By AM
Before you scroll down: know that there are pictures of victims of Leishmaniasis in this document. It is important to know what this parasite is capable of but if it makes you throw up your dinner… well then, you’ve been warned. I have placed the more graphic pictures at the very bottom of this document (end of text), so you can still enjoy a good read! These are figures 4, 5, 6 and 7. Viewer discretion is advised.
I’ve always been interested in pathogenic organisms. Something about infecting a host much larger than yourself and brining it down to its knees is fascinatingly terrifying. Through my microbiology classes, I’ve had the easiest time memorizing the names of pathogenic organisms and their mode of transmittance and infection. So, when we were assigned to write a short report about a eukaryotic microbe, I knew what category of microbe I was going to look into. Eukarya (plural of eukaryote) are cells that have a nucleus and their name literally translate to true nucleus from Greek. The other major cells type is prokaryotes, those without a nucleus. Eukarya are cells like us (animals), plants, fungi (mold+ mushrooms) and a large protist group; the most famous member of which is the common amoeba. Out of the infectious eukarya, Plasmodium (causative agent of Malaria)was the first organism that came to my mind, but I wanted to use this opportunity to learn about a new organism. At least to me, Plasmodiumand a lot of other famous eukaryotic microbes are a dead horse; I already know so much about them that I can just write a short essay without any research. After about thirty minutes of googling, I chose the organism Leishmania major, though I’ll spend a good chunk of time talking about all Leishmania as well.It is an infectious eukaryote, single celled, it has a cool rather apocalyptical name and it is quite prevalent in our world. Although I suspect I had heard or read about the disease it causes, leishmaniasis, I virtually had no other information about this infectious organism.
Evolutionary speaking, we are inclined to naturally fear large predators. Even small, but still potentially dangerous insects can cause a sever reaction in some people. But there is another danger, lurking in an invisible world. Though evolution may not have readied our minds for this world, it did for our bodies. As we speak, there are millions of microbes and viruses desperately trying to invade you and turn you into a microbe-making factory. Can you blame them? Just like the argument humans make when committing an atrocity: “I’m doing it for our children”, they want their progenies to live happily off your body. Sadly for them, your immune system relentlessly destroys almost all of these hardworking microbes. The few that survive either do no harm or are barely scraping by. But then why do we get sick? How can a sickness kill us if our immune system is so good? How some tiny brainless cells can bring a complex organism such as ourselves down? Even with our awesomely equipped immune system? So, sit tight and grab a warm blanket as we dive deep into the cold, remorseless world of one of these pathogens; where no rules except those of Leishmania are abided.
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| Figure 1: This figure shows the parasite Leishmania donovani, causative agent of visceral leishmaniasis. Though this the most dangerous kind (look below), most Leishmaniaspecies look very similar. Note the thread like structure at the end of each parasitic cell, which is a flagellum that allows for motility. Picture was taken at 400x magnification using brightfield microscopy. (From here.) |
Leishmania are parasitic eukarya that are present across the globe (fig 1). There are more than 20 species of Leishmania that can infect humans with varying severity depending on species type. The Leishmaniagenus belongs to the order ofTrypanosomatida.This order hosts other human pathogenic species such as Trypanosoma brucei that causes the sleeping sickness and Trypanosoma cruzi that causes the Chagas diseases (2). Like leishmaniasis, both these diseases are transferred by an insect vector to the human host. The severity of infection in other animals varies, though all Trypanosomatida can infect a range of mammals with varying success rates. Frequently, small mammals such as mice can serve as a reservoir for these parasites. The target species here, L. majoris only present in the eastern hemisphere. This species is mostly restricted to Asia, Africa and parts of Europe (1). Regardless of species, humans get infected by the bite of a female sandfly for uptake of a bloodmeal (fig 2 and fig 3). The parasite has a sexual cycle that can only be completed within the female sandfly (fig 2). Don’t think the female sandfly is very happy to host this unwelcomed guest. The immune system of the female sandfly is also overwhelmed, and its salivary glands are invaded by the parasite (1). However, the sandfly doesn’t get “sick” as it still needs to be able to fly and eat, just to pass the parasite to fresh hosts. We won’t talk about the growth cycle of the parasite, although it is worth noting that it is complicated, and each stage is quite specialized (fig 2). Upon bite of the sandfly, three types of infection are possible: cutaneous, mucocutaneous, and visceral (1). These infection types are restricted by the species type though the demarcations are still blurred as some species seem to cause two or even all three types of infections (1). Now, we will discuss the infection types and the species that can cause them.
