GENETIC AND DEVELOPMENTAL DISEASE
Genetic abnormalities often affect a critical process of fetal developmental and are lethal, leading to early fetal death and are never detected. Some genetic problems lead to serious conditions in which the death of the fetus occurs at or soon after birth. Other genetic abnormalities may arise which are not fatal and have varying degrees of impact on appearance and the animals peformance. In livestock species if genetic abnormalities are present or suspected the animal is usually euthanized.
Commonly, genetic problems are recessive traits so a heterozygous animal may be phenotypically normal while carrying a mutated gene. The carrier state of a heterozygous sire or dam will go undetected until crossed with another heterozygous animal bearing the same recessive gene. Some of the offspring are likely to be homozygous for the recessive gene (25%) and will be phenotypically abnormal. Carriage of a recessive gene can be determined by backcrossing a sire to his daughters. Through extensive selective breeding and culling recessive genes problems have become less common.
Some genetic defects are sex-linked. What does this mean? The recessive gene associated with the condition is situated on the X or female chromosome. Thus the condition may be transmitted through the female line who can be carriers but will be phenotypically normal. An examples of a sex-linked abnormality in domestic species is hemophilia in dog (see sex-linked diseases in genetics section).
Abnormalities may also arise due to errors in chromosome number or assortment. Chromosomes may break or be duplicated. In a closed flock or a herd the probability of genetic problems increases with interbreeding.
Development problems may also arise in utero for non genetic reasons. For example exposure to a toxic chemical in feed. Inadequate maternal nutrition, lack of vitamins, diseases may lead to malformation or abortion. The environment in utero and the consequent development can also been influenced by other factors. An example is the freemartin in cattle. If a cow delivers twins of mixed sex (one male and one female) there is an 85% chance that the female will be infertile. Around 85% of the time the fetal circulations of twins are fused. Under this situation the sex hormones produced by the male fetus in utero abnormally affect development of the female sex organs and impair subsequent fertility. Such heifers are called freemartins.
NUTRITIONAL AND METABOLIC DISEASE
Usually caused by an unbalanced diet (too much grain/too little forage; excessive non protein nitrogen) an inadequate supply of a nutrient (e.g. white muscle disease due to inadequate selenium) or with excess of a nutrient causing toxicity e.g. copper. There are a numerous nutritional diseases of domesticated animals.
Some important examples that you will be expected to know:
Milk fever. Also known as hypocalcemia/ or parturient paresis. This is a problem around parturition mainly in the dairy cow. A lactating cow is excreting around 1.25g Ca/l in her milk. May be losing 25-50g Ca per day. Leads to a fall in blood calcium level hence hypo (low) calcemia ( calcium). Milk fever occurs less frequently in newly calved heifers compared to older cows. It is more common among some breeds, particularly the Jersey. Occurs soon after calving. Symptoms are depression, loss of appetite, loss of consciousness, recumbency, head turned back. Treated by intravenous injection of calcium salt usually calcium borogluconate. This is given slowly to avoid affecting heart. An important prevention is to ensure a low calcium in the dry period. High calcium availability in late pregnancy will reduce the cows subsequent ability to mobilize calcium from bone to counteract loss in milk. Also high vitamin D corrcet calcium/phosphorus ratios in the ration are important.
Ketosis. Also known as Acetonemia or Pregnancy disease. Common problem in lactating dairy cattle and periparturient sheep. Characterized by low blood sugar. Usually occurs in early lactation in cows, 1-2 weeks before lambing in the sheep. Signs are loss of appetite, fall in milk yield, smell of acetone on breath, grinding of teeth, trembling, eventually collapse and death. Increased nutrient requirements are not being met, particularly inadequate supply of energy as carbohydrate or fat. Animal begins to mobilize body fat reserves too rapidly without sufficient carbohydrate in diet. Treated by IV glucose to counteract low blood sugar or by feeding sodium propionate.
