PROFILES: Trypanosomal Disease: Description, Mechanisms of Infection, Treatments

Summary

Trypanosomes are a genus of protozoa (a type of single-celled organism with a nucleus) that are parasitic to animals and humans (Turnbull, 2001). Relatively common around the world, only some species of trypanosoma are harmful to their hosts (University of Bristol, 2009).

Trypanosomes are transmitted by hematophagous insects. They are the cause of African sleeping sickness and Chagas disease which both have a latent and a chronic infection stage that attacks the nervous system. Treatment administered is aggressive and encompasses a mortality risk, although there aren’t many other alternative treatments. Supportive care is a better and cheaper option for financially challenged patients or for sufferers in developing countries (Wiser, 2011).
Trypanosomes are Zooflagellates; typical protozoans that feed by absorption or engulfing their food (Rogers, 2011). They have a flagellum for motility, and a kinetoplast. These are organelles that house the DNA of the “power stations” of the cell - the mitochondria.
Trypanosomes have several developmental stages in their life cycle (Aus. Society for Parasitology, 2010):

1. Amastigote- No flagellum or undulating membrane (Aus. Society for Parasitology, 2010).
2. Promastigote- Kinetoplast is located in the rear of the cell, and there is a developing flagellum (See Figure 1) (Aus. Society for Parasitology, 2010).
3. Epimastigote- Kinetoplast is located in the rear of the cell. There is a developed flagellum and undulating membrane. This phase is prominent when the organism is in the vector’s salivary gland (Aus. Society for Parasitology, 2010).
4. Trypomastigote- Kinetoplast is located at the front of the cell. There is a developed flagellum and a long undulating membrane. This phase occurs when the organism is in the host (Aus. Society for Parasitology, 2010).

Human African Trypanosomiasis

Probably the most well-known form of Trypanosomal disease, HAT was first documented and diagnosed in 1901 (Turnbull, 2001). There are several species of trypanosoma that cause the disease, Trypanosoma gambiense being identified first in Central Africa (Turnbull, 2001). T. rhodesiense was later discovered in Zimbabwe, when a person who had never travelled to a T. gambiense endemic region was shown to have a trypanosomal infection in a blood smear test (Turnbull, 2001). These two trypanosomes are often classed as sub-species of T. Brucei (which causes a similar disease in animals, Nagana), as all three are identical in morphological terms (Wiser, 2011). Despite this being the case, genetic techniques have shown that the species in question are assuredly different from one another on a molecular level (Wiser, 2011).

Studies suggest the following mechanism of infection takes place when the vector, the tsetse fly, feeds on a potential human host. The epimastigote trypanosome swims out of the fly’s salivary gland, obtaining entry into the bite site (Turnbull, 2001). Once there, it begins to replicate rapidly, forming a painless sore called a chancre (Turnbull, 2001). T. gambiense is slow in reaching the next stage of its life cycle - the infection can sit in the sore for years before migrating into the body-systems – whereas T. rhodesiense’s infection develops at a much faster rate from this point (Moore, 2013). Either way, at this point in the infection the host may only suffer mild symptoms (e.g. muscle aches and a fever).
The next stage of infection involves the protozoa developing into trypomastigotes and migrating into 
the circulatory system through the local lymph tissue (Turnbull, 2001). These tiny organisms can then be found all over the host’s body.
Due to the infection of the lymph system, the nodes swell up (Winterbottom’s Sign) (Aus. Society for Parasitology, 2010) from a massive influx of B-lymphocyte immune system cells. Trypanosomes burst and release their toxic compounds, causing other immune system cells known as macrophages to release chachetin, a tumour-necrosis factor, causing weight loss, fatigue and a muscle wasting (Wiser, 2011).
Just like the progression from the chancre, the time it takes for the parasite to cross into the Central Nervous System (CNS) varies between species. T. rhodesiense’s invasion is very fast, in most cases taking less than a month (Aus. Society for Parasitology, 2010).
Sometimes patients die from other causes (e.g. swelling of the heart) before the parasite reaches the CNS due to the rapid progression of the species. T. gambiense, however, takes over the brain and nervous system slowly causing insomnia, general mood changes and brain inflammations; haemorrhages and cerebral oedemas. This strain of the disease progresses over years, and eventually leads to a coma or death through other minor intervening diseases (Aus. Society for Parasitology, 2010).
When the trypanosome population in the host reaches the stationary phase of their growth, they change back into epimastigotes to prepare for the infection of another tsetse fly (Martin, 2003). When the fly bites the infected host the trypanosomes swim up its proboscis and into the mid-gut, meaning the parasite can spread to other hosts (Wiser, 2011).
Now in the vector, the amino acid Proline is used alongside glucose in aerobic cell-respiration, and the volume of mitochondria increases. These changes are referred to as procyclism (Wiser, 2011). It takes around a week for these procyclic trypanosomes to pass through the vector, towards the salivary glands. Once there they transform back into epimastigotes and use their flagella to cling on to epithelial cells, ready to enter another host all over again (Wiser, 2011).

