Sporidesmin toxicity (facial eczema)

Sporidesmin toxicity (facial eczema)

Managing this disease involves consideration of the four factors below:

1. Detecting cases of liver damage:

  • Species – Sheep, cattle, camelids, deer
  • Specimen – Serum for GGT and GLDH
  • Container – Gel or red top tube
  • General information –  Serum GGT and GLDH rise quickly following sporidesmin injury and are almost always high when signs of liver injury (e.g. photosensitivity and jaundice) appear. At 2-4 weeks following ingestion of sporidesmin, there is a good relationship between the GGT level and degree of liver damage. Cases can be graded as mild, moderate or severe at this stage but this has not yet been proven to have prognostic potential. Testing for GLDH helps differentiate other causes of liver disease. In cases of facial eczema, GLDH levels are usually lower than GGT levels.

 2. Determining if zinc intakes are adequate for protection

  • Species – Sheep, cattle, camelids, deer
  • Specimen – Trough water, drench, feed
  • Container -Yello top pot
  • General information – Determination of zinc levels in drinking (trough) water or drenches can detect whether levels are toxic or adequate for facial eczema protection. Zinc drenches can also be tested for proper mixing of the zinc additive. Zinc in feed can also be tested. Serum zinc levels for facial eczema prevention (see Minerals section of Handbook) are general guidelines as clinical and subclinical sporidesmin toxicity can still occur with serum zinc values within this range depending on level of toxin challenge, other trace element intake e.g. Copper and other host or environment factors. Serum zinc assay is however useful in cases of suspected zinc over-dosage/toxicity – see below

 3. Diagnosing zinc toxicity

  • Species – Sheep, cattle, camelids, deer
  • Specimen – Live animal: Serum in plain or LH tube for zinc and GGT levels, EDTA for a full CBC if there is anaemia; Dead animal – Fixed abomasum, liver and pancreas for histopathology. Fresh liver and kidney in yellow top pot for zinc determination
  • General information – Diarrhoea, weight loss, and metabolic disease in cattle are the main clinical signs of zinc toxicity. A few will also develop a haemolytic anaemia. In the live animal, measuring serum zinc concentrations is a valid way to check for possible zinc toxicity. In those animals that appear anaemic or have red urine, a CBC is also recommended. Overdosing zinc can suppress copper and iron absorption.  At necropsy, the lesions to look for are abomasal ulceration and pancreatic atrophy and fibrosis. Abomasal ulceration occurs when zinc sulphate is used for drenching, as this triggers closure of the oesophageal groove, diverting the drench directly into the abomasum. For this reason, zinc sulphate drenches are not recommended. Pancreatic changes will occur with poisoning due to any form of zinc salt but are a chronic change and often hard to detect. The more common lesion is hepatobiliary necrosis due to sporidesmin toxicity. It is worth stressing that most cases of suspected zinc toxicity turn out to be sporidesmin toxicity due to under dosing with zinc.

 4. Spore counting for facial eczema

Pasture spore counting

  • Specimen – Pasture from at least 5 locations
  • Container – Paper bag
  • General information – Selecting the sampling site: This depends on the type of farm and the management policy. If all the animals are in one group, e.g. a dairy farm, only the paddock the animals will graze the next day need be sampled. If set stocking is practised, then it is necessary to sample the dangerous paddocks.

Tips to help predict the most susceptible paddocks for spore counting:

  • Spore counts on north and west facing slopes are usually higher than east and south facing slopes.
  • Flats generally have lower counts than slopes above them as cool air flows downward at night.
  • Paddocks with a lot of pasture litter and those that are well sheltered often have higher counts.

A useful system is to use a warm slope as an indicator site and to sample it regularly, at least three times a week. When spore numbers on it start to rise, the other paddocks should be checked to define the spore pattern over the farm. Using this system, a bank of information will accrue and susceptible paddocks will be identified. However, it is not safe to assume only the same paddocks will be susceptible every year.

 Method for obtaining a pasture sample for spore counting:

  • Using shears or scissors, cut a handful of pasture leaves from about 1 cm above ground level at not less than five places, which are at least 10 m apart, and submit in paper bags not plastic bags so as to avoid ” plant sweating”.
  • Avoid parts of paddocks, which are sheltered by trees or hedges. Take separate samples if you need to know spore numbers under hedges.
  • Sample from an area of even slope.
  • If you sample the same site regularly, follow the same route across it.
  • Take samples at least 3 times a week and more often if the weather favours spore production. Spore numbers will rise in the absence of rain if the weather is humid, particularly late in the season. Numbers do not always rise immediately after rain and the peak may occur up to a week after the last fall.

 Faecal spore counting

Faecal spore counting represents more accurately what the animals are ingesting and therefore more accurately reflects FE risk. Faecal samples are also easier to collect. Faecal spore numbers are  approximately double the pasture levels but this is very dependent on grazing pressure. The ratio may be less than 2:1 in cattle because of their lower faecal dry matter. As a tentative recommendation levels should be considered dangerous when faecal spore counts approach 75-100,000 spores per gram of faeces.

As you are interested in the spore intake of the herd or flock rather than individual animals, pooling of 5-10 samples is recommended. These are best pooled at the laboratory. Fresh faeces may be removed from the pasture, yards or from the rectum for this purpose.