Conditions of the establishment of parasites in a host

WfADOMOśCI 1-'ARAZYTOLOG[CZNE T. vn, NR 1/5i6, 1961

CONDITIONS OF THE ESTABLISHMENT OF PARASITES IN A HOST

BY

WITOLD STEFAŃSKI

Department of Parasitology Polish Acaderny of Sciences.

Warsaw

It would appear that the potential parasite wanting to seltle on the external covering of i-ts host should not meet with any great difficulties if the hosts morphological characteristic do not present any great obstacle.

The ectoparasite however, does not change its habitat which surrounds it up to the present. Should the

host for whatever reason change its surroundings, the parasite not adapted to them dies. We know of many examples of this kind of phenomena. Travelling from the sea to the river, salmon in a short time lose their ectoparasitic crusta- ceaus. Migratory bil,ds on their flights from the south to the north change partly their acarologic fauna, and so on. This all proves that ectoparasites have not been able to isolate themselves from iheir external surroundings but are susceptibk'!

ito

a high degree to their influence.

Establishment of the ectopara- site even in a host living in the same surroundings is not by any means a simple matter. Saprozoic

nematodes (Rhabditis s trongyloi-

fot. J. Waluga Paper read at the 7th Congress of the Polish Parasitological Society, Olsztyn- Kortowo, 11-14 VI 1961.

cles)

settle sometimEs in the form of larva in the skin of dogs and other mamrn.als, causing a state of inflamation with eruptions and mange. In any case these larvae nevEr matura in thei,r hosts but do so in the litte~·

of the dogs kennel. The culture of these nematodes does not present any diffic:ulty under laboratory conditions. For instance they are easily cul- tured on a dish covered with mould and bits of raw meat. However all cur atternpts to infect dogs with them by methods which included, amongst others, putting the culture larvae on the carificated skin of the dog. were fruitless. (Slusarski, Markiewicz,

Stefański,

1956).

There exists therefore unknown factors conditioning the invasion of these Jarvae, factors which were absent in our experiment.

Ofte;~1 this factors is the state of nourishment of the organism of the host, which in a way unknown to us, eHects the settling even of actual parasites. Infection with lice and ma1lophaga in well-fed horses never reaches the degree it does as in horses fed with chaff domin:1ted feed. So if the potential parasite needs for its settlement on the cxternal covering of its host not only the presence of these same conditions in which it has existed, but moreover conditions dependant on the host itself. :t is obvious that the demands of the endoparasite are still more complex.

To get inside the host the parasite has two main ways to choose from: the skin and the mouth.

Sometimes the organism itself creates conditions enabling the penetra- tion of the parasite. Such oonditions, for instance, are created by plants for the spores of fungi. ,,In a drop of water or dcw on a leaf salts dis- solve, chiefly carbonates of potassium and calcium carried from the inside of the plant by the transpiration current of the water. In this way the reagent of the water changes into a weak alkaline favourable to promoting the germination of the fungi. Leaves diffuse phosphorous compounds and growth substances which are no:1rishment for the ger- minating spore of fungi. Even gas substances secreted by leaves induce the growth of the fungi. These substances act chemotropically on hypha.

In coming into contact with the epithelium of a plant the hypha sweUs, and in two to four hours develops appressiorium whose wall becomes m:ucosal and adheres to the plant epithelium, so that even heavy rairi will not wash it away.

(,,Wszechświat·',

1960, B. Starmachowa).

ft is difficult to trace a similar example among zoogenous parasites.

It is only possible to speak here of the tactility causing the cercaria tCJ

be attracted more intensively through the mucous of the snail hosts,

,Nhich are intermediates hosts of the given trematodes, than by the

m:,1cous of foreign snails.

PROBLEM PASOŻYT-ŻYWICIEL

The Lrsi obstacle which the potentia! parasite meets in penetrating the host is the skin . Whereas however adult parasitical worms have sometimes strongly arnied anterior ends so thei, r larvae in

generał,

are devoid of such adaptations by which they could penetrate through the skin only on a mechanical way. On the other hand the larvae of para- sitic worms are equipped with a more effective means in the form of glands which secrete hystolyt:c enzymes. In miracidium we know them as penetrating glands, whereas in nematodes as pharyngeal glands.

Unfor-iunately the m echanism of the action of these glands is far from known. Analogously to bacteria a search was made in the secretion of the glands for the diffusion factor hyaluronidase, which has the ctbility to depolymerize glucoproteins. The presence of this enzyrne in protozoa is uncertain , it has been found however, in the larva of many parasites.

