Additional testing has confirmed that an imported thoroughbred mare in the Waikato region is positive for Theileria equi, a protozoan that causes piroplasmosis.
Theileria equi is one of the two protozoa known to cause the blood infection.
It is the first case of equine piroplasmosis recorded in New Zealand, and was found during routine health testing required before the horse was to be shipped to Australia to be mated with a stallion there.
The horse had been imported under New Zealand protocols last year from France as a maiden mare and has been resident in the Cambridge area since arriving in February 2019.
She had been bought at auction in Europe in December 2018. She was negative for piroplasmosis in a pre-sale test, was negative before going into quarantine, and was negative under New Zealand protocols, before departing from Britain.
Biosecurity New Zealand and the Ministry for Primary Industry are investigating in the expectation it will be an isolated case.
Testing began on Friday morning of all mares at the affected stud. It is understood that the mare has had direct contact with a very small number of horses.
Equine piroplasmosis is spread by ticks. Although New Zealand has ticks, they are not the species known to transmit the protozoa.
The disease is not directly infectious between horses, but in rare instances it can also be transmitted between animals through contaminated medical equipment, such as needles or other invasive items.
Infected animals present with severe acute disease characterized by high fever, lethargy, poor appetite, swelling in the extremities, an enlarged spleen, destruction of red blood cells resulting in anaemia, a rapid heart rate, urine discoloration, and occasionally death.
Animals who recover from a primary infection remain recessive carriers, with fluctuating levels of the parasite. Infectious carriers are not always symptomatic.
The disease is widely distributed in Asia, Europe, Africa and South America. It causes economic loss and affects international horse movements.
It is understood that the positive mare has not shown any signs of ill health while in New Zealand.
Some countries that import horses from New Zealand, including Australia, require certification that New Zealand is free of piroplasmosis.
The positive test result means that authorities cannot currently provide that assurance.
For this reason a shipment of horses to Australia that was scheduled for the evening of May 19 was put on hold and a later shipment was also delayed.
Officials have been working closely with Australian authorities to agree on alternative assurances to allow exports to continue.
Good progress is reportedly being made on the development of a testing regime to support this.