Vector-borne infections

Reading: pg. 383 – 399

Introduction:

•      Vector-borne infections

–   transmitted by blood-feeding arthropods

–   Pathogens: viruses to worms

•      2 groups: comprise largest number of vectors

–   Insects

–   Ticks and mites

•      Vectors effective means of transmission in sparsely populated areas

•      Disease control via vector control

–   Control of West Nile virus by spraying for mosquitoes

Arbovirus

•         arthropod-borne viruses

•        multiply in salivary gland of vectors

•        Pass to vertebrates during blood meal

•         only a few cause human disease (multisystem)

•         yellow fever

–        flavivirus, only one antigenic type

–       mosquito vector

–       restricted to Africa, Central and South America and the Caribbean

–       Transmission

–       symptoms can be mild

–       resultant liver damage can be fatal

–       Diagnosis is clinical, no specific treatment

–       Prevention: vaccine, vector control (insecticides), insect repellent or nets

•             Dengue fever

–           Flavivirus with four antigenic types

–           mosquito vector

–           occurs in South East Asia, the Pacific, India and the Caribbean

–         “jungle” dengue fever

–         Mild form – malaise, fever, headache, arthalgia, nausea, vomiting, rash

–         (fig. 27.1) Severe form – dengue hemorrhagic fever shock syndrome with mortality of 10%

–         No antiviral treatment, no vaccine

•             Arbovirus encephalitis (fig. 27.2)

•            Most infections mild, few lead to fatal encephalitis

•            West Nile virus

•             major causes of hemorrhagic fever in endemic areas

•            (fig. 27.3) arboviruses that cause hemorrhagic fevers

Rickettsial infections

•            small bacteria

•          Gram negative rod

•          Rickettsia, Coxiella, Ehrlichia, Orientia, Bartonella à rickettsial diseases (fig. 27.5)

•          fig. 27.4: typical events in rickettsial infection

•            Infections tend towards persistency and latency

•            symptoms are fever, headache and rash

•            laboratory diagnosis is primarily serological

•          Complement fixation

•          Culture difficult and dangerous

•            all are susceptible to tetracycline

•            some vaccines available

•            Rocky Mountain spotted fever

–         transmitted by dog ticks or wood ticks

–         10% mortality in 40-60 year olds – due to delayed diagnosis

•             Mediterranean spotted fever

–           transmitted by dog ticks

–           mortality can be 10%

•             Rickettsial pox transmitted by mites

•            Disease is mild

•             epidemic typhus transmitted by body lice

•            R. prowazekii – multiply in louse and transmitted in feces

•            Untreated epidemic typhus – has a 20-60% mortality rate

•             endemic typhus transmitted by rat fleas

•            Cause R. typhi

•             scrub typhus

•            Cause Orientia tsutsugamushi

•            Transmitted by trombiculid miters (chiggers) during feeding

•            Only found in Far East

•            Bacterium transmitted vertically in mites

Borrelia infections

•         relapsing fever

–      Cause Borrelia recurrentis

–      epidemic form transmitted by body lice

–      fig. 27.7 transmission

–      endemic form transmitted by ticks

–      Bacteria endemic in rodent populations and transmitted by ticks

–      fig. 27.8, fig. 27.9 course of event in relapsing fever

–      diagnosed by laboratory culture and Giemsa stains of blood

–      treated with tetracycline to prevent relapses

•             Lyme disease

•           Cause Borrelia burgdorferi

–         transmitted by ticks

•         Fig. 27.10 Transmission to humans

•         (fig. 27.11) Characteristic rash of Lyme disease

•         Short-term symptoms:

–       fever, headache, myalgia, lymphadenopathy

•         Long-term:

–       neurologic (meningitis, encephalitis)

–       cardiologic (heart block, myopericarditis)

–       Arthritis

–         Diagnosed serologically and treated with antibiotic

Malaria

•            mosquito vector

•            life cycle

•          (fig. 27.12) 4 Plasmodium species cause disease in humans

•          fig. 27.13 lifecycle in mosquito and human

•            fig. 27.15 Clinical features: fluctuating fever and drenching sweats

•          Symptoms: fever à cerebral and renal disease

•          Pattern of fever coincides with lifecycle of parasite

•            Complications (fatal)

•          Include cerebral malaria, severe anemia, hypoglycemia, lactic acidosis, glomerulonephritis

•            Immunosuppressive

•          Due to reduced T cell cytotoxicity

•          Fig 27.16 immunity to malaria

•            diagnosed by seeing parasites in red blood cells in blood smears

•          Treatment with quinine

•      Trypanosomiasis (protozoan disease)

–  Sleeping sickness: transmitted by tsetse fly

•   Neurologic infection à coma and death

–  Chagas’ disease: transmitted by kissing bug

•   Attack heart muscles à fatal heart disease

•      Leishmaniasis

–  Transmitted by sandflies

–  Causes liver failure