TUGAS PARASITOLOGIRANGKUMAN

oleh:

Anindhita Dyah Sekartaji NIM 132010101086

FAKULTAS KEDOKTERANUNIVERSITAS JEMBER

JEMBER2015

BAB 110

ZOONOSES

Trenton K. Ruebush II

Zoonoses are infections ihat are transmitted in nature between vertebrate animals and humans.

They include those infections that man acquires from lower animals as well as infections that can

be transmitted from man to lower animals. Also usuallv included are infestations with

ectoparasites, such as scabies and myiasis, in which the arthropod burrows into or penetrates the

body of the vertebrale host.

To understand the epidemiology of zoonotic diseases, it is first necessary to have a

comprehensive understanding of the cycle of transmission of the organism in natuie. as wiih any

infectious disease, four factors must be considered in the transmission of zoonotic diseases: the

infective agent, the host, the route of transmission, and the enviionmenr. However, as a group of

diseases, the zoonoses tend to demonstrate much more complexity and variability in their

epidemiology than do most infectious diseases. This is primarily due to the fact that a minimum

of two hosts, the natural reservoir host and man, are involved in the transmission of all zoonotic

diseasei. As a result, a greater variety of host-infectious agent interaction may be established, and

the epidemiology of the disease may vary with each host-agent combinalion.

Infective Agent and Reservoir Host.

More than 150 different zoonotic diseases have been recorded. The natural or reservoir host of a

zoonotic disease may be a wild animal, a domestic animal, or both. Human beings are not

necessary for the transmission of zoonotic diseases in nature. They become infected when

they accidentally come into contact with some part of the narural transmission cycle between the

organisms animai reservoir host. Arthropods may also act as a reservoir for certain infections

agents. The rickettsiae of Rocky Mountain spotted fever can be maintained for several years in

the tick vector by-transstadial and transovarial passage in the absence of infected vertebrate

hosts. Tiansoiarial transmission of some virus species also occurs in mosquitoes. Babesia microti

apparently overwinters in the tick vector and then is transmitted io susceptible rodent hosts

during the following spring.

Transmission. The transmission of an infectious agent

from the reservoir host to man may take place by an of the four basic routes of diseaie

tranimission contact or by the airborne, vector-borne, or common vehicle route. However, the

route of transmission of ar: organism among the natural animal hosts and betrveen the reservoir

host and man is not necessarily identical. With tularemia, for example, tick transmisiion is the

major _route of spread among wild animals, but man generally acquires infection by direct

contact with the tissues of infected animals.

Environment. Environmental factors play an important role in the epidemiology of all zoonotic

diseases but particularly for those who contact with wild animal reservoirs. The general term

"landscape epidemioiogy'" has been used to emphasize the epidemiologic importance of the

environment in which a disease occurs and to indicate that the physical characteristics of a

locality may suggest the likelihood of a given disease being present or not.

Control. Zoonotic diseases present special problems in the planning of control and eradication

programs. Control measures may be directed against any of the links in the chain of

transmission-the infective agent, the natural host, or the route of transmission. Because of the

variety of host animals involved in many zoonotic diseases, however, control of the infectious

agent in one or two animal species alone is often not sufficient to eradicate the disease. This is

particularly true in the case of zoonotic diseases that have a wild animal reservoir, such as

sylvatic plague, leishmaniasis, or yellow fever. Efforts directed against the reservoir of an

infective agent offer a greater chance of success lvhen the major reservoir host is a domestic

animal. Control programs for bovine tuberculosis, anthrax, and brucellosis have been particularly

successful. Even with these diseases, however, it is often not economically feasible to eliminate

all infected reservoirs. As a result, the infection may persist at lorv levels in one or more

reservoir hosts, and unless controI measures are continued, further outbreaks will occur. Public

health education and protection of food supplies are generally effective in controlling zoonotic

diseases such as trichinosis, brucellosis, and salmonellosis, in which infection is acquired by

ingestion of the causative agent. In the case of occupational diseases, measures ciirected toward

reducing liuman exposure to the infectious agent through healtn education, improvement of

working conditions, and vaccination of highrisk individuals may be the most satisfactory

approach to disease control.

BAB 111

MOLLUSKS INVOLVED IN DISEASE TRANSMISSION

William A Sodeman, Jr.

Some mollusks transmit infection when eaten because of bacteria or viruses concentrated by

filter feeding, such as the bivalves, ovsters, mussels, and clams. Many parasites utilize the snail

as a first and/or second intermediate host for their larval stages. Transmission to humans can

occur with ingestion of raw or poorly cooked snails, but most often the parasite leaves the snail

and penetrates the skin of the host or encysts on another food item that is eaten raw. Snail

identification primarily depends upon morphology, including distinctive shells, radula (tooth)

patterns, and soft-part anatomy, principally the anatomic variations oI the genital apparatus. In a

few cases, biochemical. inrmunologic, or cytogenetic characteristics must be asccrtained to

distinguish betu,een morphologicalli, sirnilar. closely related species. Shell morphology is often

helpful in separating potential internrediate hosts from the manv unsuitable mollusks. There are

many areas where shell morphology alone can serve as an adequate guide to the prcsence or

absence of suitable intermediate-host snails, but in some circumstances, the definitive species

identification requires at least the study of soft-pan anatomy and radular structure. The class

Gastropoda is divided into two subclasses: Streptoneura readily identifiable by the presence of an

operculum and the Euthyneura, of which the freshwater pulmorrtes are the primary concern.

