Arthropod-Borne Diseases

Arthropods can transmit viruses, bacteria and parasites. To date, there is no known arthropod-borne fungal infection.

Mosquitoes transmit more disease, and a greater variety of disease, than any other arthropod. However, other biting flies, fleas, ticks, lice and mites are also responsible for transmission of some infections. Most of these infections are zoonotic; animal and human hosts may exhibit disparate behaviour and inhabit differing environments, thus the arthropod vector is essential at bridging the ecological gap between the two. Arthropod-borne animal infections may adapt to transmit to humans and directly from human to human, making arthropods important contributors to the emergence of some infectious diseases.

Some mosquito species, such as Aedes (Stegomyia) aegypti , are increasingly cosmopolitan and are found throughout the tropical world. The ability of Ae. (Stg.) aegypti to breed in dirty urban water containers has resulted in large outbreaks of dengue in tropical metropolises. Rapid airline transport of mosquitoes or infective human hosts may result in large outbreaks of disease in areas previously free of infection but with competent arthropod vectors. South America has been afflicted by this twice in the last 5 years; chikungunya arrived in 2013 and Zika virus in 2015.

Some pathogens, particularly parasites, are dependent upon arthropod host physiology to complete their life cycle. Most pathogens can multiply within the arthropod vector and some, particularly viruses, may mutate to more virulent or transmissible forms of disease. A single nucleotide polymorphism in Venezuelan equine encephalitis virus increased its infectivity to Culex , resulting in an outbreak of disease in Mexico in 1996.

With the exception of Japanese encephalitis and yellow fever, there are few effective vaccines for arthropod-borne diseases. At the community level, prevention relies upon vector control through environmental management and chemical agents. In recent years, there has been increasing insecticide resistance worldwide, threatening control programmes. There is also increasing resistance to essential drugs used to treat these diseases, such as artemisinin which is used to treat malaria.

This chapter is divided into four sections. The first describes the main mosquito vectors of infection and the following three sections describe arthropod-borne viral, bacterial and parasitic infections.

Mosquito Vectors

The main characteristics of the principal mosquito vectors of infection are presented in the following figures.

Fig. 1.02, Female Anopheles gambiae probing.

Fig. 1.03, Aedes (Stegomyia) aegypti taking a blood meal.

Fig. 1.04, Aedes (Stegomyia) albopictus taking a blood meal.

Fig. 1.05, The expanding geographical range of Aedes (Stegomyia) albopictus .

Fig. 1.10, Third-stage larva of Anopheles spp.

Fig. 1.11, Third-stage larvae of Culex quinquefasciatus and Aedes (Stegomyia) aegypti .

Fig. 1.13, Heads of adult culicine mosquitoes.

Fig. 1.14, Heads of adult Anopheles mosquitoes.

The Arboviruses: Arthropod-Borne Viral Infections

The arboviruses are a diverse group of viral infections spread by arthropods. Following a blood meal from an infected vertebrate host, the virus multiplies within the arthropod, before being transmitted to a vertebrate host.

Most arboviruses are zoonotic; the exceptions are dengue and o’nyong’nyong where humans are the primary vertebrate host. Mosquitoes are the most common vectors, but ticks and sandflies are also implicated in the transmission of some viruses. The global distribution of some mosquito vectors and the rapidity of international travel has allowed certain viruses to become endemic in areas previously free of disease following movement of viraemic hosts or mosquitoes. Gene mutation may also have an effect on transmission; chikungunya moved rapidly through Asia following a single amino acid change in its envelope protein, which greatly increased transmissibility by a secondary mosquito vector, Aedes (Stegomyia) albopictus .

Although arboviruses cause a wide range of clinical symptoms, it is conceptually simpler to consider them in syndromic groups. Some, such as dengue and chikungunya virus, cause a fever, arthralgia and rash syndrome. Others, such as West Nile virus and St Louis encephalitis virus, cause an encephalitic syndrome. Viruses such as Crimean-Congo haemorrhagic fever and yellow fever may cause a haemorrhagic syndrome. There is some overlap between these syndromic groups; West Nile virus causes fever, arthralgia and rash in mild infection and encephalitis in severe infection. Table 1.1 summarises current arboviruses of clinical significance, updated to December 2016.

