Chapter 5Chapter 5

Zoonotic and Vector-Borne Diseases

Learning ObjectivesLearning ObjectivesBy the end of this chapter the reader will be able to:• Discuss the impact of human environmental change

on emerging infectious diseases in animals and man

• Indicate how a disease may be transmitted from an animal reservoir to humans

• Define what is meant by a vector-borne disease and a zoonotic disease

• List three human diseases transmitted by arthropod vectors and three caused by zoonoses

• Discuss methods used to control vector-borne and zoonotic disease

ZoonosisZoonosis• Refers to “an infection or infectious

disease transmissible under natural conditions from vertebrate animals to humans.”

• Methods of transmission:o Contact with the skino A bite or scratch from an animalo Direct inhalation or ingestiono The bite of an arthropod vector

ZoonosisZoonosis

Bacterial ShapesBacterial Shapes

Bacteria vs. VirusBacteria vs. Virus

VectorVector• Defined as “an insect or any living carrier that

transports an infectious agent from an infected individual or its wastes to a susceptible individual or its food or immediate surroundings.”

• Exampleso Various species of rodents (rats and mice)o Arthropods (mosquitoes, ticks, sand flies, biting midges)

Vector-Borne DiseasesVector-Borne Diseases• Generally spread by blood feeding vectors

(Biological transmission)• Increased through environmental factors• Examples:

o Malariao Leishmaniasiso Plagueo Lyme diseaseo Rocky Mountain spotted fever

MalariaMalaria• A disease found in more than 100 countries, with

more than 40% of the world’s population at risk• Endemic regions include Central and South

America, Africa, India, Southeast Asia, the Middle East, and Oceania.

• Annual death toll for malaria is more than 1 million persons.

• Global direct economic costs incurred by malaria are estimated to be $12 billion U.S. annuallyo The direct costs include those for treatment and

prevention of the disease o Lost productivity, lost earnings, and negative impacts

upon travel and tourism

TransmissionTransmission• Principle vector for malaria is the Anopheles

mosquito which carry a plasmodium (unicellular parasite)

• Involves the complex life cycle of mosquitoes (the vector) and human hosts (with human liver and human blood stages).

Figure 5-3 Figure 5-3 Female Female Anopheles gambiae Anopheles gambiae mosquito feeding.mosquito feeding.

SymptomsSymptoms

• Fever and flu-like symptoms such as headaches, muscle aches, fatigue, and shaking chillso 9 to 14 days after bite of an infected

mosquitoo Life-cycle Figure 5-4

• Other symptoms to follow can be jaundice, kidney failure, seizures, mental confusion, coma, and ultimately death.

Figure 5-4 Figure 5-4 The life cycle of the parasites of the genus The life cycle of the parasites of the genus PlasmodiumPlasmodium, causal agents of , causal agents of

malaria.malaria.

Control of MalariaControl of Malaria• During mid-20th century, efforts to control malaria

by spraying with DDT and administering synthetic antimalaria drugs were found to be effective.

• Thought to be harmful to wildlife; use of DDT was opposed by many developed nations, especially the U.S.

• South Africa has (and to some extent still) used annual spraying of DDT inside of homes.

LeishmaniasisLeishmaniasis• The reservoir for the cutaneous form of

leishmaniasis includes wild rodents, human beings, and carnivores (e.g., domestic dogs).

• Transmitted from the reservoir to the human host by a sand fly known as the phlebotomus fly.

TransmissionTransmission• Caused by the transmission of a protozoal

organism that exist in two formso Promastigotes and amastigotes

• Present in the countries around the Mediterranean and endemic in a total of 82 countries

• The annual incidence is around 600,000• The reason for increasing cases are:

o Movement of the human population into endemic areaso Increasing urbanizationo Extension of agricultural projects into endemic areaso Climate change due to global warming

TransmissionTransmission

Plague Plague • The bacterium Yersinia pestis is the infectious

agent for plague, a condition that infects both animals and humans.

• Transmitted by the bite of a flea harbored by rodents.

• Historians believe that the plague epidemic during the Middle Ages (the “black death”) was caused by fleas from infested rats.

Source: Adapted and reprinted from the Centers for Disease Control and Prevention. Plague: CDC Plague Home Page. Available at: http://www.cdc.gov/ncidod/dvbid/plague/index.htm. Accessed February 28, 2010.

Figure 5-14 World distribution of plague, 1998.

