A M . ZooLocrs-r, 8:139-149 (1968).
Some Aspects of Nutrition in Parasites
CLARK P. READ
Department of Biology, Rice University, Houston, Texas 77001
SYNOPSIS. Some selected examples of food-getting mechanisms in animal parasites are reviewed.
The range of adaptations includes: (1) mechanical devices for biting or sucking plus internal
or external digestive capacity, (2) internal digestive capacities only, or (S) no mechanical feeding
mechanisms and no digestive capacities. In the latter case, feeding mechanisms are restricted to
absorptive capacity. Available data on absorption are briefly reviewed.
Questions concerning the food of parasites in hosts are posed, and the following examples are
analyzed to demonstrate the difficulties in answering these questions: Fasciola, hookworms,
Trichuris, and some nematode parasites of vertebrates.
The nutrition of parasites is of significance to zoologists not only for its bearing
on comparative biochemistry and physiology, but also because nutrition is the primary consideration in the ecology of those
organisms which utilize other living systems as habitat and food. The nutritional
relationship with a host is usually regarded
as the hallmark of parasitism. However, in
preparing this paper, the author has tried
to maintain the view that examining the
nutrition of parasites is actually a biased
examination of a more general aspect of
parasitism, namely, the assumption by the
host of regulatory functions which maintain the integrity of the parasite. All parasitic organisms depend on the host for
maintenance of homeostasis. Nutritional
specializations are obvious cases of this
more general characteristic. The author
has discussed this in more detail elsewhere
(Read, 1968).
Various authors have reviewed special
aspects of the cultivation of animal parasites and should be consulted for a more
complete discussion of some of these topics:
Shorb (1964); Guttman and Wallace (1964);
Guttman (1965); von Brand (1962, 1966);
Silverman (1965); Read (1966); Moulder
(1962); Dougherty, et al. (1959); Weinstein
(1966). Others will be cited in context.
The feeding of an animal in nature is
often more complex than simply furnishing
it with a mixture of required substances.
House (1959) suggested that the term, nutritional requirements, should be restricted
to the chemical factors making a diet ade-
quate; the term, chemical feeding requirements, should be restricted to the chemical
factors important to normal feeding behavior; and the term, physical feeding requirements, restricted to requirements in
dietary texture, position, intensity of light,
and other physical factors that influence
feeding behavior. In examining the available information on animal parasites, it is
plain that we know more about nutritional
requirements than about chemical or
physical feeding requirements. In this
paper, I shall review some of the gross aspects of the nutrition of parasites. The
topics have been selected to indicate the
lacunae in our understanding of some well
known parasitisms.
FOOD-GETTING MECHANISMS IN
THE DEFINITIVE HOST
The primary concern of organisms, in
general, is to obtain food in sufficient
quantity and of satisfactory quality for the
maintenance and reproduction of the species. Since many parasitic organisms are
literally living in a pool of food, it is of
interest to examine the modes of feeding
in some of them. Many parasites have adaptations for chewing or biting the tissues
of the host, and some of them may have
enzymatic mechanisms for getting about
through host tissues; such enzymes may
also be involved in the obtaining of nutrients. Organisms which bite or suck food
from the host are found in several phyla.
The trematodes, many nematodes, some
arthropods, hirudinean annelids, and some
139
140
CLARK P. READ
others show adaptations of this sort. The
hookworm nematodes show extreme adaptation for a biting and sucking pattern of
nutrition. A relatively large volume of
blood is passed through the bodies of these
parasites from their sucking activity on
the intestinal mucosa. Much of the material passes out of the anus of the worms
as undigested blood. It has been suggested
that this serves a respiratory function as
well as a nutritional one. In some groups,
the sucking activity is not a specialization
associated with parasitism. Among the muscoid flies, for example, sucking is a common mode of feeding. Those that have
become suckers of blood have evolved
mechanisms for penetrating the skin of
hosts but the sucking mechanism is quite
similar to that in non-blood-sucking species which feed on liquid food. In the
laboratory there are frequent difficulties in
studying the feeding of parasites. For example, in the usual saline medium used for
studying the metabolism of nematodes,
Nippostrongylus does not ingest the liquid.
The physical and/or chemical feeding requirements are apparently not satisfied in
such media (Roberts and Fairbairn, 1965).