SUBCLASS STREPTONEURA. Snails within the subclass Streptoneura are operculate, with

the opercu1um, a trap door covering the aperture of the shell, fixed to the top of the foot behind

the aperture. The operculum can be calcified, but in most freshwater families it is made of an

organic noncalcified material. Soft-part anatomy shows a characteristic figure-eight twist to the,

visceral nerve and in the aquatic form, anterior gills. There are three orders, one of which, the

Mesogastopoda contains snails of medical importance.

Order Mesogastropoda. Snails of the order Mesogastropoda are responsible, as intermediate

hosts, for the transmission of an extraordinary collection of trematodes that can produce disease

in man. Eight families are of medical importance. These are the Ampullariidae (Pilidae),

Viviparidae, Pomatiopsidae, Bithyniidae, Thiaridae, Pieuroceridae, Potatimididae, and

Littorinidae.

Family Viviparidae. The families Ampullariidae and Viviparidae can be distinguished by the

appearance of the operculum, which grows with a concentric rather than a spiral pattern.

Viviparidae are large, globular snails (Fig. 111-1) that are ovoviviparous (bear live

young). They are widespread in lakes and rivers. A single species, Viviparus javanicus, serves as

a second intermediate host for Echinostortta ilocanum in Java. Family Ampullariidae.

Ampullariidae are also large. globular, egg-laying snails that have a worldrvide disiribution. Two

of the many genera in thrs family are of medical signi0cance. Pila luzonica and P. conica sert'e as

second intermediate hosts for E. ilocanum in the Philippine Island of Luzon. Marisa cornuarietis

(Linnaeus) is an aquatic carnivorous snail that has been utilized in the attempted biologic control

of Biomphalaria species that can transmit Schistosoma mansoni in South America and the

Antilies. Family Thiaridae. Although these snails have a worldwide distribution, their medicai

importance rs principally in the Orient and Southeast Asia. One African genus is involved in the

transmission of disease. The genus Thiara have tall, turreted shells measuring 2.5 to 5 cm as

adults. There is a prominent axial beaded sculpture. Thiara granifera (Lamarck) (Fig. 111-i) in

Taiwan and the islairds of Southeast Asia and noll, introduced in Central America, the Antilles

and the United States; Melanoides tuberculata in both Africa and Asia; and Brotia asperata

(Lamarck) in the Philippines all are reported to serve as first intermediate hosts for the lung fluke

Paragonimus westermani.

SUBCLASS EUTHYNEURA. This subclass is separated into four orders, three of which_the

Basommatophora, the Stylommatophora, and the Systellommatophora have representatives of

medical importance. Order Basommatophora. This order has four families with genera of

medical importance, all for their role as intermediate hosts of tremitodes. These are the families

Ancylidae, Lymnaeidae, physidae, and planorbidae. Ferrissia Family Ancylidae.(Bourguignant).

A single genus has been of the implicated Ancylidae, as the intermediate host for Schistosoma

haematobium in India. Family Lymnaeidae. This fanrily is large and geographically

widespread. The liver flukes Fasciola hepatica, Fasciola gigantica urilize genera of this family as

intermediate hosts. Several species of Trichobilharzia that can cause schistosome deimatitis in

man also use Lymnaeidae as hosts. Nine species of. Lymnaec serve as intermediate hosts of F.

hepatica.

CONTROL

One obvious means of eliminating snail-related diseases is to control ihe intermediate host. The

success of control measures is variable, and many factors affect their use. As a consequence,

there is no simple protocol for control measures. Each attempt at control needs to consider the

host, intermediate host, infecting agents, and physical setting, as well as the relationships among

these four variables, if a workable approach is to be identified. Control methcds teke the form of

protection from exposure to the detinitive host and the intermediate host, as well as antisnail

measures.

EXPOSURE CONTROL. Logical approaches to controlling mollusk-transmitted diseases

include restriction of human exposure to the mollusk and/or restriction of exposure of the

mollusk to infecting organisms, Control of human contact with infected mollusks is largely

accomplished by quarantine and posting of infected patients. This is effective, realistically, only

in dealing with commercial food-processing operations. Regulation of shellfish beds is effective

in the control of oyster- and cian-transmitted viral hepatitis as well as of transmission.