(Schistosoma mansoni, Strongyloides ratti, Ancylostoma caninum, Stron- qylus equinu.s, S. edentatus and others). The metods used in the detection of these enzymes are of such little precision that further research is ne- cessary before giving a

finał

opinion as to the significance of hyaluroni- dase as a fact01· faeilitating penetraiion of the larva of the parasite worms through the skin, and its traveling through the tissues of the host. Not quite certain too is the role and mechanism of the action of the colla- genase and other enzymes depolymerizing glucaproteins, sealing, as is known, the intercellular spaces.

At any rate it should be expected that on the activity of these en- zymes depends the decisive moment of penetration to the organism of the host . Independant from the activity of the penetrating glands, larvae whi· ch penetrate to the improper host die, often even before they have been able to perforate the skin of the host.

In this respect the penetration of the parasite by means of the

orał

cavity appears 'easier. One way or another, with food or water, the para- site usually encysted gets into the alimentary canal. At this moment however, the potentia! parasite finds itself in surroundings completely different to those of its former habitat.

The

degree of these differences depends on the position of the host in the system of the zoogenous world.

The body temperature of cold-blooded animals does not differ greatly from that of its surroundings, as likewise osmotic pressure of inverte- brates living in sea water.

Saprozoic nematodes penetrating together with earth into the alimen-

tary canal of the earthworm remain within the same temperature of the

surroundings, whereas the potentia! parasite of constant body-temperature

vertebrates must be adapted to a change into a higher temperature. As

matter of fact a number of independant living nematodes stand high

temperatures, some of them can live in springs of temperatures over

40''.

We know also of several saprophytic nematodes which havi-ng got into the alimentary canal of mammals live in them several days, and die presumably owing to the unfavourable chemistry of the surroundings . There exist also parasites whose hosts are warm-blooded and cold- -blooded vertebrates. To such belong a range of trematodes living on fish and mammals. A known example of this kind is Rossicotrema donicum , a parasite of preying fish, dog, cat and fox (Malczewski in litt.) found in the alimentary canal of a fox several such trematodes the adaptability of which to changes of temperature lies in wide ranges.

There are however parasites for which temperature is a decisive factor. To s:1ch belong, for instance, Trichinella spiralis, a mammal para- site, which can reach sexual maturity in the intestine of reptiles and amphibea only after the surrounding temperature has been brought to over 25°. (Goujou, 1866,

Trawiński,

1937, Kocylowski, 1945, Matoff, 1953). Though experimentally it is possible to infect almost all mammals with it, we know here of characteristic exceptions. Namely, one doet;

not succed in infecting these parasites on bats, body temperatures of which during hibernation reach 5°, and even in summer time does not often exceed 20° (unpublished report Chute and Covalt, 1960). Blachard and Blaton (1907) stated that marmots fed on hairworm during hiberna- tion were not liable to infection, and Chute (1961) confirmed the same experimentally with hamsters kepf from 24-72 hours in a temperature of 5°.

A high body temperature does not suit hairworms also, proof of which is the difficulty of infecting birds with these parasites.

The changes in osmotic pressure do not in

generał

according to Rogers in (1955) present an obstacle in the establishment of a potential parasite.

This does not apply in any case to sea animals in which osmotic pres- sure is dose to that of the surroundings.

Especially hardy in this respect are nematodes. Osche (1952) quotes many examples of saprozoic nematodes which adapted themselves to changed osrnotic pressure together with the advance of decomposition of medium on which they were cultured. Also many parasitic nematodes stand a great change in osmotic pressure whereas others are particularly sensitive to such a change, as • each of us could prove by transferring some of the parasitic nematodes to water. Research however, on the permea- bility of worm cuticle showed that the cuticle is permeable in water.

Chloride seems to pass easily through the cuticle but its concentration in

the per.:visceral fluid is less than its concentration in its surrounding

habitat. Moreover all research workers agree that the cuticle is not

permeable to glucose. (Robson, A. D., Stevenson , W. and Beadle, L. C.,

1952).

PROBLEM PASOŻYT-ŻYWICIEL

Changes in the saltiness of water influence the change of the parasite fauna, most likely in an indirect way. We will return to this question.

It would appear that a serious physiological adaptation on the part of the potentia} parasite is demanded by the change in the oxygen pres- sure in the organism of the host, in comparison with the external sur- roundings. Indeed the intestines of the majority of vertebrates contain little oxygen. Particularly low oxygen press:ire is marked in the first stornach of ruminants and in the large intestine and bile duet of Perisso- dactyla animals. In fact saprozoic nematodes in this respect are un- demanding, which is why they can be considered as potential parasites, however a large number of potential parasites must before penetrating into the hosts intestines change their aerobic for anaerobie.

