Parasites: Tales of Humanity's Most Unwelcome Guests: January 2017

Tuesday, 31 January 2017

Is Your Cat Causing Your PMS? Actually, No

Late January 2017 brings a flurry of online articles about cats and premenstrual syndrome (PMS). Cats, it seems, cause PMS, or severe PMS, or a cat parasite causes PMS, or a cat parasite makes PMS worse, or women infected with the parasite are more likely to have PMS. Once again, cat owners are looking askance at their furry friends and wondering if it’s time to switch to a goldfish.

All this results from an article published in the Journal of Clinical Medical Research in October 2016 (online August 30, 2016): “Toxoplasma gondii Infection and Premenstrual Dysphoric Disorder: A Cross Sectional Study.” The authors of the study conclude that infection with the parasite might be associated with more severe symptoms of premenstrual dysphoric disorder, and that further research would help confirm or disprove the link. Do they think cats cause PMS? No.

PMS Is Not the Same Thing as Premenstrual Dysphoric Disorder

According to the Mayo Clinic, 75% of menstruating women experience PMS to some degree. It brings both physical and emotional symptoms ranging from fatigue to depression. This is not premenstrual dysphoric disorder, and it isn’t the group that the researchers studied. They report that only 1.3% to 8% of women experience premenstrual dysphoric disorder, a severe form of PMS, which has serious psychological symptoms. Thus, the study had nothing to say about the PMS that the majority of women experience, and did not link PMS to Toxoplasma gondii (or cats!).

Are Women Infected with Toxoplasma gondii More Likely to Have PMS?

Toxoplasma gondii forms microscopic cysts
in the tissues of cats, humans, and other hosts

The study didn't address this question; however, in this study group, the reverse appears to be true for premenstrual dysphoric disorder. The authors report that 10 out of the 151 women subjects tested positive for either antibodies to the parasite, or the parasite itself. That's 6.6%

The prevalence of infection in humans varies widely from country to country, however in Mexico, where this study was done, 15% to 50% of the general population tests positive, depending on the region. There’s no evidence here that infection causes premenstrual dysphoric disorder.

Does Toxoplasmosis Make Premenstrual Dysphoric Disorder Worse?

The researchers looked at 59 distinct signs and symptoms experienced by women suffering from premenstrual dysphoric disorder. Their results suggested that the parasite might make two of them worse: feeling overwhelmed, and feeling out of control.

These results are preliminary at best. The sample size of 151 women was small, and there was low prevalence of T. gondii infection in the group. It’s inaccurate to draw conclusions about all women with both toxoplasmosis and premenstrual dysphoric disorder based on 10 women. It’s possible the parasite is unfairly maligned here, let alone the cat.

If You Are Infected With Toxoplasma gondii, Did You Get it From Your Cat?

Infective oocysts of Toxoplasma gondii
contaminate the environment via cat feces

Cats, and cat litter boxes, can be the direct source of T. gondii in humans, but there are quite a few other ways to catch toxoplasmosis. The most common source of infection varies from place to place, but most of us are more likely to catch it by eating undercooked meat or contaminated food or drinking contaminated water.

That’s not to say there isn’t a cat involved in the story somewhere because infected cats do cause contamination of food and water, but it’s not really fair to draw a straight line from a house cat to its owner. Feral cats and wild felines play a significant role.

Things to Remember About Cats and Toxoplasma gondii

A cat with toxoplasmosis will only pass oocysts in its feces for a couple of weeks when it’s first infected. For the rest of its life, the cat has the parasite but isn’t infectious (unless you eat your cat without cooking it first; I doubt this happens often).

Cats that don’t go out are less likely to become infected with T. gondii. Cats, too, catch it from eating raw or contaminated food (mice etc.).

Cats often become infected with
Toxoplasma gondii by eating infected prey

Is T. gondii a cat parasite? Cats are the only animals that host the sexually reproductive stage of the parasite (and pass oocysts). Hundreds of other species of animals host the parasite in tissues, and are infectious if eaten without thorough cooking.

If T. gondii does turn out to cause worse symptoms in premenstrual dysphoric disorder, should we blame the unfortunate cat, caught between a ubiquitous parasite and people who have spread millions upon millions of feral cats all around the globe? I think that’s misleading, to say the least.