ANTISNAIL MEASURES. Four mechanisms for direct snail control may be employed. These

include snail removal by mechanical or manual means, snail control by environmental

manipulation, the use of molluscicides, and the use of biologic control. Snail Removal by

Mechanical Means. This is largely a measure of the past. No selective mechanisms currently

exist to concentrate snails for removal. Environmental Manipulation. This has proved to be a

useful tool in snail control, but costs as well as local engineering problems severely limit its

application. Where transmission of infection involves irrigation schemes or fish farms, it is often

possible to plan for intervals of desiccation. Molluscicides. Four compounds are currently

employed in the chemical control of mollusks. These compounds have been subjected to

extensive irial and are commonly available in quantity. Copper Sulfate. This compound has been

used the longest and is effective only against aquatic snails. Its effect is considerably reduced by

organic matter in the environment, and it currently has limited application. Sodium

Pentachlorophenate (NaPCP), This compound is available in many formulations, some of which

have slow-release residual capability. NaPCP is absorbed by mud and is broken down by

sunlight, It is a toxic substance and must be employed with attention to the safety of workers.

Niclosamide. Niclosamide (Bayluscid) is effective at a much lower concentration than NaPCP. It

may be applied as a powder or an emulsion and is effective against amphibious snails. N-

Tritylmorpholine, N-Tritylmorpholine (Frescon) is available in many forms, including bait. It is

less soluble than the other compounds mentioned. It has excellent stability in the environment

and is effective in low concentration

BAB 112

TICKS AND MITES IN DISEASE TRANSMISSION

Daniel E. Sonenshine and Abdu Fahrang Azad

Ticks and mites are members of the subclass Acari; one of the dominant subclasses of the

Arachnida (chelicerate arthropods). Arachnids are believed to have appeared during the late

Archeozoic or early Paleozoic eras.. i.e., during the period of proliferation of bottom feeding

invertebrates (e.g., Trilobites). Adaptive radiation followed colonization of the terrestrial

environment, with explosive multiplication of new life forms. Virtually all of the arachnid taxa,

including the Acari, are believed to have evolved during this warm, moist period (Savory, 1977).

Arachnids, including the ticksa and mites, are distinguished from insect by the lack of clearly

defibes head, by chelicerae instead of mandibles, by the absence of antennae. and bv the

presence of 4 pairs of walking legs (except in larvae of the Acari). The body is subdivided into

the anterior prosoma, bearing the appendages, and the posterior opisthosoma. The Acari are

readily distinguished from.other arachnids by the general absence of segmentation, so that the

prosoma and optsthosoma are fused (in all but a few adult mites), and by the presence of the

gnathosoma (i.e., capituluii in ticks), a unique structure at the anterior end of the body bearing

rhe mouthparts.

TICKS (Suborder Ioxodida)

All ticks are obligate bloodsucking parasites. Most ticks are relatively large, i.e., 5 to 10 mm

long in adults, as compared with mites, which usually measure less than 1 mm in length. There is

a single pair of respiratory pores, or spiracles. The hypostome is prominent and covered with

retrose teeth for anchoring the tick to its host. This is the primary holdfast for attaching the tick

to the host’ body. In ioxodids, copious quantities of cemment, secreted during the first few hours

of attachment process, surround the hypostome and secure the tick to host skin. Genus Ixodes.

This is the largest genus of hard ticks, with approximately 245 species.These ticks are readily

recognized by the anal groove, which curves anterior to and encloses the anus. In the males, the

ventral surface is covered by sclerotized ventral plates. Most species of the genus are nest- or

burrow-inhabiting parasites with cryptic, nonfeeding males. Several species, however, are non-

nidiculous,distributed widely throughout wooded or grassy environments. All are 3-host ticks;

each life stage drops off after feeding to molt on the ground or in the nest. Genus Amblyomma.

This is one of the largest of the ixodid tick genera, found mostly in tropical and subtropical

regions of the world. these ticks are easilr. recognized by their remarkably ornate multicolored

scutum and their unusually long mouthparts, particularly palpal.article two, which is about twice

as long as article three. All species are 3-host ticks. In Africa, A. hebraeum and A. variegatum

are important vectors of animal disease, especially heartwatei, and are also important as pests of

livestock. Congo-Crimean hemorrhagic fever (CCHF) has also been rlcovered from A.

variegatum on many occasions, and specimens have also been found to be infected with the

yellow fever virus. In the United States, A-. americanumi an important pest of livestock, deer,

and humans, is a known vector of Ricketsia rickettsii, the agent of RMSF.

MEDICAL AND VETERINARY IMPORTANCE OF TICKS.

Ticks transmit a greater diversity of disease causing agents than any other group of arthropod

vectors. These include protozoan parasites, e'g', the babesias and theilerias infecting livestock,

bacterial agents such as the borrelias, numerous rickettsiae, and an even greater variety of

arboviruses. In addition, some ticks cause severe toxemias and fatal paralysis of their hosts.