Cutaneous is the most common type, caused by my target species: L. major. This disease causes serious boils and ulcers to develop on the skin, specifically where the sandfly initially bit the victim. I was surprised to learn that L. majorcan survive uptake by macrophages (3). Macrophages are immune cells in our bodies that eat invading microbes and digest them (phagocytes). In fact, studies have shown the parasite contains several receptors that facilitate its uptake by a macrophage (3). The parasite literally wants to be eaten! Upon ingestion, the parasite survives digestion by the macrophage and turns the tables around: it starts eating the macrophage alive! The parasite will eat and replicate within the macrophage until the macrophage dies. Then, the parasite will lyse the old macrophage and thousands of new progenies will be released into the bloodstream of the host (usually a human). These will all look for fresh hosts: new macrophages that will be infected and the cycle will continue. Luckily, L. majorrestricts itself to the dermis tissue (skin) and does not migrate elsewhere in the body and hence, is not lethal (1). Unfortunately though, the parasite leaves life lasting scars on its host skin which will often entirely deform facial features or other body parts (fig 4 and fig 5). Usually, the victims will face social stigma for the rest of their lives for their disfigured features.
The second form, mucocutaneous, is caused by L. tropicaI. As the name suggests, this parasite is predominantly located at tropical regions (1 and 4). Mucocutaneus leishmaniasisfollows a very similar pattern to L. major(cutaneous type) with the exception that mucosal surfaces are also affected along with skin. This is one of the nastiest forms of leishmaniasis (in terms of visible damage done) as the boils and ulcers primarily affect the face (where mucosal surfaces are abundant) and will cause gross disfigurement of lips, mouth, nose and eyes (which may lead to blindness). Additionally, unless controlled quickly, the damage done is often permanent. I have decided against sharing any pictures of mucocutaneous leishmaniasis as they are simply too gruesome for most human beings (myself included). You can look it up online if you desire but you are seriously warned against it!
Although the two types of Leishmania I have discussed here so far are nasty, they are not lethal. Though very effective drugs treatments are not available for either the cutaneous or the mucocutaneous form (no cure), our body’s immune system will wipe out the parasite population in about six months to two years. Additionally, the drugs that are currently available can help speed the recovery process and more or less control the damage done by lesions, if administered quickly. However, the third and final form is the most dangerous. This form is one of the most lethal infectious diseases in humans with a mortality rate of near 100% if left untreated. Though malaria takes throne of being the world’s leading eukaryotic parasite killing humans, visceral leishmaniasis comes in second place. This disease is caused by the species Leishmania donovani. In this type, the parasite travels to internal organs such as the spleen, liver and bone marrow (1). Here, the parasite will continue to multiply which will cause massive enlargement of liver and particularly the spleen, which can become even bigger than the liver in some cases (fig 6). If you’ve ever seen a liver and a spleen side by side, the size of the spleen is about 1/3 of a healthy liver. So, patients with this disease often have extremely enlarged abdomens (fig 6). Additionally, the parasite may also actively infect the skin and cause nasty lesions, just like the cutaneous form of leishmaniasis (fig 7). However, the true lethality comes from infection of the bone marrow. The bone marrow is the source of new red blood cells, white blood cells (immune cells) and platelets (blood clotting factors). By lounging at the bone marrow site and quite literally eating the marrow, the parasite severely depletes the body of red blood cells, platelets and most importantly, immune cells (1). The patients will often become anemic, lethargic with sudden bouts of fevers. These symptoms are similar to a malaria infection which may cause a malaria misdiagnosis to be made; which is often a death sentence for the patients. It doesn’t help that the geographical areas that hosts L. donovani often host malaria parasites too, which only increases the chances of a misdiagnosis to be made. However, as mentioned, the immune cells of the body are heavily depleted due to activities of the parasite in the bone marrow. Though it might seem a little unrelated at first, the most famous virus in the world, HIV, shares some similarity with visceral leishmaniasis. HIV kills its victims indirectly by depleting the body of a particularly important immune cell. In HIV infection, it is opportunistic infections that kill the patient not the HIV virus itself. Even the virus of common cold can be deadly if you are HIV positive and not under control. Hence, very much like an HIV infection, in visceral leishmaniasis, the hosts die from opportunistic infections rather than the activities of L. donovani directly. Visceral leishmaniasis is a bit different than HIV infection as unlike the HIV virus, L. donovani can destroys all immune cells and does not focus on one specific type. As you have already understood, visceral leishmaniasis is very deadly. I don’t believe I need to tell you that HIV virus is also deadly and has killed millions of people. It is interesting for me to see that two completely different organisms, one a virus and one a eukaryote, get their super lethality and ability to kill their hosts by targeting the same system: the immunity. Of a particular concern to WHO is a visceral leishmaniosis/HIV super infection. As both these infectious diseases suppress and destroy the immune system, the host is almost guaranteed to die. Sadly, the areas that hosts L. donovani often have prevalent rates of HIV infection too; which only increases the chances of getting the super infection.
