Download Freeman 1e: How we got there

CHAPTER 27
Animal-Transmitted, ArthropodTransmitted, and Soilborne Microbial
Diseases
Animal-Transmitted Diseases
Rabies
• A zoonosis is an animal disease
transmissible to humans, generally by direct
contact, aerosols, or bites.
• Rabies occurs primarily in wild animals and
is an important endemic zoonotic disease in
developing countries.
• In the U.S., rabies can be transmitted from
the wild animal reservoir to domestic animals
or, very rarely, to humans (Figure 27.1).
Vaccination of dogs and cats is important for
the control of rabies.
Hantavirus Pulmonary
Syndrome
• Hantaviruses occur worldwide in rodent
populations and cause serious diseases such
as hantavirus pulmonary syndrome (HPS)
when accidentally spread to humans. In the
United States, hantavirus infections were not
recognized until 1993.
Arthropod-Transmitted Diseases
Rickettsial Diseases
• Rickettsias are obligate intracellular
parasitic Bacteria that are transmitted by
arthropods.
• Rickettsias cause a variety of diseases in
humans and animals, of which the most
important are typhus fever, Rocky Mountain
spotted fever, and ehrlichiosis.
• Most rickettsial infections are readily
controlled by antibiotic therapy, but prompt
recognition and diagnosis of these diseases is
still difficult.
Lyme Disease
• Lyme disease is now the most prevalent
arthropod-borne disease in the United States
(Figure 27.8).
• It is transmitted from several mammalian
host vectors to humans by ticks. Prevention
and treatment of Lyme disease are
straightforward, but accurate and timely
diagnosis is a major problem.
Malaria
• Malaria is a widespread, mosquito-borne
infectious disease occurring mainly in
tropical and subtropical regions of the world.
It is a major cause of morbidity and mortality
in developing countries.
• Malaria has been a selection factor for
several resistance genes in humans. Sickle
cell anemia is a genetic trait that confers
resistance to malaria but causes a reduction in
the efficiency of red blood cells by reducing
the oxygen-binding affinity of hemoglobin.
• Thalasemia is a genetic trait that confers
resistance to malaria but causes a reduction in
the efficiency of red blood cells by altering a
red blood cell enzyme.
• The disease is preventable with a
combination of public health and
chemotherapy measures.
• Figure 27.10 illustrates the life cycle of the malaria
parasite, Plasmodium vivax.
West Nile Virus
• West Nile fever is an emerging mosquitoborne viral disease (Figure 27.13).
• The natural cycle of the disease involves
infections of birds by the bite of an infected
mosquito. Humans and other vertebrates are
occasional terminal hosts.
• Although most human infections are
asymptomatic and undiagnosed,
complications in diagnosed infections can
cause up to 3% mortality due to encephalitis
and meningitis.
Plague
• Plague is largely confined to individuals
who come into contact with rodent
populations that are endemic reservoirs for
Yersinia pestis.
• A disseminated systemic infection or a
pneumonic infection leads to rapid death, but
the bubonic form is treatable with antibiotics
if rapidly diagnosed.
• Figure 27.15 shows the epidemiology of plague
due to Yersinia pestis.
Soilborne Diseases
The Pathogenic Fungi
• A variety of soilborne fungi produce disease
in humans. Superficial, subcutaneous, and
systemic mycoses are infections that are
difficult to control because of a lack of
specific antifungal drugs and the ubiquitous
nature of the pathogens.
• Fungal infections may cause serious
systemic disease, often in individuals with
impaired immunity, such as AIDS patients.
• Table 27.2 lists some pathogenic fungi and the diseases they cause.
Tetanus
• Clostridium tetani is a ubiquitous soilborne
microorganism that can cause tetanus, a
disease characterized by toxin production and
rigid paralysis.
• Tetanus is preventable with appropriate
immunization. Treatment for acute tetanus is
generally unsatisfactory, with significant
morbidity and mortality.