Chp 20 Diseases of the blood (Bacterial & Viral Diseases Transmitted by Arthropod Vectors)

See Chp 5 for parasitic/fungal/helminthic infections



1.    Plague – Yersinia pestis; zoonosis; spread from infected rodents to humans by fleas; incidence of disease is low; bacilli travel in lymphatics to lymph nodes where they cause enlargements caused buboes (especially in armpits and groin); hemorrhages turn skin black (“Black Death”); deaths can be prevented with antibiotic treatment.


2.    Tularemia – Francisella tularensis – zoonosis; reservoir:  usually cottontail rabbits (number of causes rises during rabbit-hunting season); first isolated in Tulare County, CA in 1911; low incidence in US; disease is an hazard for taxidermists, hunters, trappers; transmission:  breaks in skin, bite from arthropod vector (tick, deer fly), inhalation of aerosols during skinning, consumption of contaminated meat; clinical signs:  ulcer formation at the entry site, fever, severe headache, and buboes (enlargement of lymph nodes); one form can lead to a septicemia that resembles typhoid fever; vaccine does exist but frequent boosters are required.


3.    Relapsing FeverBorrelia recurrentis (spirochete) and other species of this same genus; transmitted by ticks and human body and head lice; lice are crushed and their body contents scratched into the skin; ticks transmit bacteria in their salivary secretions several days of high fever, respites, and shorter periods of fever due to changes in organisms’ antigens; can cross placenta; treat with antibiotics; syphilis patients were once purposely infected with relapsing fever to kill syphilis bacteria.


4.    Lyme DiseaseBorrelia burgdorferi (spirochete); transmitted by deer ticks (Ixodes damini); white-tailed deer are a reservoir; develop flu-like symptoms; the rash erythema chronicum migrans characterizes the disease (“bull’s-eye” rash – concentric rings around initial site of tick bite); other symptoms include arthritis, myocarditis, etc.; there is a vaccine for dogs; treat with antibiotics.

     5.    Typhus Fever – occurs in a variety of forms:


a.       Endemic (murine) typhus – murine refers to rats and mice; Rickettsia typhi (obligate intracellular parasite); does occur in Texas; transmitted by fleas (fleas defecate while biting, infecting the humans); fever, chills, crushing headache; disease is self-limiting and lasts about 2 weeks if untreated; mortality is low.


b.      Epidemic typhus (classic or louseborne typhus) Rickettsia prowazekii (obligate intracellular parasite); epidemics were halted with discovery of pesticide DDT during WWII; transmitted by body lice (when a louse bites, it defecates; as victim scratches bite, they inoculate the microorganisms into the wound); fever and headache; rash starts on trunk and spreads to extremities (rarely affects palms or soles); vaccine is available; treat with antibiotics; mortality can range to 40% if untreated.


c.       Brill Zinsser disease (recurrence of epidemic typhus) – disease has milder symptoms, in shorter in duration, and often does not cause a skin rash; caused by reactivation of latent microorganisms harbored in lymph nodes.


6.    Rocky Mountain Spotted FeverRickettsia rickettsii (obligate intracellular parasite); transmitted to humans by dog ticks; fever, headache, weakness; rash begins on ankles and wrists and progresses toward trunk (reverse of progression in typhus); rash is prominent on palms and soles; blood vessel damage cause hemorrhages throughout the body; treat with antibiotics.


7.    Rickettsial poxRickettsia akari (obligate intracellular parasite); transmitted by mites fond on house mice; disease is mild and lesions resemble those of chickenpox; often misdiagnosed as chicken pox.



1.    Yellow Fever – monkeys are reservoirs of the disease; transmitted to humans by mosquitoes; many victims suffer severe liver damage and become jaundiced.


2.    Encephalitis – discussed under nervous system diseases; transmitted by mosquitoes.