Module 8 - Arthropods and Disease Flashcards
What are disease vectors?
Carry a pathogen between susceptible hosts
- arthropods can act as disease vectors for hosts such as humans, animals, and plants
What are etiological agents?
Organisms that directly cause disease in another organism
- the simplest way in which arthropods can impact animal health
- act as etiological agents in a variety of ways
What 2 types of human psychological disorders are associated with insects?
- Phobias
- Entomophobia and arachnophobia are severe but illogical fears of insects and spiders, respectively. Individuals are terrified of these arthropods, even when presented with one they are aware is completely harmless. Can lead to unnecessary overuse of chemical insecticides - Delusional parasitosis
- Occurs when an individual is convinced that they are being attacked by insects or other parasites, when they are not.
What is the movement of parasites and pathogens between hosts and vectors called? What 3 components are involved?
Transmission cycle
1. A host, usually a vertebrate, infected with a disease-causing organism
2. An arthropod vector capable of acquiring and transmitting the disease-causing organism
3. Another host that is susceptible to infection by the disease-causing organism
How do we categorize hosts?
Based on their role in the life history of the disease-causing organism
What is a definitive host?
Where sexual reproduction by the disease-causing organism typically occurs
What is the intermediate host?
The host in which the disease-causing organism does not reproduce sexually
- asexual reproduction may occur
Are vertebrates and arthropods definitive or intermediate hosts?
They can be either definitive or intermediate hosts based on the stage of the disease-causing organism they carry
When does the transmission of disease-causing organisms between vertebrate hosts occur?
During the arthropod vector’s normal feeding activities
What two basic mechanisms can arthropod vectors transmit diseases by?
Mechanical disease transmission and Biological disease transmission
How does mechanical transmission of disease-causing organisms occur?
Through direct physical contact between the vector and vertebrate host, whereby the disease-causing organism does not undergo any development or reproduction on or within the vector.
Why does the number of infectious units present in or on the vector decline over time with mechanical transmission?
Since the disease-causing organism is not developing during mechanical transmission. This results in a reduction of mechanical transmission over time post-contamination.
How can mechanical transmission occur?
Through feeding by a vector with contaminated mouthparts, or regurgitation by the vector while feeding on a vertebrate host
How does biological transmission of disease causing organisms occur?
Always involves some development or reproduction of the disease-causing organism within the arthropod vector.
With biological transmission, why do infectious units in the vector typically increase over time?
Since reproduction can occur within the vector; the likelihood of successful disease transmission also increases over time.
What does biological transmission usually involve?
Specialized blood-feeding arthropods, such as mosquitoes, biting flies, and ticks, that introduce disease-causing organisms through their saliva, which is injected into the host during feeding.
What is the difference between mechanical transmission and biological transmission in terms of number of infectious units?
Mechanical transmission: # of infectious units decrease with time
Biological transmission: # of infectious units increase with time
How can we further categorize biological disease transmission?
Based on the pathway taken by the disease-causing organism
1. Horizontal transmission
2. Vertical transmission
Horizontal Transmission
Involves the movement of disease-causing organisms between hosts and vectors, or between vectors within the same generation.
Vertical Transmission
Involves movement of disease-causing organisms between generations of either the vector or host. Occurs when disease-causing organisms are passed from an adult female arthropod vector to her developing offspring. The disease-causing organisms may be maintained in the offspring’s body throughout development.
Vector Competence
A vector’s ability to acquire, maintain, and transmit the disease-causing organism in question
- Arthropod disease vectors must have high vector competence to effectively transmit disease
- Disease-causing organisms that replicate within an arthropod vector may be more likely to be transmitted, but their replication is not required for vector competence
When can barriers to vector competence occur?
At any stage of pathogen acquisition, maintenance, or transmission
Emerging diseases
Those that have recently appeared in a population and may threaten to increase in severity.
- Trade and travel have impacted the incidence of emerging diseases; so has deforestation and other human activities
Morbidity vs Mortality
Morbidity refers to the state of being sick, while mortality is a measure of the number of deaths within a population
Medical entomology
Studies the impacts of insects and other arthropods on human health by investigating factors such as the behaviour, ecology, and biology of arthropod disease vectors
Medical entomologists also study the EPIDEMIOLOGY of arthropod-borne diseases- what is this?
