Basic biological aspects of Tritrichomonas foetus of re-levance to the treatment of bovines suffering of tricho-moniasis

doi:10.4236/ojas.2011.13015

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Vol.1, No.3, 112-120 (201 1)

doi:10.4236/ojas.2011.13015

Open Journal of Animal Sciences

Basic biological aspects of Tritrichomonas foetus of

relevance to the treatment of bovines suffering of

trichomoniasis

Newton Soares da Silva1,3*, Susane Moreira Machado1, Fernando Costa e Silva Filho2,

Cristina Pacheco-Soares1,3

1Laboratório de Bi o l o g ia Celular e Tecidual, Instituto de Pesquisa e Desenvolvimento, Universidade do Vale do Paraíba, São José dos

Campos, São Paulo, Brasil; *Corresp onding Author: nsoares@univap.br

2Instituto de Biofísica Carlos Chagas Filho, Universidade Federal do Rio de Janeiro, CCS - Bloco G, Rio de Janeiro, Brasil;

3Laboratório de Dinâmica dos Compartimentos Celulares, Instituto de Pesquisa e Desenvolvimento, Universidade do Vale do Paraíba,

São José dos Campos, São Paulo, Brazil.

Received 8 June 2011; revised 14 July 2011; accepted 25 July 2011.

ABSTRACT

Tritrichomonas foetus is a flagellate protozoan

and the etiological agent of bovine genital

trichomoniasis [1], which is an infectious vene-

real disease. This parasite is usually found as-

sociated with the mucosal surface of the uro-

genit al tract in females or the male preputial and

penile membranes. In females, the clinical

manifestations may include abortion, with repe-

tition of estrus at irregular intervals, vaginitis,

cervicitis, endometritis, and pyometra. Parasi-

tized males may have a discharge with small

nodules in the preputial membrane. After that,

the bulls have no clinical symptoms, and are

thus an asymptomatic carrier that may spread

the infection. Considering that a bull could

cover up to twenty females [2], bovine genital

trichomoniasis is a serious medical and veteri-

nary problem, with economical repercussion for

beef and milk production. As T. foetus is an

amitochondrial and aerotolerant organism, en-

ergy production under low O2 tension in the

protozoan is done via hydrogenosome, which,

as the name suggests, is the organelle where H2

is generated [3,4,5]. The molecular machinery of

mitochondrial cell death is, therefore, absent in

this parasite and the mechanism that activates

of cell death program is not clear. This review

seeks to understand the characteristics of the

protozoan parasite T. fo etu s in order to propose

new therapies for animals suffering from this

infectious and contagious agent.

Keywords: Reproduction; Protozoan Parasite;

Tritrichom on as foetus

1. INTRODUCTION

Tritrich omonas foetus, Reidmüller 1928 emend. Kirby

1947, is a flagellate protozoan that lives in oxygen-poor

environments, such as the bovine reproductive tract [6].

This protozoan is found in the urogenital mucosal sur-

face of males and females, causing bovine trichomoni-

asis, an infectious disease with venereal transmission,

which causes infertility and abortion in cattle increasing

herd management expenses [7,8]. Trichomoniasis is pro-

bably the third most common cause of abortion in cattle

(after Brucellosis and Leptospirosis) [7]. This disease

has worldwide distribution [8], and is endemic especially

in regions with poor sanitary control or where the use of

natural mating for reproduction is extensive [9].

T. fo etus is physically associated with the epithelium

lining the urogenital cavities of cattle [10,11]. In bulls,

the parasite can be detected in the preputial cavity and

urethra, as well as in the urogenital canal [11,12]. In cows,

it inhabits the vagina and uterus, in which the parasite

can inhibit the attachment of embryo or rupture its mem-

branes after attachment, leading to abortion [10,11,13].

Despite the economic importance attributed to bovine

trichomoniasis, there have been few studies about the

molecular basis and chemotherapy of disease, thus it has

failed to be eradicated, or even rigorously control. There-

fore, the disease continues to cause problems for Brazil-

ian and other cattle [9].