Disease Relationships of the Ixodidae (Hard Ticks)

Lyme Disease. In the United States, the most prevalent tick-borne disease is Lyme borreliosis,

also known as Lyme disease. This dise.ase is caused by a spirochete and is transmitted by

certain.species of the geius lxodes. The disease was first described from the vicinity of Old

Lyme, Connecticut, -United States, but has spiead and is now reported-in 43 states throughout

the country. Approximately 5000 cases were reported in 1988, especiallv in southeastern New

York state and eastern Connetticut. The disease is now regarded as epidemic. In the eastern and

central United States, the major vector is the deer tick. Rocky Mountain Spotted Fever. Until

the epidemic rise of Lyme disease, RMSF was the most prevalent tick-borne disease in the

United States This disease is caused by Rickettsia rickettsii. Despite its name, most cases of

RMSF occur in the eastern regions of the country. The primary vector is the American dog tick,

Dermacentor variabilis, although a variety of other species can also transmit the rickettsiae.

Ticks are the reservoir hosts, and the organism persists in the vector ticks from generation to

generation by transovaria1 transmission. Thus, rickettsia-infected ticks inoculate mice and other

small mammals on which they feed, spreading the disease and providing opportunities for

uninfected ticks to acquire the infection by simultaneous blood feeding on rickettsemic hosts.

Humans acquire the disease when biten by rickettsia-infected, human-biting ticks. Congo-

Crimean Hemonhagic Fever. CCHF is a viral disease that is widespread throughout large areas

of central and western Asia, Europe, and Africa. CCHF is a true tick-borne arbovirus, because it

passes trans-stadially and transovarially within the tick population and survives interseasonally

in these vectors. At least 25 tick species and subspecies have been reported to serve as vectors

and/or reservoirs for this virus. The 2-host vectors Hyalomma marginatum rufipes and other

subspecies are especially important because the immatures may feed on migratory birds as well

as hares and hedgehogs, whereas the adults select artiodactyls and, when available, humans.

Thus, these ticks are important in disseminating the virus intercontinentally along the migratory

routes followed by migratory birds. Omsk Hemorhagic Fever (OHF). This is an acute, febrile

disease with a distinct hemorrhagic syndrome, It is caused by a virus of the genus Flavivirus,

family Togaviridae. This virus is in the same category as the causative agents of the closely

related tick-borne encephalitis (TBE) (i.e., RSSE), Powassan, Louping ill, and Kyasanur Forest

disease and other tickborne Togaviridae. These agents were formerly known as group B viruses.

Relapsing Fever. Perhaps the most important human disease transmitted by argasid ticks is

relapsing fever, a spirochetal disease caused by species of. Borrelia. In East Africa, relapsing

fever is transmitted 6y species of the Ornithodoros moubata complex (sensu Walton). Relapsing

fever outbreaks have alio occurred in the western United States, particularly in rodent- and tick-

infested cabins or similar shelters for hikers or campers.

MITES

In contrast to the ticks, mites exhibit much greater diversity in their body structure and biology.

Most mites are small when compared with ticks, although a few reach sizes as great as 7 mm

long. This vast assemblage of tiny arthropods consists of more than 200 families and over

30,000 species; thousands, perhaps tens of thcusands, more ipecies remain to be described. In

contrast to the ticks, the chelicerae of mites are quite variable, but usually scissorIike, with their

cutting edges most often located on the medial facets. Mites occupy an exceptionallv diverse

habitats. Numerous species live entirely in the soil, feeding on fungi, bacteria, other

microorqanisms, or decomposing organic matter. Others li'.e on the ground, especially in the

upper layers of the soil. ai the interface with the duff, deiritus, incli or ieafr laver that

characterizes the top of the root zone oi the ve-qetation. These may include predaceous species.

dung-feeding species, and others feeding on ingestion. Numerous species are phytophagous

feeding on a wide variety of plants or stored foods.-Many miteJare parasitic, including some that

have obligate parasitic life cvcles. Many are parasitic on insects, others on vertebrates. Some,

such as the itch mite, Sarcoptes scabei, live their entire lives in the tjssues of their host. Although

most could be classified as ectoparasites, living in the skin, in feathers, or in the fur of-their

hosts, others, such as the nasal mites (Halarachnidae), are. endoparasitic, living in the nasal

passages of seais and walruses or in the lungs, bronchi, tiacheae. or

sinuses of various mammalian hosts. The typical life cycle includes the egg, proton.v-mph.

deutonymph, tritonymph, and the adult. Often. one or more of the immature stages is omitted as

development is accelerated. In the insect-parasitizing hay itch mite. Pyemotes ventricosus, the

female does not iay eggs but reproduces viviparously. The opisthosomal region of the female

swells during feeding, forming a billoon-like structure. Young mites develop within this sac and

emerge when they are sexually mature adults.

