Dr. James Evermann
Lecture 1: Animal Diseases
Diseases and microbial infections of livestock are serious impediments to significant worldwide expansion in the production of human foods and other valuable products of animal origin. Uncontrolled epidemic plagues such as rinderpest virus in Africa, foot and mouth virus in Latin America, and hog cholera virus in Europe are so economically disruptive that they alone can prevent establishment of commercially viable livestock industries.
In the past 5 years there have been multiple examples of how diseases of livestock can have severe economic consequences. Some of these diseases are bovine spongiform encephalopathy (BSE) in Great Britain and other European countries, recent outbreaks of vesicular stomatitis virus (VSV) in New Mexico and Colorado and equine infectious anemia (EIA) in Utah.
Infections of livestock need not cause disease in order to have an economic impact. An example is the food borne bacteria, Escherichia coli 0157:H7. The occurrence of these undesirable microorganisms in the food chain alters consumer preferences and in the long run severely impacts the livestock industry.
The objectives for the first series of lectures are:
Four key questions will be addressed:
Question 1: Where is the infectious agent or material when not causing disease? The answer to this question is ecology. Ecology pertains to the location and survivability of an agent on the farm. It says nothing about disease. The ultimate control measure is to find out where the infectious agent is located when it is not causing disease.
Question 2. What factors contribute to the infection/disease process? This is the study of epidemiology. Epidemiology is the disease progression. Example: Vesicular stomatitis (VSD) causes vesicular lesions in horses after a 4 day incubation period. The entire farm is quarantined for a period of four weeks. (That is the epidemiology) the ecology of VSD is where the virus is when it is not causing disease. Any thoughts? The ground?, water supply?, manure?. We don’t know the reservoir for vesicular stomatitis when it is not causing disease. There has been speculation that it might be insects. Viruses usually need a living host. They don’t survive very well in the soil or in water. They can be transmitted by water or soil, but they do better in a living host. The true carrier for vesicular stomatitis is currently not know and might be some kind of an insect.
Question 3. What factors can we control to minimize or eliminate the infection/disease process? It is most important to know what factors you can control to minimize the disease process? Knowledge of the ecology and epidemiology of a disease allows you to more effectively control the disease or minimize the degree of infection.
Question 4. Is the infection/disease zoonotic.. a public health risk? When evaluating diseases like vesicular stomatitis you must know if the infectious disease is zoonotic; is there risk to public health? Dr. Stone’s series of lectures will focus entirely on the zoonotic potential of on the farm infections.
Disease potential is important for several reasons:
Impaired production can have economic repercussions. For example, we ship cattle between the US and Japan. If there is any occurrence of vesicular stomatitis, that animal does not go to Japan. Thus we are unable to export cattle and that becomes an economic concern.
Additionally, as a producer, you care about the quality of your stock, so you have an emotional tie to the health of the cattle as well.
It is very important to understand how diseases impact livestock and production medicine and then look at how you might be able to control the disease. Rinderpest in Africa, hoof and mouth disease in Latin America, and hog cholera in Europe are all infections that have been eradicated in the US. The reason they have been eradicated is because of their economic impact.
In the past five years there have been examples of diseases of livestock that can bring about detrimental economic consequences:
It is important to think about how disease affects the total picture; with a combination of economic, emotional, and public health repercussions.
What are the human health impacts of these infectious diseases of livestock? The human health or public health impact is the zoonotic potential of a disease. The public health impact of animal disease is really an important concept to understand.
It is important to recognize that not all infections of livestock cause disease in the host animal. An example is food borne infection with E. coli 015787. This agent is carried in cattle as a normal gut microbe. It is not until the agent gets into the food source that it can cause disease in humans.
The differentiation of infection from disease is really important. One must distinguish between infection of an animal and the disease that the particular agent can cause. In some cases an infected animal may not show signs of disease but it may be a source of human infection. It is also important that you understand the ecology and epidemiology of a particular infection.
Differentiating Infectious From Non-Infectious Diseases
Diseases may occur from infectious or non-infectious origins.
Infectious diseases are caused by one or a combination of the following pathogenic agents.
A few of the agents here that we will be covering in this class. Bacteria such as E. coli, salmonella, and clostridia are common infections in cattle. They don’t always cause disease, but when they do, they raise a lot of havoc. Vaccinations exist for several of these agents.
Some of the viruses that will be discussed include: Blue tongue, rabies, and vesicular stomatitis. Rabies and Vesicular Stomatitis Disease are zoonotic.
