All About Zoonoses

In recent decades, emerging diseases have cast a wide net of fear with new infections breaking into human populations. Ambiguity on what comprises zoonoses and how they spread persist.

This section answers general issues of concern as also provides a comprehensive detailing of what zoonoses are, how they spread, what can one do to protect oneself, what is the India situation and what are the prevention and control mechanisms.

What are Zoonoses?

Zoonotic infections or zoonoses are infections naturally transmissible between vertebrate animals and humans. They constitute nearly 60% of all known human infections and over 75% of all emerging pathogens. Zoonoses may be bacterial, viral, or parasitic or may involve unconventional agents such as prion diseases (Bovine spongiform encephalopathy).

Apart from being a public health problem, many of the major zoonotic diseases prevent the efficient production of food of animal origin and create obstacles to international trade in animal products. Animals (thus) play an essential role in maintaining zoonotic infections in nature.

Since the 1980s, infectious diseases of zoonotic origin have become the focus of public health attention. Several large and highly visible outbreaks of emerging zoonotic infections that have affected both human and animal populations and commanded attention worldwide include Mad Cow Disease or Bovine Spongiform Encephalopathy (BSE), West Nile virus fever, HIV, Severe Acute Respiratory Syndrome (SARS), avian influenza A(H5N1) and dengue fever outbreaks.

Diverse zoonotic pathogens
Zoonoses are caused by a diverse group of microorganisms. Infectious syndromes caused by zoonotic pathogens are equally diverse. Hence classification of zoonoses is difficult. Diseases are classified by the nature of the pathogen, animal host, mode of transmission between animals to humans and geographic range of host or clinical syndrome (systemic disease or specific organ system of infection).

Why are Zoonoses important?

In addition to contributing to significant infectious disease burden, zoonoses have major implications on economic, labour and health productivity of a nation, accounting for a majority of new and emerging infectious diseases. Preventing and controlling zoonoses is even more critical today in the context of globalisation of international trade, changes in agricultural practices and global warming, since all these factors are inherent risks for emergence, spread and persistence of zoonoses.

Zoonoses could present themselves as small outbreaks (anthrax, nipah) and also be under the category of ‘neglected zoonoses’ such as brucellosis, rabies, cysticercosis and bovine tuberculosis. Neglected zoonoses are endemic in population groups/commodities and do not normally present as outbreaks. Rather, they continue to severely impact productivity of both humans and animals thus contributing to aggravation of poverty.

What are the key issues in Zoonoses prevention and control?

Lack of awareness: In the absence of a concerted approach towards advocacy and credible sharing of information across platforms, there is ambiguity on how zoonoses impacts socio-economic conditions of affected and at-risk population/s. There are veterinarians who may be less aware of the importance of zoonoses; medical clinicians who encounter zoonoses in human patients but fail to recognise them or concentrate on treating individual patients rather than look at efficient disease control; and general population who is not aware of how prevalent zoonoses are and how they should protect themselves. This situation is further exacerbated in situations where there is inadequate communication between veterinarians, health care professionals and public health organisations.
Weak surveillance systems: Zoonoses are often undiagnosed, therefore untreated, ignored and/or under-reported which makes surveillance and control a huge challenge
No man’s land: Infections that affect animals and humans fall in ‘no man’s land’. Here, a single department is unwilling or unable to take responsibility for the infection. Additionally, in human-animal episodes, for instance, neither the human nor the veterinary health systems have the capacity to deal with an outbreak that involves cross-transmission
Lack of intersectoral approach: From the public health perspective zoonoses, unlike some of the other communicable diseases are not an organised effort in national programmes. Resultantly, it is only when an outbreak occurs or disaster strikes that governments and communities wake up to the fact that risk assessments, early warnings, laboratory capacity for diagnosis, monitoring and treatment are competing priorities. All sectors concerned with human health and development must work together to recognise and combat zoonotic infections

What makes India more vulnerable?