Hookworms do not appear to ingest liquid
media in the absence of serum (Warren and
Guevara, 1962; Fernando and Wong, 1964).
Among the parasitic protists there are
forms which take in solid food by phagocytic mechanisms. Some of these, e.g., Evtamoeba histolytica, show specializations for
the ingestion of cells but may also live on
other solid materials in the digestive tract
of the host. Many of the forms which bite
and suck food from the host must have
enzymatic mechanisms for the digestion of
the materials, but there is little information on their precise nature. Extracts of
the digestive tract of nematodes contain
proteolytic and amylolytic enzymes, but it
has not been established that digestion occurs in the lumen of the gut in these
animals. Some parasitic arthropods possess
digestive enzymes; in one case there is
evidence that a symbiotic bacterium of the
genus, Pseudomonas, may be involved in
the breakdown of protein, the arthropod
then utilizing the digested material (Borgstrom, 1938). Some forms may be capable
of ingesting solid food in some circumstances but live entirely on liquid food in
others.
Some forms rely entirely on the absorption of nutrients in solution and are actually incapable of digestion. The cestodes
and acanthocephalans, which lack an intestinal tract, may be in this category. It is
reasonably certain that the tapeworms, at
any rate, do not produce amylolytic or proteolytic enzymes for digesting food in the
environment. The hemoflagellate Protozoa
may be in the same category, because the
search for digestive activity in them has
consistently failed. Absorptive mechanisms
may, therefore, be of great importance in
parasitism. This will be discussed below.
There are some parasites which can ingest the liquid portion of the host juices
and can carry out some digestion but are
not capable of biting or breaking into host
tissues to any significant extent (e.g., some
of the adult ascarid nematodes). Probably
these parasites have some digestive functions but these may well be specialized.
This is a relatively uninvestigated subject.
The enzymes presumably involved in the
migration of certain parasites might also
be construed as digestive in function. The
growth which occurs during the migrations
of some species implies that they feed during these movements, and some ingestion
of liquefied host tissue must occur.
Extracorporeal digestion occurs in at least
some nematodes parasitizing plants. These
forms possess a mouth stylet which is used
in penetrating host substance. The dorsal
gland is thought to produce a digestive
secretion which flows out through the
mouth. Liquefied food is then ingested
(Linford, 1937). This mode of feeding requires further study.
The mechanisms by which malarial parasites utilize hemoglobin, or rather the protein portion of hemoglobin, are not clear.
The known properties of cell membranes
make it unlikely that the protein could
enter the parasite intact. Several workers
hypothesized that the parasites could se-
NUTRITION OF PARASITES
141
crete digestive enzymes and hydrolyze hemo- histochemical procedures. In a number of
globin as an initial step, and studies with cases this constitutes good circumstantial
extracts indeed showed that the parasites evidence that digestion is carried out in
contain such enzymes, (Moulder and Evans, the gut. However, there seems to be a lim1946; Cook, et al, 1961). One of the diffi- ited number of instances in which more
culties in this interpretation was the fre- direct evidence is available.
quently observed fact that the malarial pigEvidence that an enzyme extracted from
ment is found inside the parasite rather tissue of a parasite's digestive tract may
than out in the cytoplasm of the host.
actually function as a digestive enzyme
Light was shed on the problem by Rud- was provided by Thorson (1956a) who
zinska and Trager (1957, 1959) who studied studied a proteolytic enzyme extracted
the ultrastructure of Plasmodium lophurae from the esophagus of the hookworm, Anand P. berghei. Thin sections revealed cylostoma caninum. The extracted enzyme
vacuolar processes which were interpreted was specifically inhibited by serum from
as evidence of phagotrophy. Similar proc- a dog made immune to Ancylostoma by reesses were reported in the related proto- peated infection. This was direct evidence
zoan, Babesia rodhaini (Rudzinska and that the dog had been in contact with this
Trager, 1962). Aikawa and his colleagues antigen at the tissue level. Thorson (1956&)
(1966) have described a cytostome in the was also able to produce some immunity
erythrocytic stages of eight species of Plas- to Ancylostoma by injecting dogs with an
modium. This cytostome appears to ingest esophageal extract containing the proteocytoplasm of the host cell. Vacuoles con- lytic activity.
taining ingested cytoplasm are pinched off
Lee (1962) directly observed the secretion
from the filled cytostome. Presumably, di- of an esterase in the gut of Ascaris. This
gestive enzymes are secreted into the vacu- is a merocrine secretion, in which droplets
oles since the heme pigment granules are coalesce and pass through the boundary of
observed to collect in them.
intestinal cells into the lumen.
