The word disease refers to any structural or functional deviation from the normal in the system of an animal. Livestock diseases have direct and indirect impacts on the health, socioeconomic status, and well-being of humans. Diseases reduce the amount of livestock products available as animal protein, which can lead to malnutrition (kwashiorkor and marasmus), while some animal diseases are also transmissible to humans via direct contact or their food products (zoonoses).
Livestock producers, pet owners, veterinarians, processors, retailers, and consumers of food of animal origin are all crucial in the surveillance efforts of these diseases. More so, in this age of globalization, most diseases do not respect geographical boundaries (transboundary animal diseases), hence the national and global surveillance efforts of the World Health Organization (WHO), the Office des Internationales Epizooties (OIE), and the Food and Agriculture Organization (FAO).
Disease Prevention, Control, and Eradication
The application of various measures to exclude diseases from an unaffected population is known as disease prevention. It is the first line of defense against disease. While measures directed at reducing the frequency of existing disease to a level where the infection has no significant public health or economic consequences are termed disease control, efforts directed at eliminating the disease agents from a geographical area are termed disease eradication.
These measures are usually applied either at the individual animal or at the population level and can also be at local (e.g., farm), national, or global levels.
The general features of a disease that enable appropriate intervention include:
- The existence of sufficient detriment that eradication is considered economically justifiable;
- The existence of features that enhance easy detection and surveillance and
- The existence of at least one tool that is effective in altering disease transmission.
Reasons for Disease Prevention and Control
- Mortality: Disease may cause premature death of animals, and this can have devastating effects both on individual families and on the entire country, especially when such animals experience high mortality during their peak of production.
- Reduced Productivity: The presence of a disease causes a reduction in the yield and quality of products and thereby leads to decreased availability of animal products, decreased availability of animals for transport, for games, for companionship in the case of pets such as dogs and cats, and for clothing (in some parts of the world).
- Disruption of herd structure and productivity (population dynamics): When a disease results in a high rate of premature death or involuntary culling, the herd will often have lower productivity and economic returns. In a dairy herd, this may have negative effects on productivity because diseased animals will not remain in the herd long enough to adequately achieve their full genetic production potential, which occurs at approximately the fifth lactation. Another effect of culling is increased demand for and, hence, the high cost of replacement animals, as well as an increased risk of introducing diseases to the farm.
- Public Health: Diseases of animals affect human and animal welfare. Zoonotic diseases such as rabies, tuberculosis, and trypanosomiasis can directly affect human health. The health of animals, particularly if they are pets, can have a significant impact on the mental and physical health of man. In addition, using various chemicals to combat animal pests and diseases can result in the deposition of residues in edible animal products such as meat, milk, eggs, or the environment, and this may directly affect human health by causing anaphylactic reactions in humans to antimicrobial agents or indirectly by causing increased resistance to antibiotics. Antibiotic resistance is a major global health concern due to its misuse in both humans and animals.
Transmission of Diseases
The three common methods of transmission of infectious agents to animals are contact, vehicle, and vector. The understanding of these methods is crucial to the adoption of appropriate strategies for the prevention, control, and eradication of diseases.
1. Contact Transmission
Direct contact denotes physical contact between the infected and susceptible individual, whereas indirect contact transmission denotes contact between the infected and susceptible individual by means of fresh secretions, e.g., a leptospiral organism in urine contaminating drinking water, or by means of aerosol droplets resulting from coughing or sneezing. For instance, contact with recently infected litter in poultry is usually a more important means of transmitting coccidiosis and Newcastle diseases.
2. Vehicle Transmission
Involves inanimate substances, e.g., feed, water, dust, and equipment. Vehicular transfers can be mechanical or biological. Mechanical transmission involves a physical medium of transfer to the host, e.g., truck tires contaminated with the Foot and Mouth Disease virus and poultry feathers contaminated with the Newcastle Disease Disease virus. While in biological vehicles, the agents undergo further development or multiplication in the medium; an example would be the transmission of bacteria in milk.
3. Vector Transmission
This denotes invertebrate animals carrying infectious agents between vertebrates. Such transmission can be purely mechanical, e.g., the transmission of the equine encephalitis virus by mosquitoes, or biological, e.g., the development of the larval stages of the dog earthworm Dirofilaria imitisin by Culex species of mosquitoes, which must occur for the larvae to become infective to animals or humans.
Disease Prevention and Control Strategies
Several factors contribute to the occurrence, distribution, and spread of diseases. Hence, there is more than one approach to preventing or controlling a disease.
Common disease prevention and control measures are aimed essentially at some or all of the following:
- Destruction or removal of disease-causing organisms or conditions (e.g., nutrition) in the environment;
- Interference with the possible transmission of disease agents to susceptible animal hosts; and
- Making the animal hosts resistant to disease-causing agents.
Strategies employed in disease prevention and control include:
1. Management and Husbandry
Good husbandry and management practices are the primary and crucial aspects of disease prevention. Sound husbandry practices reduce stress on the animals, thereby reducing the possible host susceptibility to infections. Good husbandry practices include:
- Stocking of the foundation and replacement animals that are free from genetic abnormalities or vertically transmitted infections;
- Isolation of all newly purchased animals, separated for some days under veterinary supervision before they are mixed with the old stocks;
- Adequate ventilation, a consistent temperature, and avoiding drafts and excessive humidity;
- Avoiding overstocking by allowing adequate floor space per animal.
