Chapter 29 - Nematodes:
Filaroidea
Generalized Life Cycle
• The long thread-like, adult filarial worms are found in the lymphatic glands,
tissues and body cavities of the host
• Females are ovoviparous, with larvae hatching in the uterus
• At the time of larviposition,
the larvae, known as
microfilariae, are less well
developed than typical first
stage larvae and are
considered prelarvae or
advanced embryos
• Once deposited,
microfilariae migrate into the
blood vessels (via the
thoracic lymph duct or by
penetrating the walls of the
lymph vessels) to invade
neighboring small blood
vessels
Generalized Life Cycle cont.
• Larvae can survive in
the blood for years until
ingested by the insect
vector
• Blood inhabiting
microfilariae are usually
sheathed, retaining the
flexible eggshell as an
outer covering
• However, in tissue
dwelling species, the
sheath is usually
sloughed and the larvae
is said to be unsheathed
Generalized Life Cycle cont.
• After being ingested in a blood meal of a suitable insect vector,
microfilariae develop in the digestive tract of the insect into the first stage,
rhabditiform larvae
• These larvae penetrate
the midgut into the
homocoel and migrate
into the thoracic muscles
where the molt and
metamorphose into the
infective 3rd stage
filariform larvae
• Filariform larvae
migrate to the proboscis
of the insect and get into
the human blood stream
while the insect is
feeding
Generalized Life Cycle cont.
• In the human, larvae
undergo molts and
metamorphose into adult
worms during their
migration to the
definitive site of
infection
• Within about 6 months
microfilariae appear in
the blood stream
Periodicity
• There is usually a nocturnal surge in the microfilarial population in
the peripheral circulation
• The nature of the periodicity varies among species and even
among strains
• For example, while microfilariae of W. bancrofti in the Caribbean,
SA, Africa, and Asia are nocturnal, those of the strain endemic to
the South Pacific Islands are diurnally subperiodic (they are
present in the peripheral circulation throughout a 24 h period, their
numbers increase during the day time)
• When the microfilarial worms vacate the peripheral circulatory
system, they accumulate in the small vessels of the lungs and liver
Periodicity cont.
• Evolution of the phenomenon is of obvious survival value since it
enhances opportunity for microfilariae to be ingested by the insect
vectors at certain times
• It is not surprising that the surge of microfilariae coincides with the
active feeding periods of the various insect vectors
• The mechanism responsible for microfilarial periodicity is
uncertain
• However, it is known that it is dependent on the 24 h activity
pattern (=circadian rhythm) of the definitive host
• The sleeping period of the host is characterized by decreases in
body temperature and oxygen tension, increases in carbon dioxide
tension and body acidity, etc.
• Any of these physiological changes may trigger the rhythmic
behavior of the microfilariae
Wuchereria bancrofti
• This filarial worm is parasitic only to humans
• It is characterized by extensive enlargement of
extremities
• Ancients likened the thickened skin to that of
elephants, hence the misnomer elephantiasis
(which literally means “caused by elephants”
rather than “like elephants”)
Life Cycle
• Adults lived coiled
together in the major
lymphatic ducts
• Here the lymph
vessels and glands
become blocked
causing edema
• In time, the
accumulation of
connective tissue
cells and fibers
contributes to the
enlargement of
limbs, scrotum, and
other extremities
Life Cycle cont.
• Sheathed
microfilariae,
following deposition,
utilize mosquitoes
(Culex, Aedes,
Anopheles) as vectors
• Once introduced
into the definitive
host, larvae migrate
to various lymphatics
of the groin glands
and epididymis of
males and labial and
mammary glands of
females
Epidemiology
• Widely distributed: Nile delta, central Africa, Turkey, India,
southeast Asia, Pacific Islands, and parts of SA e
• Human infections are closely related to the ecology of the
mosquito vectors as well as to human habits
Clinical Features
• Lymphatic filariasis most often consists of asymptomatic
microfilaremia.
• Some patients develop lymphatic dysfunction causing lymphedema
and elephantiasis, frequently in the lower extremities (including scrotal
elephantiasis).
Laboratory Diagnosis
• Identification of microfilariae by microscopic examination of blood is
the most practical diagnostic procedure.
Treatment
• Use of antibacterial cream on wounds stops bacterial infections and
keeps swelling from worsening.
• Diethylcarbamazine (under an investigational New Drug Protocol
from the CDC Drug Service) and ivermectin are effective for the
treatment of filariasis.