1.1. Morphofunctional Characteri stics of T.

foetus

1.1.1. General Aspect s

Tritrichomonas foetus Kirby, 1947 belongs to the

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113

family Trichomonadidae, order Trichomonadida Kirby,

1947, class Zoomastigophora, subphylum Mastigophora,

Sarcomastigophora, phylum Sarcomastigophora, sub-

kingdom Protozoa, and kingdom Protista [14].

Tritrichomonas foetus, Tritrichomonas suis (parasite

of the digestive tract and nasal cavity of pigs) and Tri-

trichomonas mobilensis (parasite of the digestive tract,

especially the cecum and colon in monkeys) were found

after analyzing gene sequences to probably be strains or

variants of the same species [10,15,16].

T. foetus is a monogenic and monoxenic protozoan,

which is aerotolerant and actively mobile. It reproduces

by binary division and in vitro, axenic culture. The pre-

dominant form of trophozoite is approximately 10 to 25

m long by 3 to 15 m wide. However, both in vitro and

in vivo or in situ, the parasite has morphology that varies

from pyriform to fusiform or even round. It characteris-

tically has three free anterior flagella and a quarter, an-

teroposterior projection , which is occasionally associated

with the plasma membrane, resulting in a stru cture known

as the undulating membrane. This, in turn, is associated

internally to coast. The axostil, an organelle formed by

parallel bundles of microtubu les and anteroposter ior pro-

jection, projects itself externally in the form of terminal

spine [10]. Axostil and pelta, parts of the cytoskeleton,

together with the coast and flagella are jointly involved

in motor functions and cell division of the parasite

[10,17]. The other intracellular structures of T. fo etu s are

the parabasal body (Golgi apparatus and parabasal fila-

ments), the oval nucleus (located in the anterior third of

the body), and hydrogenosomes (functional replace-

ments of the mitochondria). The cytoplasm also has a

wide variety of vacuoles and vesicles related to the en-

cytosis, digestion, and transport processes, as well as

rough endoplasmic reticulum, free ribosomes, and gly-

cogen granules [17]. The nutrient uptake in T. fo etu s can

occur by diffusion as well as by phagocytosis and pino-

cytosis, resulting in the formation of cytoplasmic vacu-

oles of various sizes [17]. In vitro in ax enic and acellular

culture, T. foetus uses glucose as the principal energy

precursor. Even in vivo, where the supply of sugar is

almost nonexistent in the initial infection sites, the para-

site makes use of precursors in the synthesis of poly-

amines, such as arginine, to produce the energy needed

to survive in the host [18,19].

1.1.2. Particular Point, the Cell Surface

The first region the parasite contacts the host envi-

nment is on its surface. Therefore, this organelle plays a

crucial role in the etiology of bovine trichomoniasis.

From a structural viewpoint, it is defined as a cell surce

organelle formed by the plasma membrane and exterlly

associated carbohydrates. Studies that focus on tricomo-

ds-host cell interaction have shown that the cytotoxicity

of T. foetus can be triggered from physical contact be-

een the surfaces of the parasite and host cell, resulting in

the secretion of extracellular proteases and glycosidases

[20-22]. Such enzymes, in turn, seem not only to induce

cytolysis or the disruption of epithelial junctions, but to

remodel the surrounding extracellular matrix (ECM)

[23]. Thus, mechanochemical monitoring of the ex-

tracellular environment by the surface of the parasite

would co-opt the host’s ability to constitute a niche and

thus to survive in various anatomical sites.

From a molecular standpoint, the surface of tricho-

monads is a mosaic composed of cytopathic and cyto-

toxic enzymes, lectins, adhesins, receptors, and co-recep-

tors for ECM components, as well as other molecules

involved in adhesion and toxicity of host cell trichomo-

nads [22-28].

Adhesins and cysteine proteases on the surface of T.

foetus are directly involved in parasite interaction with

host cells. A 100-kDa protein characterized as adhesin

has been observed throughout the cell surface of T. fo e -

tus, mainly in the region of the flagella [29]. Cysteine

proteases, soluble or associated with the surface of T.

foetus, are have more cytotoxicity than the cytoad hesion.