There are marked differences in the
Very little is known about specific digessize of the cytostome among the species of tive enzymes in intestinal trematodes (see
Plasmodium investigated, the cytostomes Jennings, 1968, for a comprehensive review
of species living in birds having diameters of digestion in flatworms). Mention may be
about three times larger than those of spe- made of a protease extracted from schistocies from mammals. According to Aikawa, somes which has a very high specificity
et al. (1966), this, as well as differences in for hemoglobin and globin, as measured by
the chemical nature of the pigment formed, the release of tyrosine residues (Timms and
causes significant differences in the size of Bueding, 1959; Senft, 1965). Since a hemopigment residues found in the different chromogen has been demonstrated in the
species of Plasmodium. Rudzinska, et al. schistosome gut (Rogers, 1940 a), and hemo(1965) studied digestion in several species globin disappears from media in which
of malarial parasites, and found two pat- schistosomes have been incubated in vitro
terns: in Plasmodium falciparum, diges- (Halawani, et al., 1949), there is circumstantion takes place within the food vacuole tial evidence that these worms have a proand the pigmented residue accumulates teolytic digestive enzyme acting on hemothere; on the other hand, in P. vivax, vesi- globin. However, it is not known whether
cles are pinched off from the food vacuoles this enzyme is secreted into the lumen of
and digestion and the accumulation of the schistosome gut.
residues takes place in the small vesicles.
Erasmus and Ohman (1963) furnished
The literature on digestion in helminths direct evidence that the adhesive organ in
is scanty (see Lee, 1965, for a review). Sev- strigeid trematodes secretes hydrolytic eneral hydrolytic enzymes have been identi- zymes and is probably of significance in
fied in extracts of helminth gut tissues by extracorporeal digestion. Thorsell and
142
CLARK P. READ
Bjorkman (1965) showed that Fasciola car*
carry out extracorporeal digestion of gelatin. Gelatin was digested not only by direct
contact with the flukes, but also by saline
solution in which flukes had been incubated.
There is no evidence that tapeworms can
elaborate digestive enzymes, although intracellular hydrolytic enzymes most certainly
occur in these worms. I have attempted
to demonstrate hydrolytic digestive enzymes
in intact Hymenolepis diminuta but without success. The worms have been incubated for periods up to four hours with
glycogen, starch, serum albumin (both rat
and ox), or casein. No measurable amount
of these substrates entered the worm, nor
was there measurable hydrolysis. In addition, the worms have been incubated with
C14-labeled algal protein or serum albumin
which allowed a very sensitive determination of the liberation of acid-soluble or
ethanol-soluble material. No hydrolysis was
detected. It was reasoned that, since some
vertebrate digestive enzymes act primarily
on denatured rather than native protein,
denaturation of the protein might be necessary for the action of worm enzymes. Serum
albumin was denatured by several procedures, including treatment with urea followed by dialysis, heat, and treatment with
mild acid. These preparations were then
introduced to intact worms, but no hydrolysis could be detected.
that materials are absorbed through the
external body surface. Even the liver fluke,
Fasciola, can apparently absorb glucose
through the external surface (Mansour,
1959). Ligating the oral opening of this
worm had no effect on the rate at which
glucose was taken up from the medium.
Phifer (1960 a, b, c) studied the absorption
of glucose by the tapeworm, Hymenolepis
diminuta, using very short periods of incubation and isotopically-labeled sugar. He
found that glucose accumulated against a
concentration-difference, and that at least
94% of the absorbed sugar could be recovered in a chemically unaltered form. Absorption was non-linear with respect to the
concentration of the sugar, and the rate
of absorption was lowered by starving the
worms or previously treating them with
metabolic inhibitors. All evidence points
to a process of active transport. Certain
sugar analogues were found to be competitive inhibitors. These observations are
consistent with the interpretation that there
is specificity of the locus of absorption
(Read, 1961). Von Brand and his colleagues
(1964) reported that the tapeworm, Taenia
taeniaeformis, absorbs glucose by a specific
sodium-requiring membrane mechanism.