India’s status as a developing country combined with cultural notions involving farming practices and the role of animals, places it at greater risk of zoonotic infections. In the absence of a strategic focus among all the concerned sectors, prevention and control effort of zoonotic infection continues to fall short of global standards.

With a poorly defined human-animal interface, the current strategies fail to target at-risk populations, such as the urban and rural poor, populations involved in agriculture and the immunocompromised.

Though commonly occurring in rural areas, urban areas are also prone to zoonotic infections. With India’s urban population expected to double by the year 2020, there is need to protect the urban poor from zoonotic infections. Regardless of where the disease occurs, zoonoses have a consequential impact on productivity.

Current zoonoses control and prevention programmes do not adequately attend to the public health, environmental management, public safety and veterinary medical spheres of the disease nature. This, including technical capacity constraints, lack of research-based policy making and irregular surveillance and response end up adding to the challenges of control and prevention regarding zoonoses.

Zoonoses prevention and control requires an interdisciplinary disease management approach that can help develop stronger linkages between the human, domestic animal, wildlife and environmental sectors.

Veterinary medicine, human medicine, wildlife, microbiologists and public health practitioners have to come together to develop and implement a coordinated response to effectively prevent and control zoonotic infection.

Where do Zoonoses come from?

In recent decades, emerging diseases have cast a wide net of fear with the emergence of new infections that are breaking into human populations. Numerous reasons have been cited for this trend, most significant being the increase in number of people on the planet and their movement across locations.

In 1900, there were 750 million people on Earth. Now there are 6.2 billion. This staggering growth has pushed people deep into previously untouched corners of the world, where they are encountering new animals and their indigenous pathogens. This population growth has led to farmers raising a lot more livestock in crowded spaces and adding new animals to the food supply since traditional stocks are being depleted. In many countries people and animals are intermingling in large urban markets which are usually in restricted spaces, increasing chances of infection and disease outbreak.

Wild animals can be “cute”, interesting and great company for animal lovers who go to great lengths to acquire, tame and bring in exotic pets into their living spaces. But not many of them know this can lead to a zoonotic infection, in some cases, fatal and with potential of spreading. Increasing global trade in exotic animals has to an extent led to recent monkey pox outbreaks in certain parts of the world.

For most zoonotic pathogens, people are dead-end hosts. An animal reservoir provides a permanent home and people are only infected incidentally, as appears to have been the case with monkey pox. But sometimes, a virus "jumps" to people from other animals and starts to thrive in human populations. The virus that caused SARS, for example, infected civet cats and then the people who handled them in a market that supplied restaurants in China's Guandong province.

The increased mobilisation of humans and pets and increased use of livestock and animal products have led to new concerns for the international health community. Apart from the development of new diseases, there is a recurrence of old diseases, which if transmitted to humans, have the potential to create pandemics, like the recent H1N1 "swine flu" virus. Besides causing a direct health hazard to humans, they affect the world's food supply by requiring destruction or quarantine of millions of dollars in livestock. The huge international trade in animal products for food, plus the large scale movement of people through tourism, have increased the opportunity for these pathogens to mutate and cross transmit between species.

SARS, Ebola virus, H5N1, West Nile virus and H1N1 are a few that have crossed the species barrier recently and now infect humans, sometimes with devastating effects. The mutated pathogens can be transmitted via the food chain, contact with infected animals or can even be airborne or waterborne in some cases.

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How do Zoonoses spread?

Zoonotic diseases can be transmitted through aerosols, bites and scratches, direct contact, arthropod vectors or contaminated food and water.

Transmission of zoonoses between human and animals occurs all around us in a variety of settings - urban, rural and wild areas. Rural areas due to their poor sanitation and animal rearing practices have always been vulnerable. Rapid expansion of urban areas and subsequent unplanned urbanisation have created conditions suitable for transmission of zoonoses in these areas as well - water borne infections due to contamination and scarcity of water supply, vector borne infections due to breeding of vectors, food borne infections due to contamination of open foodstuffs as well packaged foods. Contact with domestic animals continues to be frequent; and pets are a major reservoir and source of zoonoses, especially for children.