With the exception of parasites living in the bronchi or in the blood, all others do not oxi<lise fully the substance from which they draw their energy. In this process it is

possibłe

to differentiate two types of fermentation: aerobic and anaerobie. (V. Brand, 1959).

As V. Brand (1959) rightly observed the place of establishment of the parasite is not the only deciding factor. In these same surroundings lack of oxygen pa,rasites can live next to each other, some of predominating anae robie respiration and others of predominating aerobic respiration.

The deciding factor h€I'e is the size of the parasites. The greater the para- site so the less favourable for it is the surface mass ratio in the process

o[

diffusing oxygen. For instance, ascariasis settling in surroundings scanty in oxygen live practically like anaerobi, whereas small nematodes can cover their need of oxygen in a marked degree, in the same sur- roundings. This does not apply to parasites of

speciał

adaptations. Blood sucking worrns such as Ancylostomatidae or Haemonchus contortus cover their need of oxygen from the blood of the host. On the other hand Ni.ppostrongylus muris has in the fluid of its

pseudo-abdominał

cavity haemoglobin in sufficient quantity for its need of oxygen.

Other parasites again as for instance Trichomonas tenax, flagellata preying in the

orał

cavity of man, and so in surroundings rich in oxygen,

devełope

best in an artificial culture in anaerobie medium. V. Brand (1959) from whom is derived the greater part of information on the respiration

ol'

the parasite, assumes that the rich bac~eriological flora of the

orał

cavity consumes all the oxygen, the flagellata there must adapt them- sE lves to anaerobie life.

Why the secreted

finał

products even of parasites consuming a large quanUy of oxygen remain only partly oxygenised, is not elear. This is the case with blood parasites: Plasmodium, Trypanosoma, Schi stosoma and others.

·\i\liadomo:~ci Parazytologiczne z. 4:, 5, 6 2

Equally aerobes as with anaerobes featurise uneconomical exploitation of potential energy, contained still in a large quantity in the

finał

pro- ducts of oxygenation secrEted by endoparasites. Parasites may allow themselves such uneconomical management, drawing from the hosts organism a large quantity of original substance, which they need; namely carbohydrates. There exist indications that for saprozoic parasites the rarity of oxygen in a potential host should not create a great obstacle.

It appears that one of the most effective barriers in preventing the settlement of parasites in the alimentary canal of the host is a high concentration of hydrogen ions, since, as was demonstrated by Davey (1938) and Osche (1952) a high concentration of hydrogen ions is fatal for many free living and parasitic nematodes. This also explains the rare appearance of parasites in the stornach of animals which have a high concentration of hydrogen ions. If the parasite cannot settle in the sto- rnach it can occupy a further segment of the alimentary canal, of pH starting from 6,5.

The fatal effect of the gastric juices will be limited if the parasite passes quickly through the stornach. In a:ddition the chemical structure of the parasital worms cuticle protects it to a certain degree from the effects of excessive addity of the surroundings. This was proved by examples of provisional settlement of nematodes in the alimentary canal of vertebrates. Known in this respect is the example of Rhabditis pellio, saprozoic nematode, found many a time as a temporary parasite in the alimentaJry canal of man and frog. In spealking of gastric juices it needs to be stated, that the potential parrasite is threa:tened with greather danger from proteolytic enzymes acting in the alimentary canal.

This suggests that parasites secrete antiproteolytic enzymes. In fact Sang (1938) and Collier (1941) showed that some parasitic nematodes produce a substance protecting them from the effects of pepsin and trypsin. This experiment however, was not proved by further studies, and that is why it seems that the reason for the parasites resistance should be looked for, above all, in the actual cuticle structure of the parasite.

In this respect the best known is the cuticle of worms which is largely composed of mucoprdteins which are not subject to the action of proto- lytic zoogenuous enzymes but subject relatively easily to the action of vegetable enzymes, for example papain. Moreover according to some researchers (Trim 1944) the surface cuticle of worms is covered with a layer of lipids.

The surface layers of the cutic1e are in fact, very resistant to the

activity of the digestive juices, whereas the lower layers are digested by

pepsin and trypsin.

PROBLEM PASOŻYT-ŻYWICIEL tHi7

All this of eourse, does not explain the fact why the mor1ified para- sites are digested. This would point to an unknown mechanism operating while they are alive.