Plague Plague • Etiology• Once transmitted to humans, it can progress in

stageso Bubonic plague – fever, chills, headacheo Lymphandenitis – infected lymph nodeso Pneumonic plague – communicable through respiratory droplets

• Untreated bubonic plague has a fatality rate from about 50 – 60%

• Controlled by mainly public awareness and controlling the rodent population

Lyme DiseaseLyme Disease• A condition identified in 1977 when a cluster of

arthritis cases occurred among children around the area of Lyme, Connecticut.

• The causative agent for the disease is a bacterial spirochete known as Borrelia burgdorferi.

• Transmission of Lyme disease to humans is associated with infected black-legged ticks (Ixodes scapularis) that ingest blood by puncturing the skin of the host

Figure 5-16 Figure 5-16 (Top) From left to right: The black-legged (Top) From left to right: The black-legged tick adult female, adult male, nymph, and larva.tick adult female, adult male, nymph, and larva.

Source: Adapted and reprinted from the Centers for Disease Control and Prevention. Lyme Disease. Available at: http://www.cdc.gov/ncidod/dvbid/lyme/ld_transmission.htm. Accessed March 14, 2010.

Figure 5-17 Figure 5-17 Number of reported cases of Lyme Number of reported cases of Lyme Disease by county – United States 2007Disease by county – United States 2007

Treatment and Treatment and PreventionPrevention

• Signso Red, expanding rash

called erythema migrans (EM)

o Fatigue, chills, fever, headache, muscle and joint aches, and swollen lymph nodes

• Antibiotic therapy exists for cases that are caught early

• Preventative measures:o Take caution when clearing brush or landscapingo Be aware when you are in areas that ticks inhabit o Wear light colored clothing and inspect yourself after

leaving an outdoor area

Rocky Mountain Spotted Rocky Mountain Spotted FeverFever

• Causal agent is Rickettsia rickettsii, a rickettsial agent. o Transmitted by the bite of an infected American dog tick

or Rocky Mountain wood tick

• A febrile disease with sudden onset of high fevers that may last up to 3 weeks.

• Case fatality rate can range up to 25% among untreated patientso Much lower when treated early with antibiotics

Viral Hemorrhagic Fevers (VHF)Viral Hemorrhagic Fevers (VHF)

• Viruses that cause VHFs require an animal host or insect host (arthropod vector) as a natural reservoir.

• Examples of animal host reservoirs include rodents such as the cotton rat, deer mouse, and house mouse.

• The viruses are limited to those geographic areas in which the host species reside.

Arthropod-Borne Viral Arthropod-Borne Viral Diseases (Arboviral)Diseases (Arboviral)

• A group of viral diseases that can be acquired when blood-feeding arthropod vectors infect a human host.

• Vectors that transmit arboviruses include ticks, sand flies, biting midges, and mosquitoes.

Four Main Clinical Symptoms of Arboviral Disease1. Acute CNS [central nervous system] illness 2. Acute self-limited fevers, with and without exanthum [rash]3. Hemorrhagic fevers4. Polyarthritis and rash, with or without fever and of variable

duration

Arboviral EncephalitidesArboviral Encephalitides• Caused by a virus that produces an acute

inflammation of:o Sections of the braino Spinal cordo Meninges

• Transmitted by the bite of an arthropod vector (primarily mosquitoes)

• Cost of arboviral encephalitides is approximately $150 million per year, including vector control and surveillance activities.

West Nile VirusWest Nile Virus• Classified as a mosquito-borne arboviral fever,

the etiologic agent is a Flavivirus. • Mosquitoes are the carriers that become infected

when they feed on infected birds. • Health effects vary from no symptoms to very

severe symptomatology.o Onset between 3 and 14 dayso In severe cases, neurological effects can be permanent

West Nile Virus - 2009West Nile Virus - 2009

Emerging ZoonosesEmerging Zoonoses• Refers to zoonotic diseases that are caused by

either apparently new agents or by known agents that occur in locales or species that previously did not appear to be affected by these known agents.

• Factors in the rise of emerging zoonoseso Ecological changes that result from agricultural practices

o Other factors include changes in the human population and human behavior

Figure 5-20: Range and recognized site(s) of origin of a variety of emerging and re-emerging infections.

v-CJD = variant Creutzfeldt-Jakob disease; E. coli = Escherichia coli.

Hantavirus Pulmonary Hantavirus Pulmonary Syndrome (HPS)Syndrome (HPS)

• The causative agent is the hantavirus, part of the viral family known as Bunyaviridae.

• May be transmitted when aerosolized (airborne) urine and droppings from infected rodents are inhaled.

• Highly fatal condition transmitted mainly from four species of rodents: cotton rat, rice rat, white-footed mouse, and deer mouse.

