Ectoparasites as Vectors for Viral Spread (Week 4, Mod 7)

What is an arbovirus?

Short for “arthropod-borne” virus

• Is a virus that is transmitted to humans or animals via arthropods

  • ticks, mosquitoes, or midges

• Term refers more to mode of transmission rather than a group of viruses

Virus REPLICATES within the arthropod; depends on it for survival

What are the main differences between DNA viruses and RNA viruses?

DNA virus:

• DNA is genetic material; replicates in the host cell’s nucleus, using host’s machinery

• Makes virus more stable and less prone to mutations

RNA virus:

• RNA is genetic material; replicates in CYTOPLASM of host cell

• Mutates more frequently due to lack of proofreading mechanisms during replication

What is “biological transmission” of a virus?  What is “mechanical transmission”?

Biological transmission:

Virus replicates INSIDE vector before transmission 

• Vector plays a vital role in virus lifecycle 

• Ex: dengue virus; replicates in and is transmitted by mosquitoes 

Mechanical transmission:

Virus DOES NOT replicate within the vector; simply carried on vector’s mouthparts

• Vector is a PASSIVE carrier; doesn’t become infected itself 

• Ex: houseflies spreading viruses by interacting with contaminated surfaces

Refresher: What is vertical transmission? What is horizontal transmission?

Vertical transmission → PARENT TO OFFSPRING

Horizontal transmission → from individual to individual; this includes arboviruses

What 3 species of mosquitoes can act as vectors, and what viruses do they mainly transmit? Describe where they can be found and when they are most likely to bite.

1) Aedes species

• Transmit viruses such as dengue, Zika, chikungunya, Rift Valley Fever, and yellow fever.

• Primarily bite during the day, especially in the early morning and late afternoon.

• Lay eggs in small water containers, making urban environments ideal breeding grounds.

2) Anopheles

• Main vector for malaria (caused by Plasmodium parasites), but also transmits some arboviruses like O'nyong-nyong.

• Primarily feed between dusk and dawn.

• Prefer clean, still water bodies such as ponds, marshes, and rice fields for breeding.

3) Culex species

• Transmit West Nile virus, Japanese encephalitis, and lymphatic filariasis.

• Bite primarily at night and prefer birds as their primary hosts, but also feed on humans and other animals.

• Thrive in polluted and stagnant water, such as drains and marshes.

What are the 5 species of ticks of significance? Are they hard or soft ticks?

1) Ixodes - hard tick

2) Rhipicephalus - hard tick ; BROWN DOG TICK 

3) Amblyomma - hard tick 

4) Demacentor - hard tick 

5) Agarisidae - SOFT tick 

What virus does the Ixodes species mainly transmit? Generally describe their pathogenesis and where they can be found in the environment.

• Transmit Lyme disease and tick-borne encephalitis (TBE).

• Can attach to a host for several days to feed, transmitting pathogens in the process.

• Prefer forested and grassy areas, with high humidity levels.

What virus does the Rhipicephalus species mainly transmit? Generally describe their pathogenesis and where they can be found in the environment.

• Transmit diseases like East Coast fever (in cattle) and Rocky Mountain spotted fever in humans.

• Commonly found on livestock and dogs, where they attach for long feeding periods.

• Thrive in warmer climates, especially in Africa, and are known to infest cattle heavily.

What virus does the Dermacentor species mainly transmit? Generally describe their pathogenesis and where they can be found in the environment.

• Transmit Rocky Mountain spotted fever and tularemia.

• Can feed on a variety of hosts, including humans, dogs, and wild animals.

• Prefer grassy, brushy, and wooded areas, and are often found in these habitats in temperate climates

What virus does the Argasidae species mainly transmit? Generally describe their pathogenesis and where they can be found in the environment.

• They are vectors of tick-borne relapsing fever (TBRF), caused by Borrelia bacteria. Species like Ornithodoros moubata transmit African swine fever to pigs.