Rickettsial infections include Rickettsia rickettsii and Coxiella burnetii which is also called Q-fever. Anaplasma marginale is a rickettsia sometimes referred to as "anaplas". Chlamydia are like viruses in that they live inside cells, however they have a lifecycle like bacteria. A number of these can cause reproductive disease in sheep, such as Chlamydia ortis. Chlamydia picona causes abortion in cattle, Chlamydia psittsi causes psittacosis which is a zoonotic infection that can be transmitted from birds to humans. As a rule, all the Chlamydia family are zoonotic.
Mycoplasma are bacteria-like agents. Mycoplasma may cause pneumonia. Ureaplasma cause reproductive disease in cattle.
Protozoans are an interesting group because they can act as unicellular parasites, and can cause abortion. Additionally protozoans can be zoonotic. Toxoplasmosis can be transmitted from cats to sheep or goats and then to humans, or transmission can be directly from cats to humans.
Neosporosis is a major emerging protozoan disease agent that causes 40% of all bovine abortions. We don’t know the reservoir for neosporosis just like we don’t the reservoir for vesicular stomatitis. Neosporosis has been documented in the dog. Neospora canis normally resides as an infection in the dog population, but it usually doesn’t cause much disease. However, it causes disease in cattle. Rather than trying to eradicate dogs from the farm, maybe we should talk about controlling neospora in the dog population by inoculating with a neospora vaccination. It doesn’t cause disease in the dog, but it does in cattle, so how do we control it? Our control efforts are aimed at the carrier. That is why it is really important to know the ecology of the infection. You know the dog is the carrier so if you don't want your cows infected, do not allow unvaccinated dogs around your herd. If you didn't know the dog was the carrier you could continuously be exposing and infecting your cattle and not reallize it until they broke with the disease, and even then you would not know how they contracted it.
Fungi. These are normal soil inhabitants. Once they get ingested or inhaled they usually result in abortion, such as the case with asperigillous and cryptococcus.
Finally, we have already introduced prions in the form of Bovine Spongiform Encephalopathy. These are the most novel infectious agents because they are not even a whole virus. They are a protein.
Non-infectious diseases are caused by:
There may be a combination of a noninfectious with an infectious etiology. Etiology refers
to the cause of the disease. An important disease in the Northwest is selenium
deficiency resulting in immune deficiency, resulting in abortion. The abortion can be
caused by any of these infectious agents: chlamydia, bacteria, fungi . The selenium
deficiency causes immune deficiency allowing the animal to get an infection
that causes abortion.
Infection does not always mean disease. Animals may be infected by a microorganism but not show any clinical signs. The infection may persist in that particular animal for prolonged periods of time. An example is equine viral arteritis in stallions. This is an asymptomatic infection (no clinical signs). But the virus can sporadically shed in the semen and be venereally transmitted to mares. The stallion is the carrier for this virus. The mare is the recipient of the virus and abortion ensues. Given this transmission cycle, which one do we vaccinate, the stallion or the mare? It doesn’t hurt to vaccinate the mare, but the ultimate control is to vaccinate the stallion to prevent the shedding of the virus. Five years ago we vaccinated mares, we weren’t vaccinating stallions. Now things have changed; we are looking at how the ecology of this infection is transmitted in the environment and are now vaccinating the stallion.
Infectious Diseases - Terminology
Infection does not always mean disease. Animals may become infected by a microorganism and because of natural resistance or exposure to a low infectious dose, develop a degree of adaptive immunity which allows the agent to persist on or in the animal’s body without causing disease. This animal may subsequently "clear" the infection or become an asymptomatic carrier, and sporadic shedder of the agent to other susceptible animals in the population (e.g. stallions with equine viral arteritis (EVA) virus in the semen).
Contagious versus noncontagious disease. Contagious means capable of spread amongst animals by direct contact, e.g. Pasteurella spp. in nasal droplets. An animal may have an infectious disease that is non-contagious such as Bluetongue virus, which requires an insect vector, Culicoides spp., for transmission from sheep to sheep, or sheep to cattle. Thus you could put an infected sheep and an uninfected cow in the same stall and as long as there were no Culicoides present, the cow would not become infected with Bluetongue. In this situation the sheep is infected but is not contagious unless there is an insect bite transmitting the virus from the sheep to the cow. In this particular case culicoides acts as a biological vector. That means that culicoides is necessary in the life cycle of the Bluetongue virus. The agent replicates in the culicoides and that culicoides can transmit the infection to what ever cow, sheep or goat, it bites. It is similar to the spread of malaria in humans. The insect vector acts as a flying syringe, injecting the virus.