Laminitis or founder. Severe lameness of one or multiple feet affecting horses, cattle, and sheep. Signs are pain, reluctance to move, fever. Chronic laminitis may lead to hoof malformation. Etiology is complex and confusing. Several predisposing factors have been identified. Excessive consumption of grain lush grass or legumes, rapid changes in diet, excess protein in diet relative to forage. Probably caused by change in microbial flora in the gut and the rapid release of bacterial toxins into blood, resulting in inflammation and change in chemistry of the hoof horn. May also be precipitated by other purulent infections leading to toxemia e.g. metritis. Laminitis in the horse is usually and acute problem while in the cow it is often subacute or chronic. Acute laminitis is very difficult to cure and, because the animal is in severe pain, the animal is often euthanized. Usual treatments are prolonged rest, painkillers and anti-inflammatory drugs and carefully balanced rations.
Mineral and vitamin deficiencies
Click here to see a table summary of important mineral and vitamin deficiencies.
INFECTIOUS DISEASES
Infectious diseases are those caused by an infecting agent or organism. A disease can be infectious but not readily transmitted to others. There are several categories of organism capable of infecting domestic animals. These include parasites, protozoans , bacteria nd viruses, Some of these diseases are transmissible from animals to humans - these are known as zoonoses. Examples of zoonoses are Lyme disease, rabies, and tuberculosis.
Major Categories and examples of infectious diseases
Internal parasites These are multicellular organisms living inside the animal e.g. tapeworms, round worms, liverflukes. Some are also infectious for humans e.g. Liver flukes of sheep and some tapeworms. Liver flukes are a major problem where sheep are on wet pastures. The liver fluke causes liver damage, weight loss or poor growth. Tapeworms living in intestine also reduce growth rate. While many parasites cause reduced growth and performance some cause serious disease e.g. heartworm in dogs, lung worm in cattle. These seriously affect the performance of the heart and lungs respectively. Parasites are usually controlled by providing dewormers such as Ivermectin as a drench, bolus, or feed additive.
External Parasites. These multicellular organisms live on the surface of the animal and are visible to the naked eye. Examples include ticks, fleas, biting flies, wool maggots, mites etc. Affect all types of domestic animal. Usually controlled by pesticide as spray or by immersion.
Fungi. There are many species some of which are single celled while others form long filaments of many cells. Usually do not cause life threatening diseases, unless immune system is compromised. Examples are Candida causing mouth or genital infections, ringworm causing skin infections. Aspergillus is a fungus causing respiratory disease which can be serious and also affects humans.
Protozoans. Single cells like amoeba. Visible with light microscope. Cause a variety of diseases some of which can be serious or life threatening. An example of a protozoal diseases is Sleeping Sickness which affects cattle, other wild and domestic animals and sometimes man. Caused by a trypanosome which is transmitted between animals by a biting fly, the tsetse fly. There is no effective treatment or vaccine. Control of the disease is by controlling the fly. Sleeping sickness has made parts of Africa uninhabitable to European breeds of domestic cattle which are more susceptible than the African breeds.
Bacteria. Have many different shapes or morphologies. These include rod-shaped (bacilli), spherical (cocci), wave formed or spirals (treponemes, vibrios). Size variable, generally bigger than 1 micron (1 millionth of meter). Not visible with naked eye. Multiply by binary fission i.e. each cell divides in two to form two new daughter cells. Some are capable of dividing every 20 minutes under ideal conditions. Unlike viruses and most fungi bacteria are susceptible to a range of antibiotics which are typically used to treat infections. Development of resistance to some of these antibiotics is a cause for concern. Some examples of important bacterial diseases of domestic animals are shown below.
Anthrax. Caused by Bacillus anthracis. Can affect all warm blooded animals. Usually rapidly fatal. Highly contagious to humans handling an infected animal but not normally transmitted readily to other animals on the farm.
Brucellosis. Caused by several species including Brucella abortus, B.suis, B.melitensis. Commonly affects cattle, sheep, goats, pigs, humans. Causes abortion, undulant fever. No effective treatment for affected animals. Humans normally treated with large doses of antibiotics for a very long period. Can be effectively controlled by vaccination, combined with testing and eradication. Fairly well controlled in livestock in US. There is concern about it spreading into cattle from other sources e.g. buffalo.
