The Nature Of Virus Reservoirs

Since viruses must replicate to survive, actively infected populations are the usual source of infection. Still, some viruses such as poxviruses and SARS virus have a high resistance to desiccation. In this case a contaminated object such as a desk, pen, book, contaminated clothing, or other inanimate object can be identified as the immediate reservoir. The last documented cases of smallpox in Somalia were apparently acquired from contaminated soil. The persistence of some viruses in fecal material is also a potential long-lasting, essentially passive, reservoir of infection. Aerosols of infectious Hantavirus and canine parvovirus can be infectious for many months after secretion. Also, some viruses, especially hepatitis A virus, can be isolated from contaminated water sources for several days or even weeks after inoculation.

Even though infectious virus can be maintained for a time in a passive state, in nature the ultimate source of a viral pathogen is an active infection in another host. The two most usual reservoirs for human disease are other humans or other animals (zoonosis). The spread of a virus from its reservoir to the next individual host organism or cell requires an aqueous

Human to human infection

Respiratory

Respiratory

Zoonoses (infection acquired from animals)

Biting arthropod vector

Vertebrate reservoir

Fig. 3.1 Some transmission routes of specific viruses from their source (reservoir) to humans. The mode of transmission (vector) is also shown. (Based on Mims CA, White DO. Viral pathogenesis and immunology. Boston: Blackwell Science,

Vertebrate reservoir/ arthropod vector

Biting arthropod vector

Vertebrate reservoir

Vertebrate reservoir/ arthropod vector

Fig. 3.1 Some transmission routes of specific viruses from their source (reservoir) to humans. The mode of transmission (vector) is also shown. (Based on Mims CA, White DO. Viral pathogenesis and immunology. Boston: Blackwell Science,

environment, since the virus must chemically recognize the host cell and induce chemical processes involved in getting its genomic material into the appropriate portion of that cell to initiate viral gene expression. In the case of a virus of a multicellular organism, this requires the virus to be mechanically moved into close proximity to the cell; modes of spread of some human viruses are illustrated in Fig. 3.1, and pathogenic viruses and their reservoirs discussed in this section are listed in Table 3.1.

Some viruses with human reservoirs

A significant number of human viruses leading to either mild or life-threatening disease are maintained in human populations. The list runs the gamut from colds caused mainly by rhi-noviruses, warts caused by papillomaviruses, to AIDS caused by HIV. The mode of passage of viruses between humans (i.e., the vector) is intimately involved with human behavior. This behavior can be modified by the disease symptoms themselves. Thus, a respiratory infection leads to coughing and sneezing, which spreads an aerosol of droplets containing virus. HSV is spread in saliva requiring direct transfer of an aqueous suspension; by contrast, the closely related varicella zoster (chicken pox) virus (VZV) is spread by inhalation of a virus-loaded aerosol. Warts are spread by direct physical contact between the virus-loaded source (another wart or a passive reservoir) and layers of the skin below the keratinized epidermis exposed by small cuts or abrasions. Poliovirus is spread only by virus-containing feces contaminating food or drinks that are then ingested by a susceptible host. In the case of HIV, body fluids, including blood, breast milk, serum, vaginal secretions, and seminal fluid, are sources of infection. The virus can be spread by passive inoculation of, for example, a contaminated hypodermic syringe, by transfusion, breast feeding, or by sexual activity.

Table 3.1 Some pathogenic viruses, their vectors or routes of spread, and their hosts.

Virus

Vector/route

Host

Disease

Poliovirus

Human-fecal

Human

Enteric infection, in rare cases CNS

contamination of water

infection (poliomyelitis)

or food

Western equine

Mosquito

Horse

Viral encephalitis in the horse - occasional

encephalitis

infection of human

La Crosse

Mosquito

Squirrel, fox (reservoir),

No obvious disease in squirrel or fox; viral

encephalitis

human

encephalitis in human

Sin nombre

Deer mouse

Deer mouse, other

Hantavirus hemorrhagic respiratory distress

(Hantavirus)

rodents (reservoir);

syndrome

human

HIV

Direct injection of virus-

Human

AIDS

infected body fluids

into blood

Measles

Aerosol

Human

Skin rash, neurological involvement

Yellow fever

Mosquito

Tropical monkeys, human

Malaise, jaundice

Dengue fever

Mosquito

Human, primates

Mild to severe hemorrhagic disease

Ebola

Unknown vector, but

Reservoir unknown;

Often fatal hemorrhagic fever

nosocomial

humans and primates

transmission

Hepatitis A

Fecal contamination of

Human

Acute hepatitis

water or food

Hepatitis B

Direct injection of blood

Human

Chronic hepatitis, liver carcinoma

Hepatitis C

Direct injection of blood

Human

Acute and chronic hepatitis

Hepatitis delta

Blood, requires

Human

Acute hepatitis

coinfection with

hepatitis B

Hepatitis E

Fecal contamination of

Human

Mild acute hepatitis except often fatal to

water or food

pregnant women

Rabies

Bite of infected animal

Vertebrates

Fatal encephalitis

Herpes simplex

Saliva, other secretions

Human

Surface lesions followed by latency, rare

(HSV)

encephalitis

Varicella-zoster (VZV,

Aerosol

Human

Rash, shingles,

chicken pox)

latency

Epstein-Barr (EBV)

