Hairy caterpillars, bird-borne bacteria, and flying foxes share something in common – they all pose serious threats to the lives of horses.
How do we know these things? The stories behind these discoveries are testament to the complex detective work behind scientific research, rivalling that of a good Sherlock Holmes mystery. It’s not a one-person show though, requiring considerably more teamwork than the patient services of a long-suffering Dr Watson.
One of the most committed investigators is veterinary pathologist Kristen Todhunter. She first started hunting for causes of equine foetal loss in 2004, following two unusual outbreaks in the Hunter Valley in the Australian state of New South Wales. She spent the next 10 years, she recalls, “attached to a microscope … with circles around my eyes from staring into it so much.”
Working at a company called Vetnostics, in Newcastle, NSW, Todhunter and her colleague Angela Begg could not detect any distinct pathology that would normally be expected with sudden pregnancy losses.
“It just looked like acute abortions that were all happening within mares in the same paddocks,” she says.
The first abortions were early-term, but then more advanced pregnancies started ending. Todhunter explains that it appeared that multiple mares had been exposed to some kind of mystery agent all at the same time, but chronic inflammation affected more developed foetuses over an extended period until they could no longer survive and were expelled.
The changes took place in the foetal membrane, or “amnion.” The working group developed the case definition, Equine Amniotis and Foetal Loss (EAFL), and published a 2009 paper to describe it.
Testing for causes of failed pregnancies is a process of elimination. Investigations first aim to rule out equine herpesvirus (EHV) which is highly contagious and causes an “abortion storm,” triggering every mare in a paddock to lose her growing young.
After that – funds permitting – foetal tissue is tested to search for other viruses and is cultured under a microscope. Altogether, Todhunter and Begg tested 85 foetuses and stillbirths throughout 2004.
When they cultured the stomach contents and lungs of the aborted foetuses, they found a clue, a common factor that linked them: environmental bacteria – coryneform and gram negative rods – that are not usually seen in foetal tissue.
Armed with this insight, they joined forces with a working group comprising veterinarians and scientists, including an epidemiologist and reproduction specialist, to search for more clues. Todhunter says they were “trying to rule out all sorts of plant toxins, any sort of poisoning, viruses, anything we could”.
Their investigations eventually led them to a condition with a similar pathology, labelled Mare Reproductive Loss Syndrome (MRLS). This was first observed in 2001 and 2002 during an alarming increase in horse abortions in Kentucky, in the US. Subsequent research associated MRLS with a heavy infestation of Eastern Tent Caterpillars (Malacosoma americanum).
This was another clue. The discovery led our scientists to Australia’s processionary caterpillar, known as the bag-shelter moth (Ochrogaster lunifer) or colloquially as the itchy-grub, and the less common white cedar moth (Leptocneria reducta).
The caterpillars grow in nests that resemble huge tents in the branches, or at the base, of trees.
Each caterpillar sheds its outer covering, or exoskeleton, up to eight times. Once the larvae leave the nest, the structure falls apart and the discarded exoskeletons are dispersed by the wind and environment.
Discovering that allowed Todhunter and her colleagues to finally start unravelling the mystery.
Todhunter embarked on a PhD to investigate further, teaming up with Judy Cawdell-Smith from the University of Queensland (UQ), supervised by the university’s most senior equine expert, Wayne Bryden.
“Standing in a field of native grass in country NSW,” she says of her mentor, “he not only understood my need to find out why but also understood why we should find out how.”
The researchers challenged various groups of pregnant mares with whole caterpillars, or caterpillar exoskeletons, and compared the results with a control group, conducting thorough post-mortems “from head to toe.”
Todhunter says their experiment “definitively proved that the processionary caterpillar exoskeletons as well as the whole caterpillars can cause equine abortion.
“They caused acute abortion, chronic abortion, and “everything in between.”
She explains that the two million hairs on each caterpillar – known as setae – are like little barbs. They discovered that the hairs migrate rapidly through a mare’s intestines and into the uterus and placenta. And it doesn’t take much.
