Vectors and onchocerciasis

Question 1

What are sibling species?

Answer 1

Any of two or more related species that are morphologically nearly identical but are incapable of producing fertile hybrids. Sibling species can only be identified by genetic, biochemical, behavioral, or ecological factors, and are thought to have become divergent very recently.

Only called sibling species not a species complex with blackflies.

Question 2

Where is the distribution of onchocerciasis?

Answer 2

Africa, Neotropics and Yemen. Only one foci left in South America in Venezuela/ Brazil

Question 3

Where is the most important location for onchocerciasis and why?

Answer 3

In the SAVANNAHS of West Africa as here is where it causes the most river blindness (as opposed to other locations having more dermatitis)

Question 4

Why are there areas where there is simulium damnosum but no oncho?

Answer 4

SD are not biting humans in these areas (e.g. in E and S Africa).

The majority of sibling species in the SD complex do not bite humans.

Question 5

What are the two different manifestations of oncho in two different areas?

Answer 5

Forest: lower rates of blindness in m and f but increased rates of nodules and dermatitis/ skin disease.

Savannah: increased blindness incidence

Question 6

How do parasites develop in flies from different geographical areas.

Answer 6

Co evolved so they develop much better in flies of the same area- forest parasites dev better in forest flies etc.

Question 7

What is a polytene chromosome? how are they used for identifying species?

Answer 7

Polytene chromosomes are large chromosomes which have thousands of DNA strands. They provide a high level of function in certain tissues such as salivary glands. A way of replicating the genes needed to produce specific products e.g. to be secreted from salivary glands. Can look at banding patterns/inversions in the chromosome to separate members of a species complex.

Question 8

What is the double puff?

Answer 8

A structure that allows us to identify chromosome 3 of blackflies –> banding pattern can be used to separate species in a complex/ sibling species.

Question 9

Looking at genetics, how can we tell that mating between members of a species complex is unsuccessful?

Answer 9

Never see heterozygotes in the population, meaning that if mating does occur but is unsuccessful- no recombination! Offspring not fertile.

Question 10

What is a cytospecies/ cytoform?

Answer 10

Species separated into sibling species via chromosomal evidence (looking at the double puff banding patterns on chromosome 3).

Question 11

Describe the distribution of vector species breeding grounds Kruger found up the Uluguru mountains in Tanzania vs the distribution of onchocerciasis.

Answer 11

Bottom: nkusi at the bottom. ONLY nsuki bites humans. Oncho transmission.

Middle: kilibanum. oncho transmission but not biting people?

Top: sebwe, no oncho, no biting people

Oncho/ human biting extends to the kilibanum region despite them not biting humans. Thought nkusi breeds at lower altitudes then flies up to bite people and goes back down to lay eggs.

Question 12

Although we do not know for definite, what are thought to be the factors that affect biting preference (ie anthropophilic vs zoophilic) between members in a species complex/ sibling species?

Answer 12

• CO2 attractive to all species, but less so for savannah species.
• Forest cytospecies more responsive to visual stimuli
• Savannah cytospecies more responsive to olfactory signals (e.g. sweat from legs and feet).

Question 13

What are the factors thought to make an anthropophilic species a vector or not?

Answer 13

-Environmental factors e.g. temperature affects parasite development
Intrinsic genetic factors:
-Cibarial armature
-Peritrophic matrix (e.g. agglutanins in matrix are better at holding parasites in a blood meal in savannah species)
-Humoural and cellular immunity (antibacterial peptides, agglutanins and lecting, proteases, prophenoloxidase)

Question 14

What are subcomplexes?

Answer 14

Divisions of the cytospecies e.g. in W africa there are these subcomplexes:
Squamosum subcomplex (with basically two cytospecies)
Sanctipauli subcomplex.
Damnisum subcomplex.

Question 15

How does presentation of onchocerciasis inform control programs?

Answer 15

If we want to target river blindness, we may target cytospecies that are present in the savannah. Informs geographical areas targeted by control programs.

Question 16

Where is S soubrense found?

Answer 16

Small forest streams. Found in the St Paul river in Liberia.

Question 17

In the St Paul river in Liberia, there are more soubrense spp biting than yahense. Which was shown to be a better vector? How and why?

Answer 17

Looked at parasite potential (how many infective parasites they were carrying through a dissection). Although more soubrense were biting than yahense, yahense had 4 times as many pontential infections.

Thought to be because, looking at parous rates as an indicator of lifespan, yahense was seen to live to older ages, hence has longer to pick up, develop and transmit parasites.

Also, soubrense bites animals more frequently than humans whereas yahense is more anthropophilic.

Question 18

Describe the relationship between djodji and sanctipauli. What are their vectorial capacities?

Answer 18

Djodji form has an inversion in the short arm of chromosome one and is closely associated with the Y chromosome. Chromosome is really distinct but is probably part of the species complex.

Sanctipauli is a kuch more inefficient vector to carry oncho. Djodji 100x more infected than sanctipauli so much more efficient as a vector.

Question 19

What is vagility?

Answer 19

A vector’s ability to move around/ endence to migrate or disperse. Affects vector ability and increased vagility in djodji form may contribute to its increased vectorial capacity.

Question 20

What is CTDI?

Answer 20

Community Directed Treatment with Ivermectin. Special form of MDA.

Question 21

How far can blackflies fly looking for blood?

Answer 21

20km. Can fly even further for long range migration. Wind patterns can increase this distance. Varies between cytospecies and long distance migrants can come with the wind.

Question 22

Describe the effects on migration/ dispersion that come with the monsoon and harmattan (dry) seasons.

Answer 22

Monsoon wind predominates in rainy season and pushes upwards. Dry wind predominates in the harmattan and pushes downwards.

Question 23

What is the intertropical convergence zone?

Answer 23

Where the monsoon and harmattan winds meet. This zone is pushed north or south depending on which wind is predominating. It is responsible for carrying flies.