• They feed quickly, usually in minutes, and can take multiple blood meals from same host over time. They do not remain attached for long periods.

• Lack the hard, protective shield (scutum) found in hard ticks (Ixodidae). Their bodies are more leathery and flexible

Describe fleas as ectoparasites… where do they thrive? What species are the most significant?  

Thrive in homes, farms, and areas with close human-animal contact, posing risks in both domestic and agricultural settings.

• Important species:

  • Ctenocephalides felis (Cat Flea) primarily, which infests cats but can bite humans, transmitting diseases like cat-scratch disease and murine typhus;.

  • Pulex irritans (Human Flea) which spreads serious diseases like plague (Yersinia pestis) and murine typhus, though the main plague vector is the rat flea (Xenopsylla cheopis)

• While Ctenocephalides felis prefers cats, Pulex irritans also infests pigs, dogs, and other animals

• Flea eggs are laid on the host but fall off, with larvae and pupae developing in the environment before reinfesting new hosts

Describe biting midges as ectoparasites… where do they thrive? What diseases are they known to transmit?  

Transmit livestock viruses like bluetongue virus and African horse sickness virus, which cause severe illnesses in cattle and horses.

• They primarily feed on the blood of animals but can also bite humans, making them a pest to both livestock and people.

• Midges breed in wet, marshy areas, and their larvae develop in waterlogged environments like moist soil.

What are 3 other biting flies that can act as vectors for parasites? Briefly describe how they act as ectoparasites and what diseases they transmit.

1) Sandflies (Phlebotomus)

• Transmit LEISHMANIA and sandfly fever

• Feeds on blood during the NIGHT, thrives in desert environments 

2) Stable flies (Stomoxys calcitrans)

• Can be a vector for ANTHRAX and equine infectious anemia virus 

  • Breed in organic, decaying material on farms; known for painful bites

3) Tsetse flies (Glossina)

• Transmit trypanosomiasis (SLEEPING SICKNESS in humans) and nagana in livestock

• Feeds on blood during the DAY; found mainly in subsaharan Africa, favoring rivers

What 5 factors make mosquitoes good vectors of disease?

1) Feed on a variety of hosts, including humans, livestock, and wild animals, allowing them to transmit diseases between species (a wide host range).

2) They require blood meals for egg production, which increases their contact with hosts and the likelihood of transmitting pathogens (their blood-feeding behavior).

3) Pathogens like viruses, protozoa, and filarial worms can complete part of their life cycle inside mosquitoes, making them capable of transmitting diseases upon biting (Pathogen Incubation).

4) Mosquitoes can travel over large areas, spreading diseases over vast regions, especially during favorable environmental conditions (extensive flight range and mobility).

5) Adaptability to various aquatic habitats for egg-laying ensures high human population densities, making frequent encounters with hosts more likely (breeding habitats near humans).

What 5 factors make ticks good vectors of disease?

1) Ticks (in particular hard ticks) attach to their hosts for extended periods, allowing more time for pathogen transmission through saliva or regurgitation though some soft ticks feed frequently and live in close proximity to hosts (long feeding duration).

2) Ticks often feed on multiple hosts throughout their life cycle (larva, nymph, adult), providing opportunities to acquire and transmit pathogens across species (multiple life stages feeding on different hosts).

3) Some pathogens can persist through tick molts and even be passed to offspring, ensuring sustained pathogen transmission (transstadial and transovarial transmission).

4) Ticks can survive for long periods without feeding, allowing them to persist in various habitats and increasing their chances of encountering a host (environmental hardiness).

5) Many tick species feed on a wide variety of hosts, including humans, livestock, and wildlife, facilitating the spread of diseases across different species (broad range of hosts)

What 5 factors make fleas good vectors of disease?

1) Fleas reproduce quickly and can establish large populations, increasing the frequency of contact with hosts (high reproductive rates).