Recent studies on the rat tapeworm,
Hymenolepis, have shown that there is
specific absorption of short-chain fatty
acids. Several of these are mutually competitive in the absorptive system. There
is at least one specific absorptive system
ABSORPTION OF FOODS BY HELMINTHS
for higher fatty acids (C18 and higher) and
In nematodes, the absorption of organic the lower fatty acids (C1-C8) do not interfood almost certainly involves the gut. Car- act with this system (Arme and Read, in
bons-labeled amino acids fail to enter the preparation).
The absorption of amino acids by the
body of Ascaris when the body is ligated at
head and tail, but rapidly appear in the tapeworms, Calliobothrium and Hymenopseudocoelomic fluid in unligated worms lepis, has been studied systematically. In
(see also discussion by Roberts and Fair- both worms amino acids were transported
bairn, 1965). Fisher (1962) has shown that against a concentration-difference. A numthe gut of Ascaris transports glucose against ber of amino acids acted as inhibitors of
a concentration difference from the luminal the absorption of other amino acids, and
phase to the pseudocoelomic phase. This in all cases tested these inhibitions were
transport is blocked by the anthelmintic competitive. Further, a mixture of amino
acids acted as though it were a competitive
drug, dithiazanine.
Among the worms which lack a gut (tape- inhibitor of the uptake of any single comworms and acanthocephalans) it is assumed ponent of the mixture. However, careful
NUTRITION OF PARASITES
study showed that the absorption of amino
acids by Hymenolepis was considerably
more complicated than it first appeared.
Not all amino acids were taken in through
the same membrane locus; a given amino
acid may be absorbed through several different loci; and the affinities of these separate loci for a given amino acid differ.
When the worm was reared in two different
kinds of hosts, difference in the relative
absorption of different amino acids appeared. This was interpreted not as changes
in the nature of the loci but rather as
changes in the relative numbers of loci of
the different types. A mathematical expression for the interactions of amino acids in
a mixture was derived and tested. This
showed that the effects of various mixtures
of amino acids on the absorption of a single
component are predictable. Studies on
Hymenolepis citelli by Senturia (1964) and
on the Acanthocephala, Moniliformis and
Macracanthorhynchus, by Rothman and
Fisher (1964) have shown that the competitive effects of amino acids on a single
amino acid component are similarly predictable. Studies were also made of the
fluxes across the membrane of the amino
acid, methionine (Read, Simmons, and
Rathman, 1963). Urea is taken up by the
tapeworm, Calliobothrium, by what appears to be diffusion (Simmons, et ah, 1960).
Purines and pyrimidines are absorbed by
Hymenolepis through mediated processes
(Maclnnis, et ah, 1965). Absorption of nutrients by intestinal helminths has been reviewed recently (Read, 1966).
Although there has been increased attention given to the absorption of potential
food materials by parasites, there are still
major unanswered questions concerning
just what parasites eat when they are living
in hosts. A few selected examples will indicate some of the problems.
THE FOOD OF FASCIOLA
There remains broad disagreement as to
whether the trematode, Fasciola hepalica,
feeds primarily on blood or other tissues
during its life in the bile ducts of the vertebrate host, and it appears that, at times,
143
workers have attributed to Fasciola a delicacy of food selection which may not exist.
Dawes' recent studies of the early life stages
of Fasciola in the vertebrate showed that
the young worm, burrowing through the
liver, feeds mainly on liver parenchyma.
Even here, the breakdown of liver tissues
inevitably results in local hemorrhages, and
the fluke may ingest some blood (Dawes
and Hughes, 1964).
Various workers have reported seeing
erythrocytes in the gut of Fasciola taken
from the bile ducts and may have been
overimpressed by finding that "the only recognizable elements in the gut of the fluke
consisted of red and white blood corpuscles" (Stephenson, 1947). It should be
pointed out that the finding of intact erythrocytes in the gut may indicate that erythrocytes are not as efficiently broken down
into unrecognizable components of the
caecal contents as some other cells, e.g.,
duct-epithelium.