MEDICAL AND VETERINARY IMPORTANCE OF MITES

Mesostigmatoid Mites. These mites transmit a variety of human and animal diseases. Some

transmit rickettsial diseases, e.g., rickettsialpox, caused by Rickettsia akari, and transmitted by

the house mouse mite, Liponyssus sanguineus. Others are serious pests of domestic fowl, such as

the chicken mite, Dermanyssus gallinae; the northern fowl mite, Ornithonyssus sylviarum; and

the tropical fowl mite, O. bursa, which are serious pests of chickens, turkeys, and other domestic

fowl in various parts of the world. Prostigmatid Mites. These include the chigger mites, the

vectors of scrub typhus. This disease, caused by Rickettsia tsutsugamushi, is the most important

miteborne disease of humans (Chapter 25). Scrub typhus is prevalent in eastern Asia, especially

Japan, eastern China, Taiwan, Vietnam, India, the Philippines, and many islands of the South

Pacific and Australia. It does not occur in the Americas, Europe, or Africa, even though

trombiculid mites (i.e., chigger mites) occur in those regions. Astigmatid Mites. These include

many species that are serious pests or causative agents of disease and

allergy. Among the best known are the mange mites especially the families Psoroptidae,

Sarcoptidae, and Demodicidae. Species of. Psoroptes pierce the skin and suck fluids, which

congeal to induce scabbing and related growths; the scabs provide shelter for the mites, which

reproduce rapidly in the protected environment. In rabbits, the rabbit ear mite, P. cuniculi,

proliferates deep in the ear canal and can penetrate into the brain; the mites eventually kill these

animals unless the patient is treated. In human's, females of the human itch mite, Sarcoptes

scabei,.tunnel into the stratum corneum of the skin. estations are seif-limiting.

Dust Mite Allergy. An increasingly recognized rniteassociated illness is dust mite allergy.

Proteins in mite feces, rather than the mites themselves, are regarded as the primary allergen

responsible for the ailergic reactions (Chapter 105.1). In the United States, after pollen or hay

fever, house dust is the most common cause of allergic reactions. The problem may be seasonal

in northern temperate climates, declining during the heating season, but is perennial in more

mild, humid climates. Common symptoms resemble allergies to pollen and other dustlike

substances, with upper respiratory distress, swellings of nasal membranes and sinuses, sneezing,

tearing, and related symptoms. Dust mite allergies occur worldwide but are most commonly

associated with warm, humid environments. In households, mattresses, pillows, stuffed furniture,

carpets, and other household objects where human or animal dander accumulates in large

quantities may provide optimal environments where the mites find food

BAB 113

INSECTS IN DISEASE TRANSMISSION

Duane J. Gubler

An Arthrophod may transmit a disease agent from 1. person or animal to another in 1 of 2 basic

ways. Mechanical Transmission. This consists of a simple transfer of the organism on

contaminated mouth paris or feet or by regurgitation or defecation. There is no multiplication or

developmental change of the pathogen on or in the insect during this type of

transmission.Examples include a variety of enteroviruses, bacteria, and protozoa of humans that

have a direct fecal-oral transmission cycle. Insects such as houseflies may become contaminated

with these pathogens while feeding on feces and transport them directly to the food of people.

Biologic Transmission, The second and most important type of transmission by insects is

biologic. As the name implies, the pathogen must undergo some type of development in the body

of the insect vector in order to complete its life cycle. There are 3 types of biologic

transmission. Propagative Transmission. This type occurs when the

organism ingested with the blood meal undergoes simple multiplication in the body of the insect.

Examples are the arboviruses, which replicate extensively in the tissues

of the insect. Cyclopropagative Transmission. In this type of.transmission, the pathogen

undergoes a developmental cycle (changes from 1 stage to another) as well as multiplication in

the body of lhe insect. The best example of this type is malarial in which a single zygote may

give rise to over 200,000 sporozoites. over 200,000 sporozoites.

Cyclodevelopmental Transmission. In this third type of biologic transmission, the pathogen

undergoes developmental changes from 1 stage to another but does not multiply. With the

filariae, for example, a single microfilaria ingested by a mosquito may result in only one

infective larva. Extrinsic Incubation Period. In all types of biologic transmission, time is

required for development of the pathogen to the infective stage that can be transmitted. With

arboviruses, this means infection and replication in the salivary glands; with the malaria parasite,

it means invasion of the salivary glands by the infectious sporozoites; and with fitariae, it means

development of the juvenile wonns to the active slage III larvae. This period of time is called the

extrinsic incubation period and is generally 7 to 14 days in duration, depending on the pathogen,

the vector, and a variety of environmental factors. Transovarial Transmission. Some viral and

rickettsial diseases are transmitted from the female parent arthropod through the eggs to.the

offspring. This is termed transovarial transmission. The newly hatched insect larval stages are

infected with the pathogen, which is then transmitted to subsequent developmental stages of the

arthropod (trans-stadial transmission). Finally, venereal transmission of certain viruses has been

documented. Thus, male mosquitoes that become infected transovarially can transferihe infective

virus to uninfected female mosquitoes in the seminal fluid during copulation. These latter types

of transmission have obvious epidemiologic importance in the ultimate infection of humans or

other animals and in the maintenance of the pathogen in nature.

