November/December 1999

INFECTIOUS DISEASE?…………IN MY SHEEP?…………WHERE DOES ‘IT’ COME FROM?

Simply put, ‘it’ (infectious diseases) comes directly from other infected sheep (e.g., pasteurella pneumonia), from other infected species (e.g., rabies-infected skunks or raccoons, fluke-infected snails), or from the environment (e.g., from contaminated lots, water or feedstuffs). Infectious diseases of sheep and other mammals are caused by tiny, particulate, transmissible agents, including bacteria, viruses, protozoa, fungi, prions, and parasites of lung, intestine, liver and other tissues. The infected animal is called the ‘host’, because it ‘receives’ or ‘houses’ the infectious agent. (We will exclude lice and mites from this discussion).

Some infectious agents survive outside of the body of the sheep for days, months and even years in soil or water, or as resistant structures (spores of bacteria and fungi, cysts of coccidia). After these agents gain access to the sheep’s body, a variable period of time (days to months) passes before signs of infectious disease are evident. What the infectious agents are doing in that quiet interval is what they are genetically programmed to do: survive and reproduce, and thereby continue to exist as a species. Sounds familiar, doesn’t it?………That’s what we’re trying to get our sheep to do: survive and reproduce!

After a sufficient number of particles (minimum infectious dose) of the infectious agent gain access to and reproduce in the body of the sheep or lamb, the resistance mechanisms of the sheep or lamb are overcome and the sheep or lamb becomes ill. The parts of the body that are most affected may function abnormally and yield clues to the diagnosis. As an example, some bacteria live on the surface of cells that line the intestine, urinary bladder, windpipe and lung, giving rise to diarrhea, urinary disorder, cough and pneumonia, respectively; other bacteria live in tissue fluid within liver, spleen and muscle, causing fever, pain and weakness; still others live in the blood stream and cause high fever. Viruses replicate only in the interior of living body cells, either in the blood or in organs and tissues.

After being shed from the sheep’s body in aerosols, pus, urine or feces, some bacteria and viruses die within minutes to hours, while others survive as indicated above: e.g., paratuberculosis bacteria survive in moist soil for months, while hantavirus persists in deer-mouse feces and can infect humans who come in contact with hantavirus-contaminated fecal dust. The prion agent of sheep scrapie persists in the environment for long periods as well.

Some of the effects of these agents on the sheep’s body are caused by a toxin (poison) from the agent that directly damages certain cells. Clostridia kill a variety of cells by this mechanism. For example, type D clostridial bacteria secrete a toxin that, when absorbed and circulated in the bloodstream, directly causes death of nerve cells in the brain and thus causes ‘sudden death’ of an infected lamb.

However, many of the effects of infection on sheep and lambs are not a direct effect of the agent or of its toxins. Rather, the damage is a byproduct of responses to the infectious agent by protective (i.e., ‘immune’) cells in the sheep’s body. Protective cells defend themselves (and ultimately the entire body) against the agent by undergoing changes in shape, maturity, and biochemical functions. Some consume the agent with intent to destroy it. Other protective cells secrete ‘messenger’ substances or ‘inflammatory mediators’, which orchestrate a response (e.g., fever) that is detrimental to the agent and protective to the infected host (the sheep or lamb). However, there are other byproduct-responses to these same ‘messengers’ that don’t seem to be as favorable to the infected sheep, such as aches and pains, poor appetite, and sluggishness.

The misery of our own severe head colds is largely caused by our immune system getting rid of the virus of the common cold, and that is analogous to what is happening to an acutely ill sheep. During the battle against the infectious agent, there may be severe damage to body tissues from this complex chemical activity. For example, damage to heart, lung, brain, kidney, intestine or liver may occur, resulting in poor feed conversion and stunted growth of survivors. In some very ill sheep, the damage may be so severe that the sheep dies.

