How and when to tick score
Assessing cattle for their resistance to ticks is not a simple task. Tick resistance is based on the number of ticks that mature on an animal following natural or artificial infestation with tick larvae. The number of ticks on resistant cattle will be much lower than on susceptible cattle.
Tick scoring is an alternative to tick counting. Like tick counting, it can be used to provide a basis to estimate cattle resistance to ticks. Instead of accurately counting all of the ticks on one half of each animal’s body, animals are instead assigned a tick score. Tick scores range from 0 to 5 where 0 = very high resistance which means the animal has none to very few semi-engorged ticks between 4.5 and 8 mm diameter visible on one side, and 5 = very low resistance meaning the animal has lots of semi-engorged ticks between 4.5 and 8 mm diameter visible on one side. Within a mob the tick score describes animals within the observed range of tick infestation. Tick scoring requires less skill and infrastructure, as animals do not need to be in a crush to be scored, and consequently the rate of throughput of animals per day can be increased. The scoring method is the same for all beef and dairy cattle breeds.
For natural infestations scoring is timed to coincide with the tick season (spring or start of the wet season) to achieve the greatest variation in tick numbers across animals in a cohort group. Artificial tick infestations require tick-breeding facilities and skilled laboratory technicians to deliver specific quantities of tick larvae.
For tick scoring to be meaningful ensure sure that:
- Cattle must have prior exposure to ticks with time (at least 3 months) to acquire immunity before scoring.
- Animals in the cattle tick zone can be assumed to have acquired their resistance to ticks by the time they are weaned at around 6-9 months of age.
- Cattle being scored should be managed together as a single management group without treatment to control ticks for at least 6 months prior to tick assessment.
- Assess tick resistance at the start of the tick season (spring or start of wet season) when tick numbers are higher and weaners are 9-12 months of age.
- Nutritional stress should be minimised at the time of assessment because animals undergoing nutritional stress often demonstrate an impaired immune system.
- Only count ticks between 4.5 mm and 8 mm in size (see Figure 1 and Figure 2).
Figure 1. Count ticks between 4.5 mm and 8 mm in size. Do not count ticks smaller than 4.5 mm (as this is not a good indicator of the animal’s ability to resist ticks) or more than 8 mm (as they are fully engorged ticks which will fall off the animal within a matter of hours and counting those ticks may not be comparable across animals in a cohort group).
Figure 2. Tick-susceptible animal, showing a mix of immature, semi-mature and fully-engorged cattle ticks. Image credit CSIRO Rockhampton
Interpreting tick scores
It is important to note that tick counts or scores as a measure of susceptibility/resistance are relative. A number for a single animal is useless, whereas in a group, those animals with the least ticks or lowest scores can be deemed less susceptible (or more resistant) than those animals with the highest numbers or scores (see Figure 3). An additional difficulty is that since total tick counts are affected by a large number of factors (see section ‘Susceptibility to ticks'), the only reliable comparisons are those made among similar animal types, under the same environmental conditions, as outlined in the following practical guide:
- If tick counts/scores on all animals are very high, it is likely the animals are still acquiring resistance and if tick counts on all animals are very low, it is possible there is insufficient tick challenge to measure resistance. As a guide, in a group of at least 15 animals there should be at least 20 ticks per side of animals carrying the most ticks. If the maximum count is below 20 ticks, then the level of infestation is too low, or there is too little difference between the animals, to distinguish the resistant from the susceptible individuals. Once resistance to ticks has been acquired, tick resistance is relatively stable across time.
- Animals to be compared must be of a similar age and sex class i.e. steers should not be compared with bulls, young cattle should not be compared with older cattle and lactating cows should not be compared with non-lactating cows. Similarly tick counts on animals from one paddock should not be compared to tick counts on animals from another paddock, even if the paddocks are adjacent, because environmental factors (e.g. previous stocking rates) are likely to affect the tick populations in the different paddocks.
Figure 3. Summary of experimental results of artificially infesting Santa Gertrudis (a composite breed) with cattle tick demonstrate within breed variability in resistance. Tick counts from the most resistant and most susceptible animals in the mob are shown. Image credit Emily Piper.