Control and prevention of viruses

 Strategies for Control of Viral Diseases

Disease Control Through Hygiene and Sanitation

Intensive animal husbandry leads to accumulation in the local environment of feces, urine, hair, feathers, and so on, that may be contaminated with viruses; this is especially problematic with viruses that are resistant to environmental desiccation. To avoid this, intensive livestock units operate an “all in, all out” management system, by which the animal houses are emptied, cleaned, and disinfected between cohorts of animals. Hygiene and disinfection are most effective in the control of fecal–oral infections; they have much less effect on the incidence of respiratory infections. Efforts to achieve “air sanitation” are generally unsuccessful, especially in intensive animal production systems with high population densities.

Nosocomial Infections

Nosocomial virus infections are less common in large animal veterinary practices, where animals are usually treated on the farm, than in companion-animal practices. Appropriate management can reduce the likelihood of nosocomial infections, and veterinary clinics usually require that all inpatients have current immunization. Clinics should be designed for easy disinfection, with wash-down walls and flooring and as few permanent fixtures as possible. They should also have efficient ventilation and air conditioning, not only to minimize odors, but also to reduce the aerosol transmission of viruses. Frequent hand washing and decontamination of contaminated equipment are essential. Similar strategies are required in animal shelters where large numbers of cats and/or dogs may be cohoused in an environment that is highly conducive to explosive outbreaks of a wide variety of viral diseases.

Disinfectants and Disinfection

Disinfectants are chemical germicides formulated for use on inanimate surfaces, in contrast to antiseptics, which are chemical germicides designed for use on the skin or mucous membranes. Disinfection of contaminated premises and equipment plays an important part in the control of diseases of livestock.

Viruses of different families vary greatly in their resistance to disinfectants, with enveloped viruses usually being much more sensitive than nonenveloped viruses. Most modern disinfectants inactivate viruses, but their effectiveness is greatly influenced by access and time of exposure: viruses trapped in heavy layers of mucus or fecal material are not inactivated easily. There are special problems when surfaces cannot be cleaned thoroughly or where cracks and crevices are relatively inaccessible, as in old timber buildings or the fence posts and railings of cattle and sheep yards. New data on the effectiveness of standard disinfectants or the release of new products requires access to updated information on the correct use of disinfectants. 

Disease Control Through Eliminating Arthropod Vectors

Control of arbovirus infections relies, where possible, on the use of vaccines, because the large areas and extended periods over which vectors may be active make vector control difficult or, in many instances, impractical. However, surveillance of vector populations (eg, mosquito larval counts) and/or the climatic conditions conducive to vector transmissions over wider geographical areas provides the justification for local vector control, both as a preventive and as a control strategy. For example, aerial spraying with ultra-low-volume insecticides has been used to prevent the establishment of mosquito populations carrying encephalitis viruses in some parts of North America, although there are issues pertaining to increasing mosquito resistance and environmental objections. Some countries have based their emergency arbovirus control program plans on aerial insecticide spraying. This strategy is aimed at rapid reduction of the adult female mosquito population in a defined area for a very short time.

Organophosphorus insecticides such as malathion or fenitrothion are delivered as an ultra-low-volume (short-acting) aerosol generated by spray machines mounted on backpacks, trucks, or low-flying aircraft. Spraying of the luggage bays and passenger cabins of aircraft with insecticides reduces the chances of intercontinental transfer of exotic arthropods, whether infected or noninfected.

Exclusion of ticks has proven successful in the control of African swine fever in endemic regions; however, control is more difficult in free-ranging animals.

Disease Control Through Quarantine

Movement of domestic animals across international and even state borders can be regulated in countries where there are appropriate veterinary services and regulatory infrastructure. Quarantine remains a cornerstone in many animal disease control programs. A period of quarantine, with or without specific etiologic (eg, PCR) or serologic testing (see Chapter 5: Laboratory Diagnosis of Viral Infections), is usually a requirement for the importation of animals from another country, and similar requirements may be enforced within a country or region for the control or eradication of specific infectious agents.

As international movement of live animals for breeding purposes and exhibition has increased, so has the risk of introducing disease. Before the advent of air transport, the duration of shipment usually exceeded the incubation period of most diseases, but this is no longer the case. With the ever-increasing value of livestock, national veterinary authorities have tended to adopt stricter quarantine regulations to protect their livestock industries. Complete embargoes on importation are imposed for some animals by some countries. The concept of quarantine (Italian, quarantina: originally 40 days during which, in medieval times, ships arriving in port were forbidden to land freight or passengers if there was a suspicion of a contagious disease), where animals were simply isolated and observed for clinical signs of disease for a given period of time, is now augmented and often expedited by extensive laboratory testing designed to detect previous exposure to selected viruses or a carrier state. Laboratory testing requirements are set down in detailed protocols and are supported by national legislation.

Historically, the quarantine of animals has been a successful method for preventing the introduction of many diseases; however, other diseases may be introduced in animal products (eg, foot-and-mouth disease in meat products) or by virus-infected arthropods (eg, bluetongue). It must also be recognized that most countries have land boundaries with their neighbors and cannot control human and wildlife movement easily; thus, countries are expected to confirm their disease status to the World Organisation for Animal Health, which is the responsible international body. In addition to its central role in the reporting of livestock diseases globally, this organization also is responsible for harmonizing diagnostic testing and the creation of internationally agreed criteria for the safe movement of animals and animal products. However, problems persist that are often social, economic, and political rather than scientific—for example, smuggling of exotic birds may play a significant role in the introduction of Newcastle disease and fowl plague (highly pathogenic avian influenza) viruses into susceptible avian populations.

Disease Control Through Vaccination

Each of the foregoing methods of control of viral diseases is focused on reducing the risk of infection, whereas vaccination is intended to render animals resistant to infection with specific viruses. Immune animals cannot participate in the transmission and perpetuation of such viruses in the population at risk. Thus, vaccination can reduce the circulation of virus in the population at risk, as confirmed in countries where there is widespread vaccination of dogs against canine distemper and infectious canine hepatitis. Relaxation of vaccine usage, however, can have devastating consequences as, for example, in Finland in the 1990s, when canine distemper virus reemerged into a dog population in which vaccine usage had declined.

Safe and effective vaccines are available for many common viral diseases of animals. They are especially effective in diseases with a necessary viremic phase, such as canine distemper and feline panleukopenia. It has proved much more difficult to immunize effectively against infections that localize only in the alimentary or respiratory tracts.

Vaccination has been utilized extensively, and with varying success, in programs for the control and/or eradication of certain diseases.