The main host for the hantavirus is the deer mouse--Peromyscus maniculatus, which is found throughout North America.

Figure 5-24: Hantavirus pulmonary syndrome (HPS) cases, by state of residence.

Dengue FeverDengue Fever• Caused by four flaviviruses that create illness that

range from mild to severeo Dengue fever, Dengue Hemorrhagic Fever, and Dengue

Shock Syndromeo The proportion of deaths can be as high as 40% to 50%

when the disease is untreated.

• Occurs primarily in tropical subtropical areas of the world, for example, Southeast Asia, tropical Africa, and South America. (figure 5-25)

• The vector for transmission isthe Aedes aegypti mosquito.

o Resurged after the ban on DDT

Dengue Fever in the U.S.Dengue Fever in the U.S.• In the U.S., most cases of dengue are imported by

travelers who are returning from endemic areas or by immigrants.

• Southern Texas and the southeastern states are at theoretical risk for transmission of dengue and for sporadic outbreaks.

• The dramatic global resurgence is due to lack of effective mosquito control, population growth, and poor treatment of water and human waste.

Ebola Hemorrhagic Ebola Hemorrhagic FeverFever

• Severe, often-fatal disease in humans and nonhuman primates (monkeys, gorillas, and chimpanzees) that has appeared sporadically since its initial recognition in 1976.

• Incubation period for Ebola HF ranges from 2 to 21 dayso Onset of illness is abrupt and is

characterized by fever, headache, joint and muscle aches, sore throat, and weakness, followed by diarrhea, vomiting, and stomach pain.

o A rash, red eyes, hiccups and internal and external bleeding may be seen in some patients.

Rift Valley FeverRift Valley Fever• An acute, fever-causing viral disease that affects

domestic animals (such as cattle, buffalo, sheep, goats, and camels) and humans.

• Humans can get RVF as a result of bites from mosquitoes or they are exposed to either the blood or other body fluids of infected animals

• Patents have either no symptoms or a mild illness associated with fever and liver abnormalities. However, in some patients the illness can progress to hemorrhagic fever, encephalitis, or ocular disease.

Other Notable Zoonotic Other Notable Zoonotic DiseasesDiseases

• Monkeypoxo Rareo Contracted through a bite or direct contact with the

infected animal’s blood, body fluids, or lesions

• Tularemiao Potentially serious illness that occurs naturally in the

United States. o It is caused by the bacterium Francisella tularensis

found in animals (especially rodents, rabbits, and hares).

o Also, can be contracted by consuming water or food that is contaminated with the bacterium

Other Notable Zoonotic Other Notable Zoonotic DiseasesDiseases

• Rabieso A highly fatal disease of the CNS caused by a virus that

is transmitted through the bite of an affected animal.o Worldwide, dog bites are the principal source to humans.o In the U.S., most case are associated to wild animals

(bats, raccoons, and skunk)o Early symptoms include irritability, headache, fever and

sometimes itching or pain at the site of exposure. The disease eventually progresses to paralysis, spasms of the throat muscles, convulsions, delirium and death

Treatmento vaccine is usually given in the upper arm for a total of 5

injections over a period of 28 days along with a one time dose of rabies immune globulin for immediate temporary immunity

Other Notable Zoonotic Other Notable Zoonotic DiseasesDiseases

• Anthraxo Anthrax is a serious disease caused by Bacillus

anthracis, a bacterium that forms sporeso There are three types of anthrax:

• skin (cutaneous)• lungs (inhalation)• digestive (gastrointestinal)

o All three forms are treatable with antibiotics administered as soon as possible after exposure

o Symptoms can be mild , and if ignored, become severe and death ensues

Other Notable Zoonotic Other Notable Zoonotic DiseasesDiseases

• The three types of Influenza viruses areo Human influenza A and B viruses cause seasonal

epidemics of disease almost every winter in the United States.

• The emergence of a new and very different influenza virus to infect people can cause an influenza pandemic.

o Influenza A can infect a variety of animals, but certain strains can be specific to certain species

o Influenza type C infections cause a mild respiratory illness and are not thought to cause epidemics.

Control of Mosquito-Control of Mosquito-Borne DiseasesBorne Diseases

• Use sentinel chickens to monitor for presence of viruses.

• Drain standing water.• Introduce mosquito-eating fish into ponds.• Wear repellents and protective clothing.• Repair window screens.

Figure 5-35: Check for Figure 5-35: Check for AedesAedes mosquito breeding in your mosquito breeding in your

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