Hookworms

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Hookworms

Taxonomy, Common Name, Disease

 

* CLASS: SECERNENTEA

* ORDER: STRONGYLIDA (STRONGYLATA)

* SUPERFAMILY: ANCYLOSTOMATOIDEA

* FAMILY: ANCYLOSTOMATIDAE (ANCYLOSTOMIDAE)

 

Scientific name - Ancylostoma duodenale

Common name - Old World hookworm of humans

 

Scientific name - Ancylostoma caninum

Common name - dog and cat hookworm

 

Scientific name - Ancylostoma braziliense

Common name - dog and cat hookworm

 

FAMILY: UNCINARIIDAE (ANCYLOSTOMIDAE)

 

Scientific name - Necator americanus

Common name - New World hookworm of humans, the American killer

Human hookworms:

Historical

Necator americanus was first discovered in Brazil and Texas, it is

thought to have originated in Africa, India, southeast Asia, and China

and been transported by man to the New World. The existence of

Ancylostoma duodenale in England and Belgium has been known since the

first century. It is the first hookworm for which a life cycle was

elucidated. In 1896 Arthur Looss, working in Egypt, was dropping

cultures of larvae into the mouths of guinea pigs when he spilled some

of the culture onto his hand. He noticed that it produced an itching

and redness and wondered if infection would occur this way. He began

examining his feces at intervals and, after a few weeks, found that he

was passing hookworm eggs. The paper on this life cycle is considered

a classic in the field.

Distribution

A. duodenale is the indigenous hookworm of the north-temperate zone of

the eastern hemisphere. It is generally confined to temperate

latitudes north of 20 degrees north-latitude (southern Europe,

northern Africa, India, China, and southeastern Asia, U.S., Caribbean

Islands, South America). N. americanus generally occurs south of 20

degrees north latitude and north of 20 degrees south latitude.

 

The limiting factor in the distributional pattern is associated with

the ability of juveniles to survive environmental conditions,

particularly cold and arid conditions.

Life Cycle

Eggs are passed with the feces. Eggs hatch in about 48 hours under

favorable conditions (a) moist aerated soil; (b) protection from

direct sun rays; © temperature of about 25 C. Excess water, high

soil acidity, direct sunlight and drying inhibit hatching. The

first-stage juveniles feed upon bacteria in the feces for about three

days and then molt to second-stage juveniles. First and second stage

juveniles have a rhabditiform esophagus. A second molt occurs in 5 to

8 days and these juveniles are the third-stage infective juveniles,

they do not feed, and may retain the cuticle of the second-stage

juveniles which provides some protection from adverse environmental

conditions, and have a strongyliform esophagus (posterior bulb not

separated from the corpus by an isthmus). The infective juveniles are

able to survive 6-9 weeks at 25 C and 4 weeks at 15 C. At 0 C death

occurs rapidly. They move to the surface of the soil and wave back and

forth which increases the chance to contact a host.

 

Infection by human hookworm is through the skin (cutaneous) usually

the feet or hands. After penetration the juveniles are carried by

venous blood to the heart, to the lungs, break through into air sacs,

to the trachea, and upon being swallowed pass to the small intestine.

Here they attach to the intestinal mucosa or villi where they feed on

blood, and molt twice to adults. Copulation takes place and eggs

appear in the feces about 6 weeks after infection. Approximately five

weeks pass from infection to the production of eggs. Adult worms may

live 5 years or more but most live for short periods. Peak egg

production is reached about 6 months after infection. The number of

eggs laid by individual females varies with the number of worms

present but may reach 5-10,000 in light infections. A female may lay

several thousand eggs per day.

Symptoms-Pathogenicity

 

1. Itching of skin as the result of penetration by juveniles.

2. Congestion in lungs in heavy infections.

3. Anemia due to loss of blood, particularly if diet is deficient.

4. Diarrhea.

5. Persons with chronic hookworm disease are debilitated.

 

Management

 

1. Hookworm disease of man is best controlled by sanitation

involving the proper disposal of human feces.

2. Treatment of infected individuals and proper diets to correct

anemia.

3. Protective measures such as wearing of shoes in areas where

hookworm is endemic.

4. Vermox (mebendazole).

 

Diagnosis is by demonstration of hookworm eggs or the worms themselves

in feces. Larvae can be confused with those of Strongyloides. The tail

of hookworms is pointed while that of Strongyloides is notched.

Importance

Importance: Over 900 million people are thought to be infected with

hookworms. In the southeastern U.S. 4 to 15% of humans are thought to

be infected.