Among potential parasites of free living worms, nematodes it seems are best suited to the conditions for resisting the action of the gastric juices. Proof of this is the already cited fact of the ability of same sapro- zoic nematodes to stay alive for a numb er of days in the alimentary canal of mamtmals.

In the first phase of invasion into the alirnentary canal the potentia!

pa rasit€ does not meet, in most cases, with any great difficurty in nutrition. Saprozoic nematodes swallowed together with earth by earth- worms find in its alimentary canal very similar conditions of existence and nutrition - the same detritus and soil bacteria. Specialisation and conseque ntly special demands may follow later.

The long years of research by Read and his co-workers (1949-1959) led him to the ascertion, that tapeworm extracts from the intestine of its host carbohydrates which for them are a source of growth and multi- plication, besides which the majority of tapeworms are capable of con- suming only monosaccharides. Anoplocephalidae are an exception in this respect since they are able to decompose bisaccharides. The quality of the carbohydrates has a d eciding influence on the growth and abihty of the tapeworm to multiply.

How closely the parasite metabolism may be related to that of the m etabolism of its host is evident, amongst others, in the research of Goodchild and Wells (1957) on aminoacids which are inc!uded in the composition of the tapeworm Hymenolepis diminuta and its host, the rat. Chromatographic analysis shows that in the tissues of the larvae and adult tapew orrns the same 20 aminoacids as in rats, also in every case they did not exceed 16. The researchers proved precise parrallelism both

ąualitative

and quantitive of alpha-aminoacids which are in the para- site and in the tissues of the host with which the parasite comes into co_ntact. In starved rats the quantity of this acid decreased, but at the same tim~ in the same degree their number decreased in the parasites although qualitatively they remained the same.

It is al so known that parasites need a large amount of vitamins.

A classic example is Diphyllobothrium latum, in which per 1 gr. of dry substance of this parasite fall is fourd 2,3 micrograme B

₁₂

(Nyberg, 1952).

The parasites need of other vitamins is no less. For instance liver-fluke contains 0,25-0,50/o vitamin C in relation to body weight, Ascaris lumbri- coides 0,35-0,70°/ o, Taenia saginata 0,07-0,150/o {Holtz, 1961).

All thi s applies to a true parasite, whereas a „candidate" parasite may

have in this respect lesser demands.

Amongst many other fa ctors the influence of hormones should be ment:oned in particular sex hormones. Cocks for instance are more inten- sive1y infected with Ascaridia galli worm, than hens, similarly the p er- centage of infected cocks is greater than that of hens (Dar ski in litt.). The tapewo rm H ymenolepis nana does not matur e in cas trated rats. Testo- sterone inj ections in these tapeworms induce the development of gonads.

Generally known are the experiments on the development of trema- todes preying in the urine bladder of the frog. Polystoma integerimum, whose direction of development dep ends on the action of the thyroid and pituitary hormones secreted by the frog.

If

miracidium get into the gill at a time of intense secretion of these glands, then the development to sexual maturity of the paras ite occurs in a few months. Larvae, however, settling in the gills of pond tadpoles d evelope slowly, taking 3-4 years . Engelbrecht (1961) observed that in Triaenophorus nodulosus the breaking away of strobila with fertili sed ova takes place during the fis h spawning period, so it is connected with the gonadotrophic action of hor- mone ova .

Szidat (1959) notes that in fish wandering from the sea to fresh water, there is an increase in the pit'..litary action which in turn stimu- lates the thyroid fun ction. According to Szidat the gonadotrophic acti on of the pituitary glands in fish leads to the production of neotenie forms.

The „candidates" as parasites whkh penetrate into the alimentary canal must count on the bacterial florn contained in it. In the alimen- tary canal of sucklings, parasites do not find favourable conditions un- doubtedly because of the predominance, during this period of on un- suitable bacterial fl ora (Lactobacillus s pp.). Hence the infection w orms feeding on the milk of suckling pigs meet with great difficulty . Similarly worms in 5 day old chickens stay in the larva stage (Darski in litt.).

In guinea pigs Nippostrongylus muris d o not r each development in normal conditions, on the other hand these parasites d evelope in sterile guinea pigs. These last -proved adva ntageous for the deve lopment of Nematospir oides dubius, nematodes infecting with difficulty in fertile guinea pigs. Hymenol epis nana developes equally well in both categories of guinea pigs. It may be that tapeworms are less depe ndant on the flori bacterial composition (Newton, W. L., Weinstein, P. P. , and J ones, M. F., 1959) owing to their osmotic type of nourishment.

