calendar icon 3 December 2018
clock icon 6 minute read

Sometimes reputable medicinal products, which normally at correct dosages may be highly beneficial, cause a toxic or allergic reaction. This may be because they are given to the wrong species by accident, e.g. cross contamination of feed, administration of too high a dose, or are given with another medicine with which they are incompatible. They may cause a hypersensitivity reaction in an individual. If the medicament causing the problem is in the feed, analysis of feed samples should confirm the tentative diagnosis. If the feed is suspected random samples should be retained in case a liability dispute arises. (See chapter 15 Sampling Feeds )


This medicine (Mecadox, Pfizer), is used as a growth promoter at a level of 10-20ppm (10-20g/tonne) and also in the treatment and prevention of swine dysentery at 50ppm. Poisoning may commence at 200ppm in a mild form and acute disease at 300ppm or more, usually because mistakes have been made in mixing it in feed. At the lower level of 200ppm there is reduced feed intake and growth rate, hard faeces and excessive hair growth. Incoordination, posterior paralysis and death occur after 5 to 10 days of continuous consumption. There is no antidote. At post-mortem examination lesions may be found in the kidneys and adrenal glands.


Furazolidone is a member of a group of medicines called the nitrofurans that have been used over many years for treating enteric and respiratory diseases.

The medicine is used orally for treating piglet diarrhoea and by in-feed medication to control post-weaning enteric disease. Levels of up to 400ppm cause no problems. At levels above 500ppm inappetence and mild nervous signs are seen, progressing to ataxia, fits, coma and death. The medicine is fairly quickly excreted from the system and pigs return to normal 4 to 5 days after withdrawal. Newborn piglets become depressed, hypothermic and lay on their sides, paddling and frothing at the mouth. These effects are most likely to occur if the medicine is given immediately at birth rather than 6 to 7 hours after colostrum intake.

It is common to dose piglets at birth to prevent diarrhoea. Those with low birth weight (< 800g) are susceptible to poisoning


This substance is used as a growth promoter and marketed under the trade name Romensin (Elanco). It is also an effective medicine in the treatment and prevention of swine dysentery. There is a narrow gap between therapeutic and toxic levels. The maximum therapeutic level in swine is 50mg/kg or 100g to the tonne. Clinical signs are seen when levels are twice this and heavy mortality occurs when they reach 400g/tonne. Most poisonings occur in pigs either through the accidental addition of romensin or where the calculated inclusion level has been incorrect.

Clinical signs

Clinical signs of poisoning usually occur within 12 hours of intake and include heavy and difficult breathing, frothing around the mouth, loss of use of the hind legs and generalised muscle weakness. Diarrhoea may also be seen. The toxic effects of monensin are exaggerated if either of the antibiotics salinomycin at 60ppm or tiamulin at 100ppm are also contained in the feed. Some affected pigs, particularly those that have lost the use of their hind legs may take two to three weeks to recover but many of these die.


This is based on the clinical signs and post- mortem lesions, both of which are characteristic, together with the history of the use of monensin. The lesions observed at post-mortem examination include pale areas in the muscles of the diaphragm, thighs, lower shoulders, back, ribs and heart necrosis (with equal distribution on each side). The bladder may contain red-brown urine. Feed samples should be examined for the presence of monensin. A test for this is fairly quick and straight forward. Contamination of pig feed is not uncommon because the medicine is used widely as a growth promoter in cattle and if it occurs in the presence of tiamulin or salinomycin can become toxic.


There is no antidote for monensin poisoning.


This is used as a growth promoter and to control post-weaning diarrhoea. The margins between therapeutic and toxic dose are narrow. The normal level is up to 100ppm but 300ppm or more will give rise to inappetence and incoordination with progressive loss of limb function as the dose rate is increased.


Penicillin and in particular the combination of procaine penicillin and benzathine penicillin occasionally causes vomiting in individuals or small groups of pigs when given by intramuscular injection. This is particularly marked in piglets less than ten days of age where an acute sensitivity reaction sometimes occurs. Within 3 to 4 minutes of injection piglets collapse lay on their side paddling and frothing at the mouth. Fortunately this episode passes off within thirty minutes with most pigs returning to normal. Long-acting injections of penicillin given to weaners will also cause a proportion of pigs to vomit but this is of no consequence.


Salinomycin is used as a growth promoter in pigs at levels of between 30-60ppm. It should not be used simultaneously with therapeutic levels of tiamulin in feed, water or injection and there must be at least seven days between the last exposure to salinomycin and the commencement of medication at treatment level with tiamulin Studies have shown that at the preventive level of 30-40ppm no interaction occurs between tiamulin and salinomycin. (See monensin). Poisoning is associated with a marked drop in feed intake and growth rate and sudden death.

There is no antidote.


High levels of sulphonamides form crystals in the kidneys and cause damage but poisoning is uncommon because when high levels are inadvertently mixed in feed the pigs will often reject it. Some sulphonamides and in particular sulphadimidine are excreted in the faeces and urine and are recycled back into the pigs again. If recycling occurs during the withdrawal period tissue residues will appear at slaughter. In consequence some countries have either banned or restricted its use. Other sulphonamides used for treatment do not recycle to the same extent.


This antibiotic is used extensively to treat swine dysentery and enzootic pneumonia. Levels of up to 120ppm have no toxic effects on the pig but above 200ppm there have been reports that the skin may react with contaminated faeces or urine, with marked red discoloration. After prolonged use bleeding into the muscles may occur along with increased mortality. The maximum recommended level of the use of tiamulin in the UK is 100ppm and in other countries 220ppm.

Tiamulin should not be used in conjunction with monensin because it enhances the toxicity of the monensin or salinomycin. (See monensin and salinomycin). There is no antidote.

Emily Houghton

Editor, The Pig Site

Emily Houghton is a Zoology graduate from Cardiff University and was the editor of The Pig Site from October 2017 to May 2020. Emily has worked in livestock husbandry, and has written, conducted and assisted with research projects regarding the synthesis of welfare and productivity of free-range food species.

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