These proteins induce apoptosis in bovine vaginal

epithelial cells (BVECs), which suggests that this mecha-

nism of cell death may be involved in the pathogenesis

of these protozoa [30].

A surface molecule of 118 kDa enables T. foetus to

recognize laminin-1, which is component of ECM of the

base membrane of epithelia. The immobilized or solub le

fibronectin is also recognized by T. foetus via surface

glycoconjugates rich in mannose [22,27]. Thus in prin-

ciple, the T. foetus host could not only lyse epithelial

cells or break its junction al complexe s, it may recogn ize

the ECM host and destroy or reshape it in a timely basis

thereby winning in other situations.

Lectins on the surface of T. foetus are related to the

recognition of portions of oligosaccharides of glycocon-

jugates on the surface of epithelial cells [22]. Moreover,

lectins, together with fluorochromes conjugated or com-

plexed with colloidal gold, react with residues of D-

GlcNAc, N-acetyl-D-galactosamine, sialic acid, D-man-

nose, D-glucose, L-fucose, and D-galactose on the sur-

face the protozoan [20,24]. Together, these data seem to

indicate that, at least in vitro, lectins on the surface oli-

gosaccharides of T. f o e t u s could be involved in recogni-

tion and parasite adhesion to host cells.

1.2. Pathogenesis of T. foetus

1.2.1. Female Genit al Tract

After infection, virtually all of the genital tract be-

N. S. Da Silva et al. / Open Journal of Animal Sciences 1 (2011) 112-120

114

comes contaminated within twenty days. Initially, the

parasite multiplies intensively in the vagina and is sub-

sequently located mainly in the folds of the uterine cer-

vix. Infection often leads to a moderate vaginitis with

purulent discharge, with or without mild endometriosis

and transitory infertility, leading to pyometra, salpingitis,

and cervicitis [10,13,31 ,32].

Protozoa are more numerous between 14 and 18 days

after infection. The inflammatory response of the uterus

occurs between six to eight weeks after infection and is

probably responsible for the death of the embryo. The

interruption of pregnancy is usually concentrated in the

first weeks of pregnancy, but may extend until the fifth

month. The uterus in some females do not become in-

vaded, and thus they can have normal pregnancies and

births [10,13,31,32].

From the vagina to the cervix, T. foetu s then invades

the uterus and grows in fetal membranes producing pla-

centitis, detachment, and death of the embryo, by direct

action of the protozoa or from the effects of the inflame-

mation [10,31]. The pathogenic mechanisms that cause

embryonic or fetal death have not been understood fully

yet. The inflammatory process can vary between acute

and chronic, characterized by accumulation of neutron-

phils, macrophages, lymphocytes, and occasionally

plasma cells. The inflammatory reaction that develops in

the host, causing changes in the uterine environment,

and cytotoxicity mediated by lymphokine, has been

suggested as possible mechanisms [10].

The deleterious effects that are caused by T. f oe tu s di-

rectly on the embryo or fetus have been demonstrated by

identifying cysteine proteases of the parasite in cer-

vico-vaginal mucus of experimentally infected cows.

These soluble proteases are directly associated with the

digestion of host secretions (albumin, fibrinogen) and

participate in the processes of iron absorption from lac-

toferrin [33], in cytoadhesion and acquired immunity

(immunoglobulins G1 and G2) [17,26,34].

Abortion of the calf usually occurs between the first

and third months of pregnancy, may exceptionally hap-

pen after the fifth or sixth months of pregnancy. When

there is a complete release of the placenta and fetal

membranes, female naturally recover. Retained placenta

with membranes can result in chronic catarrhal or puru-

lent endometritis and may result in permanent sterility

[13,31].

After the embryonic or fetal death occurs, a period of

gradual recovery usually restores uterine fertility within

two to six months after the initial exposure to the para-

site. If the corpus luteum and macerated fetus remain,

the female may develop pyometra, which can cause

permanent infertility. The corpus luteum is maintained

active, probably due to lack of prostaglandin secretions

by the endometrium [17].

