As we brace for another single-digit weekend in southeast South Dakota, I’ve got Florida on my mind. It might be due to the fact that I just booked my flights to Orlando for the National Pork Industry Forum and the American Association of Swine Veterinarians Annual Meeting in March. The sunny 80-degree forecast for the “Theme Park Capital of the World” this Saturday does sound pretty nice.
But most likely, the “Sunshine State” is on my mind, because it is just one of the many warm, arid places in the United States where Ornithodoros soft ticks reside. As many in the swine industry are familiar, that particular arthropod can play a role in transferring African swine fever, and ever since my conversation with Peter Fernandez about Spain being the first documented case where ASF was associated with ticks, it got me thinking … if ASF should cross our border, what role would the soft tick play here?
The infectious disease epidemiologist says this is an area we should consider.
“I don’t think it is something that should be a priority, of all the risk pathways that we have, but it should be on the list of things we need to look at as a potential factor for ASF spread and maintenance in the U.S., if it should get in,” Fernandez says.
Historically with ASF, its translocation from one place to another has almost always been associated with the movement of infected pork products, but Fernandez, a former Animal and Plant Health Inspection Service veterinarian, says soft ticks can play a role in very special circumstances if the disease gets into a country.
ASF is only transmitted by one genus of soft ticks and these ticks are very different from hard ticks. They usually dwell in nests and burrows. They feed very quickly, for only about 30 to 60 minutes at a time on their host, before hiding again. Ornithodoros soft ticks are the only known transmitters of ASF, can expand five to 10 times their original size and the females lay multiple batches of eggs.
This genus of ticks is also known for being very hardy — going months between feedings — and for their enormous longevity. The life cycle of a soft tick can be 10 to 20 years. For example, the Plum Island Animal Disease Center maintained a soft tick colony there for more than 30 years and all they did was feed them once every six months.
While it has been demonstrated that the Ornithodoros tick can transmit ASF in the lab, the conditions in the field are much different.
“As much as these ticks have some host preference, their real preference has to do with their habitat,” Fernandez says. “These ticks will feed on any number of different species, even chickens, but they have to have a habitat that allows them to survive.”
Through studies in the 1980s into the 1990s, it was determined that four species of Ornithodoros ticks found in the “New World” can become infected with ASF. Lab investigations done at Plum Island found U.S.-based O. coriaceus, O. parkeri and O. turicata were capable of becoming infected with ASF, and all except O. parkeri were competent vectors in transmitting the virus to naïve domestic swine. According to the researchers in “A Review of African Swine Fever and the Potential for Introduction into the United States and the Possibility of Subsequent Establishment in Feral Swine and Native Ticks,” O. turicata is concerning because it is found in Arizona, California, Colorado, Florida, Kansas, New Mexico, Oklahoma, Texas and Utah — states that also provide suitable habitat for large numbers of feral swine. The researchers say this presents an opportunity for the maintenance of ASF in the event of a viral incursion with the involvement of feral swine
In addition to the species found in the United States, O. puertoricensis is found in the Caribbean, specifically Jamaica, Haiti, the Dominican Republic and Panama, and can be infected with and transmit ASF to susceptible domestic swine. This species could also be troublesome as the border between the United States and Mexico could provide a bridge for the soft tick vector to enter the States.
Fernandez believes the soft tick species the United States also must be concerned about is O. coriaceus, which is predominantly found in California and down into Mexico. It is associated with hosts such as mule deer and range cattle, and usually resides in more open locations such as deer beds around scrub oak and brushy regions.
“The question becomes could that be a risk for feral swine?” Fernandez says. “Feral swine are not like wart hogs, they don’t take shelter in burrows. Feral swine in the U.S. lie up and out in the open and are not as predisposed to those soft ticks that are usually found in burrows, nests and crevices. Having said that, we do have one tick associated with scrub oak and deer beds, so the question we need to look at is what are the risks for feral swine if they enter these habitats.”
While researchers focused their attention on those four species of Ornithodoros ticks, Fernandez says he has put together an additional list of other Ornithodoros ticks that call the United States home. The majority of those ticks are associated with bats and bat caves and are probably not going to get in contact with feral swine, but Fernandez says it’s worth examining if these soft ticks can also carry the disease. He has sent this list for review to the curators at the U.S. National Tick Collection at Georgia Southern University.
It is clear that wild boar are playing a role in ASF maintenance and spread in eastern Europe. With the feral swine population increasing across Europe, in some cases by as much as 700%, and a renewed interest in hunting and maintaining game environments, Fernandez says we need to think about the U.S. feral swine population as it continues to grow.
“Since the majority of our swine are raised in intensified production in housing with solid biosecurity, they are not going to come into contact with feral swine,” Fernandez says. “The question becomes if the disease would come into the United States like it did with Belgium, it just jumps in there and gets into your feral swine population, how long will it be in that population and what measures would you have to take so it is not a risk to your domestic production?”
The United States also needs to bear in mind the feral swine population is constantly on the move and could encounter some of these tick species.
“Ticks are not likely to be the major way that this disease is transmitted and maintained, but interestingly enough they could play a role we haven’t thought of, as the Spanish found out in their ASF saga,” Fernandez says.
While Spain’s situation was very different, with housing of some pigs in stone houses where ticks like to burrow and access to wild swine, that is not the situation here in the United States. Our porcine management systems eliminate a number of risk exposure pathways.
“By eliminating risks of exposure to disease, we have eliminated risk of exposure to ectoparasites as well,” Fernandez says. “But again, many people think about the feral population which is growing all the time in the U.S. It’s definitely something to dive into.”