[MUSIC] There are two ways that insects can cause disease as etiological agents or as disease vectors. Etiological agents are organisms or compounds that directly cause disease in other organisms. So when we refer to insects as etiological agents were talking about the direct impacts of their presence and behaviors on human, animal and plant health. Disease vectors on the other hand are organism that transmit other disease-causing organisms such as bacteria, viruses and parasitic protists from one host to another. There are many ways that insects can affect hosts as etiological agents. Insects flying close to the mouth, eyes or nose of an animal can cause irritation. The mere presence and activity of insects like mosquitoes and other flies can foster anxiety in humans and cause substantial stress to livestock inducing loss of appetite and sometimes even self-injury. These nuisance insects can lower the quality of animal products such as wool, meat and milk which in turn results in economic losses for people. Bedbugs are another insect which can cause significant stress this time to their human hosts. Bedbugs as their name suggests often live on or near mattresses and bite at night. In addition to the sleep lost when humans are awoken by the biting insects significant stress is often caused by bedbugs as extermination can be exceptionally difficult. These insects are capable of rapid population growth and they often hide in small folds of fabric or corners of furniture where it is difficult to reach them with insecticides. While bedbugs don't carry human pathogens they are responsible for extreme stress and anxiety. Sometimes insects cause disease through passive food contamination. Insects can contaminate food in a variety of ways by leaving behind body fragments or through the transfer, regurgitation or excretion of pathogens on to the food. In some cases the insect may carry and transfer pathogens from infected organic materials. Cockroaches are one example of an insect that can contaminate food and food preparation surfaces. Some arthropods are not only a nuisance but also produce toxins and venom as defensive responses, which are injected into the offender with a bite or sting. Examples of venomous arthropods include many hymenoptera, some hymenotra, spiders and scorpions. Reactions to insect toxins and venom can range from mild itching to severe injury. For instance, irritants released from caterpillar seta can cause mild to serious skin reactions while toxins from blister beetles can, as the insects name suggests, cause painful blisters. In severe cases people that are allergic to insect toxins and venom can suffer from potentially fatal anaphylactic reactions. Sometimes large infestations of blood feeding insects or prolonged attack can result in blood loss or exsanguination. This can occur in moose hosts that succumb to winter tick infestations. According to research performed across North America as many as 40,000 winter ticks can be found on an individual moose over a winter season. Assuming each tick ingests approximately 0.5 milliliters of blood an infestation that consists of 40,000 ticks may result in the loss of up to 20 liters of blood. If we consider the fact that an adult male moose has an average blood volume of 32 liters heavily infested animals are left in poor physical condition after tick feeding. Infested moose also rub off fur in an attempt to scratch off the ticks. This can result in cold stress that contributes to moose mortality. Other insects such as some maggots cause disease by invading host tissues. Degrees of host invasion differ depending on the maggots species. Some species like the Bot Fly will only invade subcutaneous tissues while others will infest internal organs such as the lungs and gastrointestinal tracts if inhaled or ingested. Some species of mites also invade and feed on host skin, hair follicles, lungs and nasal passages. The effects of these infections are variable while some cause no symptoms in the host others may cause irritation or infection, especially if removal of the arthropod is attempted. Finally insects can also have psychological impacts on people through their presence or activities. There are two types of human psychological disorders that are associated with insects, phobias and delusional parasitosis. Entomophobia and Arachnophobia are severe yet irrational fears of insects and spiders respectively. In some cases the phobia can become so severe that individuals refuse to leave their residence for fear of encountering an insect or spider. Some affected people unnecessarily apply chemical insecticides which can lead to additional health consequences. The other insect associated psychological disorder is referred to as Delusional Parasitosis. Individuals that suffer from this psychological disorder are convinced that they are being attacked by parasites which they often identify as insects. These people will repeatedly consult physicians to obtain diagnosis and treatment often remaining convinced there is something under their skin. The emotional distress experienced by these individuals severely reduces their quality of life. Disease-causing organisms such as bacteria, viruses and parasites are transmitted between hosts by vectors in a process known as a transmission cycle. The transmission cycle of an arthropod borne disease typically involves three components. One, what is usually a vertebrate host infected with a disease-causing organism. Two, an arthropod vector that acquires the organism from an infected host and is capable of transmitting it to another vertebrate host. And three, another vertebrate host that is susceptible to infection by the disease-causing organism. Generally a disease-causing organism reproduces sexually only within the definitive host. An intermediate host on the other hand is a host in which the parasite doesn't reproduce sexually though it may reproduce asexually. Both vertebrates and arthropods can be definitive or intermediate host depending on whether or not they carry the sexually reproductive stages of the parasites. For example, humans are considered definitive hosts for several filarial nematodes parasites. The worms undergo sexual reproduction within the human lymphatic system. Meanwhile, the mosquitoes that vector these nematodes are considered intermediate hosts because the parasites develop within the mosquito, but do not reproduce. Arthropod vectors transmit disease-causing organisms between vertebrate host during their normal feeding activities. Disease vectors find suitable