Term Paper on Environment and Animal Health | Veterinary Medicine

Read this term paper to learn about the abiotic and biotic elements of environment that affect the health of animals.

Abiotic Elements of Environment:

Abiotic elements include the air, soil (rock), and water, plus the climate of the area. In developed countries, chemical air pollution also is a major concern from the standpoint of its effects on health and the environment. Outbreaks of fluorosis and lead poisoning have been recorded in animals pastured around fertilizer manufacturing and lead smelting plants.

Histori­cally, deaths of cattle at the Smithfield Fat Stock Show in England were early indications of the adverse effects of air pollutants; the chief pollutant being sulphur oxides resulting from the burning of coal. These deaths preceded the first documented large-scale increases in mortality in humans by a number of years.

The death of cats from mercury poisoning (Minamata disease) may similarly have predicted the adverse effects of pollution —in this case, water pollution —on humans.

Another example of the effect of unspecified air pollutants on health is the finding of more pulmonary disease in dogs living in polluted areas than in dogs living in relatively pollution-free areas (see Table 2.7). In fact, domestic and companion animals may serve as excellent sentinels of environments dangerous to man.

With regard to airborne transmission, droplet nuclei (1-2 microns in diameter) may contain living organisms or chemical pollutants. These nu­clei do not “settle out” very rapidly, and they readily reach the lung when inspired. Noninfectious protein material may be transported to the lung in a similar manner and lead to hypersensitivity-type pneumonias.

Despite the potential importance of airborne transmission of disease producing agents, two facts should be kept in mind. The nasal turbinates function to warm and filter in-coming air but apparently are not essential for the animal to have a normal lung. Pigs possessing moderately- to severely-distorted nares as a result of atrophic rhinitis appear to have only a slight increase in pneumonia over their penmates with normal turbinates.

Sec­ond, airborne transmission of some respiratory tract infections (e.g., hu­man rhinoviruses) may be a less important route of transmission than direct contact and fomite transmission. This may also be true of strangles in horses and pasteurellosis of cattle.

When cattle lower their heads to drink, large volumes of in­fected nasal mucus and discharge may drain into the water. Although little evidence exists to support the hypothesis, it may be an important source of infection for other animals in the group.

Soil type can influence the survival of living agents as well as the availability of minerals (e.g., selenium) to plants and hence to animals. Zoonotic fungi such as Histoplasma and Cryptococci survive better in soils with high organic content. Anthrax bacilli appear to survive better in soils along river valleys. Soils containing limestone and dolomite are indicative of the likely presence of leptospiral organisms.

A nationwide survey of soil in the United States for clostridial organisms has been conducted; 4 east-west transects were sampled at 50-mile intervals to ensure a representative country-wide sample. Clostridium tetani was present in approximately 30% of the samples regardless of soil type, whereas Clostridium botulinum ap­peared more frequently in some soil types than in others.

This points out the potential hazard of soil organisms including the potential for contaminating feed stuffs (such as honey) especially in areas where signifi­cant airborne soil erosion occurs. Recent large-scale outbreaks of botulism in human infants were concentrated in, but not restricted to, dry areas of the southwestern United States. (The authors note, however, that the presence of large referral hospitals for children may have influenced this distribution.)

Water may carry toxic chemicals as well as infectious organisms. The temperature and flow pattern of water can also influence the concentration of intermediate hosts or vectors of infectious agents. Under certain environmental conditions, waterborne organisms may proliferate; in the case of blue-green algae, potent toxins leak into the water when the algae die and decompose.

In other circumstances, humans and animals may defecate and urinate in irrigation ditches; infectious microor­ganisms and other parasites may thus contaminate food items, which then serve as sources of infection for other humans or animals.

Precipitation (rain or snow) “scrubs” the air, bringing infectious agents, radioactive particles, and pollutants to ground level. Contamina­tion of pasture fields and crops can occur by this mechanism. The long- term damage from acid rain, one type of pollutant distributed by this mech­anism, may be much more severe than any short-term problems.