Occupational exposure to domestic animals or animal products, especially in backyard operations remains a leading cause of zoonotic disease exposure. Recent factors that have had significant impact on emergence of zoonoses (epidemics of West Nile virus and monkey pox in North America) are human encroachment on wildlife habitat, wildlife trade and translocation and ownership of exotic pets, petting zoos and ecotourism. Traditional leisure pursuits such as hunting, camping and hiking also continue to bring people in close contact with wild animals, arthropods and sometimes contaminated water.

Zoonoses that spread through food
Worldwide, food borne diseases and more especially diarrhoeal diseases, are an important cause of morbidity and mortality creating a strong need to strengthen surveillance systems. Pathogenic microbes such as bacteria, viruses and parasites, or their toxins are present in contaminated foods.

Many microbes are commonly found in intestines of healthy food producing animals and the risk of contamination presents itself in the “farm-to-fork” cycle, on account of food contamination at different stages in the food chain:

  • During slaughter, meat can be contaminated by coming into contact with small amounts of intestinal content
  • At the food processing stage, microbes can be introduced by cross-contamination from another raw agricultural product or from infected humans handling the food
  • In the kitchen, microbes can be transferred from one food to another by a kitchen utensil used to prepare both without washing in between.
  • Proper cooking of foodstuffs kills pathogenic microbes.

Food borne infections commonly present as diarrhoeal illness. Some of the food borne diseases are classified as food borne zoonoses – diseases or infections that can be transmitted between animals and humans through food. Common food borne infections include Campylobacter, Salmonella, Listeria and viruses which enter the body through the gastrointestinal tract.

Food borne zoonotic agent  Source Mode of spread
Campylobacter lives in intestines of healthy birds These bacteria can contaminate raw poultry meat. Eating undercooked chicken, ready-to-eat food or contact with raw chicken can result in infection with this agent
Salmonella bacterium commonly found in intestines of birds and mammals It can spread to humans via foods especially meat and eggs. Salmonellosis is accompanied with fever, diarrhoea and abdominal cramps; if it invades the bloodstream it can cause life-threatening infections.
Listeria Ready-to-eat foodstuff like cheeses and fish or meat products, are often found to be at the origin of human infections although less common than Campylobacter and Salmonella, has high mortality rate among vulnerable groups like elderly/pregnant women (foetal infections, miscarriages and stillbirths).
Calicivirus (including norovirus), rotavirus and hepatitis Human and animal waste Food or water contaminated with human and animal waste cause gastro-intestinal sysmptoms. Calcivirus causes approximately 90% of epidemic non-bacterial outbreaks of gastroenteritis in the world
Recent outbreaks of food borne zoonoses

Monitoring food and livestock feed Increased global trade in agricultural products has brought more people in contact with animal diseases that have subsequently jumped species barriers. Accordingly, food and livestock feed need to be closely monitored during production, handling, processing and distribution. Intersectoral collaboration and dynamic leadership is needed because a disease outbreak in one country cannot be seen as merely a local disease but should be perceived as a global problem. No country is sufficiently isolated or protected to ensure that human and animal population is safe. Forward defenses are needed to prevent and control food-borne infectious diseases with the help of international partnerships that guarantee food quality and food safety programmes which are integrated with strategies for public health and sanitary control.

The emergence and re-emergence of zoonotic diseases may be the consequence of the new patterns of food trade or in some cases increased awareness and surveillance.
  • Enterohaemorrhagic escherichia coli was confined to North America till mid 1990s, but is now found throughout the world
  • Salmonella enteritis and multidrug-resistant form of Salmonella typhimurium via eggs has spread widely since first detected in UK
  • Bovine spongiform encephalopathy spread rapidly from UK to other countries since the 1980s and now threatens to become endemic in Europe
  • In 1997 the pig industry in Taiwan was virtually ruined by the foot and mouth disease. The implicated viral strain was closely related to strains found in Hong Kong.