Is it possible to control bluetongue? It is very difficult. We haven’t been able to come up with anything yet. What about insect control. How do you control insects? Eliminate standing water? That is impossible. Some management techniques are almost impossible to implement. Although we know the ecology of bluetongue the disease still exists. If you are raising livestock in the Columbia Basin bluetongue is endemic. Endemic means the infection is engrained in that particular area. The Columbia Basin has a lot of water and the temperature is usually rather mild, and there are a lot of culicoides. It really doesn’t matter if you’ve got cattle, sheep, or goats in the Columbia Basin, chances are they are all infected with bluetongue. If you are trying to ship cattle to Canada (a non-endemic area) and they say, "We don’t have bluetongue" then it becomes an economic problem. It is not a disease problem, but it is an economic problem.
Zoonotic Infection (Public Health). This refers to infectious microorganisms that have the potential of causing disease in humans, e.g. rabies, swine influenza, Hantavirus, and Brucellosis. Some zoonotic diseases are food borne disease such as Salmonella spp. and E. coli 0157:H7.
Zoonotic infection is an issue of public heath. There was a recent outbreak of influenza in China. This particular strain of influenza was transmitted from birds directly to humans, where as in the past it was transmitted from birds to pigs to humans.
Hanta virus is another zoonotic disease that is carried as an asymptomatic infection in mice.
Try to answer the four questions I have
posed with the following examples: a) Pine needle abortion,
b) salmonellosis, and c) shipping fever
complex.
Question 1: Where is the agent or material when it is not causing disease?
Answer: a) It is pretty easy for pine needles-it is trees. The pine needles contain a toxic substance that when ingested can result in high levels of a chemical that causes abortion.
b) Salmonellosis has a carrier. Where is the agent when it is not causing disease? It is in the intestinal tract of carrier cattle and horses. Salmonella replicates and is carried in the intestinal tract. It is shed in the feces and survives for a short period of time in the feces. It will survive for a short period of time on contaminated surfaces such as unsanitized milk bottle nipples or milk spills on other calving equipment.
c) Shipping fever complex is a multicausal/multiagent complex where the animal is infected with both a virus and a bacteria. The viruses are usually parainfluenza 3 and respiratory syncytial virus (commonly abbreviated BRSV). These viruses cause respiratory disease. Parainfluenza is strictly in the upper respiratory tract (nose, trachea). Respiratory syncytial virus is in the lower respiratory tract (lungs). Do you know where the bacteria pasteurella is normally carried? In the nose. That is the ecology of pasteurella, it lives in the nose. If your nose was swabbed fro culture or hte noses of all the cows at the WSU Dairy were swabbed for culture- all the cutures would be positive for pasturella. However, pasteurella in the lungs is bad news. What allows pasturella to move from the nose to the lungs? The viaral infections weakens the animal's immune system allowing pasturella to move from nose to lungs.
Question 2: What factors contribute to the infection disease process?
Answer: a) For pine needle abortion to occur the animal must be pregnant.
b) Factors that contribute to Salmonellosis are stress: confinement, stocking density, poor sanitation and susceptible cattle.
c) Factors that contribute to shipping fever complex include stress, confinement, increased animal density, and susceptible cattle.
Question 3: What factors can we control to minimize the infectious disease?
Answer: a) Keeping pregnant animals away from evergreens. Sometimes this is possible, and sometimes it isn't.
b) To reduce Salmonellosis, decrease stress, decrease animal density, and increase sanitation. There are also Salmonella vaccines on the market. We will talk about vaccines later. The vaccines aren’t really that good, but you can still use them. There are better and cheaper methods of control than vaccine so don’t look to Salmonella vaccinations as the only way of controlling this particular infection.
c) To reduce Shipping Fever Complex, decrease stress, reduce animal density. You can precondition the animals by against the viruses and you can give a Pasteurella vaccine. Pasteurella vaccines have become very effective over the past four years. They were not very good five years ago, but they have improved. Vaccination is a good control measure for the shipping fever complex; it is not so good for salmonellosis, and of course we don’t have a vaccine yet for pine needle abortion.
Question 4: Finally, is this infection or disease process zoonotic?
Answer: The important thing to recognize is when they are so you can take appropriate steps to protect not only public health, but your own health as well. Pine needle abortion is not zoonotic, Salmonellosis is zoonotic, and shipping fever complex is not zoonotic.