Climate is an important determinant of many diseases. Adverse weather may affect the management and care of animals, stress the animal directly, or provide conditions suitable for survival of microorganisms and parasites or their vectors. Unfortunately, unraveling the effects of weather is not easy.

Reasons for this include: its components are often very indirect determinants of disease; it may have multiple effects because there are a large number of weather components (e.g., minimum, maximum, and mean temperature; diurnal temperature fluctuations; day-to-day fluctua­tions; rainfall; humidity; wind-speed); and it is difficult to separate the ef­fects of various weather components.

Further, the general macroclimate (for which data are available) may be quite different than the microclimate (i.e., weather within a barn or at ground level).

Data on microclimate within various types of shelters are not readily available, and few studies on the effects of microclimate on disease and productivity have been reported. One study of microclimatic effects conducted in California dairies con­firmed previous macroclimatic studies of the association between weather and the health status of calves.

Despite the difficulties, even a cursory examination of data on respira­tory disease in humans or animals indicates the potential impact of weather on disease occurrence. In California, where most calves are raised out­doors, adverse weather was shown to significantly increase calf mortality during mid-summer and mid-winter.

Although management factors ap­parently accounted for most of the large variation in mortality rates among farms, the effect of weather was still apparent, even when the average level of mortality on a farm was low. Many feedlot owners and veterinarians believe weather exerts a significant effect on the health and productivity of their animals.

Formal analyses tend to support this theory, although the percentage of disease explained by weather is small. Certainly, intensively reared animals (poultry, swine, or cattle) require care­ful control and manipulation of their microclimate to remain healthy and productive.

Knowledge of the exact microclimatic requirements and the benefits of different types of housing and ventilation systems are lacking, however, partly because of the paucity of formal studies on this subject.

Another example of the effect of climatic factors is the demonstration of windborne spread of foot-and-mouth disease virus in England. Veteri­narians in many European countries also believe that introduction of this and other infections into their countries may be due to windborne transmis­sion. It is thought that wind is an important factor in spreading and pro­longing outbreaks of Newcastle disease virus and infectious bronchitis virus of poultry; however, these theories remain to be adequately tested.

The importance of accurate and complete documentation of the geo­graphic pattern of infected premises is demonstrated by the 1967 outbreak of foot-and-mouth disease in England. It might generally be thought that windborne spread would transmit the agent from infected premises to nearby farms.

Although this pattern was present, it was also found, subse­quent to the outbreak, that a meteorological phenomenon known as lee waves may have accounted for the 18- to 20-km downwind distances be­tween clusters of infected farms. Had the outbreak not been well documented, in terms of time and location, the appropriate data to identify the lee wave spread would not have been available.

Bioclimatograms:

A useful graphic method for investigating the relationship between two climatic factors and survival of parasites is the bioclimatogram. For example, temperature may be plotted on the Y or vertical axis and precipitation on the X or horizontal axis.

For each month of the year the average temperature and precipitation are plotted as one point. Each of these points is joined by a line beginning at January, connecting with February’s point, and continuing to completion at December’s point.

If the temperature and moisture requirements for the survival and/or devel­opment of an agent or its vector are known, the bioclimatogram can pro­vide a visual display of the months when the temperature and precipitation requirements are sufficient to allow survival and/or development of the particular agent.

As an example, the rate of disease each month could be displayed directly on the bioclimatogram (or with the use of transparent overlays) to visually assess if the occurrence of the disease might be asso­ciated with temperature and precipitation. This knowledge could be ap­plied, for example, to design housing for calves in a manner to lower the incidence of enzootic pneumonia.

By plotting the average temperature and humidity requirements for the survival of an agent (such as mycoplasma) one could plan housing so that the temperature and humidity within the barn were consistent with conditions necessary to maintain calf health, but inconsistent with the environmental survival of mycoplasma agents.

Biotic Elements: Flora and Fauna:

Because veterinarians focus their attention on only a few species of animals, it may be easy to forget that a large number of diseases in humans and animals are a result of a complex interplay between animal and plant species.

