One out of four humans will die because of a sexually transmitted disease. This stark statistic reminds us how crucial it is to protect ourselves, yet many overlook the fact that we’re not the only species battling STIs. In the animal kingdom, a surprising array of creatures harbor their own version of the 10 sexually transmitted infections, ranging from familiar viruses to bizarre cancers. Below we dive into the wild world of these infections, ranking them from ten down to one.
10 Sexually Transmitted Infections Overview
10 Papillomavirus
Some marine mammals are celebrated for their intelligence and even for rescuing humans from shark attacks, but they also have a knack for picking up genital warts caused by papillomavirus. The most frequently observed cases appear in Atlantic bottlenose dolphins, where unprotected mating leads to the same sort of warts humans get from HPV.
Among mammals, dolphins are prime candidates for hosting STIs, and the rise in waterborne pollutants has weakened their immune defenses, making them especially vulnerable. This environmental stress has turned the dolphins into a living laboratory, showing scientists how similar papillomaviruses affect both humans and their cetacean cousins.
Interestingly, while these dolphins develop noticeable genital warts, they rarely progress to cancer—a stark contrast to the oncogenic potential of certain human HPV strains that can cause cervical cancer in women.
There is currently no evidence that dolphin‑derived genital warts have leapt to humans, yet researchers continue to monitor the distinct viral types present in dolphin populations, keeping an eye on any possible cross‑species transmission.
9 Syphilis
Fluffy, rapidly reproducing rabbits are more than just cute companions; they also serve as carriers of a syphilis‑like disease that shares a name with the human infection. Each year, roughly twelve million new human cases surface, making the study of animal syphilis valuable for both veterinary and public health.
The bacterium Treponema paraluiscuniculi responsible for rabbit syphilis differs from the human‑infecting strain and cannot be transmitted from rabbit to person. Nevertheless, large rabbit populations have become infected, and the disease spreads vertically—from mother to offspring during pregnancy or birth.
In New Zealand, the dual surge of human syphilis and infections in white‑tailed rabbits has drawn scientific attention. Historically, African baboons experienced a syphilis outbreak in both wild and captive settings during the late twentieth century, though the strain was distinct from the human version and has since dwindled to only a few cases.
Just as antibiotics can clear syphilis in humans, timely veterinary treatment can cure infected rabbits, underscoring the importance of early detection across species.
8 Herpes
Herpes isn’t a human‑only problem; it lurks throughout the animal kingdom—from elephants and reptiles to fish and even oysters. Primates, including rabbits, can host herpes viruses that may jump to humans via bites or scratches, and the reverse transmission has also been documented.
Baboon herpes strains differ from the human simplex virus, yet they can still be transferred to people through bite wounds. Researchers note that each primate species harbors its own herpes variant, with baboon strains resembling those found in monkeys more than those in chimpanzees or gorillas.
Scientists speculate that the close similarity between human and chimpanzee herpes viruses points to a common ancestor millions of years ago. In Africa, baboons are currently battling a baffling herpes outbreak that has infected roughly two hundred individuals; while no human cases have been confirmed, the potential for cross‑species spread remains.
7 HIV
Human immunodeficiency virus (HIV) isn’t exclusive to people; close relatives such as chimpanzees and gorillas harbor similar viruses. Recent research suggests that great apes may have been the original reservoirs of HIV, with the virus jumping to humans through activities like hunting and meat consumption.
Gorillas and humans share about 98 percent of their DNA, and a particular strain of HIV circulating in Cameroonian gorillas affects an estimated twenty million people. While sexual contact spreads HIV among primates, the primary route to humans appears to involve blood‑to‑blood exposure—think bites or ingestion of infected meat.
A study of West African chimpanzees revealed that roughly ninety percent tested positive for a virus closely resembling human HIV, indicating active transmission within ape populations. Curiously, infected chimpanzees seldom develop AIDS‑like symptoms, despite their genetic similarity to us.
6 Brucellosis
Brucellosis, also known as undulant fever, ranks among the most common sexually transmitted ailments in the animal world, affecting everything from livestock to wild deer. Its prevalence threatens animal health and can also spill over to humans, leading to chronic complications.
