Cattle Breeding for Tick Resistance

Natural animal resistance has the biggest economic impact on tick control. Breeding for more resistant animals via tropical breed selection, or by within breed selection provides a low cost, permanent solution to ticks.

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).

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.

Breed selection

Breed differences

All modern cattle breeds descend from two ancient lines, Bos taurus and Bos indicus, which humans domesticated more than 10,000 years ago.

Bos indicus or Zebu breeds:

• Adapted to tropical environments due to southern Asian origin.
• Milk producing breeds include Sahiwal and Red Sindhi.
• Beef production breeds include Brahman and Nellore.

Bos taurus breeds:

• Adapted to temperate environments due to British and European origin.
• Milk producing breeds include Holstein-Friesian and Jersey.
• Beef production breeds include Angus, Hereford and Charolais.

Tropically adapted taurine breeds:

• Retain some of the productive attributes of Bos taurus breeds, but are better adapted to tropical environments.
• Southern African Sanga breeds Afrikaner, N’guni and Tuli.
• West African humpless breeds like N’dama,
• Criollo breeds of Latin America and the Caribbean such as Romosinuano.

Breeds adapted to tropical environments perform better than temperate breeds under a wide range of tropical environmental stressors including resistance to cattle tick. Bos indicus breeds show the greatest resistance to ticks followed by tropically adapted taurine breeds then theEuropean Bos taurus breeds.

In temperate environments there are substantial productivity differences (growth, milking ability, reproduction, product quality) between different cattle breeds. However in cattle grazed on pasture in tropical environments, the differences in performance are generally masked by the effects of environmental stressors on those productive attributes.

In the tropics, comparisons of performance should be made across general breed types or groupings (Bos taurus – British and European; Bos indicus; and tropically adapted taurine) rather than across specific breeds.

Any breeding program designed for cattle grazed at pasture in tropical environments should consider the impacts of both productive traits (growth, fertility, meat quality) and adaptive attributes (resistance to environmental stressors such as heat, drought and parasites), even though these traits are generally difficult to measure.

Choice of breed

Unless there are contrary economic reasons, it is strongly recommended that breeders in tropical and sub-tropical regions should make their selection of preferred breed (or breed type or composite breed) from amongst the wide range of tropically adapted breeds simply because it is much simpler and less expensive to use cattle breeds that are suited to the environments in which they are expected to perform, than to modify the environments to enable poorly adapted animals to perform to their potential. Contrary economic reasons to this recommendation could include for example the availability of significant price premiums for products such as meat or milk that have special attributes (e.g. high levels of marbling in Bos taurus beef breeds or higher milk production from Bos taurus dairy breeds). Even if those contrary economic reasons exist, economic modelling should be undertaken to determine whether it would be more cost-effective to re-locate the farm business to a more suitable environment, rather than modifying a tropical or sub-tropical environment, or incur the expense of a tick management program, to accommodate poorly adapted cattle.

Other factors impacting on cattle resistance to ticks

There are a number of additional animal and environmental factors which impact on resistance of cattle to ticks that should be considered during the design of any cattle breeding program that measures cattle tick resistance. These factors are briefly summarised under ‘Susceptibility to ticks’.

Within breed selection

Breeding cattle that are better adapted to parasites can reduce the reliance on chemicals and other management interventions, and improve productivity and animal welfare.

Heritability

To genetically improve any economically important trait through breeding, the trait(s) being selected for need to be under some degree of genetic control i.e. they must be heritable.

Heritability (H) of a trait is defined on a scale of 0 – 1 where:

H = 0 the trait is not heritable, but based on environment.

H < 0.1 there is low heritability of the trait.

0.1 < H <0.4 the trait is moderately heritable (Resistance to cattle tick falls here for beef and dairy cattle)

H > 0.4 the trait is highly heritable.

Resistance of cattle to ticks is moderately to highly heritable so it is possible to directly improve resistance of cattle to ticks through crossbreeding and within-breed selection. Preliminary evidence suggests that resistance to cattle tick may also provide resistance to other ticks.

Identify resistant and susceptible animals

To breed for any economically important trait, breeders should record all animals within a contemporary group (i.e. those that have been managed together, ideally since birth or weaning) for the trait of interest (in the case of tick resistance, undertake a tick count or a tick score – see sections ’How to count cattle tick’ and ‘How and when to tick score’). Animals can then be ranked on the basis of lowest (most resistant) to highest (most susceptible) tick count, see Figure 1.