Other evidence that blood is a major
food of Fasciola is as follows: In an heroic
experiment, Railliet (1890) injected a mixture of 500 g of plaster of Paris, 45 g of
blue dye, and 1000 g of water (not 100 g as
quoted by Stephenson) into the carotid
artery of a sheep and later observed blue
material in the caeca of Fasciola from this
host. This cannot be considered a conclusive demonstration of the sanguinophilic
nature of Fasciola. The worm has been
observed to feed on clotted blood in vitro
(Weinland and von Brand, 1926; Stephenson, 1947). However, this may have little
similarity to life in a host, since Stephenson
showed that the flukes will also feed on
each other in vitro. Some workers, accepting the idea that Fasciola is a blood-sucking
worm, have used radioactive tracers to determine the amount of blood taken up
by the flukes. These experiments, utilizing
P32-labeled erythrocytes or I131-labeled protein (Jennings, et ah, 1956) and Cr61-labeled
erythrocytes (Pearson, 1963), showed that
Fasciola acquired small amounts of the
radioactivity after the compounds were intravenously administered to rabbits or
sheep. Unfortunately, none of these work-
144
CLARK P. READ
can absorb sugar through the tegument,
this must be considered as a probable feature of the nutrition of Fasciola in the host.
Normal bile contains amino acids and a
number of other compounds of low molecular weight and potential nutritional significance (see Campbell, 1960). If Fasciola is
capable of absorbing these through the
tegument and if these are present in the
bile of hosts harboring Fasciola, we must
alter our ideas of what and how Fasciola
eats. The clinical features of fascioliasis
even suggest that the nutritional interactions of Fasciola and its host are more complex than the simple devouring of blood
by the worm. On the basis of the evidence
available we may conclude that Fasciola
has a mixed mode of nutrition. The worms'
devour cells which are available to them in
the bile ducts (including mainly epithelial
cells and such blood cells as may become
Dawes presented direct evidence that available), digest them in the caeca, and
Fasciola feeds on the thickened epithelium take the soluble products into the body.
of the bile duct. His photographs of sec- In addition, they absorb organic comtioned feeding flukes seem convincing. pounds through the tegument. It is plain
Dawes showed that during the migration of that some careful physiological and chemiFasciola, but before it enters the bile duct, cal experimentation is needed to shed light
there is a marked hyperplasia of the bile on the relative importance of these comduct epithelium; the fluke begins to feed ponents in the nutrition of Fasciola.
on this tissue shortly after entering the
duct. This work has been reviewed by
THE FOOD OF HOOKWORMS
Dawes and Hughes (1964).
There is rather clear evidence that
Mansour (1959) showed by direct experimentation that Fasciola can absorb glucose hookworms are bloodsucking organisms.
through the external surface. Flukes, with This has been directly observed by various
the oral sucker ligated, metabolized glu- workers and blood cells have been demoncose from the suspending medium at the strated in the worm's gut. Several investisame rate as unligated animals. Pearson gators have pointed out that a considerable
(1963) erroneously stated that "Mansour portion of the red cells ingested pass
(1959) claimed that neither absorption nor through the gut of Ancylostoma caninum
excretion of nutrients takes place in the in an apparently intact state. This was
gut . . .". Study of Mansour's work does also observed when A. caninum was alnot indicate that he readied such a broad lowed to suck blood through a rubber
conclusion but was concerned with the ab- membrane in vitro (Roche and Martinez
sorption of a nutrient of low molecular Torres, 1960). Wells (1931) measured blood
weight in solution. Recent unpublished loss by sucking into a calibrated pipette
studies in our own laboratory have shown the blood ejected from the terminal gut
that Fasciola carries out the mediated trans- of A. caninum while feeding in an anestheport of the non-metabolized sugar, 3-0- tized dog, and estimated that a single worm
methylglucose, and that its transport is could remove from the host as much as
competitively inhibited by glucose.
0.8 ml of blood per clay. Nishi (1933) used
Since there is evidence that the worm similar methods. Roche and Martinez Torers determined whether the radioactivity
was still in the labeled component in the
blood at the end of the experiment. This
is a necessary precaution when using isotopes to study blood volume in healthy
animals and most certainly should have
been done in studies on animals with fascioliasis. A significant amount of radioactivity was found in the duct bile from infected sheep, which adds to the ambiguity.