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| Figure 2: Life cycle of Leishmania species. Note the different names and shapes at each stage of growth. The parasite changes its cellular morphology and gene expression to be more specialized for an infection at each stage. (From here.) |
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| Figure 3: A sand fly biting a human finger. The size comparison allows you to see how small the sandfly is compared to a human. This introduces a particular difficulty in preventing bites as sandflies are small enough to squeeze through most mosquito nets and are difficult to see/feel. (From here.) |
I can’t lie, before I started my research, I didn’t think much of leishmaniasis. Maybe it’s even apparent towards the beginning of this report (while my knowledge was still very incomplete) that I was clearly underestimating Leishmaniaand what it can do. There are lots of pictures depicting the true horrors Leishmania patients undergo, but I must warn you of their graphic nature before you look them up. By being such a nasty, deadly disease, you would except Leishmaniasis to have received abundant funding and research. Sadly, Leishmaniasis is considered a neglected tropical disease and therefore, drugs, treatments and preventive methods are either non-existent or extremely ineffective. Here, I will discuss treatments and prevention for this parasite.
To this date, there are no effective vaccines for Leishmaniasis. A couple of vaccines were used in the past but were stopped as the vaccine site itself (usually on the buttocks) would turn into a nasty, slow to heal sore (1 and 2). Hence, the best method is to prevent getting bitten by the sand fly. Recommendation include using insect repellent products, sleeping in closed areas with nets and regular usage of insecticides to kill any insects. Additionally, staying indoors at dawn and dusk may be helpful as sandflies are most active at these hours. As you can clearly see, none of the preventive methods mentioned are truly effective. People that suffer from this disease often live in rural areas with few technological advances. They do not have access to nets, insecticides or insect repellents. Additionally, their houses are not fully enclosed and are easily accessible to insects, especially ones as tiny as the sandfly (fig 3). Staying indoor at dusk/dawn is not an option for these people either as those are the hours that they need to go to work and return back home. The drug amphotericin b deoxycholate, which is actually an antifungal drug has shown to be effective for treatment of visceral leishmaniasis and the two other kind as well. This drug is the only FDA approved drug for treatment of leishmaniasis (4). The drug has to be given everyday for a period of 2-3 months in some cases to prevent relapse of the disease. In visceral leishmaniasis, the drug has to be administered within a period of 30 days to be effective (5 and 6). Unfortunately, the mode of action and the side effects of this drug are poorly understood (1). Renal failures (which often lead to death) have been observed in some patients which may be due to the drug or leishmaniasis (5 and 6). Hence, this drug is far from perfect and requires more research. Additionally, newer drugs are necessary too as some parasite have started gaining resistance to amphotericin b deoxycholate (5 and 6).
To summarize, why is this parasite so dangerous? What is about leishmaniasis that can prevent our complicated immune system from launching an attack? We have to look into morphology and pathogenicity to answer these questions. There are two main factors that gives Leishmania the ability suppresses human immune system: 1- The ability to move and 2- infecting phagocytotic (macrophages) cells in the body. It is worth noting that Leishmaniacells are all motile by their flagellum (fig 1). Once they lyse their host cells, they can quickly move about the body and infect new cells and tissues. It is this motility that allows them to infect different tissues of the body and move from skin to deep organs like spleen. This motility brings me to my second point, ability to infect immune cells. By moving about quickly, they can evade most immune cells and quickly infect them; it’s more I catch you before you catch me! Most intracellular parasites (the ones that hide within cells) are protected from the immune system to some degree as they are enclosed within the host cell. Our immune system fails to recognize these infected cells until they start encoding wrong cell surface receptors. This parasite has taken this protection to the next level, where it actually hides within the cells that should be responsible for its destruction. The phagocyte, which is usually a macrophage, attempts to do its job and ingest the parasitic cell. This is when its normal mechanisms are hijacked by the parasite and the parasite starts living within the macrophage. Furthermore, the parasite suppresses normal interleukins and other chemicals secreted by the phagocytes. For those who haven’t taken immunology, these interleukins serve as bridge between adaptive and innate immunity. Essentially, they are like little letters (messages) from small guns of immunity (innate immunity) that let the big guns of immunity (adaptive immunity) know it’s time to go nuclear. Our body, like any sensible defense system, uses small guns for small skirmishes and only moves to big ones when it really feels threatened. By blocking the release of these messengers, which would normally be released by a macrophage upon ingestion of a dangerous microbe, the parasite slows down body’s response. That’s the reason it takes such a long time for a strong immune response to be launched and for the parasite to be wiped out (up until a year). As discussed, we saw this slowed response can be deadly in some cases. Another important piece of information is infection of dendritic cells. You can see dendritic cells as postal services of the body. They carry the necessary messages to initiate a massive attack and take it to a place where the big guns of immunity can read it and be activated. Dendritic cells simply chill in the body (at different locations) and ingest different materials (messages) and microbes until they are needed to take certain messages to activate the big guns of immunity. Therefore, by targeting dendritic cells, this parasite essentially paralyzes the postal services of immunity and in turn the adaptive immune system (big guns) for a long time. It’s really genius in a cruel way.