In females, the infection is self-limiting, lasting an

average of 90 - 95 days, but during that period they may

continue transmitting the parasite to bulls through

breeding activity [13]. Infected cows harbor the parasite

for several estrous cycles or after the termination of

pregnancy. T. foetus is then removed from the uterus,

cervix, and vagina, because of a specific infection in-

duced immune response. However, such immunity is not

permanent and females are subject to subsequent rein-

fections in coitus [17].

1.2.2. Male Genit al Tract

In the bull th e parasite is found on the p enis, preputial

cavity, and in some cases, the urethral opening. Penile

mucosa and adjacent areas of the preputial mucosa have

a large concentration of protozoa, which are not invasive,

situated in the surface mucosa, in secretions and the

glandular light [10,12,35,36,37].

Infected bulls serve as carriers of the parasite and in

older animals the infection becomes chronic, possibly

because of the increase in the number and depth of the

villi of the prepu tial epithelium in these animals [10,38].

Younger males are less likely to become permanent car-

riers of the infection in relation to older, nevertheless,

they are capable of transmitting the parasite to suscepti-

ble females [10].

Typically, males exhibit no macroscopic evidence of

infection due to the presence of the parasite, but can re-

main as carriers throughout their lives, unless they are

medicated. There are rare cases of balanitis and acro-

bustite. These problems, if present, are more of cones-

quence of associated infections than from the action of

the parasite. Histopathological examination of the subepi-

thelial area of the penis and foreskin reveals the presence

of a moderate infiltration of neutrophils, macrophages,

and lymphocytes, which are often accompanied by an

increased number of plasma cells [10,12,36].

1.3. Immunological Aspects

Studies on the immune response to infection by T.

foetus in bulls are scarce. The fact that young animals

are more resistant to infection is related more to the mi-

croscopic structure of the lining of the penis and foreskin

than an effective immune response, which is probably

absent in these animals [10,38].

Females are able to develop an effective immune re-

sponse against T. foetus. The parasite stimulates a mild

inflammatory response associated with the interruption

of pregnancy. Inflammation is mediated by an immune

mechanism that often eliminate the infection [17]. In

carrier cows, this immune mechanism probably fails,

maintaining the infection in the herd [32].

N. S. Da Silva et al. / Open Journal of Animal Sciences 1 (2011) 112-120

115

After the initial infection with T. foetus, bovine fe-

males respond with the local secretion of immunoglobu-

lins IgG and IgA in cervico-vaginal mucus secreted by

the uterus. The elimination of infection is probably me-

diated by specific immunoglobulins, since the organism

is an extracellular parasite. Monoclonal an tibodies cause

agglutination and complement mediated lysis that pre-

vent the adhesion of parasites to the epithelial cells of

the vagina, thereby facilitating phagocytosis of T. f o e t u s

by monocytes. The association between specific anti-T.

foetus antibodies and complement potentiates the death

of the parasites by polymorphonuclear leukocytes

[10,39,40].

1.4. Treatment

Females with T. f oe t us are able to eliminate the infec-

tion without therapy, after the adoption of sexual absti-

nence for ninety days and normal uterine involution.

Females who manifest post-coital pyometra should re-

ceive appropriate care aimed at the corpus luteum re-

gression and elimination of uterine content [8]. Animals

with untreated pyometra can become permanently sterile,

which would justify their disposal [17].

The treatment of bulls infected with T. fo etu s has been

proposed by some researchers as a complementary

measure to eradicate the disease in the herd. However,

currently there is no effective drug approved for the

treatment of males and females infected with T. foetus

and the results have not always been satisfactory [8].

Two different forms of treatment are recommended

for infected males: topical of the preputial and penile

membrane and oral. However, treatments are usually

lengthy, often requiring many repetitions, and have

varying degrees of side effects such as tissue destruction,

loss of appetite, and digestive disorders in orally admin-

istered pro ducts [8,17].

Employment of trichomonicidal agents, preferably in

bulls, is justified by the fact that these animals are the

disseminators and hosts of the parasite. In females, sex-

ual abstinence for ninety days can eliminate T. foetus.

Topical, drugs have been tested with several active in-

gredients in the elimination of T. fo e t u s , such as acrifla-

vine associated with tripaflavine and acriflavine [17].