Saliva

Human

Infectious mononucleosis, latency

Influenza

Aerosol

Human, many vertebrates

Flu

Smallpox

Aerosol

Human

Variola

Myxoma

Insect bite

Rabbits

Variable mortality, skin lesions

Rhinovirus

Aerosol

Human

Colds

Coronavirus

Aerosol

Civet cat (for SARS CoV);

Colds; SARS

human

Rubella (German

Aerosol

Human

Mild rash, severe neurological involvement

measles)

in first-trimester fetus

Adenovirus

Aerosol, saliva

Human

Mild respiratory disease

Papillomavirus

Contact

Human

Benign warts, some venereally transmitted,

some correlated with cervical carcinomas

HTLV (human T-cell

Injection of blood

Human

Leukemia

leukemia virus)

Tomato spotted wilt

Thrip

Broad range of plant

Necrosis of plant tissue, destruction of

(bunyavirus)

species

crops

Cadang-cadang

Physical transmission via

Coconut palm

Coconut palm pathology

(viroid)

pruning

Prion (protein

Ingestion or inoculation

Human, other mammals

Noninflammatory encephalopathy

pathogen)

of prion protein

have specific types,

cross species spread

possible

Plant rhabdoviruses

Leaf hoppers, aphids,

Broad range of plant

Necrosis of plant tissue, destruction of

plant hoppers

species

crops

Some viruses with vertebrate reservoirs

While many human viral diseases are maintained in the human population itself, some important pathogens are maintained primarily in other vertebrates. A disease that is transmissible from other vertebrates to humans is termed a zoonosis. Rabies is a classic example of a zoonosis that affects humans only sporadically. Because humans rarely transmit the virus to other animals or other humans, infection of a human is essentially a dead end for the virus. The rabies virus, which is transmitted in saliva via a bite, is maintained in populations of wild animals, most generally carnivores. The long incubation period and other characteristics of the pathogenesis of rabies mean that an infected animal can move great distances and carry out many normal behavioral patterns prior to the onset of disease symptoms. These symptoms may include hypersensitivity to sound and light, and finally, hyperexcitability and frenzy. Except in rare instances of inhalation of aerosols, humans only acquire the disease upon being bitten by a rabid animal; however, the fact that the disease can be carried in domestic dogs and cats means that when unvaccinated pets interact with wild animal sources, the pets become potential vectors for transmission of the disease to humans. Vaccination of pets provides a generally reliable barrier.

Viral zoonoses often require the mediation of an arthropod vector for spread to humans. The role of the arthropod in the spread can be mechanical and passive in that it inoculates virus from a previous host into the current one without virus replication having occurred (a favored route with animal poxviruses), but the arthropod's role as a vector can be dynamic. For viruses with RNA genomes that are transmitted between hosts via arthropods (such as those responsible for yellow fever, a number of kinds of encephalitis, and dengue fever), virus replication in the vector provides a secondary reservoir and a means of virus amplification. This makes spread to a human host highly efficient since even a small inoculation of the virus into the arthropod vector can result in a large increase in virus for transmission to the next host.

VIRUSES IN POPULATIONS

Most (but certainly not all) virus infections induce an effective and lasting immune response. Some of the basic features of this response are described in Chapters 7 and 8, Part II. An effective immune response means that local outbreaks of infection result in the formation of a population of resistant hosts — often termed herd immunity. This means that any virus that induces protective immunity must maintain itself either in another reservoir or by dynamically spreading in "waves" through the population at large. If enough members of the susceptible population become immune, virus cannot spread effectively and it becomes extinct. This herd immunity is a major factor in both gradual and abrupt changes in the virulence of many viruses resulting from the random acquisition of genetic alterations.

Viral epidemiology in small and large populations

The occurrence of mild respiratory infections (such as a common cold) in isolated communities provides graphic examples of the process of virus extinction. For example, when scientists visit the Antarctic research stations at the beginning of the Antarctic summer, they bring in colds to infect the resident population. When scientists stop arriving with the onset of winter, the prevailing respiratory diseases run their course and disappear. Figure 3.2 charts a classic epidemiological study of respiratory illness in an isolated fishing and mining population on

Fig. 3.2 Occurrence of respiratory illness in an arctic community (Spitzbergen Island, Norway) that is isolated during the winter months. Following the last boat communication with the European mainland, the number of respiratory illnesses declines from a low number to almost nil. With the first boat arriving in the Spring, new serotypes of respiratory viruses are communicated from the crew and passengers and a "mini-epidemic occurs." As the virus passes through the population, resistance builds and infections decline to a low level. (Based on data originally published by Paul JH, Freese HL. An epidemiological and bacteriological study of the "common cold" in an isolated Arctic community (Spitsbergen [sic]). American Journal of Hygiene 1933;7:517.)