“If you put your finger and thumb together in a circle, if you fill that space with caterpillar exoskeleton, which isn’t a huge amount from a nest, that will cause abortion,” she says.
The caterpillar’s exoskeleton is made of nature’s most abundant substance, chitin – which, in turn, is broken down by several species of bacteria.
Todhunter speculates that these microbes wreak their havoc after following the setae into the uterus. Once there, the placenta and foetus lack any immune defence to prevent the bacteria proliferating.
Although caterpillar-induced horse abortions in New South Wales have been intensively studied, the phenomenon is not restricted to the area.
It was a rude shock for Heather Paix at the Patchwork Park Equestrian Centre in Snake Valley, Victoria, last year when one of her mares lost its foal six weeks before it was due.
Paix says she has had horses “forever”. Before moving to Victoria she had been based in South Australia, and encountered no problem. Snake Valley, however, is in a declared area of processionary caterpillars – something she didn’t know.
She and her partner decided to breed. “And of course it went horribly wrong,” she recalls. Locals told her “good luck with that, because you won’t get past the caterpillars.”
It’s awful, she says, to lose a foal after all that effort.
Paix’s property is covered in black wattle trees, one of Australia’s natives that attract the caterpillar. She describes the purse-like sack that holds them as a “horror movie silk thing that looks like it’s got something very demonic within it”.
She and her partner have dedicated considerable efforts this year to felling the trees. In the meantime, she says, they theorised that if the mares didn’t eat grass, they wouldn’t eat the caterpillar setae. They took this season’s mares off grass six months into their pregnancies and fed them hay. She is jubilant to report one foal on the ground, and two more due in January, so far holding well.
Horse breeding season is a nerve-wracking time, says Kate Williams of Coffs Harbour, NSW. A show horse breeder, she lost several foals to caterpillars after moving to land containing black wattle trees. Beck Groen from Miner’s Rest in Victoria has lost three foals this season. She has one left and is “medicating the mare like a crazy woman praying I don’t lose her foal”.
“It’s devastating,” she says. She “found a tonne of the hairy little pricks,” and is certain the caterpillars killed her babies.
Up to 10% of mares “slip”, or lose their pregnancy, in Australia each year, causing immense financial and personal loss to breeders and horse owners. Equine pregnancy veterinarian, Joan Carrick of Equine Specialist Consulting in Scone, country NSW, explains that “you have to wait over 11 months to get the foal and there’s often a fairly big investment for anyone. And then there’s this emotional investment.”
If a mare loses her pregnancy or the foal is born dead, Carrick says it’s very distressing for small breeders. In the horse racing industry, she adds, pregnancies can be worth over $1 million each, so it’s no small loss financially.
“Even in the industry I work in, for the people who are doing it as a profession, it’s an emotionally devastating experience,” she says. “It’s just awful, they feel dreadful. So apart from the financial cost, it’s just sad.”
Back in Newcastle, when Todhunter discovered the caterpillar link, she found it accounted for 32% of all abortions in the region. She suspects the incidence is actually higher, because horse owners often can’t afford to submit all equine pregnancy losses for examination. Other known causes of pregnancy loss include congenital abnormalities, problems with the umbilical cord or cervix, twins, stress and trauma.
Just this year, researchers at the University of the Sunshine Coast (USC) in Queensland stumbled upon another potential trigger of failed horse pregnancies in regional NSW.
Microbiologist Martina Jelocnik’s research focus is Chlamydia pecorum, a pathogen that infects Australia’s iconic koala populations. In 2014, a related bacterium, Chlamydia psittaci, was discovered as the source of a spate of respiratory illnesses among people working in a NSW veterinary school.
All the victims had been exposed to abnormal foetal membranes from a mare. This was surprising, because birds are the common carriers of C. psittaci. This was the first recorded transmission of the bacterium from horses to humans in Australia.
