Emerging threats at the borders

Three pathogens are pushing into new territory this year: screwworm from the south, a tick-borne blood parasite moving west, and a fast-evolving PRRS variant in the Corn Belt. Alongside them, drug-resistant parasites keep eroding the tools small ruminant producers depend on. I thought a brief synopsis of where things currently stand across all four issues might be useful.

New World Screwworm returns to Texas

New World screwworm (Cochliomyia hominivorax) is back on U.S. soil. On June 3, 2026, USDA confirmed larvae in the navel wound of a three-week-old calf near La Pryor, Texas, the first U.S. livestock case in about 60 years. As of October 22, USDA had confirmed 61 animal cases in Texas and New Mexico since June, in cattle, goats, sheep, horses and dogs. That total includes the first U.S. wildlife case, a hunter-harvested elk on a Pecos County game ranch. Fly-trap detections are counted separately. Cases picked up again in October after a summer lull, and on October 24 USDA confirmed four more: two cattle in Hudspeth County, Texas, and two animals, including a ram, in Doña Ana County, New Mexico. The U.S. eradicated the fly in 1966 with sterile insect releases, but cases in Panama climbed sharply after 2023 and spread north through Central America into Mexico by late 2024.

A 2026 review in PLOS Neglected Tropical Diseases links the resurgence to illegal cattle movement, clearing of forest for pasture, rising cattle density, and a warming climate that lets the fly reclaim its historic range. The authors also stress the human health risk. Modeling in Scientific Reports shows suitable habitat across much of the southern U.S., overlapping areas with large cattle and horse populations.

The practical message is simple. Females lay eggs only in open wounds, so castration, dehorning, branding, calving navels and even tick bites are entry points. Inspect animals often, treat wounds promptly, and report any suspected maggot infestation to your state veterinarian.

References

  1. Of cattle and humans: A narrative review on the re-emergence of the New World screwworm (Cochliomyia hominivorax) and its threat to human health. PLOS Negl Trop Dis. 2026. doi:10.1371/journal.pntd.0014727.
  2. The reemergence of the New World screwworm and its potential distribution in North America. Sci Rep. 2025;15:23819. doi:10.1038/s41598-025-04804-9
  3. Congressional Research Service. New World Screwworm: Background and Issues for Congress (R49124). August 11, 2026.
  4. Entomological Society of America. Special Collection: Re-invasion of the New World Screwworm in Central and North America.
  5. Screwworm cases reach 61, including first wildlife detection in Texas. Houston Public Media. October 22, 2026 (updated October 23).
  6. 4 new screwworm cases confirmed in Hudspeth, Doña Ana counties. Border Report (KTSM). October 26, 2026.

Theileria orientalis Ikeda: A tick-borne anemia moving west

Bovine theileriosis has gone from a novelty to an established problem in parts of the South and Midwest. The cause is the protozoan Theileria orientalis, Ikeda genotype, first identified in the U.S. in a Virginia beef herd in 2017 and described in Emerging Infectious Diseases. It has since been confirmed in at least 16 states, following the spread of the invasive Asian longhorned tick (Haemaphysalis longicornis). That tick reproduces without mating, so one female can found a population. Missouri now reports cases in 59 of its 114 counties and considers the disease endemic. An interesting article related to the speed with which this tick can spread is detailed in a Bovine Veterinarian article entitled, “They Exploded Overnight: East Tennessee Producer Battles Asian Longhorned Tick Outbreak.”

The parasite infects red and white blood cells, causing anemia, lethargy, labored breathing, jaundice and abortion. It is easily mistaken for anaplasmosis but differs in important ways: it affects cattle of any age, mortality is usually lower, though it can be substantial in naive herds, and it does not appear to respond to antibiotics. Recovered cattle stay carriers. It also spreads mechanically on shared needles and surgical tools.

There is still no approved treatment in the U.S., but a vaccine option arrived in September 2026. Medgene’s prescription platform vaccine targets the parasite’s major piroplasm surface protein (MPSP). It is a killed product given as two doses four weeks apart, sold only through a veterinarian with a valid VCPR, and it does not protect against anaplasmosis. Platform vaccines are licensed without the field efficacy data required for conventional vaccines, so set expectations accordingly. Control still relies on whole-herd tick control, pasture and brush management, a new needle for every animal, disinfected instruments, testing purchased cattle, and blood testing to confirm suspect cases.

