[MUSIC] So, we're here at the University of Illinois College of Veterinary Medicine, and we're in the Food Animal Hospital, and today we wanted to talk about respiratory disease. And it's a very important infectious disease for the cattle industry and for the pig industry as well. So, it's very important in understanding how that disease affects production, and the sustainability of production in knowing how a pathogen, how an infectious agent actually causes that disease. And how the host, herself, responds to that disease. So, this calf here is about four months old, she's a calf that will be a breeding and a milk producing animal for the owner. She is quite thin, and she has quite advanced respiratory disease. But we wanted to use her to illustrate what are the normal paths of the body. That help the animal defend against respiratory infections. And then, how do those pathogens overcome those normal defenses? So really, when we think about the respiratory system, we can think about it in two main parts. The upper respiratory system, so the nose, the sinuses, the trachea, which is the windpipe, which conducts the air down to the lungs, and then the lungs themselves. And there's various mechanisms along that respiratory tract that protect it from invasion of pathogens. It's a remarkable system. It has to take in air and then take the gases out of that air, and those gases, the oxygen, contribute to the health of the animal as they would with us. They do with production animals as well, and a good respiratory system is important for them to grow, it's important for them when they're producing milk as well. So, the volume of respiratory system is remarkable. Even though we can't see from outside, each time this animal breathes, it takes in one and a half liters of air. But the lining is about 120 square meters. Something like 10 times her surface area. And then if you look at her breathing she's breathing at about 20 times a minute. So, she's taking in 20 or 30 liters of air every minute. Now, that's great if that's healthy air. The trouble is if that's full of bacteria or full of viruses that's a huge amount of pathogen to be exposed to the body. So, the lungs and the respiratory system itself has to be thin enough and permeable enough to allow gas exchange but thick enough and robust enough to make sure that those bugs don't get into the body system. In this calf, those mechanisms have gone wrong slightly. So, how does her body make sure that those pathogens, potential pathogens, don't access into her body. Firstly, up here in the nose and the nasal passages there's a system of mucous and hair. And that performs a protective barrier, which stops the microbes contacting directly the cells themselves. The cells lining the respiratory tract are very specialized, they have very small hairs called cilia and they beat. So when there's mucous, with this mucous, it's not just a physical barrier, but it's a moving physical barrier and these little hairs they beat in such a way that they move mucus from the lower parts of the respiratory tract up to the upper parts of the respiratory tract. And it's a remarkable, they're specialized epithelial cells. And as the microbes get trapped in that mucus, these waves of cilia form an escalator and the mucus carries out particles such as straw and dust, but also the microbes themselves up into the upper airways. From there, that's when once that mucus is detected in the upper airways in the throat, it will produce a cough In the nose it will induce a sneeze, the microbes are removed by coughing or sneezing as well. And sometimes we'll see this animal cough, and she actually has excess mucus, because she's fighting an active infection. So, that's really how the upper airways work. Mucus specialized lining cells that take the mucus, and it's expulsed from the upper respiratory shrank by coughing and sneezing, when we get then to the lungs it's a bit of a different story, but there's a remarkable set of very specialized cells that line the airways and the terminal last sacs at which the gas exchange takes place and they're called Alveolar Ephithelial Cells. The terminal sacs themselves are called alveoli. They're like small, little balloons. And they have specialized cells. Now those cells, there's two main types. One of the cells produces a substance called surfactant. And that surfactant stops these balloons from collapsing. Every time the animal breathes these terminal balloons open up and close. And as they open up, they fill up with air, the lining becomes very thin, and that's where the gas exchange takes place. But that's also where microbes can be exposed to the thinnest parts of the respiratory tract membrane. So the surfactant, which is the lining chemical that keeps those balloons open also contains proteins and chemicals, which kill bugs themselves. So, the whole respiratory tract is lined with a chemically rich mucus and surfactant, which kills microbes themselves. The surfactant is also very, very sticky, and it sticks to the microbes and sticks the microbes together. And that allows other cells, macrophages, neutrophils, and dendritic cells that we've spoken about in the introduction to immunology, allows those cells to ingest microbes, not just individually, but also in groups. They form nets of cells in the lungs, and those nets of cells capture microbes. Ingest them themselves, they are killing cells, but they also direct the rest of the immune response. If more cells are needed, If more mutant cells are needed, they give out signals, and they recruit, neutrophils, and lymphocytes into those lung fields, which do additional killing. So, the respiratory tract has a set of chemicals, a lining, a physical structure, and a cellular population, which does its best in normal circumstances to prevent microbes from accessing the body. [MUSIC]
