Plasmodium – Protozoa (Hematologic Infection)

00:01 Hello, and welcome to Parasites: Plasmodium.

00:06 We're going to be going a bit deeper into diseases caused by several individual types of parasites.

00:14 Just to remind you, protozoa are single-celled eukaryotes, and we're going to be talking about protozoan parasites in the next two lectures.

00:27 And these include the plasmodium species, the causative agents of malaria.

00:35 There are a variety of different plasmodia that cause human malaria, plasmodium falciparum for example and this blood smear stain to visualize two parasite containing red blood cells.

00:48 The pictures are slightly different for each organism.

00:52 Plasmodium vivax is another malaria-causing protozoan parasite, plasmodium malariae, and finally, plasmodium ovale, the disease is caused by these different plasmodium species.

01:09 Have overall features in the similar fashion, but they do have some differences and there's slight geographic differences as to the location of the parasites.

01:19 This global map shows you the regions of the world in which infection with the plasmodia species occur.

01:28 The malaria, as you can see most of Africa is affected in red, a good part of Asia, Central and South America.

01:38 So, the regions essentially boarder the equator, these are where the mosquitos are found that transmit the infection.

01:47 Malaria is a life-threatening disease that is preventable and curable.

01:51 In 2019, there were approximately 230 million cases worldwide, and approximately 400,000 deaths.

02:00 Children under 5 years of age, account for two-thirds of all malaria deaths worldwide.

02:06 In 2019, 94% of the malaria cases and deaths occurred in Africa.

02:13 99% of those by plasmodium falciparum.

02:17 As a consequence of multiple infections with these parasites and multiple bouts of malaria in the lifetime of these individuals, the effect is a reduction in both economic and social development.

02:31 If we could rid the globe of malaria, we'd make so many more people able to have productive lives and contribute to the human experience.

02:41 Humans are the only reservoir for those four species of plasmodium that I showed you at the beginning.

02:48 There are animal variants of these malarias, and as far as we know, they don't cause human disease.

02:57 There's some suggestion that these maybe changing but for the most part, the four species I showed you, humans are the only reservoir.

03:04 They are transmitted among humans by the bite of an infected female anopheline mosquito. Alright, female mosquito.

03:16 Now, can you tell me, do male mosquitos bite you? I'll give you a few seconds. The answer is no.

03:24 Whenever you're bitten by a mosquito, it's a female mosquito doing that and that's because she needs the blood in order to lay eggs, interesting little observation, so all the diseases transmitted by mosquitos, whether they be parasitic or viral, it's all spread by female mosquitos.

03:43 Let's take a look at the life cycle of plasmodium falciparum, both in the mosquito and in the human host.

03:52 There are slight differences between falciparum and the other species but this will suffice to illustrate the general pattern.

04:01 We start at the top of the slide with the mosquito injecting parasites, we're gonna continue through the liver stage, the blood stages, and eventually, we're gonna get back into the mosquito.

04:14 So, this is a cycle that is maintained among people by the mosquito vector.

04:20 So, let's look at step one, the mosquito, again, it's a female mosquito is biting someone and delivering the form of the parasite called a sporozoite.

04:31 Sporozoites come out of the salivary gland of the mosquito and the mosquito of course is taking a blood meal so the mosquito inserts its proboscis into the skin, it actually injects some saliva because that saliva contains a variety of compounds to make it easier for the mosquito to draw a blood meal, like anti-coagulants and other things as well, and of course, if the mosquito is infected, it will deliver sporozoites as well, so the simple fact that the mosquito has to take a blood meal injects those sporozoites and of course the mosquito will withdraw a blood from that host.

05:14 Those sporozoites are then entering into the circulatory system of a human.

05:20 Within 30 minutes, those sporozoites go to the liver, it's gonna be their main stop on their trip to the human body and they become a form called cryptozoites.

05:34 Now, many parasite go through multiple stages in their life cycles with various names, sporozoites, what's in the mosquito; cryptozoite is what the sporozoite becomes in the liver.

05:47 The parasite replicates in the liver and it does so inside an infected cell and that cell eventually produces many, many merozoites, the cell ruptures within 8 to 14 days after the initial mosquito bite and those new merozoites are released into the blood stream.

06:13 In the blood, the merozoites then infect its second target which is the red blood cell.

06:21 They enter the red blood cell in which they multiply.

06:25 Now, here, we're showing you several of the stages that occur within the red blood cell starting from the top, the free moving red blood cell that's infected with what we call a ring forms, you can see those two in the upper left.

06:39 And then below it are the different forms of the infective stages within the red blood cell.

06:44 If you do smears of infected individuals, you'll see all these forms in the blood, and in the blood the merozoites multiply and at some point the blood cell ruptures and releases more merozoites into the bloodstream.

06:59 This kind of multiplication that we've been talking about both in the red blood cells and in the liver is asexual, its simply binary fusion.

07:09 But, at some point, in the reproduction, the sexual stages are made and this is an organism that can reproduce in both ways, sexually and asexually, so at some point during the replication, both male and female gametocytes, that's the name for the sexual stage of the organism, are made, and now these will be picked-up by a new mosquito if this person who's now full of malaria parasite is bitten and of course, if you're in an area endemic for malaria, this is likely that you'll be bitten thousands of times a day, so mosquitos will deliver malaria parasites and they'll pick up new ones from infected people.

07:52 Now, when a mosquito takes a blood meal, the blood goes to the gut of the mosquito, of course, cuz that's where it is digested, and so go the male and female gametocytes.

08:06 After they are ingested by the anopheles mosquito during a blood meal, Marked by a green circle in the middle of this life-cycle diagram, the male gametocytes or microgametocytes and the female gametocytes or macro-gametocytes are collected in the midgut or stomach.

08:23 While in the mosquito stomach the microgametes penetrate the macrogametes to form zygotes.

08:29 The zygotes become ookinetes, which invades the midgut wall of the mosquito where they develop into oocyst.

08:37 The oocyst grow, rupture and release sporozoites which make their way to the mosquito salivary glands.

08:44 Inoculation of the sporozoites into a new human host perpetuates the malaria life-cycle.

08:51 Note that on the top right side of the cycle in the human liver stages, delayed multiplication in the liver may cause periodic relapses.

09:01 One to two years for a plasmodia ovale, and three to five years for plasmodia vivax.

09:08 The parasites multiplication in the mosquito is known as the sexual or sporogonic cycle, and it's multiplication in the human is known as the schizonic or asexual cycle.