Table 1.1

Arboviruses of clinical significance, updated to December 2016

Virus or disease	Main vector(s)	Main reservoirs/amplifying hosts	Geographical distribution	Clinical syndrome
Family: Togaviridae
Genus: Alphavirus
Chikungunya	Aedes	Monkeys, ? rodents	Africa, Asia, South America	FAR
Mayaro	Aedes, Anopheles	Wild vertebrates	South America	FAR
O'nyong'nyong	Anopheles	Humans	Africa	FAR
Ross River	Aedes	? Wallabies	Australia	FAR
Sindbis	Culex	Birds	Africa, Asia, Australia, Europe	FAR
Venezuelan equine encephalitis	Aedes, Culex	Rodents/horses	North/South America	Encephalitis
Western equine encephalitis	Culex	Birds/horses	North/South America	Encephalitis
Eastern equine encephalitis	Culiseta	Birds/horses	North/South America	Encephalitis

Family: Flaviviridae
Genus: Flavivirus
Dengue 1, 2, 3, 4	Aedes	Aedes/humans	Worldwide tropics	FAR
Zika	Aedes	Humans/non human primates	Africa, South America, Asia, Pacific	FAR, congenital syndrome
Kunjin	Culex	Birds	Australia, Indonesia	FAR/encephalitis
Japanese encephalitis	Culex	Birds/pigs	East Asia/Australasia	Encephalitis
Murray Valley encephalitis	Culex	Birds	Australia	Encephalitis
Rocio	Culex	Birds	Brazil	Encephalitis
St Louis encephalitis	Culex	Mosquitoes, birds	North/South America	Encephalitis
West Nile	Culex	Birds, bats, horses,	Africa, Europe, North America, Middle East	Encephalitis
Louping ill	Ixodidae	Rodents, birds	UK	Encephalitis
Powassan	Ixodidae	Rodents	North America	Encephalitis
Tick-borne encephalitis	Ixodidae	Small mammals/ruminants	Europe	Encephalitis
Russian spring-summer encephalitis	Ixodidae	Rodents, birds	Russia	Encephalitis
Kyasanur forest disease	Ixodidae	Forest rodents/monkeys	India	Haemorrhagic
Omsk haemorrhagic fever	Ixodidae	Ticks, rodents	Russia	Haemorrhagic
Yellow fever	Aedes Haemagogus	Mosquitoes/monkeys	South America, Africa	Haemorrhagic

Family: Reoviridae
Genus: Coltiviridae
Colorado tick fever	Ixodidae	Rodents	North America	FAR/encephalitis/haemorrhagic

Family: Bunyaviridae
Genus: Nairovirus
Crimean-Congo haemorrhagic fever	Ixodidae	Ticks, sheep, goats, cattle, rodents, birds	Parts of Africa, Asia, Europe, Middle East,	Haemorrhagic
Genus: Bunyavirus
California encephalitis group	Aedes	Rabbits, rodents	North America	Encephalitis
La Crosse	Aedes	Chipmunks, squirrels	North America	Encephalitis
Oropouche	Culicoides	Monkeys, sloths	South America	FAR, encephalitis
Genus: Phlebovirus
Rift Valley fever	Anopheles, Aedes, Culex	Cattle, sheep, goats	Africa, Middle East	Fever, encephalitis, haemorrhagic
Sandfly fever (Naples, Sicily)	Phlebotomus	Unknown	Africa, Asia, Europe	FAR
Sandfly fever (Toscana)	Phlebotomus	Unknown	Countries bordering Mediterranean Sea	FAR, encephalitis

FAR: fever, arthralgia, rash syndrome

Dengue

Fig. 1.16, Breeding site of Aedes (Stegomyia) aegypti in a Thai cemetery.

Fig. 1.17, Clinical course of dengue infection.

Fig. 1.19, Confluent rash with islands of sparing.

Fig. 1.20, Linear petechiae in a patient with dengue.