To summarize, susceptible sheep acquire infectious agents from:

A. Direct contact with other sheep that are sick, such as through face-to-face contact with pinkeye chlamydial agent in tears,

B. Feedstuffs that are contaminated with feces and urine that contain the infectious agent,

C. Aerosolized, inhalable particles of Pasteurella haemolytica that are delivered into the environment from sheep or lambs with pasteurella pneumonia,

D. Soil- and water-borne agents deposited in feces and urine, such as salmonella, E. coli and leptospira,

E. Contaminated surfaces (‘fomites’; i.e., inert objects that are not infected, in the usual sense, but that are contaminated with the agent) such as feed-bunk surfaces and crevices, bedding, and tack that "carry" or hold the agent in a form that is contagious to susceptible sheep………………Come to think of it, contaminated food and contaminated hands on food servers do the same to people!

F. Club lamb fungus spores are acquired by sheep, lambs, children and adults from fungus-infected sheep and lambs, as well as from fomites.

G. Brucella ovis in infected semen is transferred to the ewe’s vagina during breeding, and to other rams through mounting behavior while the rams are grouped in a pen.

H. Infectious agents are also acquired by sheep and lambs by penetration through the skin during bruising injury, and by thorns and stickers, ear-tags, needles, or surgery, in each case by penetration through contaminated skin.

BEFORE GOING ON, WHAT ABOUT THE TAR BABY? Uncle Remus, a fictional person made famous by Joel Chandler Harris, told a story of Br’er Rabbit. When he came upon the Tar Baby (a tar-covered doll), Br’er Rabbit became incensed when it did not respond to his conversation. Enraged, Br’er Rabbit swatted the Tar Baby, and became tangled up in the tar himself!

Disease agents are sort of like the tar, in that the agent is spread virtually everywhere on sheep and throughout their environment during an outbreak of infectious disease! The downside is that, in closely kept flocks, most if not all of the flock-members become exposed to the agent. The upside is that some, even most, of those sheep that do not become sick may acquire some degree of immunity as a result of exposure to the infectious agent. After the disease outbreak is over, the number of infectious particles in sheep and in the environment decreases over time, under the influence of sunlight, air exchange, and drying.

However, most infectious agents remain somewhere on the property, at the very least in chronically-infected sheep that survive the outbreak. That’s because the immune response is not fully protective against, for example, infectious footrot, and even less so against OPP, ram epididymitis, Johne’s disease, and caseous lymphadenitis. However, these chronic diseases can be excluded from a flock, and through the use of several strategies and a lot of effort, a defined health status flock can be created,as described below.

HOW CAN I HAVE A DEFINED HEALTH STATUS FLOCK?

The short answer is: Grow your own or buy one. Let’s start with growing your own.

STEP ONE: Obtain solid diagnostic evidence of infectious disease. This will require tests on blood or feces, and in some instances, submission of a live animal(s) suspected to have the disease. Find a veterinarian who is willing to go down this hard road with you, and commit to each other for a definite period of time. Develop a plan together and review your progress after six months and then quarterly thereafter. We’re all human; either may want out for whatever reason.

Complete transparency in sharing information between the two of you is vital; nobody will win if one party holds back. Divide the labor; have your veterinarian handle communications with diagnostic sources, while you handle communications with industry sources. Agree on a time-based fee schedule up-front, and get a fee estimate while you’re at it so that there are no surprises later.

STEP TWO: When you know which disease/diseases are causing major economic losses in your flock or which limit your marketing opportunities, it’s time to sit down and plan. Work with your veterinarian to develop a specific, detailed written plan by which to reach your goal. The plan should clearly state the goal, e.g., which diseases are to be eradicated from your property, including the time-line, and enough detail so that everyone who works with your sheep can understand it and sign on to the plan. Each of you is responsible for tapping into whatever additional expertise is needed to make your plan as successful as possible. You are clearly the decision-maker; your veterinarian is your risk-management advisor; each of you contributes knowledge.