Equall y distinctly is shown the dependance of protoz oa on bacterial

flori and culture in vitro. In this respect in most of the would b1:o

r esea rches on the culture of histol ytic

Ęntamoeba

it was cofirmed that

the last mentioned develops well in th e prescnce of Bacillus cereus,

Proteus sp. , Ps eud om onas sp. or Alkaligen2s j'ecalis. We have data to

as~;ume that similar relation s exist likew ise in the organism of the host.

PROBLEM PASOŻYT-ŻYWICIEL

The b:ggest obstacle to the settling of parasite is congenital resistance of the host.

Hcre are

several

known examples of either congenital or natural resistance.

Cysticereus bovis or C. cellulosae can develop only with adult tape- worm in the intestine of man, b

ecause all animals show a congenital resi-

stance to these t

apeworms. The swallowed larva

of dog nematodes, To x ocara canis, wandering in various tissues of man does not develop beyond the third

stage.

H

orse

tapeworm dev

elopes only

in hoofed an:mals.

Hen Coccidae arc

specific only

in these birds and rabbit Coccidae contrary to farmer belief d

o

not infect hares.

This kind

of example of parasite speci:fity

may be quated in g

reat

numbers, as well as examples of their

small specifity.

The proble m of specification is not the

subject

of m

y

report,

or

does it link up?

I will try to darify this in further consideration.

The bladderworms of the tapeworm

subjected

primarily t

o t

h

e

a,·tion

of

the gastric juices, where the pH :s an average of 3,5 in m

amm1ls,

to a certain degree prepares the cyst covering it for the action

nf

the bile, pancreatic juiccs and

of

the secretion of the walls of the small intes tines. The bile salts appeared to be at least in vitr

o,

the decisive for the activaiion of the tapeworm larva of rod

ents and are an aid to

dige~tion of the s urrounding cyst. From the degr ee of concentration of the

salts,

dep

ends

the speed in freein

g

the larva from the cyst. However, the larva of Taenia taeniae formis, d

eveloping in

cats is not activated by bile

salts,

but on th

e

contrary, a hig h concentration of the salts bring about the r

everse effect the

larva r

epeatedly

retreating into the cyst

s.

Rotz- man (1959), to whom we acknowledge the inclusion here of this data, read also de Wa

eles

(1934) r

esearch

concerning the larva Taenia pisifor- mis. According to the latter this larva is inactivated and killed by gly- cocholates contrary to Taurocholate which activates it. So according to Waele T. pisiformis can not d

evelop

in

an

animal, which has gly cocholi-- nate s in its bile

,

it can however infect the dog, Which is in fact devoid of th

ese s

alts.

Read and his co-workers give other

examples

influencing th

e esta-

blishment of p

arasites.

In

carti

lagenous Esh concentration

of

th

e

u

rea

in the spiral valves is equal to concentration of ur

ea

in blood (ab

out 2000

mg

⁰/o).

S

o

the species, which exist in these fish are not insensitive

to urea in vitro, whereas tapeworms and Trematodes of other vertebrate

g-roups die quickly :n such concentration

.

It may be that in this way it

is poss ible to explain the lack of true trematodes (Trematoda-Digenea) in

the instestine of cartilageno:1s fish.

Cochoristica symmetrica cannot settle in animals with a strong concen- tration of hydrogen ions in thei· r gastric juices. Similarly O. symmetrica and T . taeniaeformis die in the alimentary canal of a host which has a high concentration of bile salts.

Finally I would like to draw attention to one more factor. According to Rogern (1957) the casting off of skin by larva of nematodes of the third stage is conditioned on one side by its endocrinological mechanism, which morphological substratum is contained in the base of the larvas pharynx, and on the other side by the release of this mechanism through conditions prevailing in the alimentary canal of the given host. These conditions of course, differ in different species of animals. Similarly they influence in various ways the moulting of the individual species of ne- matodes.

According to Rogers and Sommerville (1957) the most effective action is by sti~ulants at a low oxidation reduction potential, the optimal con- centrati:on of hydrogen ions for Haemonchus contortus being 6,5 - 7,5, for Trichostrongylus axei 5-6, and for Trichostrongylus colubriformis pH was b€low 4.

It follows that the conditions for parasite settlement are rather spe- cialized.

I have enumerated many of them but obvio:isly not all. Do not those already known to us show us sufficiently the way of research which should

be

foHowed for the understanding of the essence of specifity? In this relation namely the idea of specifity looses its mystery, becomes a complex of conditions for which the candidate parasite, burdened with the whole of the philogenetic past, was able to adapt itself.

Author's address : Warszawa, Pasteura 3