Water borne Zoonoses
Water borne disease in both epidemic and endemic forms continues to occur in developed and less developed countries. Concern for waterborne disease is dominated by pathogens transmitted by faecal–oral route and by drinking water.

Water borne transmission includes diseases transmitted by faecal droplet inhalation (adenoviruses) and exposure through contact (recreational/occupational). It is interconnected with consumption of seafood (shellfish and other harvest fisheries outputs) and through indirect exposure to water in foodstuffs when water is used in irrigation, food processing, or as an ingredient.

For most water-related pathogens, the organism enters the environment via human or animal faeces deposited on land/water. Some organisms enter the environment via human or animal urine (Leptospira and schistosomes). Other organisms are directly discharged into water (guinea worm larvae) exiting via the skin. Some organisms live within mosquitoes that breed near water. Under favourable conditions, some water-related pathogens can multiply in the aquatic environment. and may reach high concentrations. The final element in this transmission process is the infectious dose or degree of water exposure necessary to transmit the infection.

Depending on type of agent and transmission, water borne zoonoses are classified as:

  • Water borne infections: In typhoid and cholera an enteric microorganism enters the water source through faecal contamination and transmission occurs by ingestion of contaminated water. These infections can be controlled by improving microbiological water quality through water treatment or source protection
  • Water-washed infections: Include diseases due to poor personal and/or domestic hygiene and lack of readily accessible water. This limits washing of contaminated hands and utensils and permits transmission of infectious agents, such as Shigella. Their control is through provision of greater quantities of water and easier access to water and education to improve personal and domestic hygiene.
  • Water-based infections: Include worm infections where pathogen spends part of its life cycle in an aquatic environment. The prototype infections in this category are dracunculiasis, due to ingestion of water contaminated with guinea worm (Dracunculus medinensis) and schistosomiasis transmitted by contact with water contaminated with species of trematode genus Schistosoma. Humans become infected when free-swimming infective larvae penetrates skin during water contact. Control of dracunculiasis and schistosomiasis is through protection of the water source and the user by limiting skin contact with water and by eradication of intermediate hosts.
  • Infections with water-related insect vectors: These are transmitted by insects that breed in water, such as mosquito vectors of dengue and chikungunya or insects that bite near water, like tsetse flies that transmit sleeping sickness. Control of these infections is through application of pesticides, destruction of breeding grounds and construction of piped water supplies.

Zoonoses that spread through Vector

Zoonotic Disease Main Vector  Where does the vector breed? How to prevent and control?
Japanese Encephalitis Culicine mosquitoes mainly Culex vishnui group (Culex tritaeniorhynchus, Culex vishnui and Culex pseudovishnui) are chief vectors of JE in India  Epidemics occur when virus is brought into peridomestic environment by mosquito bridge vectors where there are pigs, which serve as amplification hosts, infecting more mosquitoes which then may infect humans - Reduce vector density; use insecticide treated mosquito nets

- JE vaccine - Make anticipatory preparation for timely availability of medicines, equipment and trained medical/nursing/paramedics

- Keep piggeries 4-5 km away from human dwellings
Dengue and chikungunya Aedes aegypti is a small, black mosquito with white stripes and is approx 5 mm in size.   Desert coolers, drums, jars, pots, buckets, flower vases, plant saucers, tanks, cisterns, bottles, tins, tyres, roof gutters, refrigerator drip pans, cement blocks, cemetery urns, bamboo stumps, coconut shells, tree holes and places where rainwater collects/is stored. - Personal Prophylatic Measures (mosquito repellant creams, bed nets, wearing full sleeves)

- Source reduction: Not allowing fresh water collections in domestic, peri-domestic and commercial areas (clean coolers etc at least once a week, cover overhead tanks, discard used tyres).