hosts using cues such as body heat, carbon dioxide, color, shape and odor. The vectors have well developed sensory organs to respond to this variety of host cues. There are two basic mechanisms of disease transmission mechanical transmission and biological transmission. Mechanical transmission occurs through direct contact between the vector invertebrate host. The disease-causing organism doesn't undergo development in the vector during the process. Common avenues of mechanical transmission include contaminated mouth parts or regurgitation by the vector during feeding. As disease-causing organisms do not undergo development or reproduction in the vector during mechanical transmission the number of infectious units present in or on the vector generally declines over time. This means that a vectors ability to mechanically transmit the disease lessens with time post contamination. House flies vector diseases through mechanical transmission. A house fly can transmit a disease by contaminating the host food with the disease-causing organism carried on the tarsi, body hairs or other parts of the fly. During biological transmission disease-causing organisms always undergo development or reproduction within the insect vector. During biological disease transmission the number of infectious units present in the vector can increase over time and so does the likelihood of successful transmission. Many important human diseases are transmitted biologically by arthropod vectors such as Malaria, Dengue, Chagas disease, Lyme disease, filarial worms and Zika virus. Biological transmission usually involves blood feeding arthropod vectors that have specialized mouthparts to access blood meals from the host. Biological transmission occurs in insects such as mosquitoes, biting flies and fleas as well as other arthropods like ticks. These arthropods typically introduced disease-causing organisms with saliva, which is often injected into the host during feeding. Unlike other diseases which are transmitted biologically through direct feeding activities of arthropods, Chagas disease is transmitted not by feeding but through excretion. Chagas disease is transmitted when feces contaminated with the parasitic protozoans from blood feeding kissing bugs are accidentally scratched into the open wounds caused by the blood feeding insects. Biological disease transmission can be further categorized into either horizontal or vertical transmission. Horizontal transmission involves the movement of disease-causing organisms between hosts and vectors as well as between vectors. Vertical transmission though occurs when disease-causing organisms are passed between generations of the vector. The disease-causing agents are passed from an adult female vector to her offspring. There are usually transmitted through infected ovarian tissues to the developing embryos. The disease-causing organisms can be maintained throughout the offsprings development into adulthood. Vector competence is the vectors ability to acquire, maintain and transmit the disease-causing organism in question. Some disease-causing agents can even replicate or continue development within the arthropod vector, though this is not required for vector competence. Other disease-causing agents are simply acquired, carried and transmitted by the arthropod vector. In either case arthropod disease vectors must have high vector competence to be effective at disease transmission. Even if an arthropod picks up a pathogen during feeding, it may not be able to transmit this to another host. There are barriers to vector competence that can occur at any of the stages of pathogen acquisition, maintenance or transmission. For example, maintenance of the disease-causing organism will be interrupted if it is attacked by the vectors immune system resulting in low vector competence. Increased global trade and travel have resulted in the global movement of insects and other arthropods including disease vectors. These vectors can become invasive in their new range and may carry diseases from their region of origin or transmit diseases present in the new range. In the case of invasive disease vectors, the vector may not have the same level of competence in the expanded range. The Asian Longhorned tick is native to Eastern Asia and has since spread to other regions like Australia and New Zealand. It has established in these regions where it vectors several human and animal diseases caused by multiple species of bacteria and viruses. The Asian Longhorned tick has recently been found in the Eastern United States. It is crucial to determine the extent and potential expansion of the new range of this potential vector and its competency as a vector of disease in its new range. Information about the ecology of the tick such as habitat and feeding preferences can help inform scientists and health agencies about its potential risks as a vector. Emerging diseases are a major burden on global economies and public health. Reports of emerging infectious diseases have risen in the past seven decades and approximately 22% of these cases are vector-borne diseases, many of which are carried by arthropods. Although not all of them are recent developments the number of diseases transmitted by ticks for example is remarkable. Factors that drive at the emergence and spread of arthropod vector diseases primarily come from human activities such as deforestation that cause significant changes in disease transmission dynamics. A major contributor to emerging diseases is the movement of goods and people around the planet. An individual can acquire a disease-causing organism in one region and move to a distant region before symptoms are even apparent. Human-driven climate change is another variable that contributes to the emergence and intensity of vector-borne disease outbreaks. Changes in temperature and humidity can influence the abundance and distribution of disease-causing organisms, vectors and hosts. They can also affect activity in development rates of vectors in disease-causing organisms potentially leading to increases in the vector and the disease. In this lesson we learned about the ways in which insects and other arthropods can act as etiological agents and disease vectors. We examined some of the many ways in which insect vectors can transmit disease-causing organisms. We also discussed how human activities have contributed to the spread of insect vectors as well as emerging diseases. In the next video, we will examine some examples of well-known arthropod borne diseases from human history.