Under natural conditions, the evolution of plant species directly influences the number and types of animals present in a defined ecosystem. This is less true today in our highly manipulated agricultural ecosystems, where the majority of foodstuffs may be grown some distance from the animal industry and transported to farms by truck and train.

i. Flora:

Plants may be important as causes of disease because they form the basis of the ration or diet of most animals. The selection and processing of plants and their products to form a nutritious diet at minimal cost is now a highly specialized and competitive industry. Also, the availability and cost of major ration components (such as corn) may dictate the expansion or contraction of animal industries.

Not all plants are edible however. Plant toxicities (e.g., alkaloid toxici­ties from lupin species, Japanese yew, and Crotalaria or Senecio genera) occur commonly.

Deficiency diseases (e.g., hypomagnesemia resulting from prolonged feeding of oats and/or barley; acute vitamin A deficiency in beef cattle resulting from grazing on inadequate pastures, and poor reproductive efficiency in cattle being fed inadequate amounts of energy, protein, and phosphorous) are well recognized.

Dry hay may be a better roughage than corn silage for starting stressed calves because of the much higher levels of potassium in hay, and it is believed that the requirement for potassium is increased during periods of stress.

Plants may also be indirectly causally associated with a number of diseases. Facial eczema in sheep results from eating pasture heavily contam­inated with fungi (Pithomyces chartarum). Sheep also become infected with metacercaria of liver flukes encysted on plants, as well as the larval forms of Echinococcus granulosus. Similarly, cattle may ingest the larval forms of Dictyocaulus from contaminated herbage.

Thermophilus fungi contaminat­ing hay may lead to interstitial pneumonia in humans and cattle (called farmer’s lung); whereas other fungi produce toxins (often hepatotoxic) such as aflatoxin or ochratoxin as well as estrogenic substances such as zearalenone.

Dicoumarol production by moldy sweet clover was at one time a major source of poisonings in North America. Today, low courmarin cultivars may allow renewed production of this very high-yielding legume. The pollutants may settle onto fodder crops and be ingested in large doses.

Decaying plants may produce disease through the formation of toxic gases. Examples include silo-filler’s disease (caused by the production and release of nitrous oxides in fermenting silage), and the effects of toxic gases such as methane, hydrogen sulphide, and ammonia released from decaying manure.

The chronic effects of these gases on the health of livestock and the role they may play in predisposing the respiratory tract to infectious agents are of interest and concern for intensively reared livestock such as poultry, swine, and beef cattle.

Finally, feedstuffs of plant origin may serve as vehicles for a variety of microorganisms and parasites. Examples include Listeria monocytogenes in corn silage (perhaps because the organism grows well in silage, or because of the rodent concentration in silage), and the spread of toxoplasma cysts in grain, due to the habit of cats defecating in granaries while purportedly keeping the rodent population in check.

ii. Fauna:

With respect to animal species, and for most infectious diseases, any particular group of animals may be at risk of infection from other members of its own species or from members of other species of animals or inverte­brates.

The zoonoses (infectious diseases common to humans and animals) provide a good illustration of the complex way different species may com­bine to ensure the survival and transmission of infectious agents. For pur­poses of presentation, the zoonoses have been classified on the basis of their cycle of perpetuation as direct, cyclo-, meta-, and saprozoonoses.

Direct Zoonoses:

Direct zoonoses may perpetuate in a single host species. Examples include bovine brucellosis and tuberculosis; rabies in wild, domestic, and companion animals; and pseudo-rabies in swine. Al­though these diseases can survive in one species, there may be local excep­tions.

Before pursuing this, a brief discussion of the distinction between res­ervoirs and carriers is in order. A reservoir is the species without which the agent is unlikely to perpetuate. A carrier, on the other hand, may silently (since it is sub-clinically infected) transmit the organism, but it is not neces­sary for the perpetuation of the agent.

Thus, many species are carriers. For example, many species (including dogs, cats, and sheep) are susceptible to B. abortus infection but they are carriers only, not reservoirs, and do not sustain the infection for prolonged periods. Bovine tuberculosis essentially depends on the family Bovidae for survival, although local potential reser­voirs (such as the badgers in England and opossums in New Zealand) are recognized.