Fall

Winter

Spring

Fall

Winter

Spring

Spitzbergen Island in the Arctic Ocean. Note, that after the last contact with the "outside world," the incidence of such viral-borne respiratory infections rapidly declines to an undetect-able level.

In large populations the rate of virus spread greatly surpasses the limitations of the generation of herd immunity and the introduction of a novel pathogenic virus leads to epidemic spread of disease. The recent outbreak of SARS in China and its spread to Canada provides an important case study of this process, as well as providing examples of effective and ineffective public health measures set up to deal with it. The SARS virus is a member of the corona virus family and distantly related to one that causes mild colds in humans. The virus appears to have been maintained in wild animal populations in southeast China and was introduced into humans in Guangdong Province and the city of Guangzhou (Canton) through the custom of using such animals as dietary delicacies. While human infection is characterized by flu-like symptoms, the persistence, severity, and relatively high death rate suggested that this was a novel type of infection — a novel virulent form of influenza or an uncharacterized respiratory virus. Current evidence suggests that the Chinese government, in hopes of avoiding loss of tourist and business travel revenues, suppressed news of this outbreak.

The disease was spread by a physician who had treated infected individuals in China and then traveled to Hong Kong on business — as the first identified source of infection, he was termed the index case. He contaminated the registration desk of the hotel in which he was registered and this desk served as a source of infection for a number of tourists from other parts of the world including Toronto, Canada who happened to be staying in the same hotel. The disease spread into individuals in Hong Kong and was eventually described and quarantined there, but not before other infected individuals traveled back to Canada, and, in lesser numbers, to the United States.

In Toronto, the index case of the local epidemic was a woman who infected her immediate family members upon returning from Hong Kong. She and one son subsequently died, but not before being admitted to the hospital where a physician treating them as well as other members of the hospital staff were infected. This illustrates a continuing conundrum of modern medicine — the concentration of individuals suffering from an infectious disease in a hospital can serve as a potent reservoir for the spread of that disease through the staff attending them, and, subsequently, others. Such nosocomial infections are a major occupational hazard for hospital personnel as well as patients suffering other maladies, yet hospitals are obviously necessary for the treatment of the severely ill.

The Canadian public health authorities were reluctant to initiate stringent quarantines for infected individuals in the hospital where the first patients were housed, and the hospital served as a source of infected individuals who spread the disease to others both through family and social contacts and through contact in other workplaces. By contrast, in the United States, the infection was initiated somewhat later. By that time sufficient information concerning the disease, its spread, and its control lead to rapid quarantine of SARS patients, especially among health workers. These control methods were successful in the United States and Europe, as well as in Hong Kong, and the virus never spread beyond the first intimate contacts.

Day 1

Day 3

Day 5

Day 7

Index case (died)

Case A

(Admitted to hospital A, then died)

Day 10-15

Index case (died)

Case A

(Admitted to hospital A, then died)

Day 10-15

Fig. 3.3 Fictionalized time-line of the spread of SARS virus following its introduction into Toronto, Canada from Hong Kong in early 2003. The data for this figure are based on material presented on the CDC website (http://www.cdc.gov/ncidod/sars/) and in the February 2004 issue of the Journal of Emerging Infectious Diseases, which was dedicated to studies on the SARS outbreak of late 2002—early 2003.

HRHR W

24 persons 9 persons 21 persons 15 persons 4 persons 7 persons

Fig. 3.3 Fictionalized time-line of the spread of SARS virus following its introduction into Toronto, Canada from Hong Kong in early 2003. The data for this figure are based on material presented on the CDC website (http://www.cdc.gov/ncidod/sars/) and in the February 2004 issue of the Journal of Emerging Infectious Diseases, which was dedicated to studies on the SARS outbreak of late 2002—early 2003.

A fictionalized sequence of events based on the Canadian SARS outbreak is shown schematically in Fig. 3.3. Without the intervention of public health and other government agencies, the spread would continue through a susceptible population for an extended period of time. Further, it is clear that rapid recognition of symptoms and effective quarantine of affected individuals is the key to stopping spread. In the case of SARS, the suppression of information concerning its appearance until it was potentially out of control in Asia could have lead to a widespread epidemic there and in neighboring countries.

Many have suggested that only the lucky fact that SARS is not particularly efficient at spreading between individuals saved us from a much more serious situation. Further, it has been argued that SARS provided a testing ground for public health response strategies, which worked reasonably well. Other examples of serious virus epidemics have not had as felicitous outcomes — for example, the HIV epidemic, which continues to grow and consume increasingly significant public health resources. Several other potentially lethal epidemics threaten the human population in the next few years, including a strain of avian influenza (H5NI), which has the potential of being truly devastating.

General features of these diseases and the viruses that cause them will be discussed in Part III, and an overview of the potential threats of virus disease in the future will be briefly addressed in Part IV. It suffices here to note that the dynamics of virus spread are not the problem, rather it is coordinating political, public health, medical, and scientific resources targeted at the control of infection in a timely and efficient manner that is and will continue to be major challenges.

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