2) Fleas bite multiple hosts to obtain blood meals, which increases their potential to spread diseases, such as plague or typhus (biting behaviour).

3) Fleas can infest mobile hosts like rodents, allowing pathogens to spread over wide areas (host mobility).

4) Flea larvae can thrive in the environment, particularly in animal bedding or nests, maintaining a ready supply of vectors in close proximity to hosts (environmental hardiness).

5) Fleas can parasitize a wide range of hosts, including both mammals and birds, increasing their potential to spread zoonotic diseases (adaptability to wide host range)

What 5 factors make biting midges good vectors of disease?

1) Their small size makes them difficult to detect, allowing them to feed unnoticed on their hosts (Tiny size and silent feeding).

2) Biting midges often exist in large swarms and are found in diverse environments, which increases the likelihood of pathogen transmission (high density and widespread distribution).

3) Biting midges can carry and transmit a range of pathogens, including viruses like Bluetongue and African Horse Sickness (can transmit a variety of pathogens).

4) Short reproductive cycles allow quick population growth and frequent contact with hosts (rapid reproductive cycle).

5) Some Culicoides species target specific hosts like livestock, enhancing the vector-host relationship and pathogen transmission efficiency (preference for specific hosts). 

What 4 factors make sandflies good vectors of disease?

1) Sandflies feed at night, avoiding host defenses and improving blood meal success. 

2) They have a broad host range, feeding on mammals, birds, and reptiles 

3) They have a wide ecological range and can thrive in both rural and urban environments, increasing their geographic reach. 

4) Leishmania parasites develop in their digestive tract, ensuring efficient pathogen transmission

What 4 factors make stable flies good vectors of disease?

1) Stable flies bite frequently, increasing their potential to spread blood-borne pathogens. 

2) They also mechanically transmit diseases like trypanosomiasis by contaminating their mouthparts. 

3) They are highly adaptable and can thrive in both urban and rural environments. 

4) Their widespread breeding in decaying organic matter makes them resistant to control efforts

What 4 factors make tsetse flies good vectors of disease?

1) Tsetse flies feed exclusively on blood, making them effective transmitters of Trypanosoma parasites. 

2) Their long lifespan (1-3 months) and preference for large mammals, including livestock and wildlife, enhance their transmission efficiency. 

3) Can pass pathogens to offspring, ensuring long-term disease persistence. 

4) Tsetse flies thrive in specific habitats, like riverine areas, influencing disease spread

What are the CHEMICAL options for control for each of these vectors?

Insecticides/Acaricides:

• Mosquitoes: Indoor residual spraying (IRS), larvicides in breeding sites, and aerial space spraying 

• Ticks: Acaricides on livestock/pets.

• Fleas: Topical treatments (fipronil, imidacloprid) on pets.

• Biting midges: Insecticides sprayed near animal shelters.

• Sandflies: Residual spraying on walls in human habitats.

• Stable fly: Insecticide sprays in barns or fly traps.

• Tsetse fly: Insecticide-treated targets or aerial spraying.

What are the BIOLOGICAL and GENETIC methods of control for each of these vectors?

• Mosquitoes: Larvicidal bacteria (Bacillus thuringiensis), predators like fish.

• Ticks: Entomopathogenic fungi targeting tick larvae.

• Fleas: Nematodes introduced in infested areas.

• Stable Fly: Release of parasitic wasps to reduce larvae.

• Tsetse Fly: Sterile insect technique (SIT) to reduce populations.

What are the ENVIRONMENTAL and PHYSICAL methods of control for each of these vectors?

• Environmental Management:

  • Mosquitoes: Removing standing water to prevent breeding.

  • Ticks: Clearing vegetation to reduce tick habitats.

  • Stable Fly: Manure management to eliminate breeding sites.

• Physical Barriers:

  • Biting Midges, Sandflies, Mosquitoes: Use of insecticide-treated nets, fine mesh screens.

  • Tsetse Fly: Traps and targets to lure and kill flies.