DISEASE TRANSMISSION BY MAJOR INSECT GROUPS

FLEAS (ORDER STPHONAPTERA)

Fleas make up the order Siphonaptera. The adults are small, wingless, laterally flittened, obligate

bloodsuckers that parasitize a wide variety of vertebrate hosts. The larvae are normally free

living, legless, eyeless, and wormlike. They generally live in the nest or habitat of the host and

feed on- organic matter. There are over 2000 species of fleas; with representatives on all

continents, including the Arctic and Antarctic. The majority (94%) of species parasitize

mammals, with the remainder parasitizing birds. Only a relatively few species are of importance

in transmitting disease to humans. The-developmental cycle of fleas from egg in adult normally

takes place in the nest of the hosi. Eggs are generally laid in the nest, where the elongate-l-arvae

feed on organic material such as scales, diied blood, ard fe-ces deposited by the adults, After 2 to

3 weeks, the fully grown larvae spin a cocoon and pupate. The pupal stage may last from 1 to 2

weeks, after which the adult emerges, often after the stimulus of movement or vibration caused

when a host enters the nest. The entire period.of development may last 3 to 4 weeks or longer,

depending on temperature in the nest. The larvae-require high hurnidity. Disease Transmission.

Fleas are important natural vectors of 2 diseases of humans-plague and murine typhus.

In.addition, they have been implicated as, but not proved to be, vectors of a variety of other

diseases such as tularemia, pseudotuberculosis, erysipeloid, hemorrhagic nephrosonephritis,

boutonneusi fever, and a fever and are known intermediate hosts for at least 2 tapeworms.

Plague. Caused by Yersinia pestis, plague is a typical zoonosis of rodents and small mammalJ

that exists in much of the world in a flea-rodent-flea transmission

cycle. Large epidemics of plague, which in the past caused millions of deaths, no longer occur.

Although it is a less important means of transmission, fleas have also been known to transmit

plague bacilli in their feces. In this case, the organism is rubbed into the bite wound, other skin

abrasions, or mucous membranes. Transmission has also been reported when infected fleas are

crushed between the teeth. Finally, pneumonic plague is transmitted from person to person

by aerosol. Murine Typhus. murine or flea-borne typhus is a rodent zoonosis caused bv

Rickettsia typhi (mooseri). This is a disease primarily of rats and mice and has a u'orldqide

distribution, mainly in the tropics. Clinically, it is similar to epidemic or louseborne typhus, but

somewhat milder, The rash is the same. Humans become infected incidentally by a flea that has

strayed from its host, Rickettsiae are ingested by the flea with a blood meal from an infected rat.

The organisms multiply within the gut and are passed in the feces of the flea. The mechanism of

transmission is by rubbing infected feces into skin abrasions or by transfer to mucous

membranes. Transmission may also occur by inhalation of dust contaminated with infected flea

feces. Cestode Infections. In addition to serving as vectors, fleas also act as intermediate hosts

for at least 2 tapeworms that may infect humans, Dipylidum caninum of dogs and Hymenolepis

dimhura of rats. Eggs of both parasites are passed in the feces of their respective vertebrate hosts

and are ingested by larval fleas feeding on detritus in the nest, They hatch, and the cysticeroids

develop in the body cavity of the immature ffea. The adult flea is thus infected at the time of

emergence from the pupa, and transmission occurs when the flea is ingested or crushed between

the teeth of a dog, rat, or person.

SUCKING LICE (ORDER ANOPLURA). Sucking lice are small, wingless, obligate

ectoparasites of mammals belonging to the order Anoplura. The body is flattened, and the legs,

in part, are adapted for clinging to hairs and feathers, Most species are very host speciflc, and the

entire life cycle is spent on one host. The life cycle of all 3 species is incomplete, takes about 3

weeks, and is completed on the human host. The head and crab lice glue their eggs to hairs,

whereas the body louse lays eggs in the seams of clothing. Epidemic (Louse-Borne) Typhus.

Epidemic typhus is caused by Rickettsia prowazekii. The disease has a wide distribution in

Europe, Africa, Asia, and the Western Hemisphere. Large epidemics are generally associated

with cooler temperatures during times of war, famine, and natural disasteis where people are

crowded together in conditions of poor sanitation and hygiene. Epidemic typhus has a human-

louse-human cycle. A person is rrsually infectious for the lice during the febrile period, and lice

become infected when taking a blood meal at that time. The rickettsiae enter the epithelial cells

of the midgut and multiply to such an extent that the cells rupture in 3 to 5 days, releasing large

numbers of rickettsiae into the lumen of the intestine, from which they are then passed in the

feces of the louse. People become infected when infectious feces are rubbed into abrasions of the

skin caused by scratching or into mucous mernbranes. Less commonly, infectious rickettsiae can

be released from lice by crushing. Lice feces may dry in the clothing and can remain infectious

for 60 to 90 days. As a result, the feces may become airborne, causing transmission by

inhalation. Transmission of R. prowaekll does not occur by the bite of lice. reservoir hosts of R.

prowezekii. Trench Fever, Trench fever is caused by Rickettsia quintana. It takes its name from

the trenches of World War I, where it was first described and where it was a major problem. It

was reported again in Eastern Europe during World War II. Trench fever has been reported from

Europe, Africa, Mexico, and Central and South America but is an uncommon disease today.