So, what have we learned so far? Leishmaniasis is a nasty disease and it kills and disfigures our fellow humans with no regards or mercy. But fear not as WHO and other agencies are slowly working their ways to control and eradicate this disease. As with all of our problems in this world, education is the answer. For many people, they don’t understand how leishmaniasis works, what they can do to avoid it or how to diagnose it in themselves or their animals before things get out of hand. If you want to make a change, there are three options you can take: 1- you can go and study Leishmania for a career. This means you’d have to go to graduate school or medical school and that’s hard and expensive. But if you have the means to do so, it will be worth it as your efforts will lift generations from suffering. 2- Or you can look up leishmaniasis on WHO and see which of their options of preventing this disease fits your interests the best. 3- Which is the last but not the least method: education. Familiarize yourself and those around you with this disease. Don’t scare them by showing them the nasty pictures but do urge them to let a physician have a look at that bug bite which is turning nasty. Remember Leishmaniasis is truly dangerous when diagnosed late. If we can understand it, we can destroy it. Us humans have eradicated and destroyed many obstacles in our history, from travelling in air to smallpox; nothing has stopped us. Leishmania does not stand a chance, I’m sure of it. It will take some time, but we will prevail, we always have and always will.
Graphic Figures below!
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| Figure 4: children affected by Leishmaniasis which mostly seems to be cutaneous (the type that affects skin). The sores can usually leave permanent scars and disfiguration which leads to heavy social stigma. (From here.) |
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| Figure 5: child suffering from cutaneous leishmaniasis, the less severe kind affecting only skin. (From here.) |
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| Figure 6: Girl affected by visceral leishmaniasis, caused by L. donovani and the most serious kind. The bloated abdomen area is a sign of inflamed liver and most especially, the super inflamed spleen. (From here.) |
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| Figure 7: Dog affected by Leishmaniasis. The agent here is believed to be the L. donovani (visceral Leishmaniasis). Not only will the animals themselves suffer, but they can also serve as a reservoir for the parasite and eventually pass it on to humans. (From here.) |
References:
1. Ready, P. D. (2014). Epidemiology of visceral leishmaniasis. Clinical epidemiology, 6, 147.
2. Jaskowska, Eleanor; Butler, Claire; Preston, Gail; Kelly, Steven (2015): A maximum likelihood protein sequence phylogenetic tree of trypanosomatids.. PLOS Pathogens. Figure. https://doi.org/10.1371/journal.ppat.1004484.g002
3. Turco, S. J., & Descoteaux, A. (1992). The lipophosphoglycan of Leishmania parasites. Annual review of microbiology, 46(1), 65-92.
4. Aronson, N., Herwaldt, B. L., Libman, M., Pearson, R., Lopez-Velez, R., Weina, P., ... & Magill, A. (2016). Diagnosis and treatment of leishmaniasis: clinical practice guidelines by the Infectious Diseases Society of America (IDSA) and the American Society of Tropical Medicine and Hygiene (ASTMH). Clinical infectious diseases, 63(12), e202-e264.
5. Thakur, C. P., Singh, R. K., Hassan, S. M., Kumar, R., Narain, S., & Kumar, A. (1999). Amphotericin B deoxycholate treatment of visceral leishmaniasis with newer modes of administration and precautions: a study of 938 cases. Transactions of the Royal Society of Tropical Medicine and Hygiene, 93(3), 319-323.
6. Das, A., Karthick, M., Dwivedi, S., Banerjee, I., Mahapatra, T., Srikantiah, S., & Chaudhuri, I. (2016). Epidemiologic correlates of mortality among symptomatic visceral leishmaniasis cases: findings from situation assessment in high endemic foci in India. PLoS neglected tropical diseases, 10(11), e0005150.