A science that has to do with the incidence, distribution and control of diseases in populations. This info is used to help develop control and monitoring programs for arthropod-vectored diseases.
What do arthropod-borne diseases have major impacts on?
Human health, influenced the outcomes of war, and possibly even destroyed entire civilizations
Up until pesticides became popular in WW2, what were most soldiers dying from?
Disease! Many of these were arthropod-borne diseases, which thrived in the unhygienic conditions of a battlefield
Epidemic typhus
- Has influenced the outcome of several famous battles and wars
- The disease is vectored by the human body louse, Pediculus humanus
- The disease-causing bacteria, Rickettsia prowazekii, is an intracellular parasite that damages cells lining blood vessels
- High mortality rate
What is Rickettsia prowazekii classified as? Why?
Classified as a bioterrorism agent due to the severity of this disease and the potential for the bacteria to be aerosolized.
What is the most famous example of an arthropod-borne disease epidemic?
PLAGUE
- caused by the bacteria Yersinia pestis
- transmitted by the feeding activity of up to 31 species of fleas
- the most common fleas to have played a role in the spread of this disease are the Oriental rat flea, Xenopsylla cheopis, and the human flea, Pulex irritans
Simplified Plague Transmission Cycle
- Rats become infested if fed on by infected fleas <—> Fleas become infected after feeding on infected rats
- Humans become infected if fed on by infected fleas
How does Yersinia pestis invade humans?
The bacteria invade the human lymph nodes, damaging an important component of the human immune system. The lymph nodes swell causing the characteristic bubonic plague. The lymph nodes may break open, spreading the bacteria into the bloodstream or lungs to cause the more deadly septicemic or pneumonic plagues respectively.
Today, what regions of the world are the most severely impacted by arthropod-borne diseases? Why?
Tropical regions
- tropical conditions easily sustain large populations of insect vectors within large populations of susceptible hosts
What two significant diseases today are vectored by arthropods and cause high morbidity and mortality respectively?
- Lymphatic filariasis (elephantiasis)
- a disfiguring disease caused by the presence of parasitic filarial nematodes, or roundworms; worms are transmitted by female mosquitoes
- infection causes extreme tissue swelling and thickened skin on the limbs, chest, and genitals
- high morbidity can have major socioeconomic impacts
- common drug treatments are in place (preventative chemotherapy and deworming drugs) which may be more effective than vector control efforts in mitigating the spread - Malaria
- caused by protozoan parasites from the genus Plasmodium, this disease is vectored by female mosquitoes in the genus Anopheles
- high mortality in children and infants, but pregnant women and HIV patients are also high risk
- causes anemia and multiple-organ failure if treatment is delayed, leading to death
- treatable and potentially preventable
- WHO has made significant efforts to reduce cases of malaria worldwide by attempting to interrupt the malaria transmission cycle
Modern medicine has come a long way, so why do new arthropod-borne diseases continue to surface?
Pathogens may mutate, or travel may facilitate the movement of arthropod vectors, and thus a disease that was previously rare or mild may become a significant concern and exert a major strain on public health resources
What is an example of a virus where there was an increase in cases globally likely due to transmission from travellers?
Zika virus
- discovery of an association of the disease in pregnant women to microcephaly in newborns resulted in the fear of a possible epidemic
- symptoms are generally mild but when a pregnant woman is infected, Zika can cause congenital brain abnormalities and other severe disorders in a developing fetus
- range expansion of mosquitos has caused an increase in cases and has impacted global distribution
The northern’s hemisphere’s harsh climate hosts a relatively small abundance of arthropods, but what new medically important arthropod-borne disease has emerged in this region?