2. Reduction of Contact
Disease prevention and control can also be achieved by reducing or preventing physical contact between infected and non-infected animals via aerosols, physical barriers (solid partition pens), or by building separate facilities. All-in and all-out systems of husbandry in poultry, swine, feedlot cattle, and fish production ensure that new stock do not have any contact with the old and possibly diseased stock. Adequate ventilation in the building is necessary to reduce aerosol transmission in intensively housed poultry, swine, and cattle systems.
3. Modification of Host Resistance
The resistance of the host to infection may be modified by increasing genetic resistance, e.g., by selecting strains of poultry resistant to Mareks disease, by transferring passive immunity, e.g., by ensuring that all newborn animals are fed the colostrum within the first three days of their delivery, and by stimulating acquired resistance, e.g., through mass vaccination.
In modern usage, vaccination is synonymous with immunization. It is the art of inoculating an animal with living or dead antigenic preparations in order to protect it against attacks from some specific diseases. Vaccination programs are developed for different animal species to offer protection during their lifetimes against prevalent or emerging diseases.
4. Quarantine of Suspected Animals
This is the enforced physical separation from the healthy population of infected or potentially infected animals, their products, or items that they have contaminated. For example, imported cattle are usually placed in quarantine stations for a defined period prior to being released to the owner to ensure that, through clinical and/or serologic monitoring, they are not infected with such diseases as rinderpest, contagious bovine pneumonia, and foot and mouth disease.
Similarly, dogs are usually quarantined for a period to ensure that they are free of rabies before being admitted into most countries. To be effective, the period of quarantine must exceed the usual incubation periods for important diseases for the species.
5. Use of Chemicals
Disinfection may be used to reduce the risk of transmission of infectious agents, e.g., the use of formaldehyde between batches of eggs in hatcheries. Pesticides may be used to reduce or eliminate vector populations and hence aid in the control of diseases, e.g., by dipping the lips of cattle into arsenic or organophosphate compounds to prevent tick and flea attacks.
Drugs such as sulphonamides applied to the drinking water of poultry prevent or control coccidiosis; anthelmintic drugs are also administered to eliminate intestinal worms in animals; and antibiotic therapy using chloramphenicol, furpirinol, oxytetracyline, and sulphamerazme is efficacious in treating diseases in animals and fish.
Malachite green, copper sulfate, prydylmercuricacetate (PMA), and potassium permanganate are anti-parasitic agents that are common chemicals used in fish pond management against diseases.
Safety precautions are required for the administration of chemicals and drugs, usually under the prescription and supervision of veterinarians, to prevent toxicity to the animals, resistance by the disease agents, drug residues in meat, eggs, and milk, as well as human health dangers to people handling the chemicals.
6. Environment: Health, Sanitation, and Hygiene
Most diseases result from an ecological imbalance between the host and its environment. If the environment can be altered to reduce the severity or likelihood of such imbalances, the result will be a reduced level of disease. Examples of such measures include improving the ventilation and lighting in an animal house.
Environmental hygiene also includes the physical cleaning of animals and their environment, which may be combined with the use of disinfectants. It also includes such management schemes as pasture rotation.
The aquatic environment also plays a vital role in disease prevention and control in fish husbandry since the host and the parasites are both found in the same environment. The water must be free of pollution, with the physico-chemical and biological characteristics such as temperature, dissolved oxygen, nitrite concentration, turbidity, alkalinity, and carbon dioxide concentration that may affect survival, growth, and reproduction remaining in the normal range.
The control of such diseases as fin rot, vibriosis, and columnaris disease is best achieved by routine pond management such as the maintenance of water quality, high dissolved oxygen and low ammonia levels, good husbandry, and low stocking densities.
7. Slaughter of Infected or Suspected Animals
This is the deliberate killing of infected, susceptible, or contact animals in an attempt to stamp out disease and prevent it from spreading to a healthy population. This can be done selectively (on an individual animal basis) or by complete depopulation (on a herd, flock, or area basis).
Selective slaughter has been used in a number of campaigns against animal disease. This involves a method of case finding, usually by means of an immunological screening test, and then the killing of test-positive animals; hence the name test and slaughter. For instance, the Mad Cow disease case in Europe. The main disadvantages of this approach include the high cost as well as the risk of environmental pollution since the carcasses are either burned or buried very deep with at least six feet of soil above them.
8. Biological Control
This control method utilizes living things that are considered to be reasonably non-detrimental to control disease-causing agents. The measures may be aimed directly at agents of disease or indirectly via the control of vectors or reservoir populations.
An example is the use of sterile male flies to control screwworm disease in cattle in the Southern United States. Male flies are grown in large numbers in captivity, sterilized by irradiation, and then released to alter the reproductive cycle of the fly. Since the females mate only once, mating with an irradiated male leads to no offspring and hence a subsequent reduction in the number of adult flies.
The incidence of Columnaris disease in fish might even be reduced by adding a significant number of competitive bacteria, like C. freundi, to susceptible fishponds.
9. Extension Education
Enlightenment of livestock farmers and the public in general on the need for disease recognition and the various ways to prevent or control them begins with their notification of appropriate animal health experts and authorities. Education of the public, usually through veterinary extension services, should therefore be an integral part of disease prevention, control, and eradication efforts.
Extension education helps to drastically reduce the economic and social impact of epidemic diseases, especially zoonotic infections. In veterinary practice, there are several classic examples where educational programs have been integral parts of disease control programs. These include the campaign to eradicate rinderpest in 1962–72, rabies in 1982, and recent efforts against African swine fever in 1999–2000 and avian influenza in 2006.