Characteristics

Much similarity of morphology and biology exists among the hookworms

(Families: Ancylostomatidae and Uncinariidae). Most species are rather

stout, and the anterior end is curved dorsad, giving the worm a

hooklike appearance. The buccal capsule is large and heavily

sclerotized and usually is armed with cutting plates, teeth, lancets,

or a dorsal cone. A dorsal gutter extends along the middorsal wall of

the buccal capsule, emptying the dorsal esophageal gland into it. Lips

are reduced or absent. The esophagus is stout, with a swollen

posterior end, giving it a club shape. It is mainly muscular,

corresponding to its action as a powerful pump. The esophageal glands

are extremely large and are mainly outside of the esophagus, extending

posteriad into the body cavity. Cervical papillae are present near the

rear level of the nerve ring. Males have a conspicuous copulatory

bursa, consisting of two broad lateral lobes and a smaller dorsal

lobe, all supported by fleshy rays. These rays follow a common pattern

in all species, varying only in relative size and point of origin;

consequently, they are important taxonomic characters. The spicules

are simple, needlelike, and similar. A gubernaculum is present.

Females have a simple, conical tail. The vulva is postequatorial, and

two ovaries are present. About 5% of the daily output of eggs is found

in the uteri at any one time; the total production is several thousand

per day for as long as 9 years.

 

Necator americanus has a pair of dorsal and a pair of ventral cutting

plates surrounding the anterior margin of the buccal capsule. In

addition, a pair of subdorsal and a pair of subventral teeth are near

the rear of the buccal capsule. Males are 7 to 9 mm long and have a

bursa diagnostic for the genus. The needlelike spicules have minute

barbs at their tips and are fused distally. Females are 9 to 11 mm

long and have the vulva located in about the middle of the body. The

anterior margin of the buccal capsule of Ancylostoma duodenale has two

ventral plates, each with two large teeth that are fused at their

bases. A pair of small teeth is found in the depths of the capsule.

Adult males are 8 to 11 mm long and have a bursa characteristic for

the species. The needlelike spicules have simple tips and are never

fused distally. Females are 10 to 13 mm long, with the vulva located

about a third of the body length from the posterior end.

References

 

1. Schmidt, G.S., and L.S. Roberts. 1989. Chapter 26. Order

Strongylata: Bursate Phasmidians. in Foundations of Parasitology.

Times Mirror/Mosby College Publishing. St. Louis. 750 pages.

2. Markell, E.K., M. Voge, D.T. John. 1992. Medical Parasitology.

W.B. Saunders Company, Philadelphia. 463 pages.

3. Maggenti, A.R. 1981. General Nematology. Springer-Verlag, New

York. 372 pages.

4. Chandler, A.C. and C.P. Read. 1961. Introduction to

Parasitology. John Wiley & Sons, Inc. New York. 822 pages.

5. Noble, E.R. and G.A. Noble. 1973. Parasitology The Biology of

Animal Parasites. Lea & Febiger, Philadelphia. 617 pages.

6. Georgi, J.R. and M.E. Georgi. 1990. Parasitology for

Veterinarians. W.B. Saunders Company. Philadelphia. 411 pages.

 

Hookworms of Other Animals:

Other species of hookworms are associated with a variety of carnivores

and ruminants including dogs, cats, foxes, wolves, swine, cattle,

sheep, goats, alpaca, llama, deer, seals, and elephants. The life

cycles of these hookworms are all similar to those of human hookworm

with some exceptions relative to the mode of infection. The principal

effect upon the host is associated with loss of blood.

 

A. caninum - is the most common hookworm of dogs and cats in the

northern hemisphere. Humans are not a normal host and a condition

called creeping eruption or cutaneous larva migrans occurs when humans

become infested. Oral infection of cats and dogs is common with this

hookworm. The ingested juveniles pass to the small intestine where

they may penetrate the intestinal wall and proceed as in cutaneous

infection, or they may enter the intestinal mucosa and then return to

the lumen and develop to adults.

 

A. braziliense - is common in dogs and cats in the tropics. Infection

is usually cutaneous and the migration through the body is as in human

hookworm. It also causes creeping eruption in humans. Prenatal

infections are common in dogs. If pregnant females are exposed to

infection some of the juveniles pass through the placenta and infect

the fetus. The worms in the fetus do not mature until the pups are

born. Death may occur in prenatally infected pups. There is evidence

indicating that older dogs are less susceptible to hookworm infection

than young dogs. Also older dogs may develop immunity due to earlier

infections.

 

The young of fur seals become infected through the mother's milk. This

has also been reported for dog hookworm, but is not common.

Management

Clean quarters, removal of feces, exposure of inclosures to sunshine,

gravel surfaces, anthelmintic treatments to rid animals of infections,

adequate diets. Animals on deficient diets are more susceptible to

infection.