References

  1. Oakes VJ, Yabsley MJ, Schwartz D, et al. Theileria orientalis Ikeda genotype in cattle, Virginia, USA. Emerg Infect Dis. 2019;25(9):1653–1659. doi:10.3201/eid2509.190088.
  2. Dinkel KD, et al. A U.S. isolate of Theileria orientalis, Ikeda genotype, is transmitted to cattle by the invasive Asian longhorned tick, Haemaphysalis longicornis. Parasit Vectors. 2021.
  3. Loftin K. Bovine Theileriosis Update. University of Arkansas Cooperative Extension Service; 2026.
  4. Carrico J. Theileria spreads in the U.S. DTN/Progressive Farmer. March 2026.
  1. Stump Denton A. New cattle vaccine offers first line of defense against Theileria and Asian longhorned tick. Bovine Veterinarian. September 24, 2026.
  2. Stump Denton A, Bickelhaupt H. They exploded overnight: East Tennessee producer battles Asian longhorned tick outbreak. Bovine Veterinarian. June 23, 2026.

PRRSV variant 1C.5.32 takes over the U.S. herd

One PRRS virus variant now dominates U.S. swine production. Variant 1C.5.32 belongs to the L1C.5 lineage, often typed as RFLP 1-4-4, and began spreading quickly through the Midwest in late 2024. The Swine Health Information Center now calls it the most concerning strain in circulation, with Iowa and Minnesota at the center of activity. The L1C.5 lineage made up more than 62% of wild-type PRRSV detections reported in 2026 through June 19, and 1C.5.32 alone hit 411 newly affected sites in the six months before the June 2026 variant report.

Losses are heavy. In herds that were AASV status IA or IB before the outbreak, median losses with 1C.5.32 reached 3,545 piglets per 1,000 sows, compared with 1,245 for outbreaks caused by the older L1A lineage. A challenge study in Viruses confirmed that the L1C.5 variant is highly virulent and may transmit more readily than other lineage 1 isolates.

The lineage keeps changing, with new variants first detected in Arkansas, North Carolina and Pennsylvania over the 2025–26 winter. Tighten biosecurity against both airborne and mechanical introduction, sequence outbreak isolates, and follow the Morrison Swine Health Monitoring Project’s monthly variant reports for regional trends.

References

  1. In vivo and in vitro characterization of the recently emergent PRRSV 1-4-4 L1C variant (L1C.5) in comparison with other PRRSV-2 lineage 1 isolates. Viruses. 2023;15(11):2233.
  2. Swine Health Information Center. SHIC monitoring emerging PRRSV variant 1C.5.32 causing industry concern. July 8, 2026.
  3. Morrison Swine Health Monitoring Project. National Surveillance Report: PRRSV Variants Under Monitoring. University of Minnesota; updated monthly.
  4. PRRSV 1C.5 rapidly expanding, diversifying across U.S. swine herd. National Hog Farmer. February 25, 2026.

Dewormer resistance in sheep and goats: New evidence, same message

Drug-resistant barber pole worm (Haemonchus contortus) remains the leading parasite threat to sheep and goats. A study of 46 farms across eight southern states, Puerto Rico and the U.S. Virgin Islands found resistance widespread enough that the authors recommended routine resistance testing on every small ruminant farm. Worldwide, resistance to benzimidazoles, macrocyclic lactones and levamisole is now documented in many nematode species, most often Haemonchus, Trichostrongylus and Teladorsagia.

Two 2026 papers add new angles. In Pathogens, researchers genotyping the benzimidazole resistance mutation (F200Y) found homozygous resistant worms in 60% of sheep and about half of the roe deer sampled. In BMC Veterinary Research, a Brazilian team tracked benzimidazole and levamisole resistance markers every six months after deliberately replacing a flock’s resistant worms, testing whether drug susceptibility can be restored.

For producers, the advice holds steady. Confirm drug efficacy with a fecal egg count reduction test. Deworm selectively using FAMACHA scores rather than treating the whole flock. Leave some animals untreated to keep susceptible worms in the population, and use pasture management to cut exposure.

References

  1. Howell SB, Burke JM, Miller JE, et al. Prevalence of anthelmintic resistance on sheep and goat farms in the southeastern United States. J Am Vet Med Assoc. 2008;233(12):1913–1919.
  2. Detection of Benzimidazole-Resistant Haemonchus contortus in Domestic and Wild Ruminants in Bosnia and Herzegovina. Pathogens. 2026;15(1):113.
  3. Resistance reversion after Haemonchus contortus replacement in a sheep flock: monitoring of benzimidazole and levamisole resistance-associated polymorphisms and impact on lamb survival. BMC Vet Res. 2026;22:91.
  4. Anthelmintic resistance in gastrointestinal nematodes of sheep and goats: a systematic review. Adv Vet Res.

Drafted with assistance from Claude (Anthropic); reviewed and verified by the author.