Fig. 1.21, Subcutaneous haemorrhage in dengue haemorrhagic fever.

Fig. 1.22, Intracranial haemorrhage in severe dengue.

Fig. 1.24, Guppy fish used for vector control in South East Asia.

Yellow Fever

Fig. 1.27, Temperature chart of a patient with yellow fever.

Fig. 1.28, Upper gastrointestinal bleeding in the terminal phase of yellow fever.

Fig. 1.29, Section of liver from a fatal case of yellow fever.

Fig. 1.30, Angolan child vaccinated during the 2016 yellow fever outbreak.

Chikungunya and Other Alphaviruses

Fig. 1.31, Acute arthritis of the interphalangeal joints in a patient with chikungunya.

Fig. 1.32, Eastern grey kangaroos, vertebrate host of Ross River virus.

Fig. 1.33, MRI changes in a fatal case of eastern equine encephalitis.

Zika

Fig. 1.35, Brazilian infant with microcephaly.

Fig. 1.36, CT scan abnormalities in congenital Zika virus syndrome.

Fig. 1.37, Urine and semen; diagnostic specimens for Zika virus polymerase chain reaction.

Japanese Encephalitis and West Nile

Fig. 1.38, Geographical distribution of the West Nile and Japanese encephalitis viruses.

Fig. 1.39, Grey heron, a potential carrier of West Nile virus (WNV).

Fig. 1.40, Maculopapular rash in West Nile virus (WNV).

Fig. 1.41, A section of human brain from a fatal case of encephalitis caused by West Nile virus (WNV).

Fig. 1.42, MRI of an immunocompromised patient with West Nile virus (WNV).

Fig. 1.43, Control of culicine vectors of West Nile virus (WNV).

Fig. 1.45, Cycle of transmission of Japanese encephalitis virus (JEV).

Fig. 1.47, Rice paddy fields in South India.

Fig. 1.49, Wasting following acute flaccid paralysis secondary to Japanese encephalitis virus (JEV).

Fig. 1.50, MRI changes in acute Japanese encephalitis virus (JEV).

Rift Valley Fever and Crimean-Congo Haemorrhagic Fever

Fig. 1.51, Distribution of Rift Valley fever virus (RVFV) and Crimean-Congo haemorrhagic fever (CCHF).

Fig. 1.52, Farmer with cattle in East Africa.

Fig. 1.53, Retinitis in a patient with Rift Valley fever virus (RFVF).

Fig. 1.55, Immature Hyalomma marginatum rufipes feeding on an African dove, Streptopelia senegalensis .

Fig. 1.56, Ecchymoses in a patient with Crimean-Congo haemorrhagic fever.

Fig. 1.57, Bilateral subconjunctival haemorrhage.

Fig. 1.58, Subarachnoid haemorrhage in a patient who died of Crimean-Congo haemorrhagic fever.

Arthropod-Borne Bacterial Infections

Arthropod-borne bacterial infections are predominantly transmitted by ticks, lice, mites and fleas. The exception to this is Francisella tularensis, which can be transmitted by mosquitoes, and Bartonella bacilliformis, which is believed to be transmitted by sandflies.

Rickettsiae multiply within vascular endothelial cells resulting in a vasculitis-like systemic disease of varying severity. These infections are summarized in Table 1.2 . Borrelia species are expert at evading the immune system; B. burgdorferi is one of medicine’s great imitators whilst episodic antigen variation in other Borrelia species gives rise to periodic fever relapses. Yersinia pestis has caused some of the greatest plagues in history, including the Black Death, which killed up to 200 million people between 1346 and 1353.