For genetic improvement purposes, this ranking is best done in conjunction with Estimated Breeding Values (EBVs) as calculation of EBVs also accounts for genetic linkages or relationships and other effects impacting on different animals in the contemporary group and therefore provides a more accurate assessment of the ranking of those animals. Although more research is needed, resistance to most environmental stressors appears to be largely independent of productive traits such as growth, reproduction and product quality.

The process to assess resistance of cattle to ticks is the same for all cattle breed types (beef and dairy cattle and across Bos taurus and Bos indicus breeds). Cattle need to be exposed to ticks and allowed time to acquire their level of resistance before assessments of tick resistance are undertaken. Cattle which are reared on pasture in tick-endemic areas can be assumed to have acquired their resistance to ticks by the time they are weaned at around 6-9 months of age. Weaning tends to occur around the start of the tropical ‘dry season’, to ensure cows are not lactating during periods of nutritional stress. The start of the ‘dry season’ also tends to coincide with the start of cooler months when tick infestations are lower and less reliable. Hence it is recommended that assessments of cattle tick resistance be undertaken from the commencement of warmer periods in post-weaning cattle (e.g. in the southern hemisphere that would equate to calf weaning from May to July and tick resistance assessments undertaken from August or September, when the calves are 9-12 months of age).

Using genomic information to improve selection for tick resistance

As increasing genomic information becomes available for cattle it has become clear that resistance to cattle tick is not controlled by a simple presence or absence of a single genetic trait. Instead 100’s to 1,000’s of genes appear to be involved. Breeding values for cattle tick resistance have not yet been achieved. The resistance status of a very large number of cattle populations need to be determined first. These populations are currently being set up for study in Australia and several other countries.

Assuming the measurement of host resistance becomes more feasible, future animal breeding approaches using genomic information should be able to improve host genetic resistance to cattle ticks.

Using other traits for indirect within breed selection

• Direct genetic control refers to traits (such as tick resistance) that are heritable.
• Indirect genetic control refers to traits that associate with other heritable traits, they are inherited together.
• Cattle tick resistance can be selected for indirectly by finding other traits that associate with it such as low rectal temperature under heat stress, or low worm counts.

To improve traits through breeding, the traits being selected must be under direct or indirect genetic control. Direct genetic control is assessed by estimating the heritability of traits. Indirect genetic control relies on the heritability of the traits as well as the favourable or unfavourable associations (genetic correlations or co-heritabilities) between different traits. The ‘Within breed selection’ and ‘How and when to score’ sections outline the processes needed to genetically improve tick resistance using direct genetic control methods, based on the moderate to high heritabilities estimated for cattle resistance to ticks.

Selecting cattle directly for their resistance to ticks is generally the easiest and preferred method of achieving genetic improvement, but might be enhanced by indirect selection. This section describes the possibilities of using indirect selection for resistance to ticks in cattle.

In tropical composite beef cattle grazed at pasture in the tropics, genetic correlations (associations) have been found between tick and worm counts, as well as tick count and rectal temperature (Figure 1). Lower rectal temperature has also been favourably associated with production measures such as weight gains, birth weight, pregnancy rate and days to calving. A study that selected for higher growth rates in cattle resulted in improved resistance to ticks in the tick-susceptible cattle but not the tick-resistant cattle.

These correlated reductions in tick counts as a result of selecting animals with lower EBVs for rectal temperatures suggest that an alternative approach to genetically improving tick resistance might be to select cattle for reduced rectal temperatures under conditions of heat stress. It can be concluded that selection to improve resistance to any one stressor of tropical environments will improve resistance to other stressors. This is particularly true for resistance to ticks, worms and heat stress, where genetic correlations have been consistently moderately positive, suggesting the same or closely-linked genes affect all three traits.

With the exception of heat stress, resistance to ticks and most environmental stressors appears to be largely independent of productive traits such as growth, reproduction and product quality.

