Sudden death of heifers due to Tick Fever
Unusual presentation:
Eleven heifers became lethargic and weak, and then died within 24-48 hours.
Above: Post mortem found jaundice throughout the carcass, an enlarged liver and abnormal spleen.
Below: Timing of management events and deaths.
Time and location: May to July 2023, Queensland.
Case definition: Heifers that become weak, lethargic and disoriented – followed by recumbency and death within 24-48 hours.
Disease mapping: Eleven first-calf heifers, from a mob of 400, died in 9 weeks in May-July 2023. The heifers were mid-way through their calving period, grazing black soil country (with some yellow-wood present) and being supplemented with a urea-based dry lick. They were fully vaccinated for Botulism and 7in1. Most of the deaths had occurred within 200 metres of a water point. Deaths shown on timeline (above).
Gross findings: A recumbent heifer was euthanized for post-mortem examination. Jaundice was seen throughout the carcass and the blood was very watery. The liver was enlarged and the spleen looked abnormal. Samples submitted to the laboratory included: bloods (clotted and EDTA), blood smear, aqueous humour, brain smears, and fresh and fixed tissues(heart, lung, spleen, liver, kidney, small intestine, rumen and brain). The differential diagnoses considered were: tick fever (most likely based on post mortem signs), urea toxicity, Bovine Ephemeral Fever (3 day sickness), poisonous plant (yellow-wood), anthrax and TSE.
Laboratory findings: The blood and brain smears were both positive for Anaplasma marginale, confirming a diagnosis of Tick Fever.
Animal / management / environment risk factors: It is likely these heifers have had very little exposure to ticks in their lives, meaning no natural immunity to tick fever. Moving cattle to a part of the property closer to the river increases risk of tick exposure. It is possible the tick treatment in late May prevented the onset of the main outbreak until July.
Recommendations:
- While waiting for lab results, treat any other sick animals with Imidox. Because of the close proximity to soak yards, suggest mustering ASAP, very slowly (even if it takes all day to get them there) and leave stragglers behind (injecting with Imidox when you do). Once yarded, treat all for ticks to prevent further infection in healthy animals.
- To prevent further outbreaks, give animals in this mob a few weeks to recover then treat again for ticks with a long acting tick treatment and vaccinate with blood on the same day. Also implement this strategy with any other breeders that have not been vaccinated for tick fever as there is a chance the outbreak may spread.
- From this point forward, vaccinate all weaners with blood at the end of weaning.