Table 1.2

Clinically relevant rickettsial diseases, updated to 2016

Disease	Causative agent	Vector	Animal reservoir	Geographical distribution
Typhus
Epidemic typhus	Rickettsia prowazekii	Pediculus humanus corporis	Humans Flying squirrels– New World Unclear elsewhere	Ethiopia/Rwanda/Burundi Rural South/Central America
Endemic (murine) typhus	Rickettsia typhi	Xenopsylla cheopis ∗ Ctenocephalides felis	Rat (cat, opossum-USA)	Worldwide
Spotted fevers
Rocky Mountain spotted fever	Rickettsia rickettsii	Various tick spp. ∗	Many small mammals	North America, patchy distribution in Central and South America
Mediterranean spotted fever/Fièvre boutonneuse	Rickettsia conorii	Rhipicephalus sanguineus ∗	Dogs, small mammals	Mediterranean, sub-Saharan Africa
African tick typhus	Rickettsia africae	Amblyomma spp. ∗	? Rodents	Sub-Saharan Africa, Eastern Caribbean
Lymphangitis associated rickettsiosis	Rickettsia sibirica subsp mongolotimonae	Dermacentor spp. ∗	Farm and wild animals	France, Portugal, South Africa
Transitional group rickettsiae
Queensland tick typhus	Rickettsia australis	Ixodes holocyclus ∗	Rodents	Australia
Rickettsial pox	Rickettsia akari	Mites ∗ Liponyssoides sanguineus	Mice	Korea, central and southern Africa, Ukraine, eastern USA
Scrub typhus
Scrub typhus	Orientia tsutsugamushi	Trombiculid mites	Small mammals, rodents, birds	South and South East Asia, Australia, Pacific
Anaplasmosis
Human monocytic ehrlichiosis	Ehrlichia chaffeensis	Amblyomma americana	White tailed deer, raccoon, opossum	Unclear – cases reported from North America, Europe, Asia, Africa
Human granulocytic anaplasmosis	Anaplasma phagocytophila	Ixodes scapularis	White-footed mouse, other rodents, cattle, sheep, horses	North America, North and Central Europe, China, Russia

∗ Vertical transmission can occur in vectors.

Table 1.3

The filarial diseases

Species	Clinical disease	Location of MF (periodicity)	Sheath	Main characteristcs	Distribution
Wuchereria bancrofti	LF	Blood (nocturnal ∗ )	+	Tail - pointed tail, nuclei not to tip. Body - 275-300 x 9μm, smooth curves. Sheath - stains pale mauve with Giemsa.	All tropics
Brugia malayi	LF	Blood (nocturnal ∗∗ )	+	Tail - tapers irregularly, 2 nuclei at tip. Body - 200-275 x 6μm, kinked. Sheath - stains bright pink with Giemsa.	Southeast Asia
Brugia timori	LF	Blood (nocturnal)	+	Tail - tapers irregularly, 2 nuclei at tip. Body - 290-325 x 6μm, kinked. Sheath - stains poorly with Giemsa.	Timor-Leste
Loa loa	Eye worm	Blood (diurnal)	+	Tail - rounded tip, nuclei to tip. Body - 250-300 x 9μm, kinked. Sheath - does not stain with Giemsa.	West Africa, forested areas
Onchocerca volvulus	River blindness	Skin (no periodicity)	–	Tail - long and pointed, nuclei not to tip, crooked. Body - 240-360 x 8μm. Head - spatulate. No sheath.	Africa, Central and South America, Yemen
Mansonella perstans	Most asymptomatic	Blood (no periodicity)	–	Tail - nuclei extend to tip of rounded tail, large nucleus at tip. Body - 190-240 x 5μm. No sheath.	Sub-Saharan Africa, Central and South America
Mansonella ozzardi	Most asymptomatic	Blood/Skin (no periodicity)	–	Tail - long pointed tail with no nuclei. Body - 150-200 x 5 μm, anterior nuclei positioned side by side. No sheath.	Central and South America, Caribbean
Mansonella streptocerca	Asymptomatic Dermatitis	Skin (no periodicity)	–	Tail - nuclei extend to tip of rounded, hooked tail. Body - 180-240 x 5μm. Single file anterior nuclei. No sheath.	Central and West Africa

LF , lymphatic filariasis.

∗ In the South Pacific, microfilariae may be diurnally sub-periodic.

∗∗ In the Philippines, microfilariae may be nocturnally sub-periodic.

All of these bacteria are capable of causing severe disease, requiring antibiotics for resolution. Most respond to doxycycline, although the addition of further agents including aminoglycosides or chloramphenicol is often beneficial.