Estimated Breeding Values (EBVs) for tick resistance

As described in the ‘Within breed selection’ section, EBVs are more accurate assessments of an animal’s ranking (relative to other animals in the cohort group) for tick resistance. However due to the difficulties of measuring tick resistance using either tick counts or tick scores (see the ‘How and when to tick score’ section), EBVs are not currently included in routine genetic evaluations for either beef or dairy cattle in any country. Research is underway to develop simpler and more cost-effective measures of tick resistance. In the meantime though, if breeders wish to evaluate their cattle for tick resistance, it is suggested they should discuss the options and opportunities with their genetic service provider (e.g. BREEDPLAN in Australia) and/or their cattle breed society.

Keep ticks and tick borne diseases out

Ticks spread diseases and cause damage during feeding. So it is important to keep them under control. But not all ticks are the same. Although all ticks go through the same life cycle stages (egg to larva to nymph to adult), they differ in that some ticks live on only one host, whereas others may need multiple hosts to complete their life cycle. The ticks that are of concern to livestock owners are cattle ticks, paralysis ticks and bush ticks. These ticks vary in their distribution across Australia, and some are easier to control than others. It is important to be able to identify the different types of ticks.

Cattle tick control

Cattle tick are a 1-host tick meaning they live their whole life cycle on the one host. This makes it possible to eradicate cattle ticks. Bush ticks and paralysis ticks are 3-host ticks and each life cycle stage can live on a different host, including wildlife, so with them it is about control not eradication.

Keeping cattle tick out of your herd starts with making sure that animals don’t stray off the property and pick up tick larvae somewhere else. Secure boundary fences are essential to stop cattle straying on or off the property. It is also important to make sure that any animals you bring onto your property are not carrying cattle tick. Cattle, buffalo and deer are the animals that cattle tick likes to live on but they can also be found on sheep and goats, horses and camelids in small numbers. Any new animals should be treated before introduction to the property.

Fortunately cattle ticks mostly occur in the higher rainfall areas of northern Australia (Qld, NT and WA), they are not normally found in more temperate areas in southern Australia.

Cattle tick is notifiable in all the non-endemic areas (south of the tick line) which means that when you suspect or detect cattle tick you must notify a government biosecurity officer so that they can be eradicated from the herd and to protect neighbouring herds as well.

In areas in northern Australia where cattle tick are endemic you should vaccinate the herd against tick fever which is spread by cattle tick and can kill cattle quickly. Some breeds of animal are more susceptible to ticks than others.

See also how to manage other hosts of cattle tick.

Bush and paralysis ticks

Controlling bush tick and paralysis ticks is harder because wildlife can easily move across boundary fences and drop ticks onto the ground during travel. In addition, each life stage of a 3-host tick only spends around 5 to 7 days on the host animal, so ticks can be easily be missed if cattle are only inspected occasionally.

Paralysis ticks produce a toxin during feeding that causes muscle paralysis that requires an antidote treatment from a veterinarian.

Bush ticks can carry a blood parasite called Theileria which can cause anaemia in cattle and has been seen in many areas of southern Australia particularly in introduced cattle.

Controlling these ticks needs a combination of regular treatments to kill any ticks on the animal and avoiding grazing animals in paddocks with ideal habitat for wildlife such as undergrowth or blady grass pasture at times when these ticks are active.

It is sound biosecurity practice to quarantine introduced animals to prevent pests and diseases entering the property in a separate area for 3 weeks before they are mixed with the other animals on the property. During this time they should be inspected for ticks on several occasions and treated with tick killing chemicals to reduce the likelihood that they will bring in chemically resistant ticks, or ticks that carry diseases such as tick fever, or Theileria.

How to manage other hosts of cattle tick

To keep cattle tick out of your herd it is important to make sure that any other animals on your property are not carrying them. Cattle ticks prefer to live on cattle, however, buffalo, bison and deer can also be heavily infested. To prevent cattle tick associated with other hosts from impacting your property you should:

• Secure boundary fences to stop infested cattle and feral animals from straying onto your property and dropping seed ticks onto the pasture.
• Include feral deer control in your cattle tick management plan, if they are a problem, and time their control with the cattle tick season.
• Treat any new animals prior to their introduction to the property.
• Inspect and treat low risk hosts that share pasture with your cattle. This includes horses, camels, goats, sheep, llamas, vicuña, guanacos and alpacas which may carry cattle tick in low numbers.