Cattle may be infected with the virus of pseudo-rabies, but again, they appear to be short-term carriers and usually develop clinical disease (“mad-itch”) and are dead-end hosts.

The major reservoir for rabies appears to vary with locale; for example, foxes are the reservoir in con­tinental Europe, foxes and skunks in central Canada, and the raccoon in the southern United States. Bats (both insectivorous and bloodsucking) appear to be the primary reservoir of rabies in areas such as Mexico.

Cyclozoonoses:

Cyclozoonoses require more than one vertebrate species for survival. Examples include the taeniad and echinococcal para­sites. Hydatid disease, discovered fortuitously less than 20 years ago in California, depends on the dog-sheep cycle for survival. However, in Cali­fornia and probably other western states, the disease now has established itself in wildlife, particularly the coyote-deer cycle.

Metazoonoses:

Metazoonoses require a vertebrate and an inverte­brate host for perpetuation. There is a long list of these diseases; chiefly parasitic, viral, rickettsial, and, less frequently, bacterial agents are in­volved. Examples of parasitic diseases include African trypanosomiasis with its devastating effects on animals and humans, and canine heartworm in North America.

Heartworm has been recognized as endemic in the southeastern United States for many years, but only recently it has also been found to be hypo-endemic in southern Ontario, Canada. In Ontario, mosquitoes are the presumed vectors and will sustain development of the parasites, although no locally trapped mos­quitoes have been found to be infected.

Viral metazoonoses include eastern and western equine encephalitis and bluetongue. Avian species are the reservoir of the equine encephalitic viruses and bird-to-bird transmission is achieved by mosquitoes. It is fortu­nate for both humans and animals that these mosquitoes prefer to feed on birds.

Had agricultural systems not encroached on the natural marshland ecosystem of the reservoir avian species, these viruses would likely have remained as only silent infections of birds. Bluetongue is currently a per­plexing problem in North America because cattle are probably functional reservoirs. However, cattle are not unduly affected, and the virus is spread by biting insects, such as Culicoides. On the other hand, sheep develop severe clinical disease.

Plague is perhaps the most interesting of the bacterial metazoonoses. This infection is endemic in many ground squirrel colonies in the south­western United States. It is spread primarily by fleas who prefer the ground squirrel to other species.

Sporadically, however, dogs, cats, and humans may be infested and bitten by fleas, and hence become infected with bu­bonic plague. Outbreaks of plague may be observed subsequent to massive die offs in the squirrel colonies.

Saprozoonoses:

Saprozoonoses require a non-animal site, usually soil or water, to develop and/or survive. Many of the mycotic sapro­zoonoses do not require a vertebrate for their perpetuation, whereas most parasitic saprozoonotic agents require a vertebrate for at least part of their cycle of perpetuation.

Examples of mycotic saprozoonoses include histo­plasmosis, coccidiomycosis, blastomycosis, Cryptococcus’s, and aspergillo­sis. Parasitic saprozoonoses include coccidiosis, visceral larva migrans, an-cylostomiasis, and ascariasis.

Although presented here to complete the classification of zoonoses, the survival and multiplication of the agents of saprozoonoses often is highly dependent on the structure and composition of the soil.

Within-Species Infections:

Despite the importance of the zoonoses, the greatest problem facing the private veterinary practitioner is the threat and spread of infection among members of a species of animal. These diseases (some of which are zoonoses) greatly reduce the productive efficiency of domestic animals and threaten the health of companion animals.

Examples include rinderpest, foot-and-mouth disease, brucellosis, and mastitis in cattle; strangles, corynebacterium pneumonia, and infertility in horses; distemper, parvovirus enteritis, kennel cough, and pneumonitis in companion animals; Haemophilus and Mycoplasma pneumonia in swine; and infectious laryngotracheitis and Newcastle disease in poultry.

Although all the above diseases have an agent as the proximate cause, feeding, housing, and management (including the use of quarantine) are probably important components of the causes of these diseases.