CHAPTER 27 Animal-Transmitted, Arthropod- Transmitted, 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 illustrates the life cycle of the malaria parasite, Plasmodium vivax.

West Nile Virus West Nile fever is an emerging mosquito- borne 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 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.