These types of plants are from the plant family Alliaceae.

>Alliaceae, family of flowering plants in the order Asparagales, with about 30 genera and more than 670 species, distributed throughout most regions of the world, except for the tropics, Australia, and New Zealand. Members of the family have corms, bulbs, or underground stems; most have long, thin leaves and clusters of varying numbers of flowers. The genus Allium contains the common onion (A. cepa), wild onion (A. cernuum), garlic (A. sativum), wild garlic (A. ursinum), leek (A. porrum), chives (A. schoenoprasum), and shallot (A. ascallonicum). Many other onion- or garlic-scented species are cultivated as ornamental border plants, such as A. moly, A. carinatum, and A. pulchellum of Europe, as well as A. stellatum and A. textile of North America. ([Encyclopedia Brittanica][1])

Many of these foods are not good for your cat or dog's health and can actually damage a pet's health, causing a type of anemia, from a  condition known as allium toxicosis. 

>Allium species toxicosis is reported worldwide in several animal species, and 
the toxic principles present in them causes the transformation of hemoglobin into methemoglobin, consequently resulting in hemolytic anemia with Heinz body formation. (1)

Onion, garlic, and many variegations of the onion plant  contain sulfoxides and disulfides, onion has higher concentrations. These chemical cause the red blood cells in cats to break up, which can result in hemolytic anemia. This type of anemia can make a cat fatigued, affect a cat's physical and mental performance and is potentially life threatening.

The signs of toxicity are:  

> **CLINICAL SIGNS**  
> In dogs and cats, clinical signs of Allium  species
> toxicosis may appear within one day of  consumption if large amounts
> of material have  been ingested. However, it is more common  the
> development of clinical signs after a lag of  several days. The
> first signs are usually of  gastroenteritis: vomiting, diarrhea,
> abdominal  pain, loss of appetite, depression and dehydration.  It
> will take a few days for the dog to display the  signs associated with
> the loss of red blood cells:  pale mucous membranes, rapid respiratory
> rate,  difficulty to breathe, lethargy, dark colored urine  (reddish
> or brown), jaundice, weakness, and rapid  heart rate. Depression,
> hemosiderin in urine,  exercise intolerance and cold sensitivity may
> also  be observed. In cases of recent ingestion, the  breath of
> the affected dog or cat may smell like  onions or garlic. (1)

There is not treatment for the toxic effects of feeding your pet foods from the onion family, animals are treated with intravenous fluids and may require blood transfusions.

A full list of the plants associated with allium species toxicity is here:  
![enter image description here][2] table courtesy of Allium species poisoning in dogs and cats, Salgado BS, Monteiro LN, Rocha NS (1)



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References:

- Allium species poisoning in dogs and cats  
 Salgado BS, Monteiro LN, Rocha NS [PDF][3] (1)

- [Foods that are dangerous for your cat][4]   
 VCA Animal Hospitals

- [DennyVet.com][5]


  [1]: http://www.britannica.com/EBchecked/topic/16346/Alliaceae
  [2]: https://i.sstatic.net/N9dVQ.png
  [3]: http://www.scielo.br/pdf/jvatitd/v17n1/02.pdf
  [4]: http://www.vcahospitals.com/main/news/pet-news/foods-that-are-dangerous-for-your-cat/508
  [5]: http://www.denneyvet.com/INFONEW.html