Tuberculosis. Caused by several species including Mycobacterium tuberculosis; M.avium and M.bovis. Affects mammals, birds and reptiles. Effective treatments are available although drug resistance is becoming a serious problem among humans. Requires long course of therapy. Affected animals react to an extract of the bacteria, called tuberculin. Herds are tested regularly by veterinarian and affected animals slaughtered. Meat and milk of infected animals can transmit disease.
Swine dysentery. Caused by Treponema hyodysenteriae. A disease of pigs which is common in intensive swine operations. Causes diarrhea and weight loss in young pigs. Controlled by antibiotics in feed. Some vaccines being developed.
Colibacillosis. Caused by Escherichia coli. Can infect all warm blooded animals. All animals have E.coli in their gut and most strains are harmless. Colibacillosis is associated with virulent strains. Some vaccines provide protection, often a problem in the young animal if immunity is inadequate. Young can be protected by immunizing dam so that she provides protective antibody in colostrum
Salmonellosis. Caused by various species of Salmonella. Typically two forms: an enteric form in which diarrhea and vomiting are present and young animals are mainly at risk of dehydration and an invasive form in which the Salmonella invade the bloodstream and cause septicemia. Which is often fatal. Some vaccines exist but are of limited efficiacy. Antibiotics and fluids are the usual treatments.
Mastitis. Infection of the mammary gland by various species of bacteria. Affects all mammals reducing milk yield and quality. Prevention by hygienea and therapy. A vaccine is available to reduce incidence of coliform mastitis.
Chlamydia/Rickettsias. Intermediates between bacteria and viruses both in size and properties. Most are tick borne and cause some very serious human diseases such as Typhus fever, Rocky Mountain Spotted Fever. Most important example in livestock is Q fever which is common in cattle, sheep and goats. It is transmissible to humans. Can be transmitted in unpasteurized milk. Usually asymptomatic in animals although it can cause abortion. In humans causes a flu-like illness which subsides in a few weeks. Controlled by milk pasteurization and tick control.
Viruses. Smaller than bacteria, viruses cannot be seen with light microscope but are visible with the electron microscope. Genetic material can be DNA or RNA. Propogate only in living tissue but can survive outside the body and remain infectious for varying periods of time. There are many important viral diseases of domestic animals.
Aujeskys Disease (Pseudorabies). A disease affecting young pigs. Signs are fever, convulsions, paralysis and death. Causes intense itching hence its other name "mad itch". A vaccine is available.
Hoof and mouth disease. A very highly infectious disease affecting cattle, pigs and other hoofed species. Debilitating febrile illness, usually first detected through viral sores on mouth, tongue, nose and feet. Outbreaks are devastating. USA and Canada are free of hoof and mouth although it is present in parts of South America. It is common in many parts of the world, particularly Asia and parts of Africa. A vaccine does exist and is used extensively in regions in which the disease is endemic.
Infectious Equine Anemia (Swamp Fever). The virus is transmitted by flies and mosquitos. Symptoms are a high fever, respiratory distress. It is usually fatal. Some animals recover to be carriers. Obvious abdominal swellings are often present. Infected animals can be detected by the Coggins Test. No vaccine is available.
Newcastle Disease. A highly contagious respiratory and neurological disease of poultry which is usually fatal. Vaccine available and widely used.
Rabies. Can infect all warm blooded animals. Fatal once symptoms develop. Has a furious form (La Rage) and a dumb form. Wild animals are important as reservoirs of infections e.g. racoon, fox.. Vaccination is effective and is used inncompanion and domestic species. Transmission by entry of virus through a wound e.g. a bite from an infected animal.
Stress and disease. Farm animals may be placed in environments and systems which are stressful. Overcrowding, transport, restraint, large groups, extremes of temperature, ammonia build up etc. The stress can lead to significant modifications to the immune system increasing susceptibility to infectious diseases or to somatic dysfunctions such as gastric or duodenal ulcers. Sometimes the signs that an animal is stressed are revealed through behavioral changes. Often shown as stereotypical behaviors (repetitive behaviors with no apparent purpose). Examples are bar biting, cannibalism, aggression, and pacing. See animal welfare.
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