Setaria is a genus of filarial roundworms that infects livestock and wildlife worldwide. There are several dozens species worldwide.
Abattoir surveys in India reported up to 40% of the cattle screened to be infected with several Setaria species.
The most important species for horses and livestock are the following:
- Setaria labiatopapillosa infects mainly cattle, other bovines and cervids. They occur worldwide, with variable incidence.
- Setaria digitata infects cattle and hoofed animals, mainly in Asia.
- Setaria equina infects horses and other equids worldwide.
- Setaria congolensis infects swine in Africa.
- Setaria tundra affects reindeer and other cervids in the cold regions of the Northern hemisphere.
Some species (e.g. Setaria digitata, Setaria labiatopapillosa) can occasionally infect inadequate hosts such as sheep, goats and horses and migrate to the nervous system causing live threatening mechanical damage, or to the eyes.
These worms do not affect poultry, dogs or cats.
The disease caused by Setaria worms is called setariasis or setariosis.
Are horses, cattle or other livestock infected with Setaria worms contagious for humans?
- NO: The reason is that these worms are not human parasites.
Final location of Setaria worms
Predilection site of adult Setaria worms is the peritoneal cavity. Occasionally they can get into the central nervous system or the eyes. Microfilariae (iamture larvae) are foudn in the blood.
Anatomy of Setaria worms
Adult Setaria are slender worms up to 15 cm long, whereby females are longer than males. They have a milky, translucent aspect. The body tapers towards the tail, which is coiled.
As in other roundworms, the body of these worms is covered with a cuticle, which is flexible but rather tough. The worms have a tubular digestive system with two openings, the mouth and the anus. Characteristic for this genus is a chitinous ring around the mouth. They also have a nervous system but no excretory organs and no circulatory system, i.e. neither a heart nor blood vessels.
The female ovaries are large and the uteri end in an opening located close to the mouth. Males have a copulatory bursa with two unequal spicules for attaching to the female during copulation.
Setaria females release no eggs but microfilariae that are about 250 micrometers long.
Setaria worms have indirect life cycles that are not yet completely elucidated. Main intermediate hosts are various mosquito species (Aedes spp, Anopheles spp, Culex spp).
Adult female worms release microfilariae in the abdominal cavity of their hosts. These microfilariae get into the blood stream and reach the capillaries in the skin. Mosquitoes become infected with microfilariae when they feed blood of infected hosts that contains microfilariae. These microfilariae develop to infective larvae inside the mosquitoes in 2 to 3 weeks. Infected mosquitoes transmit these infective larvae to other hosts during their blood meals.
These larvae migrate to the peritoneal cavity where they complete development to adult worms and reproduce. In inadequate hosts (e.g. sheep, goats), erratic migrating larvae can reach other organs, e.g. the central nervous system and the eyes. In pregnant animals prenatal transmission has also been reported.
The prepatent period (i.e. time between infection and first eggs shed) is 3 to 7 months, depending on the worm species and the host.
Harm caused by Setaria worms, symptoms and diagnosis
Adult Setaria worms in the peritoneal cavity of infected horses, cattle and other livestock are considered as non-pathogenic and do not cause any significant economic damage.
However, erratic larvae in inadequate hosts (sheep, goats) may reach the central nervous system (CNS) and migrate along the nerves and cause substantial damage. Young hosts are particularly susceptible. The diseases can appear overnight with muscular weakness and uncoordinated movements of all four limbs or only the hind limbs. This can lead to paralysis and death in a few days. Microfilariae of Setaria equina may occasionally reach the eyes of horses and cause blindness.
Diagnosis is based on the detection of microfilariae in blood smears. Diagnosis of migrating larvae in the CNS requires histopathological examination.
Prevention and control of Setaria infections
A key preventative measure to prevent or at least reduce cattle infection with these worms is to control the vector mosquitoes. This may not be easy to achieve because mosquitoes can feed on many other hosts and treating cattle or other livestock with insecticides is always insufficient to reduce the mosquito population, which means that there will always be enough mosquitoes around to transmit the disease. Some external parasiticides can protect livestock against mosquitoes for a few weeks. To learn more about mosquito control on livestock click here.
Many classic anthelmintics (benzimidazoles, levamisole, salicylanilides, tetrahydropyrimidines, etc.) are not effective against adults or larvae of these worms. There are reports that some macrocyclic lactones (e.g. ivermectin, moxidectin) are partially effective against microfilariae of certain Setaria species. In most countries there are no products approved for Setaria control. This means that it must be used off-label. However, for simple cost-benefit reasons, use of anthelmintics on horses, cattle and other livestock is usually not indicated
So far no true vaccine is available against Setaria worms. To learn more about vaccines against parasites of livestock and pets click here.
Biological control of Setaria worms (i.e. using its natural enemies) is so far not feasible, although it may be practicable against mosquitoes in some regions.
You may be interested in an article in this site on medicinal plants against external and internal parasites.
Resistance of Setaria worms to anthelmintics
There are a no reports on confirmed resistance of Setaria worms to anthelmintics.
This means that if an anthelmintic fails to achieve the expected efficacy against Setaria worms it is most likely that either the product was unsuited for the control of these worms, or it was used incorrectly.
Ask your veterinary doctor! If available, follow more specific national or regional recommendations for Setaria control.