Orally, imidazole derivatives have been tested, such as

metronidazole, ipromidazole, and dimetridazole, with

varying results [17,41]. Treatment is not recommended

for a large number of bulls or to be used indiscriminate

in a herd, but should be restricting to high value live-

stock.

Diseases caused by tricomonads can be cured by the

use of 5-nitroimidazoles such as metronidazole, effective

against various anaerobic and aerotolerant microorgan-

isms [42]. Via glycolysis, the decarboxylation of pyru-

vate to hidrogenosomas T. f oe tu s is coupled to ATP syn-

thesis and linked to electron transport mediated by fer-

ridoxin. This pathway is responsible for metabolic acti-

vation of 5-nitroimidazoles [3,42]. The antimicrobial

effect of these drugs depends more on its interaction

with DNA, than its metabolic reduction in the microor-

ganism and the consequent generation of free radicals

[43].

Although resistance to metronidazole did not show

any alarming clinical problem, some data related to the

subject should be taken in to cons ideration. The results of

[8] and [44] demonstrated that in vitro and in vivo

trichomonads develop resistance in a short period of

time, at low drug concentrations. This is alarming be-

cause of the possibility of trichomonads resistant strains

arising due to inappropriate treatment regimen pre-

scribed to animals carrying the parasite, which seems to

have occured with T. f oe t us: strain KV1 MR > 100 [14].

Many if not most drugs used to treat patients with the

human and bovine trichomoniasis are nitroimidazole

derivatives (2, 3, 4, and 5-nitroimidazoles). Although

these drugs have different pharmacokinetics, their intra-

cellular mechanisms of action are based on reducing the

nitro grouping [3,45,46].

1.5. Photodynamic Therapy

Photodynamic therapy (PDT) uses th e combin ation of

a dye or chromophore (photosensitizer) and a light source,

usually a LASER [47]. Although PDT was originally

developed as a cancer therapy, it also has great therapeu-

tic potential for other mucocutaneous manifestation dis-

eases such as psoriasis, fungal infections, and bacterial

infections resistant to antibiotic treatments. Photody-

namic therapy with derivatives of phthalocyanines is

proving effective in inactivating viruses such as herpes

simplex [48,49,50].

The concept of PDT originated in 1900 when Oscar

Raab [51] demonstrated that free-living protozoa of the

genus Paramecium could be killed by prior incubation

with sensitizers and subsequent white light illumination.

However, the modern PDT originated with the studies by

Lipson and Schwartz in 1960. These investigators ob-

served that injection of hematoporphyrin in tissues of

cancer patients, induced the appearance of fluorescence

in neoplastic lesions, visible during surgical procedures

[52].

1.5.1. Photosensitizers

Photosensitizers are chromophors that absorb light

energy at specific wavelengths and are able to induce

reactions in nonabsorbent molecules [52]. A variety of

synthetic photosensitizers has been proposed as the sec-

ond generation, for example, phthalocyanines and chlo-

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116

rins. Studies using these photosensitizers have been de-

veloped in order to eliminate problems associated with

the first generation photosensitizers, which have cuta-

neous photosensitivity and make inefficient use of light

at a low penetration wavelength [53].

The phthalocyanines induce sensitivity to radiation in

the wavelength range between 600 - 1.200 nm, which

can be conjugated to a variety of metals such as alumi-

num and zinc, increasing their lifespan and improving

photodynamic toxicity. The presence of side groups in

these photosensitizers can change its electrostatic charge

and solubility, thereby affecting its uptake by eukaryotic

cells. The increase in the sulfonation level of phthalo-

cyanine progressively reduces its affinity for lipid bilay-

ers, making it less phototoxic to mammalian cells. The

association of the photosensitizer with the lipid bilayer

may be more important than the amount of reactive

oxygen produced [53,54].

