Taxonomic ranks under review (cf. Encyclopedic Reference of Parasitology,
Metazoa (Animalia) (multicellular
Adenophorea (Aphasmidea) (without chemoreceptors known as phasmids)
Trichocephalida (Enoplida) (thread-head)
Trichuroidea (whipworms, anterior end long and narrow, stichosome pharynx)
Trichurid worms are known as "whip-worms"
because they have a broad short posterior end and a very long narrow
whip-like anterior end (with a stichosome pharynx) which is embedded
in the mucosa of the lower intestines of humans and domestic animals.
Heavy infections may cause dysentery, anaemia, malnutrition, and occasionally
rectal prolapse. They have simple direct life-cycles involving the faecal-oral
transmission of eggs containing infective larvae. Eggs excreted with
host faeces contaminate soil, food and water supplies and have a characteristic
barrel-shape with mucoid polar plugs at each end.
species cause trichuriasis in humans and animals]
Whipworms form three different developmental stages; eggs, larvae and
adults. The eggs are ellipsoidal to barrel-shaped, measuring 50-70µm
in length by 25-35µm in
width and have two distinct mucoid polar plugs. They are typically unembryonated
in faecal samples and develop infective larvae in the external environment.
Adult worms have elongate whip-like bodies (3-7cm long), with a long
thin anterior end that suddenly becomes thick at the posterior end.
The mouth is a simple opening without lips and the oesophagus is thin,
tubular and surrounded by glandular stichocytes (whole structure referred
to as stichosome pharynx). Adult female worms measure up to 7cm in length
and the uterus contains many lemon-shaped eggs. Adult male worms are
smaller measuring up to 5cm in length and they have a tightly coiled
posterior end and a single spicule with a spiny, eversible sheath.
Host range: The species
T. trichiura is found in human populations throughout the world,
mainly in tropical and subtropical regions. It is estimated that around
10% of the world population (800 million people) may be infected. Parasites
are very prevalent in regions where human excrement (nightsoil) is used
to fertilize vegetable gardens. Infections are typically over-dispersed,
where a few individuals harbour most of the worms. Other whipworm species
occur in a range of domestic and wild animals, including T. ovis,
T. skrjabini, T. discolor and T. globulosa
in ruminants, T. vulpis, T. campanula and T. serrata
in dogs and cats, T. suis in pigs and T. muris in
rodents. Zoonotic transmission of T. vulpis to humans has occasionally
of infection: Juvenile
worms develop in glands of the caecal and colonic mucosa where they
moult and grow. Adult worms have their anterior ends embedded in the
mucosa with their posterior ends dangling into the lumen.
Small worm burdens rarely cause disease, while heavier infections may
produce a variety of conditions, ranging from local enteric disturbances
to systemic conditions and occasionally death. The anterior ends of
the adult worms are embedded in the mucosa where they feed on fluids,
digested tissues and possibly blood. They may cause significant trauma
to the mucosa with chronic haemorrhage leading to dysentery and anaemia.
Pathogenesis has been related to host inflammatory responses, involving
markedly reduced cell-mediated responses and elevated IgE responses,
characteristic of local tissue anaphylactic responses. Persistent infections
have been associated with malnutrition, growth retardation, and reduced
cognitive function in children. Chronic infections may also cause finger
(and occasionally toe) clubbing evident as odd thickening of the ends
of the digits. Heavy infections may produce tenesmus (urgency) causing
the host to strain and possibly suffer rectal prolapse.
Whipworms have a direct developmental cycle whereby embryonated eggs
are directly infective to the definitive host. Infections are transmitted
by the faecal-oral route, involving the ingestion of eggs with contaminated
food, water or soil. Fertilized female worms produce numerous eggs (3,000-10,000
per day) which are excreted with host faecal material. The eggs embryonate
in around 10 days and develop infective larvae in about three weeks
in moist shady soil (or up to 4 months in cold conditions). Eggs are
dispersed in the environment by anthropogenic activities as well as
by wind, water and insects (houseflies can act as mechanical vectors).
When ingested, infective larvae emerge from the eggs and invade the
mucosa of the lower intestines where they tunnel, grow and moult to
form adults. Patent infections may develop in 8-12 weeks and can persist
for 1-4 years. Infections may also accumulate in hosts as they are constantly
re-infected from their heavily-contaminated environments.
Infections are routinely
diagnosed by coprological examination of faecal samples, usually following
concentration, and the microscopic detection of the characteristic eggs.
In individuals with rectal prolapse, worms can be seen macroscopically
attached to the mucosa. Colon endoscopy has also been used to reveal
the presence of worms.
Treatment and control:
Whipworms are resistant
to many anthelmintic treatments due to their relative inaccessibility.
Mebendazole and albendazole have proven effective, and pyrantel/oxantel
pamoate and flubendazole have some activity. Thiabendazole is also effective
but has unpleasant side-effects. Prevention of infections is best achieved
by thorough washing of vegetables, salads and fruits with clean water
prior to consumption. Control measures include education programmes
to improve personal hygiene and sanitary conditions, prohibiting the
use of excrement as fertilizer (or ensuring it is processed by suitable
microbial biocomposting prior to use) and regular deworming campaigns.