American Trypanosomiasis

In 1911, Carlos Chagas discovered a similar disease to African sleeping sickness in the Americas which he named Chagas’ Disease. The species was found in a child that suffered from a fever, anaemia and lymphatic swelling (Tolan, 2013) - symptoms all found in the African Trypanosomiasis. American Trypanosomiasis is found across central and southern America, spanning from southern USA to the very south of Argentina (Tolan, 2013). The disease is the number one cause of heart failure in central and southern America (Burleigh, 2013).
The parasite, T.cruzi, is mainly transmitted by the blood-sucking Triatomine bug although a variety of other hematophagous (blood sucking) insects can act as vectors (Wiser, 2011). T. cruzi’s infection process is quite different to that of the T. Brucei species (See Figure 3):
The trypanosomes reside in the vector’s gut, and rather than migrating to the salivary glands they are egested out when the fly lands on a host to feed (Wiser, 2011). Infection is caused when the faeces containing the trypanosomes are rubbed into the irritative bite wound.
The trypanosome burrows into the mucous membrane and meets the influx of macrophages and lymphocytes. Some of the parasitic cells are engulfed by these immune system cells, and others enter the local tissue through a lysosome (Tolan, 2013). The trypomastigotes escapes its lysosomal vacuole by releasing a very active hemolytic protein which disrupts the vacuole membrane (Tanowitz et al., 1992). The population of the parasites then transform into simple amastigotes (Bell, 1995), replicate within their host cells, and after 4 days they are released as trypomastigotes (Wiser, 2011). After this first round of intracellular replication, trypomastigotes are found in the blood and can reinvade other cells around the body (Bell, 1995), specifically cardiac, nervous and smooth muscle tissue (Tolan, 2013).
The heart is the main target organ for T. cruzi – the parasite can reside in the cardiac and nervous tissue for years without being detected (Wiser, 2011). This is known as the acute stage of Chagas’ Disease (Tolan, 2013). A tell-tale sign of this stage is Romana’s sign – the swelling or inflammation of the eye (conjunctivitis) due to trypanosomes in the eye’s mucous membrane (Wiser, 2011). A swelling can also occur around the vector bite site. This is called a Chagoma and is similar to African trypanosomiasis’s chancre.
The chronic stage is reached when more than 20% of the nervous tissue in the heart is damaged causing conduction problems, arrhythmias and congestive heart failure (Tanowitz et al., 1992). The stage begins with trypanosomes being found in the spinal fluid – a sign of neural invasion (Tanowitz et al., 1992). As the heart walls become thinner, an aneurysm (bulge in blood vessels) forms in the atria (Wiser, 2011), and the organ swells causing symptoms of Dilated Congestive Cardiomyopathy (Tanowitz et al., 1992). Death usually occurs through congestive heart failure, a rhythm disturbance or thrombosis (Wiser, 2011). 
Because American Trypanosomiasis targets the muscular and nervous tissue in general, and not just the heart, other autonomic systems are affected. Damage similar to the thinning heart walls occurs in the Gastrointestinal (GI) tract, causing nerve loss and problems with moving and absorbing food (Tolan, 2013). These symptoms can be recognised by high levels of muscle sensitivity and over contraction (Tubraikh & Ali, 2010).

