Parasites in deer stomachs are known as gastrointestinal (GI) nematode parasites. Some nematode parasites are specific just to deer but some affect deer, sheep and cattle. They are sometimes known as 'worms' or ‘gutworms’ and include roundworms, stomachworms and nematodes.
Table A: Deer Parasites, courtesy Merial.
Ostertagia - Type
These parasites are the most significant species in deer.
*Probably the most widespread and representative of the Ostertagia types found.
Cattle/Sheep parasites which appear to be a well adopted to deer.
True deer parasite.
True deer parasite.
May be found in deer. Possibly opportunists available through mixed grazing with sheep and cattle
What are the symptoms?
How does the problem spread?
Parasites move from paddock to paddock when their hosts (such as deer) are shifted and eggs are excreted with faeces onto the new pasture. GI parasites can also over winter on pastures. However warmth and high humidity are required for larvae to develop. The life cycle of GI parasites start when eggs are shed in deer faeces. The eggs hatch and develop on the pasture through to a larval stage. They are then ingested by the deer and continue to develop in the gut (although winter can arrest their development for a period).
Moderate to severe burdens of immature parasites in the lining of the abomasums (fourth stomach) can lead to reduced acid production. This causes raised pH and poor protein digestion, resulting in chronic weight loss. Parasitised deer will fail to thrive, have reduced growth rate and shows signs of a rough coat, soft faeces, soiled tail and diarrhoea. Severe GI parasitism will be fatal.
Low levels of protein in the blood can sometimes result in oedema (fluid collecting) under the jaw. This is referred to as bottle jaw.
Effect on deer production
This effect can be very serious, especially in Wapiti or young deer. Adult hinds can be affected when put under stress and experience weight loss. Stags can be affected after the stress of the rut. There appears to be breed differences in susceptibility to GI parasites - Fallow deer are relatively immune/resistant to GI parasites while Wapiti deer are more susceptible . Red deer sit in between
Diagnosis of GI parasitism in a live animal is difficult. This is because the relationship between faecal egg counts and total worm numbers is not well understood. The most risky stage for the deer tends to be when parasites are immature and lodged in the stomach lining. However, as they are not shedding eggs at this point, then their presence is not identified in a faecal egg count. For this reason faecal egg counts are not reliable. Faecal egg counts may be useful in fawns but only in autumn prior to them developing immunity to the parasites. In weaners visual signs of infection such as loss of condition and weight gain that is lower than expected can be a useful indicators. The lining of the abomasum is thickened and pitted and has a “Moroccan leather” appearance on post mortem.
Control and Treatment
Treatment with anthelmintics is complemented by pasture management and grazing management. For example, as Wapiti are more susceptible to parasitism they could be grazed on longer pastures than other deer, to reduce the chance of them picking up larvae off the pasture.
To help deer naturally resist the effects of parasitism, good nutrition is required (both good quality and high availability). Pastures high in legumes or chicory can provide good nutrition.
Which stock to treat
Once a problem is either suspected or predicted (either through a faecal egg count and/or visual signs, past experience of a block’s history or time of the year) then control should follow. Young deer are susceptible in autumn, winter and into early-spring.
Wapiti bulls are susceptible in late autumn/winter. . Wapiti cows are at more risk in summer/autumn while lactating. It is recommended that Wapiti have regular anthelmintic treatment targeting known times of stress
- Best practice recommendation is to use a Triple Combination drench. ML Injection (Cydetin or Exodus) at 1ml per 50kg and an oral combination of Oxfen C Plus and Oxfen C mixed in equal parts and given at 1ml per 5kg.
- Take care when using this recommended combination:-
- Mix and Use Basis. The mixture of equal parts Oxfen C Plus and Oxfen C should be on a mix and use basis. The two oral drenches are compatible and unused mixture will be fine in the short term but aim to minimise the amount not used at any one drenching episode. As with any anthelmintic the mixture should be shaken well before use.
- Default Withholding Time. The mixture of equal parts Oxfen C Plus and Oxfen C when given at 1ml per 5kg is off label use of these anthelmintics. (The dose rate of oxfendazole exceeds label dose and no levamisole containing products are registered for use in deer). A default withholding time of 91 days applies.
- Toxicity. High doses of levamisole are toxic to livestock including deer. Care must be taken to ensure the mixture of Oxfen C Plus and Oxfen C is in equal parts i.e. one for one. Dose to the heaviest in the mob butcare must be taken when there is a large variation of liveweight within the mob. Adjust the dose rate to suit.