- Chemical control (chemical larvicides like abate in big breeding containers; aerosol space spray during day time) to prevent breeding of vectors

Other control measures: -Biological control (larvivorous fish in ornamental tanks/fountains; using biocides)
Filaria C.quinquefasciatus Breeds in association with human habitation, domestic pest mosquitoes, preferring polluted waters (sewage and sullage water including cess pools, cess pits, drains and septic tanks)

They breed in comparatively clean water collections too.  
- Recurrent anti-larval measures at weekly intervals

- Environmental methods: source reduction by filling ditches, pits, low lying areas, deweeding, desilting,

- Biological control of mosquito breeding through larvivorous fish

- Anti-parasitic measures: Detection and treatment of microfilaria carriers and disease person with DEC by Filaria clinics in towns  
Other Classifications of Zoonoses

By etiological agent By mode of transmission By reservoir host
Viral zoonoses – Rabies, arbovirus infections, plague, leptospirosis, salmonellosis, lyme disease Direct zoonoses - Transmitted from infected vertebrate host to susceptible host (man), by direct contact/with a fomite/mechanical vector. The agent undergoes little or no propagative or developmental change during transmission (rabies, anthrax, brucellosis, leptospirosis, toxoplasmosis Anthropozoonoses Infections transmitted to man from lower vertebrate animals (rabies, leptospirosis, plague)
Rickettsial zoonoses – Murine typhus, tick typhus, scrub typhus, Q-fever Cyclozoonoses - Require more than one vertebrate host species, but no invertebrate hosts to complete their life cycle (ecchinococcosis, taeniasis)  Zooanthroponoses: Infections transmitted from man to lower vertebrate animals (staphylococci, human tuberculosis in cattle)
Protozoal zoonoses- Toxoplasmosis, trypanosomiasis, leishmaniasis   Metazoonoses: Transmitted biologically by invertebrate vectors, where agent multiplies and/or develops and there is extrinsic incubation period before transmission to another vertebrate host (plague, arbovirus infections)  Amphixenoses: Infections maintained in both man and lower vertebrate animals and transmitted in either direction (salmonellosis) 
Helminthic zoonoses-Echinococcosis (hydatid disease), taeniasis, schistosmiasis, dracunculiasis  Saprozoonoses: Requires vertebrate host and non-animal developmental site (soil, plant material, pigeon droppings) for development of infectious agent (aspergillosis, coccidioidomycosis, crytococcosis, histoplasmosis, zygomycosis)   
Fungal zoonoses- Deep mycosis (histoplasmosis, cryptococcosis, superficial dermatophytes)    
Ectoparasites- Scabies, myiasis     

When to suspect zoonoses?
Zoonoses must be included in differential diagnosis of most clinical syndromes. All patients with an infectious syndrome whose cause is not apparent after a standard history and physical examination should be questioned to assess possibility of a zoonoses especially in the following scenarios:
  • Whether the person has had exposure to farm animals (which may also be pets) such as horses, pigs, cattle, and fowl (chickens and turkeys). The clinician should determine the amount and degree of exposure
  • Whether the person has had an animal bite or scratch while visiting an area endemic for rabies. The clinician should probe recent travel history.
  • Whether the person has had exposure to pets: Does he own or has had recent contact with a dog, cat, bird, fish, reptile, or rodent? If contact may have occurred, the clinician should ask about a history of bites or scratches
  • Whether the person has indulged in a leisure pursuit such as hunting, fishing, hiking or camping. The clinician should assess specific animal contacts such as dressing or skinning animals, ingestion of water from streams and lakes and bites by arthropods such as ticks
  • Following increased vector density and in months of increased transmission, especially in endemic areas e.g. post-monsoons increase in incidence of dengue

Zoonoses of major public health importance in India Zoonotic diseases of major public health importance in India are Japanese encephalitis, leptospirosis, plague, rabies, anthrax, Kala-azar, Kyasanur Forest Disease, spotted fever caused by Rickettsiae and parasitic diseases like cysticercosisi, hydatid disease, trypanosomiasis and toxoplasmosis. The country also faces potential threat of exotic zoonotic infection like Yellow Fever, Hanta virus infection, Crimean Congo Heamorrhagic fever, Rift Valley fever, Ebolo and Marburg disease.

