the use of anthelmintic drugs is popular, it is important to incorporate other practices into the treatment and control of nodular worms to prevent resistance.

Controlling methods of nodular worms without the direct use of anthelmintics entail improving host resistance. This is an uncomplicated way to improve the health of your animals, and it is done via selective breeding and culling for natural resistance and resilience. As always, removal of manure as well as practicing good hygiene will lower the chances of infection. Providing a diet rich in proteins and trace elements will also improve the animal’s immune system. – Cleary Martin, Agri FARMACY SA

For enquiries or a list of references, email the author at simpsonvicleary@gmail.com.

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