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Natural tick fever immunity

Cattle can develop natural lifelong immunity to tick fever if they are exposed to enough infected ticks early in life. The age of cattle when first exposed to tick fever determines whether disease is likely to occur.

Exposing calves to enough ticks so they are infected with all 3 tick fever organisms before 9 to 10 months of age is the key to developing natural immunity to tick fever. Field research in Queensland shows that many cattle herds do not have sufficient exposure to achieve this.

Calf immunity from birth

Cows with tick fever immunity can pass temporary protection (maternal antibody) to their calves through the colostrum. This colostral protection lasts approximately 3 months.

If cows have no immunity to tick fever, their calves will also be susceptible during their first few months of life, but disease in young calves is not common in the field.

Age-related resistance

Most calves develop an age-related resistance that protects them from about 3 months until about 9 months of age. This is irrespective of breed and whether dams are immune.

Calves exposed to tick fever when the age-related resistance is present rarely show clinical signs and develop a solid, long-lasting immunity.

If immunity develops in all your calves, tick fever will not be a problem, but they must be exposed to all 3 parasites. This requires exposure to thousands of ticks because not all ticks carry the infection.

The age-related resistance gradually wanes from 9 months of age. If they haven’t been exposed to and developed immunity to tick fever, they will become highly susceptible to tick fever. If then exposed to infected ticks later in life, these cattle are likely to develop a severe, life-threatening infection.

Likelihood of developing immunity

An engorged female tick can produce more than 3,000 seed (larval) ticks, but only a very small number of seed ticks (less than 1 in 1,000) will carry the tick fever organisms. For this reason, calves do not always become infected (then protected) following exposure to ticks, even though it only takes 1 infected tick to transmit tick fever.

If you rely on natural tick exposure for long-term protection of your calves, thousands of ticks will need to bite the animals. Low tick numbers, as a result of dry seasons or tick control strategies, can delay transmission of tick fever parasites for months or even years.

More calves develop immunity to Babesia bigemina because a higher proportion of ticks carry this parasite than carry Babesia bovis. This high rate of natural infection and resulting immunity, plus the fact that Babesia bigemina is less virulent than Babesia bovis, is the reason Babesia bigemina causes relatively few outbreaks in Australia.

By contrast, Babesia bovis is transmitted more sporadically, and so weaned calves can still be susceptible as the age-related resistance wears off. Babesia bovis is a highly virulent parasite and the most common cause of disease outbreaks.

Calf exposure to Anaplasma marginale is difficult to predict. Transmission depends mainly on male ticks feeding on infected cattle and then transferring to susceptible cattle within the same life cycle.

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