Japanese Encephalitis (JE): JE was first recorded in Vellore and Pondicherry in the mid 1950’s and the first major outbreak occurred in 1973 in Bankura and Burdwan districts of West Bengal before spreading to other states. The Directorate of National Vector Borne Disease Control Programme (NVBDC) is the nodal agency responsible for controlling and preventing JE in the country.

From 2568 cases reported and 707 deaths reported in 2003, the numbers have gone up to 3473 cases and 511 deaths in 2009

Chikungunya: The first outbreak occurred in Kolkatta in 1963 followed by outbreaks in 1964 in Vellore, Madras and Pondicherry. No outbreaks were reported from India between 1973-2005. Massive multi-state outbreaks were reported in the year 2007 following the re-introduction of virus into the Indian-subcontinent.

From 2461 confirmed cases in 2008 the numbers dipped marginally to 2258 in 2009.

Dengue Fever: The first outbreak was reported in 1963 in Kolkatta followed by more than 70 outbreaks. A large outbreak of dengue hemorrhagic fever was reported in Delhi and neighbouring states in 1996 with 10252 cases and 423 deaths. NVBDCP is a nodal agency for control and prevention of dengue fever.

From 12317 cases and 184 deaths reported in 2006 the numbers have dropped to 2901 cases and 13 deaths in 2009.

Anthrax: It is enzootic in southern and eastern India but less frequent in the northern Indian states. Cutaneous anthrax is the most common presentation in India because of close proximity of livestock with humans. 18 cases and 3 deaths due to anthrax were reported in some villages of Munchingput and Chintapalli Mandal of Paderu Division of Vishakapatnam District in March, April, May 2009 (DHS, government of AP)

Rabies: An estimated 20,000 people die of rabies every year with 95% of mortality and morbidity being a result of bite by rabid dogs. Most of India’s dog population (25 million) is unprotected against rabies. These cases are seen throughout the year with 2/3rd victims being males and 40% children less than 14 years of age. An estimated 17.4 million animal bites occur annually in India and almost 3 million animal bite cases elect to receive post exposure treatment based on vaccine ultilisation. NICD is the nodal agency to coordinate various activities on the rabies project that was started in 2008.

Leptospirosis: Coastal states have reported sudden upsurge. From 5313 cases and 237 deaths in 2004 the numbers have gone down to 1879 cases and 86 deaths in 2009 in the six coastal states of Andamans, Maharashtra, Karnataka, Tamil Nadu, Kerala and Gujarat.

Plague: Last human case was reported in 1966.The disease reemerged in 1994 when cases of bubonic plague were reported from Beed district in Maharashtra. Regular surveillance of rodents and vectors combined with lab and clinico-epidemiological surveillance has helped effective prevention and control of plague.

Brucellosis: The incidence of disease has been reported from various parts of the country. There is a high seropositive prevalence of Brucella in persons working in veterinary hospitals and dispensaries. Prevalence was higher in rural areas and in those 51 years and above.

Hydatid Disease: Prevalence of disease is high in food animals in India. (NICD manual has more material to refer to, in case you have time)

Rickettsial Disease: In 2004, 198 cases were found positive and in 2008, 252. (NICD manual has more material to refer to, in case you have time)

KyasanurForest Disease: The first case was recorded in 1957. Natural infections were identified in several districts of Karnataka. Prevention measures include control of tick vector, health education and vaccination.

Nipah Virus: Two outbreaks have been reported from India. The first occurred among hospitalised patients and medical staff of four hospitals in February 2001 in Siliguri and the second in April 2007 in two villages of West Bengal where fives cases and five deaths were reported. Human to human transmission is yet to be established.