It has been assumed by some workers
that the anemia frequently seen in fascioliasis is due to the simple fact that the
worms remove blood from the host. However, Sinclair (1962) showed that the anemia in sheep with fascioliasis was a norrnocytic normochromic anemia without reticulocytosis. This is not completely consistent
with the view that the anemia is due solely
to constant bleeding of the host.
NUTRITION OF PARASITES
145
res (1960) used tagged red cells and esti- that serum must be added for the utilizamated that this worm removed 15-63 mm3 tion of glucose from the external medium
per day; the results they obtained in vitro when Ancylostoma caninum is incubated
agree well with their observations on in in vitro. If serum is not present, the worms
vivo rates of blood sucking. Clark, et al. apparently do not feed (Warren and Gue(1961), using isotope-labeled red cells, esti- vara, 1962).
mated a mean loss of 0.07 ml/24 hr (max
THE FOOD OF TRICHURIS
0.12 ml) per worm in dogs.
51
Various workers have been interested in
Using the Cr -labeling technique, Roche,
et al. (1957) concluded that the human the food of the nematode, Trichuris. Blood
hookworm, Ancylostoma duodenale, pro- cells have been demonstrated in the gut of
duced a greater loss of blood than Necator, the worm (Guiart, 1908; Garin, 1913; Chitabout 0.2 ml/day/worm, compared with a wood and Chitwood, 1937; Burrows and
blood loss of about 0.01 to 0.06 ml/day/ Lillis, 1964), as well as iron ("blood") pigworm (average 0.03) in patients infected ments, and occult blood by presumptive
chemical reaction (Askanazy, 1896; Figueiwith Necator.
Roche and Perez-Gimenez (1959) pre- ra, 1919; Fernan-Nunez, 1927). Otto (1935)
sented evidence that, in humans infected reported that blood occurred in the feces of
with Necator, nearly 50% of the iron enter- 50 out of 54 humans with Trichuris infecing the gut through the feeding activity of tions. Most recently, Burrows and Lillis
hookworms was reabsorbed. Erythrocytes (1964) have referred to Trichuris as a
were doubly labeled with Cr61 and Fe59 blood-sucking worm.
As in the case of Fasciola, the finding of
and, after introducing the labeled erythrocytes into the circulation, the appearance of blood cells in the gut of Trichuris may
the isotopes in the feces was followed for lead to overenthusiastic acceptance of the
several 4-day periods. Since Cr51 was known idea that this worm feeds primarily on
to be poorly reabsorbed, the loss of iron blood. Some carefully executed studies of
was calculated from the Cr51 in the feces the pathology of the gut in trichuriasis
and compared with the loss determined must not be overlooked. Christofferson
directly from the quantity of the iron iso- (1914) described peculiar modifications of
tope in the feces of the same individuals. host cells about the anterior region of TriThe difference in the two quantities was churis in the host. Cellular transformaconsidered to represent iron reabsorbed. tions in the infected host mucosa were also
Further evidence for such reabsorption was observed by Sagrede (1925) and Lewinson
obtained by Aparcedo, et al. (1962). Evi- (1925). Hoeppli (1927, 1933) described the
dence that this reabsorption is not a spe- tunnels of the worms in the intestines of
cific feature of hookworm disease was ob- hiumans and baboons, noting alterations of
tained by feeding blood tagged with Cr51 epithelial cells; he believed that extracorand Fe69 to anemic patients without hook- poreal digestion was important in the nutriworm infections (Layrisse, et al., 1961). It tion of Trichuris. Smirnov (1936), after reshould be mentioned, however, that reab- viewing the literature and examining secsorption of iron may vary considerably with tioned Trichuris, decided that evidence for
the diet. Iron chelated with certain sugars the role of the worm as a blood feeder was
is absorbed in abnormally high amounts inconclusive. Some workers, e.g., Jung and
which may result in serious disease. Cyto- Jellife (1952), have been quite cautious in
siderosis of the African Bantu, a nutritional discussing the dietary habits of whipworms
disease related to a diet high in carbohyas these may relate to disease.
drate, low in protein, and high in iron,
On the basis of the data available, the
may be an example (Stitt, et al., 1962).
present reviewer cannot accept the verdict
In connection with the blood-feeding of Burrows and Lillis (1964) that Trichuris
habit of the hookworms, it is of interest has been shown to be a "blood-sucking"
146
CLARK P. READ
parasite nor that its blood-eating propensities explain the anemia sometimes associated with trichuriasis. It seems more likely
that Trichuris feeds on living tissues and
that blood is one of the tissues with which
it comes in contact. It obviously does not
reject blood, but its pattern of feeding appears to be quite different from that seen
in the hookworms, which are clearly bloodfeeders.