Nobel Prize winning virologist Sir Frank Macfarlane Burnet first identified C. psittaci in Australian birds in 1930, with occasional spillovers reported to humans since then.
The parasite is an extremely successful organism, according to Jelocnik. It is an intracellular pathogen that can thrive in a broad variety of hosts, including birds, mammals, and livestock.
In 2005, a high incidence of C. psittaci was reported in aborted foetuses from mares in Hungary. The USC group and officers from the Department of Primary Industries decided to test for this bacterium in equine pregnancy losses in NSW.
During 2016 they tested tissues from 199 foetuses on 54 properties. They were surprised to find C. psittaci in 21.1% of foetal and placental tissues, and 23.7% of foals with compromised health. All the foetuses had signs of severe systemic disease. Equine herpes virus accounted for 4.5% of abortions, and 28% of EAFL cases. Most of the mares were concentrated in an area about 170 kilometres inland from the mid-coast of NSW.
Using molecular DNA studies, Jelocnik confirmed the C. psittaci was identical to that found in some species of Australian parrots, such as crimson rosellas and galahs.
“We know that this chlamydia is a notorious bird pathogen,” she explains. Infected birds present with lethargy, wasting and respiratory disease. In humans, its effects can range from flu-like symptoms to pneumonia, and may require hospitalisation.
The study by Jelocnik and her colleagues was the first to show that C. psittaci might cause horse abortions in Australia. Moving forward, she says, multi-level investigations are needed to understand and address the problem, uniting researchers from microbiology and genetics to epidemiology and public health, onsite veterinarians, and primary industry.
Could this bacterium be causing pregnancy losses in other regions?
“Absolutely, yes,” Jelocnik says. “Because the parrots are all over Australia. So it could be contributing to foal deaths and people aren’t aware of it.”
The USC chlamydia team has now developed a rapid test that will help veterinarians test for the bacteria, and add to the evidence that is needed to identify its causal role, its molecular structure, ecological distribution and the risk it poses to horses and humans.
Chlamydiosis in horses manifests as pneumonia. But the processionary caterpillars don’t pose any grave risk.
“EAFL doesn’t affect the mare side of the placenta,” Todhunter explains, adding that the horse expels any inflammation along with the foetus or foal. And although they can cause itchiness, the caterpillars don’t pose a serious health threat to humans, either.
Possibly the most fateful threat to horses and humans comes from flying foxes, or fruit bats. These endearingly ugly – and ecologically important – critters from the genus Pteropus are the primary prey of the Hendra virus, of the family Paramyxoviridae. It can be passed onto horses, and there have been isolated human cases following contact with infected animals.
Although evidence suggests it is transmitted via aborted fruit bat foetuses, Andrew Breed, a veterinary scientist at the University of Queensland, says there is no clear evidence linking bat-borne diseases to mare abortions. But a Hendra virus infection is a death sentence for horses.
“If a pregnant horse was infected with Hendra virus it is unlikely either the mare or foal would survive the initial disease,” he says. And if they do, current policies require them to be euthanised because of the potentially life-threatening risk of infection in humans.
People are most at risk for infection by Hendra or the bird-borne C. psittaci through exposure to body fluids of infected horse tissue. This is particularly a risk for vets.
Todhunter says sensible precautions such as wearing gloves and masks should be observed. And while more research is needed to identify high-risk habitats for birds carrying C. psittaci, horse breeders can take steps to prevent caterpillar infestations. Methods include being aware of trees in the area, and grazing mares well away from them.
For Carrick it’s all about the foals. “I love my job, I really love what I do,” she says.
Specialising in high-risk thoroughbred pregnancies, she treasures the satisfaction of delivering a newborn foal. “Healthy foals are so cute,” she laughs. “And it’s even more fun to see them go to the race track and win.”
Natalie Parletta is a freelance science writer based in Adelaide and an adjunct senior research fellow with the University of South Australia.
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