Lyme disease
- cases have risen due to range expansion of vectors in response to climate change and other human-induced factors
- caused by the bacteria Borrelia burgdoreferi
- multisystemic disorder and disease symptoms vary between individuals; problematic bc a swift diagnosis is important for treatment
- failure to treat = severe chronic neurological and heart complications
Veterinary entomology
The study of interactions between livestock and companion animals with arthropod pests that impact their heath and insect-associated problems that affect wildlife and captive animals
Etiological Agents of Animals
Mites
- feeding activity on and within the skin tissue of animal hosts can result in persistent inflammation
- infestation causes MANGE
- mites burrow into the skin to feed, causing severe itching and sometimes lesions
- livestock must be quarantined since the disease spreads through physical contact
- mange can also be spread to humans depending on the species of mite
Vectors of Animal Disease
Viruses, bacteria, and filarial nematodes transmitted by arthropods
- Heartworm
- caused by filarial nematodes
- transmitted by an infected mosquito; larvae acquired from one host and injected into another via feeding
- larvae mature in canine host, accumulate in the heart, blood vessels, lungs, and cause blockage
- humans are dead end hosts - Bluetongue
- causes substantial losses to the livestock industry
- affects domestic and wild ruminants
- caused by RNA viruses vectored by biting midges
- swelling and cyanation of the tongue, lesions, internal hemorrhaging, and fluid buildup in lungs
- vaccines and changes in farming practices can be used to reduce infection rates - Nagana (aka typanosomiasis)
- caused by trypanosomal parasites
- vectored by tsetse flies and primarily affects cattle in southern Africa
- infect RBC and cause fever, weakness, and lethargy
- high mortality rates = major barrier to commercial livestock production
Arthropod-born plant diseases
- Plant-feeding hemipteran insects can vector plant diseases since the pathogens can be introduced into plants when they pierce the tissue to feed
- Bark beetles: spend part of their life cycle within a host tree, but adults can transport pathogens btw trees
- Other: some species of mites and non-arthropod invertebrates like nematodes
What are the 3 types of transmission of plant viruses by arthropod vectors? (named based on the continued presence of the disease in the vector)
- Non-persistent
- Semi-persistent
- Persistent transmission
Non-persistent transmission of plant viruses
- occurs when the viral particles are restricted to the mouthparts of the insect vector
- viral particles are retained on the mouthparts from a previous feeding and inoculated into plant tissue during the next feeding
- since viral particles are exposed to the outside enviro, they may remain infectious on the mouthparts for short time period
- virus can be immediately transmitted to other plants w/o an incubation period
- mostly by aphid vectors
Semi-persistent transmission of plant viruses
- does not require a long incubation period to be infectious = transmitted in a short period after being acquired
- do not circulate or replicate within the vector = longer amount of time spent feeding increases the probability of transmission
- present in more than just the mouthparts; also retained in the insect’s foregut within the cuticular layer, but do not move into tissue
Persistent transmission of plant viruses
- maintained within a vector for the remainder of its lifespan because the virus circulates and replicated within various organs of the vector, protected from outside enviro
- transmission can require extended feeding by the vector to acquire enough particles to become infectious
- virus must circulate to other tissues and requires an incubation period = vectors not able to inoculate persistently transmitted viruses into a plant immediately after feeding
- once acquired by a vector, the viruses enter the midgut and hemocoel, and colonize other tissues, such as the salivary glands and ovaries
- viruses can be maintained btw generations as they are passed from female vectors to offspring (bc repro tissues are infected)
- ex. Leafhoppers
What is an example of a common plant virus that has significant impacts on agriculture and is transmitted in a persistent manner?
Potato Leafroll Virus
- vectored by the polyphagous green peach aphid
- can cause extreme losses in potato crops
- infection causes chlorosis = purpling of the tops of plants and curling of leaves
Insects and other arthropods can also vector bacterial plant diseases. What is an example that is particularly destructive to apple and pear cultivars?
Fire blight
- caused by the bacteria Erwinia amylovora
- can colonize virtually any tissues of the plant
- infection causes tissues to wither and darken and bacterial ooze on fruit
- droplets of ooze can disperse by wind or rain to new tissues or other plants
- insects can also transmit this disease btw plants if they pick up the bacteria through feeding
- can destroy entire orchard blocks
Fungal diseases can also cause devastating effects on the plants they infect, and many are vectored by arthropods. What is a significant disease plaguing trees of North America?
Dutch elm disease
- since it is not native to North America, the American elm trees across the continent have not coevolved with the fungus and cannot tolerate infection
- most fungal plant diseases are spread via soil, wind, or rain but Dutch elm disease is vectored by a few species of elm BARK BEETLES
- caused by the fungus Ophiostoma ulmi that grows in the vascular tissues of trees
Although arthropods can be medically important as etiological agents and disease vectors, what percentage are actually considered pests in medicine, agriculture, or other fields?
1%