Treatment

Treatment of African and American trypanosomiasis involves chemotherapy (Boutielle & Buguet, 2012), although patients are rarely completely free of the parasites (Tanowitz et al., 1992).
The drugs administered can be categorised into two groups; drugs that can cross the blood barrier (Nifurtimox and Melarsoprol) and drugs that cannot cross the blood barrier (Pentamidine isethionate, Suramin) (Wiser, 2011).
African trypanosomiasis, stage 1 treatment – before the parasite invades the CNS – involves injections of either Pentamidine or Suramin into the muscle tissue, depending on which sub-species of parasite the patient has been infected with (Boutielle & Buguet, 2012). Pentamidine is administered for T. gambiense infections (Wiser, 2011). The drug works by severing and damaging DNA in the trypanosomes’ kinetoplasts (Bacchi, 2009). As previously mentioned, the kinetoplast DNA contains the information for mitochondrial replication meaning the trypanosomes carry on replicating but produce cells that are much less aerobically efficient (Bacchi, 2009). Suramin is given to patients with the T. rhodesiense infection, because Pentamidine treatment is not sufficient enough to combat the T. rhodesiense infection (Wiser, 2011). Suramin binds to the body’s Low Dense Lipoproteins (LDLs), which trypanosomes desperately need for important, structural lipids such as cholesterol (Bacchi, 2009). When the trypanosomes bind to the LDLs Suramin is shown to inhibit their enzymes required for Glycolysis in aerobic respiration (Bacchi, 2009). Although this sounds like a very effective way of dealing with the parasite, tests have shown that the inhibition process is slow and only targets newly made enzymes in the cell (Bacchi, 2009). For both drugs, treatment failures are very uncommon (Boutielle & Buguet, 2012). This being said, harmful side effects are often reported (Tanowitz et al., 1992). Anaphylactic shock – a severe, and sometimes fatal allergic reaction - is a prime example (Boutielle & Buguet, 2012).
The second stage of African Trypanosomiasis is more aggressive than the first stage, so stronger drugs such as Melarsoprol and Eflornithine are administrated intravenously (Boutielle & Buguet, 2012). Melarsoprol inhibits the enzymes such as Pyruvate kinase, preventing energy (ATP) from being produced (Gutteridge, 1985). The drug is controversially acclaimed, because of its heavy-metal arsenic group. Diarrhoea, fever and even heart and kidney damage are common amongst patients, and several treatment courses of Melarsoprol have shown 5-10% mortality rates (Boutielle & Buguet, 2012). 
Eflornithine inhibits the enzyme Ornithine decarboxylase within trypanosomes, an enzyme critical to polyamine synthesis and cell replication (Pegg, 2006). Cure rates have been claimed to be as high as 99% in some regions, although only with patients with the T. gambiense infection (Wiser, 2011). Both Melarsoprol and Eflornithine treatments are very expensive due to cost of the drugs and the need for hospitalisation during treatment (Wiser, 2011).
The main drug used for American Trypanosomiasis is Nifurtimox. This drug targets the trypanosomes in the body’s cells – stage 1 of the disease. Usually, when T. cruzi invades a cell, macrophages produce oxides (e.g. Nitric oxide) which subject the organism to oxidative stress (López-Muñoz et. al., 2010). This is where more reactive, oxygen containing compounds called free radicals are produced. These compounds can cause a lot of damage to proteins and tissue (Betteridge, 2000), including the trypanosomes’ cell tissues. In the absence of the drug the amastigote trypanosomes overcome a lot of the oxidative stress through several methods, including activating cell mediators that inhibit oxygen-intermediate synthesising enzymes (López-Muñoz et. al., 2010). Following Nifurtimox treatment and working alongside macrophages more oxidising free radicals are produced to suppress trypanosomal replication inside the cell. Many patients develop side effects from the tissue-damaging treatment, which makes Nifurtimox less effective during the chronic phase (Wiser, 2011). Because there aren’t many new drugs under development to combat the disease, treatment is mostly focused on supportive care for patients (Wiser, 2011). Treatments for easing symptoms include pacemaker fittings, bed rest, hydration and anti-arrhythmic agents (Wiser, 2011).













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