Screwworm Fly Eradication in Florida

Update: On March 23, 2017, the USDA Animal and Plant Health Inspection Service announced that screwworm has once again been eradicated in Florida. The release of sterile male screwworm flies continues to be an effective means of controlling this pest.

January 20, 2017

Since the spring or early summer of last year, Florida has been fighting an invasion of screwworm flies. The problem began in the Florida Keys, with Key deer being the animals worst affected, and a recent report of an infected dog in Homestead, Florida, tells us that it’s not over. This isn’t the first time screwworm has troubled Florida: history tells us it’s an old and familiar enemy.

Decades of Battling the Screwworm Fly

Animal and Plant Health Inspection Service

United States Department of Agriculture. "Slide of infested lamb."

Special Collections, USDA National Agricultural Library

Accessed January 20, 2017

The gruesome habits of the New World screwworm fly, Cochliomyia hominivorax, make it an unwelcome insect everywhere it occurs. The parasitic and carnivorous maggots of the screwworm fly are responsible for illness and death in animals and humans alike, and the pest used to cause millions of dollars of lost revenues in American agriculture every year.

In the late 1930s Edward Knipling, an American entomologist, realized that C. hominivorax had a weak spot, a vulnerability that might be exploited to control or even wipe out the fly. The female of the species will mate only once in her lifetime. Knipling thought that if he could somehow manipulate things so that matings were unsuccessful, the screw-worm fly would disappear.

The Sterile Fly Technique for Screwworm

Unknown. “Photograph of Dr. Edward F. Knipling (seated)

and Dr. Raymond C. Bushland.”

Special Collections, USDA National Agricultural Library.

Accessed January 20, 2017

Subsequent to Knipling’s epiphany, Raymond Bushland successfully raised screwworm flies in the laboratory, making it possible to do research on them, and Hermun Muller used radiation to render the male flies sterile. The sterile males remained otherwise healthy and mated with female flies, but no offspring resulted. Thus, these scientists were able to break the life cycle of the screwworm fly, at least in the lab. The knowledge for an eradication program was there; all they needed was a plan and the resources to carry it out.

The first area to be tested with a screwworm fly eradication plan was southern Florida in the United States—it was warm enough there for C. hominivorax to survive the winters and spread north in the warmer months, but geographically isolated enough to be a good place for a trial run. Between 1957 and 1959 hundreds of millions of sterile male screwworm flies were released in Florida, Georgia, and Alabama. It worked. By mid 1959, there were no screwworm flies in Florida.

Screwworm Fly Eradication

Unknown. [1984].

Progress in Screwworm Eradication, 1957-1984 .”

Special Collections, USDA National Agricultural Library.

Accessed January 20, 2017

Efforts to wipe out the fly were then focused on California, Arizona, and Texas. This project was repeatedly thwarted, mainly because it was impossible to prevent the reintroduction of the fly across the lengthy United States-Mexico border. Realizing that the Isthmus of Tehuantepec in Mexico might provide a northern limit for the fly that would be easier to maintain, the eradication effort incorporated northern Mexico, and the effort was finally successful. By about 1985, all of Mexico north of the Isthmus of Tehuantepec, and the Continental United States - was free of C. hominivorax.

Screwworm fly eradication was so successful that it was worth continuing the effort. The US, in cooperation with Central American governments kept pushing southward, and by 1996, the screwworm fly had retreated to the Panama-Columbia border, there to be held back by the continual release of sterile male flies. Having perfected fly rearing and eradication methods, North American specialists were also able to come to the aid of Libya when the North American screwworm was accidentally introduced there in 1988.

The good news about the current problem in Florida is that we know how to deal with it. No doubt the release of sterile males will once again free the state of this horrifyingly destructive pest.

Tuesday, 31 January 2017

Is Your Cat Causing Your PMS? Actually, No

PMS Is Not the Same Thing as Premenstrual Dysphoric Disorder

Are Women Infected with Toxoplasma gondii More Likely to Have PMS?

Does Toxoplasmosis Make Premenstrual Dysphoric Disorder Worse?

If You Are Infected With Toxoplasma gondii, Did You Get it From Your Cat?

Things to Remember About Cats and Toxoplasma gondii

Further reading