Brugia malayi
• Until 1960 several species of filarial worms with similar
microfilariae were assigned to the genus Wuchereria
• However, in that year, based on the study of adult worms, the
genus Brugia was established to designate the “malayi” group,
including Brugia malayi
• Its life cycle closely parallels that of W. bancrofti
• While B. malayi rarely affects the genitalia, the adult worms live
in the lymphatic system and otherwise cause the same symptoms as
W. bancrofti, culmanating in elephantiasis
Onchocerca volvulus
• Most prevalent in tropical Africa,
central and SA and Mexico
• Adult worms are found in fibrous
nodules located under the skin called
onchocercomas in the subcutaneous
connective tissues and viscera of
humans
• Location of the nodules differs
according to geographic areas, which
in turn may be correlated with the
biting preferences of insect vectors
• Usually there is a male and a female
coiled in each nodule, but
occasionally several worms are
observed coiled within a single
onchocercoma
Cross-section of
fibrous nodules
showing worms
Symptomology
• The microfilariae can invade the cornea and cause inflammation
of the sclera, cornea, iris and retina
• Formation of fibrous tissue usually follows, leading to impaired
vision or total blindness = river blindness
• The presence of microfilariae in the connective tissues of the
skin often produces severe dermatitis resulting from either allergic
responses (inflammatory response) or toxicity
• Affected areas of the skin become thickened, depigmented,
wrinkled and cracked
• Symptoms which resemble vitamin A deficiency, suggesting that
the parasites’s compete with the host for vitamin A
• Adult worms may also cause subcutaneous nodules, especially
over bony prominences
Symptomology cont.
Patient with river blindness
Patient suffering from
severe dermatitis
Loa loa
• Because their movement is often visible beneath the
conjunctival epithelium, these parasites are known as African eye
worms
• Humans and baboons are the only definitive hosts for Loa loa
• The microfilariae are diurnal in the peripheral blood, retreating
to the capillaries of the lungs at night
• The disease is mainly limited to African equatorial rainforests
and southern Sudan
• The intermediate hosts (mango fly genus Chrysops) breed in
muddy ponds and swamps; thus there is a high incidence of
infection in those kinds of habitats
Symptomology
• It is only mildly pathogenic
• Adult worms migrate through the subcutaneous tissues of the
body, moving through the tissues at a maximum rate of about 1.5
cm/min
• The most troublesome infection sites are the conjunctiva where
impaired vision, irritation and pain may result
• Most symptoms are general inflammatory reactions to adults and
microfilariae
• A typical manifestation takes the form of transient, painful,
subcutaneous swellings, commonly referred to as fugitive or
Calabar swellings (develop when the worm is still, disappear
when they move on)
•These are most often seen on the hands or forearms or near the
eyes
Symptomology cont.
Adult Loa loa visible under
the skin
Adult Loa loa coilied under the
conjunctival epithelium of the
eye
O. Spirurida
S.O. Camallanina
• This suborder differs in important ways from the suborder
Spirurina
• It has conspicuous phasmids with broad cavities and prominent
pores
• The esophageal glands are uninucleate
• The intermediate hosts are copepods
Dracunculus medinensis - The Guinea Worm
• Dracunculiasis (guinea worm disease)
• An ongoing eradication campaign has dramatically reduced the
incidence of dracunculiasis, which is now restricted to rural,
isolated areas in a narrow belt of African countries and Yemen.
Life Cycle
• Humans become infected by drinking unfiltered water containing
copepods (small crustaceans) which are infected with larvae of D.
medinensis .
• Following
ingestion, the
copepods die
and release the
larvae, which
penetrate the
host stomach
and intestinal
wall and enter
the abdominal
cavity and
retroperitoneal
space .
• After maturation into adults and copulation, the male worms die and the
females (length: 70 to 120 cm) migrate in the subcutaneous tissues
towards the skin surface .
• Approximately
one year after
infection, the
female worm
induces a blister
on the skin,
generally on the
distal lower
extremity, which
ruptures.
• When this lesion comes into contact with water, a contact that the patient
seeks to relieve the local discomfort, the female worm emerges and
releases larvae .
• The larvae are
ingested by a
copepod and
after two weeks
(and two molts)
have developed
into infective
larvae .
•Ingestion of the
copepods closes
the cycle .
Symptomology
• Dracunculus medinensis infections cause a broad spectrum of
nonspecific symptoms such as eosinophilia, nausea, diarrhea,
asthma, and fainting
• They are believed to result from absorption of metabolic wastes
produced by female worms during papule formation
• In addition, cutaneous ulcers caused by female worms are
common sites for bacterial infections
• Female worms failing to reach the host skin sometimes cause
reactions in deeper tissues of the body
• Commonly they degenerate or become calcified
• During these processes the worms may release strongly
antigenic molecules that can cause fluid abscesses or worms that
calcify near a joint may produce arthritis
Treatment
• Gravid female worms are commonly
drawn out mechanically from the
ulcerated area, but this approach requires
painstaking care and the worms must be
extracted slowly
• If the worm breaks during extraction
larvae escape into the subcutaneous
tissues causing severe and painful
inflammation
• The ancient method, dating back to
biblical days, of rolling the worm on a
stick is still used is parts of Africa and
Asia
• During the procedure the worm is
coiled around a stick which is turned
slowly withdrawing the worm a few
centimeters per day