BUGS (ORDER HEMIPTERA). Medically important bugs belong to 2 families in the order

Hemiptera. These are the families Cimicidae (bedbugs) and Reduviidae (triatome bugs).

Members of the order Hemiptera are the true bugs and are recognized by the characteristic

forewing, the basal half of which is membranous. The mouth parts are of the piercing-sucking

type and are segmented. The proboscis is attached anteriorly and is kept folded back between the

coxae of the first pair of legs. The life cycle is simple, with all instars requiring a meal of blood,

hemolymph, or plant juices, depending on whether the species is hematophagous, predaceous

on other insects, or phytophagous.

FAMILY CULICIDAE (MOSQUITOES). Mosquitoes are by far the most important group of

insects in terms of human disease transmission; more people die each year from mosquito-borne

disease than from any other single cause. There are over 3000 described species in 34 genera, but

only relatively few are important in disease transmission. The most important genera, Aedes,

Culex, and Anopheles, contain vector species for viruses, protozoa, and filariae of humans.

Diseases transmitted by mosquitoes are worldwide and affect millions of people

each year. In temperate regions, mosquitoes overwinter in the egg, larval, or adult stage,

depending on the species. Depending on the location, there may 6e one generation per year

(tundra), or there may be a new generation every 10 to 14 days (tropics). In temperate regions,

the cycle usually continues as soon as the ice melts; adult blood-feeding mosquitoes are usually

present from about March to October. In the tropics, mosquitoes are present_year round, but in

some areas wheie the dry season is extended, mosquitoes may estivate as adulti

or in the egg stage. Disease Transmission. At least 265 viruses have been isolated from

mosquitoes, mostly from culicines. Of these, 109 have also been isolated from humans, and there

is serolosic evidence that many more infect humans in nature. Alphaviruses. Chikungunya virus

is undoubtedly the most widespread and one of the most important alphaviruses in terms of

human health. This virus, firsf described in Africa, probably exists there in a mosquitoprimate

mosquito cycle involving forest mosquitoes. Humans are also susceptible and may carry the

virus back to the village where a person-aedes aegypti-person transmission cycle may exist. The

virus is presently widespread in the large urban areas of Asia, where it exists in this type of

cycle. periodically, transmission may occur in epidemic form. Flavivirues. Flaviviruses are

considerably more important to human health than any other group of arboviruses. Important

viruses that collectively infect millions of persons each year inclurle dengue, yellow fever, and

Japanese encephalitis viruses. All have a wide distribution and are transmitted by, species of

mosquitoes that may have close human contact. Dengue fever is an acute infection characterized

by suden onset of fever and a variety of nonspecific symptoms in its classic form. These viruses

also have the potential to cause severe and fatal disease in humans, and larse epidemics of

dengue hemorrhagic fever (DHF) are not uncommon in several countries of

Southeast Asia. There are 4 serotypes of dengue viruses (types l, 2, 3, and 4), all closely related

antigenicalIy. The viruses, have a tropicopolitan distribution that is closely linked to the

distribution of the principal vector, Ae. aegvpti. Today, there are more people (approximately 2

billion at risk for dengue virus infection than for any other arbovirus infection. Other work has

shown that there is also variation among strains of dengue viruses in their epidemic potential.

This variation among different strains of mosquito and dengue viruses could help explain why

some areas or cities that appear to be permissive to dengue virus transmission do not have large

epidemics of either dengue fever or DHF, whereas others do. Yellow fever; Yellow fever is a

flavivirus that is closely related to the dengue viruses ecologically, antigenically, and clinically.

Antigenically, there is considerable crossreaction between dengue and yellow fever viruses,

although there is not complete cross-protective immunity. Clinically, yellow fever is

characterized by sudden onset of fever and a variety of nonspecific symptoms similar to those of

dengue fever. This may be followed by a more severe disease involving jaundice, hemorrhagic

manifestations, and death. Japanese encephalitis virus: Japanese encephalitis (JE) virus is

another important flavivirus that is related antigenically to dengue viruses. It is strictly an Asian

virus, ranging from India in the west to Japan and Siberia in the east. The illness in humans

ranges from inapparent infection to severe viral encephalitis and death. It is primarily a rural

disease, but in Asia, where dense human populations occur, large epidemics occur.periodically.

Thousands of cases have occurred in India, especially in West Bengal and Bihar State, and in

Nepal. St. Louis encephalitis vlrus. St. Louis encephalitis (SLE) virus is the most important

arbovirus in North America. SLE virus is maintaincd in nature in a bird-Culex mosquito-bird

cycle. Depending on the area, the species-of bird and mosquitoes may change. In the central and

eastern United States, for example, a variety of domestic and peridomestic birds, such as house

parrows, pigeons, blue jays, and. robins, are the important maintenance and amplifying hosts.