The transport of th e phthalocyan ine, in vivo, is usu ally

performed by low-density lipoprotein (LDL). LDL is re-

cognized by receptors on the plasma membrane. The

greater the number of receptors for LDL, the greater the

incorporation of photosensitizing will be and thus of

photodynamic therapy. The incorporation of sensitizers

is higher in cells with high mitotic rate, such as tumor

and endothelial cells. The phthalocyanine-LDL complex

binds to receptors on the plasma membrane and then is

internalized via endocytosis. In the intracellular envi-

ronment, LDL-phthalocyanine is usually in addition to

the plasma membrane, in lysosomes and mitochondria

[53,55].

1.5.2. Photodestruction Mechanism

Photodynamic therapy is characterized by celular

photooxidation v ia prior sensitization of th e cell with the

photosensitizer present in the target tissue. While many

of these photosensitizers may be natural constituents of

the tissues, as the first step of treatment, they should be

introduced into tissues by direct administration. In the

second step, the tissue containing the photosensitizer is

irradiated with laser, at the wavelength of maximum

absorption of the first. The light interacts with the pho-

tosensitizer. In this state, the half life of the photosensi-

tizer is a few millionths of a second, long enough for it

to quickly energize the dissolved oxygen, resulting in

production of reactive oxygen species or ROS, such as

1O2, H2O2, OH-, and O2–. Such species are chemically

unstable and highly reactive, which generally causes

eukaryotic cell death. Various structures and organelles

can be targets for PDT, including the plasma membrane,

nucleus, mitochondria, Golgi apparatus, lysosomes, and

cytoskeletal structures [56,57].

1.6. Cell Death, Necrosis and Apoptosis.

Programmed cell death (PCD) is a genetically regu-

lated physiological process in the development and ho-

meostasis of multi and unicellular organisms [58-61].

Once scientists recognized that organisms are composed

of cells, they discovered that cell death could be an im-

portant part of life. First observed during the metamor-

phosis of amphibians, cell death was soon discovered in

the normal development of many tissues of both inver-

tebrates and vertebrates [62]. Today, cell death is known

to be a remarkable and essential event both in normal

cells of organisms and in pathophysiological processes,

which can cause diseases [63].

The term “programmed cell death” has been used to

describe cell death that occurs in predictable places and

times during development, emphasizing that deaths are

somehow programmed during the development plan of

the organism [62]. Subsequently researchers determined

that the process of cell death is the main mechanism

controlling the number of cells (differentiation, embryo-

genesis, metamorphosis, and aging). It also acts in the

defense mechanism to remove unwanted cells, which

may even potentially be dangerous to the body, and pre-

vent several diseases including cardiomyopathy and

cancers [62,64-66]. In some cases, there is degradation

of an entire cell, other times it occurs on a subcellular

scale due to lack of food in which the cell needs to de-

grade proteins and certain non-essential organelles to

recycle these compounds for reuse in the c ytosol [66 ].

The interest in the process of cell death began to grow

rapidly b y 1990 when there was a chang e in the descrip-

tion from, “the cell dies and is replaced” to a new point

of view that says, “cell death is not an incident of life,

but an important and highly controlled element of exis-

tence”. Thus, with the change of emphasis, cell death has

become recognized as an interesting biologi cal event [67].

In 1972, Kerr and colleagues (apud [65]), performed

classical ultrastructural experiments, which provided

evidence that the cells undergo at least two distinct types

of cell death: the first known as necrosis and a second

called apoptosis. This work led to interest in pro-

grammed cell death, first because it provided a visible

object (profile apoptosis) to be consistently focused on

previous experimental studies in relation to the disap-

pearance of dead cells and second ly it provided eviden ce

for the controlled events that justify th e operational defi-

nition of “pr ogrammed”.

In the last decade, apoptosis has attracted great scien-

tific interest. Significant progress has been made in un-

derstanding the factors that control survival and cell

death as well as the intracellular events associated with

the “suicide” cell. However, morphological and bio-

chemical evidence suggest that programmed cell death is

N. S. Da Silva et al. / Open Journal of Animal Sciences 1 (2011) 112-120

117

not confined to apoptosis and necrosis. In many biologi-

cal systems, cellular suicide has shown the involvement

of a lysosomal autophagic compartment. Thus cell death

associated with autophagy has been observed in the

fungus Dictyostelium discoideum during induction of

nutritional stress; physiological stages of development,

such as metamorphosis in insects,embryogenesis in

mammals with regression of webbing and in adults pri-

marily in the intestine, mammary glands, and ovarian

follicles [68].