FEEDING IN VARIOUS NEMATODES
Evidence for the tissue-feeding of a number of nematode parasites of the digestive
tract has been reviewed by Ackert and
Whitlock (1940) and by Hobson (1948).
Since these reviews, some additional work
has shed further light on these problems.
Rogers and Lazarus (1949) found that Nippostrongylus rapidly acquired inorganic
P32-orthophosphate injected intramuscularly into the host, whereas Ascaridia galli
failed to acquire significant amounts of
phosphate given to the host intravenously.
This is attributable to the very different
feeding behavior of the two worms, Nippostrongylus being a feeder on host tissues
and Ascaridia feeding on the gut contents
of the host. Esserman and Sambell (1951)
carried out similar experiments with Haemonchus, Trichostrongylus, and Oesophagostomum in the sheep. The rates at which
these three nematodes acquired intravenously administered radiophosphate clearly
indicated that they feed on the tissues of
the host. However, when labeled phosphate was given to the host by intra-abomasal injection, Haemonchus and Trichostrongylus acquired the label more rapidly
than the tissues of the abomasum or small
intestine. This may indicate that these
worms also feed on the gut content, but
will bear further investigation.
Clark, et al. (1962) measured the loss of
blood from sheep infected with Haemonchus by administering red cells tagged with
Cr51 or Fe59, and estimated an average
blood-feeding rate of 0.049 ml/day/worm
(range 0.005-0.173). This may be compared
with an estimate of many years ago by
Martin and Clunies Ross (1934), who deter-
mined the phosphorus content of eggs and
egg-output of Haemonchus. From these
data, they calculated the minimal amount
of blood which would supply the phosphorus. Doubling their minimum estimate
(to account for males and for waste), their
data indicate a blood loss of 0.03 ml/day/
worm.
Rogers (1940) made an ingenious approach by determining the zinc in the
tissues of Strongylus vulgaris and S. edentatus and in the gut tissues of horses. To
account for the amount of zinc in the
worms, Rogers estimated that 100 S. edentatus would have to eat from 400 to 2100 g
of mucosa per year. In view of later appreciation that the sloughing of the intestinal
mucosa occurs at a high rate, this figure
may not be a real representation of the
amount of living tissue devoured by Strongylus. Rogers concluded that the amount
of blood taken from the host was small;
after determining the amount of hematin
in the gut of S. edentatus, he calculated
that 0.0009-0.0002 ml of host blood would
yield an amount of pigment equivalent to
that found. Of course, as he recognized,
such calculations are provisional since no
time scale is available to judge the rate of
acquisition and loss of hemoglobin. It may
be emphasized that a visible blood pigment
in the gut of a parasite may represent a
very tiny amount of blood and may be an
unreliable criterion as to whether blood
constitutes a significant portion of the diet.
A GENERAL REMARK
The above discussion may indicate some
of the present difficulties in assessing the
nutrition of parasites. In most cases, we do
not know what or how parasites eat in the
host. Some of the more sophisticated tools
of biochemistry and physiology have not
been applied to these problems. This paper
has not touched upon the elegant nutritional studies of parasitic protozoans cultured in vitro nor on the burgeoning investigations of helminth cultivation. Further, the growing body of knowledge on
the metabolism of parasites has not been
examined. The latter area of investigation
NUTRITION OF PARASITES
is very significant from the standpoint of
the energetic relationships in parasite ecology. Many parasites carry out incomplete
oxidations, and energy balance sheets require careful evaluation of the products
of mixed fermentations (see von Brand,
1966). Virtually no data are available on
the energetic relationships of parasites to
hosts at the population level, although beginnings have been made (Whitlock, 1966).
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