Culex pipiens pipiens and Cx. pipiens quinquefasciatus, which breed in storm drains, sewage

treatment ponds, and other polluted water, are the principal mosquito vectors. Both vertebrate

and mosquito hosts bring the virus into the human habitat, where transmission to humans may

occur. Other Culex species such as Cx. resituans and Cx. salinarius have been shown to be

efficient vectors of SLE virus and are probably important maintenance hosts in locations such, us

the Ohio-Mississippi River basin, where they are involved in enzootic transmission. In Florida,

Cx. nigripalpus is the important epidemic vector. In the western United States, however, SLE is

primarily a rural infection. The principal vector there is Cx. tarialis, which brceds in a variety of

clear or foul water, irrigation ponds, ditches, and other sources of

ground water in areas fcd by irrigation systems. Although mosquitoes of the Cx. pipiens-

complex are common in western United States and may transmit SLE virus, they are not the

principal vectors in that part of the country. Protozoa. Of the prootozoan parasites of humans

transsmitted by insects. The rnalarial parasites are by far the most important. Although malaria

was nearly under control by 1970 in many parts of the tropics, it is again the most important

vector-borne disease in the world. A combination of insecticide resistance in the mosquito

vectors and drug resistance by the malarial parasites has resulted in a resurgence of malaria in

most major endemic areas of the tropics. Malaria is a disease of humans caused by 4 species of

Plasmodium. P. fatciparurn is widespread in the tropics and causes the most severe disease in

humans (malignant tertian malaria). P. vivax, which causes benign tertian malaria, is also

widespread in the tropics but occurs in temperate regions as well. P. malariae, the cause of

quartan malaria, is relatively rare but is found in temperate regions and subtropics. P. ovule,

which causes benign tertian malaria, is uncommon occurring primarily in Africa, but has also

been reported in South America and Asia. In addition, there are many other species of

Plasmodium that are natural parasites. of primates. Some of these are indistinguishable

morphologically from the parasites of humans and, in fact can infect humans. Filariasis. There

are 3 species of filaria of humans that are transmitted by mosquitoes. These are Wuchereria

bancrofti, Brugia malayi, and Brugia timori. In addition, there are many other mosquito-borne

filariae parasitizing orher animals. These inClude some species in monkeys that can also infect

humans, the common heartworm of dogs, and other species that infect a variety of mammals,

birds, reptiles, and amphibians. nd important mosquito vectors. The basic life cycle is the same

for all filarial parasites. The embryonic microfilariaeare circulating in the peripheral blood are

ingested by the mosquito when they take blood meal. The microfilariae immediately exsheath,

penetrate the midgut wall, and migrate to the thorax of the mosquito, where they enter a muscle

cell and begin to cievelop. I'he juvenile filariae molt twice during a developmeniperiod of 10 to

14 days and emerge from the muscle cells as active third-stage or infective larvae. These larvae

migrate throughout the body of the mosquito, including- the proboscis or mouth parts. When the

infective mosquito takes a subsequent blood meal, the larvae escape from the labium onto the

skin of the person and penetrate the skin.

FAMILY MUSCIDAE (HOUSEFLIES). The family Muscidae contains the common housefly,

Musca domestica, and a number of other synanthropic species that live in intimate association

with humans. Although the family contains several species that have evolved the bloodsucking

habit, none of them have been incriminated in biologic transmission cf human disease agents.

The importance of houseflies lies in their indiscriminate feeciing habits, which may include feces

of humans and animals and the food of humans. They are thus capable of mechanically

transmitting many organisms, primarily enteric pathogens belonging to a number of taxonomic

groups. These flies are strong fliers and may be feeding or resting on uncovered food only

moments after having fed on or visited feces. Transfer of the organisms can be by simple

contamination of the mouth parts, feet, and body hairs or by ingestion and subsequent

regurgitation or defecation on food. Although there is little doubt that houseflies play a role in

the transmission of a variety of human pathogens, their actual importance in the epidemiology of

the diseases is hard to assess, because most of the evidence is circumstantial.

FAMILY CHLOROPIDAE (EYE GNATS). These are small, robust flies that are attracted to a

variety of body secretions and sores of humans and animals. They are strong fliers and are

persistent in their feeding habits, continuing to return after being brushed away. Breeding sites

are usually loose, sandy soil that has a high content of organic material. The medical importance

of both genera is mechanical transmission of conjunctivitis and yaws. Although these flies are

not bloodsuckers, their habits of frequenting sores, wounds. the eye, and other secretions make

them ideal for mechanical transmission. Again, most evidence is circumstantial, but it seems

highly probable that Hippelates and Siphunculina species play important roles in the

transmission of both bacterial and viral conjunctivitis, outbreaks of which usually occur during

the fly season.

COCKROACHES (ORDER DICTYOPTERA). Other insects, such as cockroaches, are also

capable of mechanical transmission of a variety of organisms, but in general, the evidence for

this is, again, circumstantial. Furthermore, they are probably not as important as flies in this

respect because of their habits and less frequent contact with feces.