''Strongyloides westeri'', commonly referred to as intestinal threadworm, is a species of small

nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-Parasitism, parasitic nematodes also known as eelworms. They are a diverse animal phylum inhab ...

parasite in the family

Strongylidae Strongyles (from the Greek ''strongulos'', meaning round), or alternatively, strongyls, are nematode worms of the family Strongylidae, order Strongylida. They are often parasitic in the gastrointestinal tract of mammals, especially grazers such ...

. Strongyloides (from Greek strongylos, round, + eidos, resemblance) are commonly found in the

small intestine The small intestine or small bowel is an organ in the gastrointestinal tract where most of the absorption of nutrients from food takes place. It lies between the stomach and large intestine, and receives bile and pancreatic juice through the p ...

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

s (generally horses and monkeys, specifically foals), that are characterized by an unusual lifecycle (Larvae II, III) that involves one generations of free-living adult worms. They preferably infest foals, in which they cause diarrhea and dermatitis.

Description

The species is a small thin hair-like nematode. The approximate size is between 8.0-9.0mm in length and 1.0mm thick. The infectious females are between 0,1 - 9.0 mm long. Their eggs are thin walled and oval. The eggs are approximately 40x30µm to 50x40µm small.

Habitat

The species prefers a warm and moist environment and is most likely to infest

horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million y ...

s and

monkey Monkey is a common name that may refer to most mammals of the infraorder Simiiformes, also known as the simians. Traditionally, all animals in the group now known as simians are counted as monkeys except the apes, which constitutes an incomple ...

s, and particularly the proximal small intestines of

foal A foal is an equine up to one year old; this term is used mainly for horses, but can be used for donkeys. More specific terms are colt for a male foal and filly for a female foal, and are used until the horse is three or four. When the foal i ...

s. Their eggs can be found in the

feces Feces ( or faeces), known colloquially and in slang as poo and poop, are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the large intestine. Feces contain a relati ...

of fouls which are younger than two months ;

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. The ...

e can be isolated in the milk of the mare from day five to day 45 postpartum; also can occur in

liver The liver is a major Organ (anatomy), organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of proteins and biochemicals necessary for ...

lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...

s and mammary tissue; Parasitic female of S. westeri can be found submucosa of anterior

, in the lumen of intestine. Free living male ''S. westeri'' can occur in moist soil, sand, or sawdust with an acidic pH.

Life cycle

It is a representative of the

family in equines, which occurs worldwide. This parasitosis is a typical disease for young animals, as the initial infection is mainly galactogenic. From the 80th day post partum onwards, a resistance is developed, so older animals would no longer contract the disease. This is a typical parasitosis for stables, within the exogenous development of the larvae gets promoted through moist and warm environments. Characteristic for the development of ''Strongyloides westeri'' is a generation transfer of its host. The adult nematodes settle preferentially in the duodenum and jejunum and consist only of female worms, which reproduce parthenogenetically. Their eggs are excreted in the feces of the host animals, from which the rhabditiform larvae I hatch shortly. These can now either develop into the infectious larvae III via two moults, or develop into pre-sexual larvae, which become sexually mature animals and lay eggs exogenously. Then Larvae hatch from the eggs, which then develop into the infectious larvae III via two moults. Larva III can survive without a host for up to 4 months and infect the host animals percutaneously. Within the initial infection, the nematode enters the lungs via the bloodstream and lymphatic pathways; from there it travels, due to coughing, through the trachea and pharynx into the digestive tract, where it colonises the small intestine within days. The prepatency of this tracheal migratory pathway is 9 days. If the host animal is reinfected orally or percutaneously, it enters the mammary complex via the bloodstream and not the lungs. The mammary complex is considered the predilection site. The prepatency only takes 6 days. Reactivation of these larvae begins during pregnancy after which they are transferred to the foal via the colostrum.This excretion of the larvae within the mother's milk starts 2 to 4 days after foaling and lasts up to 7 weeks. Early infection treatments for foals can prevent the galaction infection altogether, but leads to disturbances in the development of their immunsystems. It also does not prevent the high risk of reinfection.

Symptoms

''Strongyloides westeri'' commonly infect young foals, they are associated with small intestine enteritis resulting in

diarrhea Diarrhea, also spelled diarrhoea, is the condition of having at least three loose, liquid, or watery bowel movements each day. It often lasts for a few days and can result in dehydration due to fluid loss. Signs of dehydration often begin wi ...

, the diarrhea could also be a result of intestinal pathogens. The diarrhea causes denuding of hair in the perianal area due to irritation caused by wet liquid feces. When infected percutaneously, skin infections, like

dermatitis Dermatitis is inflammation of the skin, typically characterized by itchiness, redness and a rash. In cases of short duration, there may be small blisters, while in long-term cases the skin may become thickened. The area of skin involved can v ...

, and reactions resulting in itching and hyperactiveness are observed, in older foals swelling of the lower hind limb can occur. Foals may become emaciated, listless, and dehydrated from enteritis when heavily infected. Coughing is only a short term effect during the migration of the ''S. westeri'' from the lung to the digestive system, the infection of the lung can result in excessive production of mucus or pneumonia.

Prevention and treatment

Deworming the mare during the last weeks of the pregnancy is an attempt to minimize lactogenic transmission, without full effectiveness. Another prevention attempt is environmental hygiene like removal of foal feces and usage of chemicals to keep the environment from getting too acidic, so that percutaneous transmission is reduced. On breeding farms foals are often dewormed at about two weeks of age, so that clinical diseases are avoided. There are only two marketed equine anthelmintics with an efficacy against ''S. westeri''. The two medications are called

Oxibendazole Oxibendazole is a benzimidazole drug that is used to protect against roundworms, strongyles, threadworms, pinworms and lungworm infestations in horses and some domestic pets. It is usually white to yellowish in appearance, and may take the form ...

and

Ivermectin Ivermectin (, '' EYE-vər-MEK-tin'') is an antiparasitic drug. After its discovery in 1975, its first uses were in veterinary medicine to prevent and treat heartworm and acariasis. Approved for human use in 1987, today it is used to treat inf ...

and both are used orally; for the treatment with Oxibendazole a dose of 15 milligram per one kilogram body weight is needed; a smaller dose of Ivermectin is needed only two milligram per one kilogram body weight.

References

{{Taxonbar, from=Q5268317 Parasites Strongylidae