The clinical implications of understanding program-

med death are significant. Identified program compo-

nents of signal transduction in PCD are potential targets

for drug development and new therapies that might acti-

vate the cell death program, thereby help ing to eradicate

pathogens by antib iotics.

Mariante [69] observed that hydrogen peroxide (H2O2)

induced activation of caspases and a mechanism of pro-

grammed cell death in T. foe tu s. The authors report that:

(1) H2O2 led to loss of motility and induced cell death, (2)

dead protozoa exhibited some characteristics similar to

those found during cell death of other organisms, (3) a

“caspase-like” protein appears to be activated during this

process of cell death. They proposed that although T.

foetus did not show any mitochondria or known cell

death pathway, it is likely to have some mechanism of

cell death. T. f oe t us exhibited morphological and physio-

logical changes in response to treatment with H2O2. To-

gether, these results suggest that a cell death pathway,

involving at least o ne member of the fami ly of caspases,

can exist in amitoco ndrials bodies such as trichomonads.

The hydrogenosome has an important role in the oxida-

tive burst in trichomonads and is a candidate to partici-

pate in this event [69 ].

In amitochondrial organisms, such as trichomonads, it

has been assumed that the machinery of PCD is absent,

but studies by [70] led to the proposal that the existence

of both caspase-dependent and caspase-independent

mechanisms of cell death may be present in trichomo-

nads.

Preliminary results of PDT in T. foetus [71] the au-

thors observed ultrastructural changes, such as project-

tions of the plasma membrane, nuclear fragmentation

with masses of heterochromatin in the periphery, prolif-

eration of endoplasmic reticulum, intense vacuolization

of cytoplasm, and complex and fragmented axostilar

pelta-internalization of flagella. These results were also

observed in [69] after treatment with H2O2, suggesting

that oxidative stress induces a mechanism of cell death

in this parasite.

2. CONCLUSIONS

In animals, the PCD is associated with the elimination

of super cells expressed in the development and eradica-

tion of others, defective [72]. Teratogenic transforma-

tions, microbial infections, lethal factors such as heat,

mutagens and oxidants agents, and toxins induce the

activation of PCD. Recent studies have shown the exis-

tence of PCD in unicellular eukaryotes of different phy-

logenetic origins, indicating that the conservation of the

molecular mechanism is relevant to the functional ro le of

this process in the biology of protozoa [69]. The possible

reasons that the apoptosis pathways in protozoa are

comparable to those of yeast and bacteria may be to (a)

protect populations of healthy cells, (b) obtain nutrients

from neighboring cells committing altru istic suicide, and

Here, we briefly review some T. foetus properties

which are relevant to the development of new treatments

directed towards bovine cattle suffering of trichomoni-

asis. Several drugs have been used to investigate the

cytotoxic effect on T. foetus, such as colchicine, vin-

blastine, cytochalasin B [27], taxol, nocodazole, griseo-

fulvin, lactacyst, and hydrogen peroxide [73]. Da Silva

et al. [71] analyzed the cytotoxic effect produced by

PDT with aluminum phthalocyanine tetrasulfonated

(AlPcS4) photosensitizer on T. foetus culture. They

demonstrated that PDT killed an amitochondrial organ-

ism such as T. foetus. In this parasite, PDT induces a

type of cell death other than necrosis. Apoptosis or

autophagic cell death after PDT in T. f oe t us may benefit

bovines by limiting the inflammatory response, which is

detrimental and could even be lethal to these animals.

We believe that the results obtained from the study of

new therapies such as PDT explained in this article will

bring benefits to the treatment of cattle suffering from

trichomoniasis. This treatment would be without side

effects, which are characteristic of antimicrobial therapy

with nitroimidazoles.

3. ACKNOWLEDGMENTS

This work has been supported by FAPESP (2008/06654-4), CNPq

(309699/2009-6) and CNPq-INCT/INPeTAm (573695/2008-3).

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