NICD manual on zoonotic diseases of public health importance
Systemic infections zoonotic – pdf
Water borne Zoonoses: WHO
“Wildlife as Source of Zoonotic Infections: Transmission Modes”: Medscape Today
“Globalisation: Catalyst for the spread of zoonotic diseases”: Dr Kwasi Bowi, President, Ghana Veterinary Medical Association in the Daily Graphic

National Vector Borne Disease Control Programme:

What can I do to prevent Zoonoses?

Zoonotic illnesses can infect humans by entering the body in a variety of ways: animal bites, insect bites, by ingestion, by inhalation, through cuts/scratches and through the eyes or contact with other mucous membranes. A combination of precautions would generally be necessary to prevent zoonotic infections:

  • Always practice good hand washing after working with animals, in animal housing areas, or when working with animal products.
  • Cook meat and eggs thoroughly and wash vegetables carefully in clean water.
  • Keep drinking water clean and protected from animals and animal feces.
  • Fight the bite- use insect bite protection techniques for both people and pets/livestock where ever possible
  • Control rats, mice and insect pests around your home.
  • Wash any animal bite site immediately with clean water and see your doctor as soon as possible
  • Keep your pets and livestock healthy, consult with your veterinarian as needed.
  • Minimise contact with animal blood, feces (poop), respiratory secretions, fluids draining from wounds and injuries. When possible take protective measures to prevent transmission of disease causing agents
  • Do not handle wildlife unless absolutely necessary

Controlling Zoonoses
Physicians, veterinarians and public health professionals must work together to recognise and control zoonotic diseases. More than 200 zoonoses exist and these illnesses are becoming more common as more humans become immunocompromised.

Approaches to the control of zoonoses differ according to the type under consideration. Because majority of direct and cyclozoonoses and some saprozoonoses are most effectively controlled by techniques involving the animal host, methods used to combat these diseases are almost entirely the responsibility of veterinary medicine. A good example is the elimination of stray dogs, for they are an important factor in of such as rabies, hydatid disease, and visceral larva migrans.

In addition, of diseases such as and tuberculosis in cattle involves a combination of methods—mass immunisation, diagnosis, slaughter of infected animals, environmental disinfection, and quarantine. Several supportive measures for of disease are useful in some cases. Air-sanitation measures are helpful in direct in which human illness is spread by droplets or dust, and zoonotic infections that are spread through a fluid medium, such as water or milk, sometimes can be controlled. Heat, cold and irradiation are effective in killing the immature forms of Trichinella spiralis, the causative agent of trichinosis, in meat; and certain antibiotic drugs help to prevent deterioration of food.

The control of metazoonoses may be directed at the infected vertebrate hosts, at the infected invertebrate host, or at both. Particularly effective in this instance has been the use of chemical insecticides to attack invertebrate carriers of specific infections, even though several difficulties have been encountered—for example, the inaccessibility of the invertebrate to the chemicals, which occurs with organisms that breed in swiftly flowing waters or in dense vegetation, and the development of insecticide resistance by the organisms. Insecticides are used to destroy the mosquitoes that spread malaria (Anopheles). Mechanical filters placed across irrigation ditches help to prevent the dissemination of the snails that transmit Schistosoma mansoni, a parasitic flatworm.

Many zoonotic agents associated with waterborne disease are also associated with food borne disease in humans. However, they frequently do not cause overt clinical disease in domestic animal populations. National and international agencies with health mandates that address control of waterborne and foodborne diseases should coordinate their activities and share resources to accomplish specific zoonotic pathogen reduction targets.

The most important component of any water treatment strategy is likely to be source protection. This can best be accomplished by taking steps to control zoonotic pathogens in animal populations, proper storage and disposal of animal wastes and limiting physical access of animals to source waters. Efforts should be focused on improving water treatment in rural areas and in small communities and improving the safety of water used in the irrigation of crops, in particular raw edible field crops.

Members of communities that are most at risk of waterborne diseases need to be identified and protected from water borne disease. Those most at risk include the very young, aged and immunocompromised individuals who are genetically most susceptible, and those with increased occupational or recreational exposure to waterborne pathogens.