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This article was published in 1983
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Drought Problems With Sheep
D.D. Salmon, B.V.Sc., Veterinary Inspector, DENILIQUIN
Under drought conditions, there is usually an improvement in the general health of sheep. This is seen because most of the significant diseases of sheep are either related to high levels of feed intake, or dependant on moist conditions for their [effect on] sheep.
In many cases this 'Healthiness' continues until the sheep have starved to death.
There are, however, quite a few significant disease problems seen during drought times. These may be as a direct or indirect result of malnutrition, or they may be subsequent to supplementary feeding.
The malnutrition resulting from drought conditions does lower the resistance of sheep to infectious agents. Fortunately most important infections of sheep depend on moist conditions for spread. When exceptions to this general rule occur, or when a condition is already present, they manifest more severely on undernourished sheep. Parasites, both internal and external, manifest themselves far more severely during droughts.
Sheep lice cause more wool damage, and probably weight loss as well, even though the number of lice may not appear any greater, in fact the hot dry conditions of drought summers should decrease lice numbers. If blowflies get a foothold on sheep weakened by malnutrition, the effects are far more severe than on well fed sheep.
Nematode and trematode parasite, when present, also cause more severe symptoms during droughts. This results from the reduced resistance of the sheep, and the fact that the nutrition the parasites obtain makes up a greater percentage of that available. Some internal parasites are also able to spread under drought conditions. The drying up of bodies of water can result in increased uptake of trematode larvae. Sheep do not normally graze in water, so the uptake in those areas is often quite low. Both Fasciola and Paramphistome burdens may become quite heavy along a retreating waterline. The infective stages of Nematodirus are very resistant to desiccation and heat. Under drought conditions with closer grazing of pasture and reduced host resistance, Nematodirus infestations may increase to pathogenic levels. A relatively uncommon worm which I have seen causing problems in drought affected sheep is Trichuris ovis. The eggs are very resistant to climatic extremes and the mature worms are long lived and usually resistant to the broad spectrum Anthelmintics commonly used. These factors produced a moderate burden of worms which exaggerated the starvation.
when the availability of normal pasture species is low, sheep often seek out plants which they do not normally graze heavily. In some cases this will result in poisoning. In my experience this is more significant in cattle, but sheep may also be candidates. The picture varies from place to place and between seasons, but Patersons Curse during dry springs or autumns and Nardoo in dry swamps are two that I have seen cause significant problems.
Nutritional deficiencies other than those of energy and, to a lower degree, protein are not common in sheep, even in extreme drought. The occurrence of Vitamin A deficiency seems poorly defined to me. I have neither seen it nor spoken to anyone who has seen it as a clinical syndrome. However the vitamin should be at very low levels after prolonged drought, especially if grain is the only supplementary feed available. Other factors - energy and protein deficiency - probably become critical before clinical avitaminosis A is seen, but there may be a subclinical effect. Vitamin E deficiency resulting in Muscular Dystrophy is occasionally seen in the Riverina when sheep are grazing poor quality straw with no grain or green feed. The syndrome is identical to that seen with Selenium deficiency.
Metabolic disturbances are not as common during drought as they are during periods of excessively lush pasture growth. This is probably because there are no abrupt changes in level of nutrition - it is consistently poor. There are, however, fairly significant losses from Pregnancy Toxaemia, and some hypocalcaemia also occurs. Both conditions are usually seen when nutrition has been very poor for a long time and the sheep are emaciated. Pregnancy Toxaemia occurs in heavily pregnant ewes, and they also have low serum calcium levels. Extra stress is not usually necessary to bring on an incident, but it will help. Hypocalcaemia can occur in any class of sheep so long as they are starved enough and subjected to sufficient stress.
Most of the significant problems I have seen with sheep during droughts result from attempts to keep them alive. It seems to be Murphy's First Law of Sheep Husbandry that sheep die no matter what you do.
In most areas, hand feeding of sheep mean[s] grain feeding. The introduction of sheep to a grain ration is fraught with danger. The ration must be introduced to the sheep gradually over a period of at least a month. High energy rations such as grain or commercial pellets are extremely likely to produce lactic acidosis until the ruminal microflora adjusts to the changed diet. When the type of grain is changed, the change must be gradual, since the ruminal microflora must change again. When commercial pellets are being fed, change of the grain used may possibly result in acidosis. The use of sodium bentonite as a ruminal conditioner appears to be very effective in preventing acidosis.
Hand feeding with grain also produces gut conditions conducive to Enterotoxaemia and unless all sheep are vaccinated it can be the cause of significant losses.
When hand feeding comprises the sole source of nutrition, many owners confine the stock to smaller areas. This is particularly the case with weaners which will not forage well. This confinement creates an environment where disease spreads more easily. External parasites and the more resistant nematodes can increase with this proximity. Coccidiosis can also build up to pathogenic levels under these conditions of close confinement. Coccidia usually only cause problems in weaners, resulting in quite severe scouring and death. Over the last few months I have seen several cases where the typical white nodules normally soon in the intestine of sheep with coccidiosis are not present. However, the sheep have every appearance of coccidiosis including histological changes consistent with the disease.
Hand feeding may be the source of a disease problem. Mycotoxins may be present in grain or hay. The degree of spoilage need not be high to produce toxic levels. The fungi of stored feed which Blood, Henderson and Radostits (1979) list as being toxic to sheep are Myrothecium spp. in hay which cause gastrointestinal haemorrhage. Fusarium spp. in barley which are oestrogenic and Penicillium spp. of 'Stored feeds' which are tremorgenic.
Infectious diseases which are related to hand feeding are Toxoplasmosis and Listeriosis. Toxoplasmosis is spread on feed contaminated with cat faeces and Listeria is associated with silage feeding. Both diseases cause abortions and each can cause nervous disease. My limited experience with abortion storms caused by either agent has been in ewes which are being hand fed at joining.
One of the main sources of concern to sheep owners during drought is the failure of weaner sheep to thrive. There is rarely any single significant disease entity which is the obvious cause of the problem. There is usually one or more of the diseases mentioned above present, but weaners are also very difficult to hand feed - they do not readily take to supplements, and almost all rations available to weaners during droughts contain insufficient protein for weaner growth.
When irrigation water is available, many sheep owners grow forage crops which will produce large volumes of feed quickly from a relatively small area and small volume of water. It is probably Murphy's Second Law that any plant which has any grazing value is also toxic, and these forage crops are no exception. The crops which are most used are millet and sorghums for summer growth and oats for autumn and winter. Sorghums contain cyanogenetic glycosides during the young rapidly growing stages and following checks in growth, millet and oats can cause photosensitisation, and all three contain toxic nitrate levels if sufficient nitrogen fertilizer is added. Careful grazing management of these crops with particular reference to the stage of growth can prevent loss.
Forage crops can also produce gut conditions which lead to enterotoxaemia, and once again vaccination is essential.
During severe droughts there are many disease problems which beset sheep. Although the overall incidence of disease is usually lower in dry times, the lowered metabolic condition of undernourished sheep exaggerates the impact of a disease incident on a mob. Most of the problems which occur are difficult to treat properly, but they can usually be easily prevented. The secret to maintaining sheep health during drought is careful planning and good management.
REFERENCE: Blood, Henderson & Radostits (1979) Veterinary Medicine (5th Edition)