Some of the strategies that you will consider together will be as follows:

A. Upgrade the flock and premise records immediately! Don’t even consider trying to define the health status of your flock without detailed individual records.

B. Remove clinically-ill sheep from the flock. This reduces the risk of exposing susceptible sheep to the infectious agent.

C. Blood-tests are available for certain diseases, by which carrier sheep can be identified, treated, isolated, kept as an infected sub-flock, or removed.

D. Establish a nucleus "clean" flock and disperse those sheep on clean ground. Surveillance and testing should be focused on the "clean" flock, because all other sheep on the premises are infected or suspect, by definition. An ongoing, aggressive testing program in the clean flock is essential because of the ‘Tar Baby’ effect; infectious agents are moved about the farm by dirty boots and equipment, by birds and dogs, and by other routes.

E. Over time (1-2 years or more), as blood tests are repeated, remove newly-diagnosed cases from the clean flock and transfer them to the infected sub-flock. The holding of an infected sub-flock is only for the purpose of managing cash flow, e.g., completing a production cycle or capturing a better short-term market. The infected flock should not be used as a breeding flock; rather, it’s a way-station to ultimate removal from the property. Serious clean-up can begin only when all infected sheep have been removed from the property.

F. Keep all sheep in age-specific groups; e.g., ewe lambs, yearling ewes, two-year old ewes, and adult ewes; ram lambs, yearling rams, and adult rams.

G. When all known infected animals have been removed from the facilities on the premises, and preferably during pasture season, begin intensive sanitary measures on the contaminated facilities including but not limited to the following: physical removal of every unattached object from within buildings and around the premises, removal of surface soil where appreciable risk exists that infectious agents are sequestered there, use of hot soapy water on wooden and metal surfaces with a brush, rinse well, treat with bleach or other sanitizing agent, dry off by giving access to sunlight and air movement, lime the surfaces of pens, rake and harrow the soil in lots, pens and paddocks, and spread with crushed rock as needed………and let the requisite time pass! Competent advice should be obtained as to the minimum time period (for each disease) during which sheep must be excluded from the premises. It may be as brief as a month or as long as a year or more, depending on which disease agent you’re dealing with.

H. Deprive an entire year’s lamb crop of colostrum by taping the teats before parturition, removing the newborns instantly at birth, and rearing them at a distant location on out-source colostrum, milk replacer and dry feeds. This strategy has been used successfully by some producers to eradicate OPP.

I. Regenerating a breeding flock from caesarotomy-derived lambs raised as in H), above, at a new location on new ground, is another approach with a greater level of security.

STEP THREE: Step back after writing up the plan, breathe deeply, and decide if the goal (exclusion of the specific disease agent(s) from your property) is worth the time and effort involved. An improved economic return from better market opportunities (e.g., seedstock and export market) must be realistically attainable within a few years, in order to make it worth your while. Also, not all infectious diseases require the rigor of every item of A-I, above.

Purchase of a nucleus flock certified to have the defined disease status that you require, and rearing them on clean ground in new facilities, may be a cost-effective option for you. Many advertisements in sheep periodicals assert that the advertised flock is free of one or more of OPP, infectious footrot, scrapie, caseous lymphadenitis, and Brucella ovis. This laudable effort on the part of many seedstock producers provides buyers an option that was not available a decade ago.

Close the defined health status flock after acquisition and use your ram genetics by AI. Later, if you choose to bring rams with the requisite defined health status into your new flock, be certain to observe a full 30-day quarantine period to reduce the risk of outbreaks of acute diseases that are not covered by your control program. N.B.: It is of paramount importance that your defined health status flock be brought onto new ground (i.e., no sheep have been on that ground for an agreed-upon time, such a year or more, depending on the disease), because bringing them onto contaminated ground on which sheep with undefined health status have been kept will negate all of the time, money and effort that you have invested in your defined health status program.