Table B: Anthelmintic option
White drenches (benzimidazoles)
Mectins and milbemycins
Products registered for deer
Oxfen C (oxfendazole)
Bomatak C (oxfendazole)
Albendazole C /Valbazen (albendazole) but may be unavailable
Cydectin and Exodus (moxidectin)
Bomectin, Ivomec, Noromectin & Virbamec (ivermectin)
Baymec, Genesis, Paramectin Oral, Paramectin Injection (abamectin)
Route of administration
Pour-on Do not use
Very short (gone in 24 hours)
Varies between active but likely to be moxidectin> abamectin> doramectin?> ivermectin> eprinomectin, although is formulation-dependent. Many farms now show limited to no persistance
Oxfendazole/albendazole > fenbendazole
moxidectin> abamectin> doramectin?> ivermectin> eprinomectin
Target stage of nematode life cycle
most effective against adult worms and less effective against immature worms
Frequency of administration
Drench regularly at intervals inside pre-patent period i.e. 21 day intervals. MUST use at 21 day intervals in autumn
Injection and Orals: Possibly use at 4 - 5 week intervals
Pour-on: Do not use
7-14 days for registered drenches
As directed by the packaging on formulations registered for deer (default 91 day period if not registered) NB default will apply to all oral and all injectible anthelmintics save Cydectin injection for which if you get a prescription from your vet this allows for treatments 49 days from slaughter. (Must talk to your vet).
Need to start early in risk period to avoid build up of parasites
Dose response trials have not been carried out on many formulations not registered for deer therefore sheep/cattle dose rates used by default but may not be optimal
NB: No injectible or oral ML formulations are registered for use in deer
Preventing worm resistance
Massey University tested the efficacy of pour-on Moxidectin, pour-on Ivermectin and oral Ivermectin in weaners on the Massey deer research unit in 2005. All drenches achieved 99.98% or better kill of lungworm and large intestinal worms, but efficacy against abomasal parasites was variable and as low as 12% for oral ivermectin. This research was based on worm counts. Faecal egg and larval count reduction tests seem less reliable in deer than other species. This is the first research report of drenches failing in deer.
Since then every deer farm investigated to date has shown some level of resistance of gut worms to ML drenches. Due to the cost of slaughter trials and total worm counts this has been less than twenty farms but due to past drench practices the issue of drench resistance is likely to be widespread. Drench resistance is a huge risk for the deer industry as there are fewer drench options for deer.
Strategies to prevent worm resistance
- Avoid high risk drench and stock management practices
- Do not under-dose
- Do not use Pour-on
- Do not drench adult deer routinely
- Quarantine drench all incoming deer
- Avoid finisher only deer blocks
- Do not use long acting drenches eg capsules or long acting injection
- Create a refugia of drench susceptible worms
Preventing the dominance of drench-resistant parasites on pastures is important. One option is to keep a number of healthy deer in a mob undrenched. They are then likely to shed non-drench resistant parasites onto pasture, diluting the proportion of resistant parasites. This is known as refugia. Avoid drenching deer immediately prior to putting them on ‘clean’ pastures as the deer will shed only resistant worms creating a population of resistant worms on these ‘clean’ pastures.
2. Use a triple combination drench
- A triple combination has been shown to be the only effective way to control worms when drench resistance is present
- When resistance is present injectable moxidectin is more effective than oral moxidectin
- Combination drenches delay the onset of resistance
- Best practice recommended Triple Combination drench is:- ML Injection (Cydetin or Exodus) at 1ml per 50kg and an oral combination of Oxfen C Plus and Oxfen C mixed in equal parts and given at 1ml per 5kg.
Take care when using this recommended combination:-
Mix and Use Basis. The mixture of equal parts Oxfen C Plus and Oxfen C should be on a mix and use basis. The two oral drenches are compatible and unused mixture will be fine in the short term but aim to minimise the amount not used at any one drenching episode. As with any anthelmintic the mixture should be shaken well before use.
Default Withholding Time. The mixture of equal parts Oxfen C Plus and Oxfen C when given at 1ml per 5kg is off label use of these anthelmintics. (The dose rate of oxfendazole exceeds label dose and no levamisole containing products are registered for use in deer). A default withholding time of 91 days applies.
Toxicity. High doses of levamisole are toxic to livestock including deer. Care must be taken to ensure the mixture of Oxfen C Plus and Oxfen C is in equal parts i.e. one for one. Dose to the heaviest in the mob butcare must be taken when there is a large variation of liveweight within the mob. Adjust the dose rate to suit.
Developing an animal health plan with a veterinarian can help ensure that parasites are being controlled through several on-farm approaches, in an integrated way.