In domesticated herds and free‑ranging deer, the disease spreads primarily through sexual contact, but cross‑species transmission can occur via contact with infected tissues, meat, or milk. Humans contract brucellosis by consuming contaminated dairy products, handling infected animals, or through direct contact with bodily fluids.
Although only about one hundred to two hundred human cases surface each year—rarely fatal—the infection can cause a persistent fever, vomiting, fatigue, diarrhea, and even visual disturbances. Untreated animals suffer more severe outcomes, underscoring the need for rapid veterinary intervention.
While sexual transmission between humans is uncommon, individuals who engage in intercourse before recognizing infection may inadvertently spread the disease to partners, necessitating medical follow‑up to prevent chronic progression.
5 Feline Immunodeficiency Virus
In 2017, nearly a million humans died from AIDS‑related illnesses, prompting questions about why HIV devastates people but not other primates. Cats face their own AIDS‑like foe: feline immunodeficiency virus (FIV), which spreads through bites, scratches, birth, and even semen.
FIV originates from the same viral family as HIV, yet it remains confined to felines. Unlike HIV, which mutates extensively, FIV comprises only five known strains that cause AIDS‑type disease in cats, and a vaccine is available for feline use.
Both viruses lead to severe immune suppression and can be fatal if untreated. The existence of a vaccine for FIV—absent for HIV—highlights differences in viral evolution and mutation rates between the two species.
4 6
Among insects, the DNA virus IIV‑6 infects cold‑blooded critters, especially crickets, turning a typical STI into a fertility‑crippling nightmare. Infected males experience a heightened urge to mate, which paradoxically spreads the virus to partners while rendering both sexes sterile.
This surge in mating drive leads to rapid transmission within cricket colonies, ultimately halting breeding as infertility spreads. The longer an individual remains infected, the greater the chance of death from the disease.
Because IIV‑6 is DNA‑based, it possesses the capacity for mutation, prompting scientists to monitor its evolution closely. Although the virus has persisted for about a decade, its prevalence continues to rise in various cricket colonies, raising concerns about potential spillover to other invertebrates.
3 Chlamydia
Chlamydia is a well‑known human STI, yet its animal counterparts present a far more stubborn challenge. The infection afflicts a broad range of species—birds, reptiles, and mammals—but in koalas it has become a major conservation threat.
Up to half of the koalas receiving care in Queensland and New South Wales test positive for chlamydia, a disease that spreads silently without obvious external signs. When the infection advances, it can cause coughing, respiratory trouble, infertility, and even death.
Although the koala‑specific strain differs from the human version, research into a koala vaccine may provide valuable insights for human vaccine development. Moreover, chlamydia can jump from birds to humans (psittacosis) and mutate when moving from birds to cats, illustrating the pathogen’s adaptability across species.
2 Venereal Tumors
While humans don’t catch cancer through sexual contact, several animal species do. Mammals, especially, are prone to venereal tumors that spread during mating. The Tasmanian devil’s facial tumor disease (DFTD) has decimated the population, killing roughly 95 percent of individuals since its emergence in 1996.
DFTD spreads when devils bite each other during copulation, transmitting malignant cells that form aggressive facial tumors. A second strain has appeared as researchers attempted to develop a vaccine, adding to the grim outlook for this iconic marsupial.
Dogs also suffer from a naturally occurring transmissible cancer called canine transmissible venereal tumor (CTVT), or Sticker’s sarcoma. Unlike the devil’s disease, CTVT remains confined to dogs and can be treated effectively with chemotherapy, showcasing a rare instance of a sexually transmitted cancer that does not cross species barriers.
1 Gonorrhea
Speculation abounds about how gonorrhea, one of humanity’s oldest and most pervasive STIs, might have leapt from cattle to people. While the disease is unlikely to be acquired through eating contaminated meat, cows are not the sole carriers; sheep, dogs, and even chimpanzees can harbor the infection.
Millions of people worldwide receive a gonorrhea diagnosis each year, making it a priority for vaccine research. The bacterium’s ability to adapt and cross species lines complicates treatment, especially as increased antibiotic use in livestock breeds resistant strains.
These resistant variants pose a heightened threat, potentially fueling larger outbreaks in both animals and humans and underscoring the need for vigilant monitoring and innovative therapeutic strategies.